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INTERNET-DRAFT Clifford Neuman
John Kohl
Theodore Ts'o
11 July 1997
The Kerberos Network Authentication Service (V5)
STATUS OF THIS MEMO
This document is an Internet-Draft. Internet-Drafts
are working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum
of six months and may be updated, replaced, or obsoleted by
other documents at any time. It is inappropriate to use
Internet-Drafts as reference material or to cite them other
than as "work in progress."
To learn the current status of any Internet-Draft,
please check the "1id-abstracts.txt" listing contained in
the Internet-Drafts Shadow Directories on ds.internic.net
(US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US
West Coast), or munnari.oz.au (Pacific Rim).
The distribution of this memo is unlimited. It is
filed as draft-ietf-cat-kerberos-revisions-00.txt, and expires
11 January 1998. Please send comments to:
krb-protocol@MIT.EDU
ABSTRACT
This document provides an overview and specification of
Version 5 of the Kerberos protocol, and updates RFC1510 to
clarify aspects of the protocol and its intended use that
require more detailed or clearer explanation than was pro-
vided in RFC1510. This document is intended to provide a
detailed description of the protocol, suitable for implemen-
tation, together with descriptions of the appropriate use of
protocol messages and fields within those messages.
This document is not intended to describe Kerberos to
__________________________
Project Athena, Athena, and Kerberos are trademarks of
the Massachusetts Institute of Technology (MIT). No
commercial use of these trademarks may be made without
prior written permission of MIT.
Overview - 1 - Expires 11 January 1998
Version 5 - Specification Revision 6
the end user, system administrator, or application
developer. Higher level papers describing Version 5 of the
Kerberos system [1] and documenting version 4 [23], are
available elsewhere.
OVERVIEW
This INTERNET-DRAFT describes the concepts and model
upon which the Kerberos network authentication system is
based. It also specifies Version 5 of the Kerberos proto-
col.
The motivations, goals, assumptions, and rationale
behind most design decisions are treated cursorily; they are
more fully described in a paper available in IEEE communica-
tions [1] and earlier in the Kerberos portion of the Athena
Technical Plan [2]. The protocols have been a proposed
standard and are being considered for advancement for draft
standard through the IETF standard process. Comments are
encouraged on the presentation, but only minor refinements
to the protocol as implemented or extensions that fit within
current protocol framework will be considered at this time.
Requests for addition to an electronic mailing list for
discussion of Kerberos, kerberos@MIT.EDU, may be addressed
to kerberos-request@MIT.EDU. This mailing list is gatewayed
onto the Usenet as the group comp.protocols.kerberos.
Requests for further information, including documents and
code availability, may be sent to info-kerberos@MIT.EDU.
BACKGROUND
The Kerberos model is based in part on Needham and
Schroeder's trusted third-party authentication protocol [4]
and on modifications suggested by Denning and Sacco [5].
The original design and implementation of Kerberos Versions
1 through 4 was the work of two former Project Athena staff
members, Steve Miller of Digital Equipment Corporation and
Clifford Neuman (now at the Information Sciences Institute
of the University of Southern California), along with Jerome
Saltzer, Technical Director of Project Athena, and Jeffrey
Schiller, MIT Campus Network Manager. Many other members of
Project Athena have also contributed to the work on Ker-
beros.
Version 5 of the Kerberos protocol (described in this
document) has evolved from Version 4 based on new require-
ments and desires for features not available in Version 4.
The design of Version 5 of the Kerberos protocol was led by
Clifford Neuman and John Kohl with much input from the com-
munity. The development of the MIT reference implementation
was led at MIT by John Kohl and Theodore T'so, with help and
contributed code from many others. Reference implementa-
tions of both version 4 and version 5 of Kerberos are pub-
licly available and commercial implementations have been
Overview - 2 - Expires 11 January 1998
Version 5 - Specification Revision 6
developed and are widely used.
Details on the differences between Kerberos Versions 4
and 5 can be found in [6].
1. Introduction
Kerberos provides a means of verifying the identities
of principals, (e.g. a workstation user or a network server)
on an open (unprotected) network. This is accomplished
without relying on assertions by the host operating system,
without basing trust on host addresses, without requiring
physical security of all the hosts on the network, and under
the assumption that packets traveling along the network can
be read, modified, and inserted at will[1]. Kerberos per-
forms authentication under these conditions as a trusted
third-party authentication service by using conventional
(shared secret key[2]) cryptography. Kerberos extensions
have been proposed and implemented that provide for the use
of public key cryptography during certain phases of the
authentication protocol. These extensions provide for
authentication of users registered with public key certifi-
cation authorities, and allow the system to provide certain
benefits of public key cryptography in situations where they
are needed.
The basic Kerberos authentication process proceeds as
follows: A client sends a request to the authentication
server (AS) requesting "credentials" for a given server.
The AS responds with these credentials, encrypted in the
client's key. The credentials consist of 1) a "ticket" for
the server and 2) a temporary encryption key (often called a
"session key"). The client transmits the ticket (which con-
tains the client's identity and a copy of the session key,
all encrypted in the server's key) to the server. The ses-
sion key (now shared by the client and server) is used to
authenticate the client, and may optionally be used to
__________________________
[1] Note, however, that many applications use Kerberos'
functions only upon the initiation of a stream-based
network connection. Unless an application subsequently
provides integrity protection for the data stream, the
identity verification applies only to the initiation of
the connection, and does not guarantee that subsequent
messages on the connection originate from the same
principal.
[2] Secret and private are often used interchangeably
in the literature. In our usage, it takes two (or
more) to share a secret, thus a shared DES key is a
secret key. Something is only private when no one but
its owner knows it. Thus, in public key cryptosystems,
one has a public and a private key.
Section 1. - 3 - Expires 11 January 1998
Version 5 - Specification Revision 6
authenticate the server. It may also be used to encrypt
further communication between the two parties or to exchange
a separate sub-session key to be used to encrypt further
communication.
Implementation of the basic protocol consists of one or
more authentication servers running on physically secure
hosts. The authentication servers maintain a database of
principals (i.e., users and servers) and their secret keys.
Code libraries provide encryption and implement the Kerberos
protocol. In order to add authentication to its transac-
tions, a typical network application adds one or two calls
to the Kerberos library directly or through the Generic
Security Services Application Programming Interface, GSSAPI,
described in separate document. These calls result in the
transmission of the necessary messages to achieve authenti-
cation.
The Kerberos protocol consists of several sub-protocols
(or exchanges). There are two basic methods by which a
client can ask a Kerberos server for credentials. In the
first approach, the client sends a cleartext request for a
ticket for the desired server to the AS. The reply is sent
encrypted in the client's secret key. Usually this request
is for a ticket-granting ticket (TGT) which can later be
used with the ticket-granting server (TGS). In the second
method, the client sends a request to the TGS. The client
uses the TGT to authenticate itself to the TGS in the same
manner as if it were contacting any other application server
that requires Kerberos authentication. The reply is
encrypted in the session key from the TGT. Though the pro-
tocol specification describes the AS and the TGS as separate
servers, they are implemented in practice as different pro-
tocol entry points within a single Kerberos server.
Once obtained, credentials may be used to verify the
identity of the principals in a transaction, to ensure the
integrity of messages exchanged between them, or to preserve
privacy of the messages. The application is free to choose
whatever protection may be necessary.
To verify the identities of the principals in a tran-
saction, the client transmits the ticket to the application
server. Since the ticket is sent "in the clear" (parts of
it are encrypted, but this encryption doesn't thwart replay)
and might be intercepted and reused by an attacker, addi-
tional information is sent to prove that the message ori-
ginated with the principal to whom the ticket was issued.
This information (called the authenticator) is encrypted in
the session key, and includes a timestamp. The timestamp
proves that the message was recently generated and is not a
replay. Encrypting the authenticator in the session key
proves that it was generated by a party possessing the ses-
sion key. Since no one except the requesting principal and
Section 1. - 4 - Expires 11 January 1998
Version 5 - Specification Revision 6
the server know the session key (it is never sent over the
network in the clear) this guarantees the identity of the
client.
The integrity of the messages exchanged between princi-
pals can also be guaranteed using the session key (passed in
the ticket and contained in the credentials). This approach
provides detection of both replay attacks and message stream
modification attacks. It is accomplished by generating and
transmitting a collision-proof checksum (elsewhere called a
hash or digest function) of the client's message, keyed with
the session key. Privacy and integrity of the messages
exchanged between principals can be secured by encrypting
the data to be passed using the session key contained in the
ticket or the subsession key found in the authenticator.
The authentication exchanges mentioned above require
read-only access to the Kerberos database. Sometimes, how-
ever, the entries in the database must be modified, such as
when adding new principals or changing a principal's key.
This is done using a protocol between a client and a third
Kerberos server, the Kerberos Administration Server (KADM).
There is also a protocol for maintaining multiple copies of
the Kerberos database. Neither of these protocols are
described in this document.
1.1. Cross-Realm Operation
The Kerberos protocol is designed to operate across
organizational boundaries. A client in one organization can
be authenticated to a server in another. Each organization
wishing to run a Kerberos server establishes its own
"realm". The name of the realm in which a client is
registered is part of the client's name, and can be used by
the end-service to decide whether to honor a request.
By establishing "inter-realm" keys, the administrators
of two realms can allow a client authenticated in the local
realm to prove its identity to servers in other realms[3].
The exchange of inter-realm keys (a separate key may be used
for each direction) registers the ticket-granting service of
each realm as a principal in the other realm. A client is
then able to obtain a ticket-granting ticket for the remote
realm's ticket-granting service from its local realm. When
that ticket-granting ticket is used, the remote ticket-
granting service uses the inter-realm key (which usually
__________________________
[3] Of course, with appropriate permission the client
could arrange registration of a separately-named prin-
cipal in a remote realm, and engage in normal exchanges
with that realm's services. However, for even small
numbers of clients this becomes cumbersome, and more
automatic methods as described here are necessary.
Section 1.1. - 5 - Expires 11 January 1998
Version 5 - Specification Revision 6
differs from its own normal TGS key) to decrypt the ticket-
granting ticket, and is thus certain that it was issued by
the client's own TGS. Tickets issued by the remote ticket-
granting service will indicate to the end-service that the
client was authenticated from another realm.
A realm is said to communicate with another realm if
the two realms share an inter-realm key, or if the local
realm shares an inter-realm key with an intermediate realm
that communicates with the remote realm. An authentication
path is the sequence of intermediate realms that are tran-
sited in communicating from one realm to another.
Realms are typically organized hierarchically. Each
realm shares a key with its parent and a different key with
each child. If an inter-realm key is not directly shared by
two realms, the hierarchical organization allows an authen-
tication path to be easily constructed. If a hierarchical
organization is not used, it may be necessary to consult a
database in order to construct an authentication path
between realms.
Although realms are typically hierarchical, intermedi-
ate realms may be bypassed to achieve cross-realm authenti-
cation through alternate authentication paths (these might
be established to make communication between two realms more
efficient). It is important for the end-service to know
which realms were transited when deciding how much faith to
place in the authentication process. To facilitate this
decision, a field in each ticket contains the names of the
realms that were involved in authenticating the client.
1.2. Authorization
As an authentication service, Kerberos provides a means of
verifying the identity of principals on a network. Authen-
tication is usually useful primarily as a first step in the
process of authorization, determining whether a client may
use a service, which objects the client is allowed to
access, and the type of access allowed for each. Kerberos
does not, by itself, provide authorization. Possession of a
client ticket for a service provides only for authentication
of the client to that service, and in the absence of a
separate authorization procedure, it should not be con-
sidered by an application as authorizing the use of that
service.
Such separate authorization methods may be implemented
as application specific access control functions and may be
based on files such as the application server, or on
separately issued authorization credentials such as those
based on proxies [7] , or on other authorization services.
Applications should not be modified to accept the
issuance of a service ticket by the Kerberos server (even by
Section 1.2. - 6 - Expires 11 January 1998
Version 5 - Specification Revision 6
an modified Kerberos server) as granting authority to use
the service, since such applications may become vulnerable
to the bypass of this authorization check in an environment
where they interoperate with other KDCs or where other
options for application authentication (e.g. the PKTAPP pro-
posal) are provided.
1.3. Environmental assumptions
Kerberos imposes a few assumptions on the environment in
which it can properly function:
+ "Denial of service" attacks are not solved with Ker-
beros. There are places in these protocols where an
intruder can prevent an application from participating
in the proper authentication steps. Detection and
solution of such attacks (some of which can appear to
be not-uncommon "normal" failure modes for the system)
is usually best left to the human administrators and
users.
+ Principals must keep their secret keys secret. If an
intruder somehow steals a principal's key, it will be
able to masquerade as that principal or impersonate any
server to the legitimate principal.
+ "Password guessing" attacks are not solved by Kerberos.
If a user chooses a poor password, it is possible for
an attacker to successfully mount an offline dictionary
attack by repeatedly attempting to decrypt, with suc-
cessive entries from a dictionary, messages obtained
which are encrypted under a key derived from the user's
password.
+ Each host on the network must have a clock which is
"loosely synchronized" to the time of the other hosts;
this synchronization is used to reduce the bookkeeping
needs of application servers when they do replay detec-
tion. The degree of "looseness" can be configured on a
per-server basis, but is typically on the order of 5
minutes. If the clocks are synchronized over the net-
work, the clock synchronization protocol must itself be
secured from network attackers.
+ Principal identifiers are not recycled on a short-term
basis. A typical mode of access control will use
access control lists (ACLs) to grant permissions to
particular principals. If a stale ACL entry remains
for a deleted principal and the principal identifier is
reused, the new principal will inherit rights specified
in the stale ACL entry. By not re-using principal
identifiers, the danger of inadvertent access is
removed.
Section 1.3. - 7 - Expires 11 January 1998
Version 5 - Specification Revision 6
1.4. Glossary of terms
Below is a list of terms used throughout this document.
Authentication Verifying the claimed identity of a
principal.
Authentication headerA record containing a Ticket and an
Authenticator to be presented to a
server as part of the authentication
process.
Authentication path A sequence of intermediate realms tran-
sited in the authentication process when
communicating from one realm to another.
Authenticator A record containing information that can
be shown to have been recently generated
using the session key known only by the
client and server.
Authorization The process of determining whether a
client may use a service, which objects
the client is allowed to access, and the
type of access allowed for each.
Capability A token that grants the bearer permis-
sion to access an object or service. In
Kerberos, this might be a ticket whose
use is restricted by the contents of the
authorization data field, but which
lists no network addresses, together
with the session key necessary to use
the ticket.
Ciphertext The output of an encryption function.
Encryption transforms plaintext into
ciphertext.
Client A process that makes use of a network
service on behalf of a user. Note that
in some cases a Server may itself be a
client of some other server (e.g. a
print server may be a client of a file
server).
Section 1.4. - 8 - Expires 11 January 1998
Version 5 - Specification Revision 6
Credentials A ticket plus the secret session key
necessary to successfully use that
ticket in an authentication exchange.
KDC Key Distribution Center, a network ser-
vice that supplies tickets and temporary
session keys; or an instance of that
service or the host on which it runs.
The KDC services both initial ticket and
ticket-granting ticket requests. The
initial ticket portion is sometimes
referred to as the Authentication Server
(or service). The ticket-granting
ticket portion is sometimes referred to
as the ticket-granting server (or ser-
vice).
Kerberos Aside from the 3-headed dog guarding
Hades, the name given to Project
Athena's authentication service, the
protocol used by that service, or the
code used to implement the authentica-
tion service.
Plaintext The input to an encryption function or
the output of a decryption function.
Decryption transforms ciphertext into
plaintext.
Principal A uniquely named client or server
instance that participates in a network
communication.
Principal identifierThe name used to uniquely identify each
different principal.
Seal To encipher a record containing several
fields in such a way that the fields
cannot be individually replaced without
either knowledge of the encryption key
or leaving evidence of tampering.
Secret key An encryption key shared by a principal
and the KDC, distributed outside the
bounds of the system, with a long life-
time. In the case of a human user's
principal, the secret key is derived
Section 1.4. - 9 - Expires 11 January 1998
Version 5 - Specification Revision 6
from a password.
Server A particular Principal which provides a
resource to network clients. The server
is sometimes refered to as the Applica-
tion Server.
Service A resource provided to network clients;
often provided by more than one server
(for example, remote file service).
Session key A temporary encryption key used between
two principals, with a lifetime limited
to the duration of a single login "ses-
sion".
Sub-session key A temporary encryption key used between
two principals, selected and exchanged
by the principals using the session key,
and with a lifetime limited to the dura-
tion of a single association.
Ticket A record that helps a client authenti-
cate itself to a server; it contains the
client's identity, a session key, a
timestamp, and other information, all
sealed using the server's secret key.
It only serves to authenticate a client
when presented along with a fresh
Authenticator.
2. Ticket flag uses and requests
Each Kerberos ticket contains a set of flags which are used
to indicate various attributes of that ticket. Most flags
may be requested by a client when the ticket is obtained;
some are automatically turned on and off by a Kerberos
server as required. The following sections explain what the
various flags mean, and gives examples of reasons to use
such a flag.
2.1. Initial and pre-authenticated tickets
The INITIAL flag indicates that a ticket was issued
using the AS protocol and not issued based on a ticket-
granting ticket. Application servers that want to require
the demonstrated knowledge of a client's secret key (e.g. a
password-changing program) can insist that this flag be set
in any tickets they accept, and thus be assured that the
Section 2.1. - 10 - Expires 11 January 1998
Version 5 - Specification Revision 6
client's key was recently presented to the application
client.
The PRE-AUTHENT and HW-AUTHENT flags provide addition
information about the initial authentication, regardless of
whether the current ticket was issued directly (in which
case INITIAL will also be set) or issued on the basis of a
ticket-granting ticket (in which case the INITIAL flag is
clear, but the PRE-AUTHENT and HW-AUTHENT flags are carried
forward from the ticket-granting ticket).
2.2. Invalid tickets
The INVALID flag indicates that a ticket is invalid.
Application servers must reject tickets which have this flag
set. A postdated ticket will usually be issued in this
form. Invalid tickets must be validated by the KDC before
use, by presenting them to the KDC in a TGS request with the
VALIDATE option specified. The KDC will only validate tick-
ets after their starttime has passed. The validation is
required so that postdated tickets which have been stolen
before their starttime can be rendered permanently invalid
(through a hot-list mechanism) (see section 3.3.3.1).
2.3. Renewable tickets
Applications may desire to hold tickets which can be
valid for long periods of time. However, this can expose
their credentials to potential theft for equally long
periods, and those stolen credentials would be valid until
the expiration time of the ticket(s). Simply using short-
lived tickets and obtaining new ones periodically would
require the client to have long-term access to its secret
key, an even greater risk. Renewable tickets can be used to
mitigate the consequences of theft. Renewable tickets have
two "expiration times": the first is when the current
instance of the ticket expires, and the second is the latest
permissible value for an individual expiration time. An
application client must periodically (i.e. before it
expires) present a renewable ticket to the KDC, with the
RENEW option set in the KDC request. The KDC will issue a
new ticket with a new session key and a later expiration
time. All other fields of the ticket are left unmodified by
the renewal process. When the latest permissible expiration
time arrives, the ticket expires permanently. At each
renewal, the KDC may consult a hot-list to determine if the
ticket had been reported stolen since its last renewal; it
will refuse to renew such stolen tickets, and thus the
usable lifetime of stolen tickets is reduced.
The RENEWABLE flag in a ticket is normally only inter-
preted by the ticket-granting service (discussed below in
section 3.3). It can usually be ignored by application
servers. However, some particularly careful application
Section 2.3. - 11 - Expires 11 January 1998
Version 5 - Specification Revision 6
servers may wish to disallow renewable tickets.
If a renewable ticket is not renewed by its expiration
time, the KDC will not renew the ticket. The RENEWABLE flag
is reset by default, but a client may request it be set by
setting the RENEWABLE option in the KRB_AS_REQ message. If
it is set, then the renew-till field in the ticket contains
the time after which the ticket may not be renewed.
2.4. Postdated tickets
Applications may occasionally need to obtain tickets
for use much later, e.g. a batch submission system would
need tickets to be valid at the time the batch job is ser-
viced. However, it is dangerous to hold valid tickets in a
batch queue, since they will be on-line longer and more
prone to theft. Postdated tickets provide a way to obtain
these tickets from the KDC at job submission time, but to
leave them "dormant" until they are activated and validated
by a further request of the KDC. If a ticket theft were
reported in the interim, the KDC would refuse to validate
the ticket, and the thief would be foiled.
The MAY-POSTDATE flag in a ticket is normally only
interpreted by the ticket-granting service. It can be
ignored by application servers. This flag must be set in a
ticket-granting ticket in order to issue a postdated ticket
based on the presented ticket. It is reset by default; it
may be requested by a client by setting the ALLOW-POSTDATE
option in the KRB_AS_REQ message. This flag does not allow
a client to obtain a postdated ticket-granting ticket; post-
dated ticket-granting tickets can only by obtained by
requesting the postdating in the KRB_AS_REQ message. The
life (endtime-starttime) of a postdated ticket will be the
remaining life of the ticket-granting ticket at the time of
the request, unless the RENEWABLE option is also set, in
which case it can be the full life (endtime-starttime) of
the ticket-granting ticket. The KDC may limit how far in
the future a ticket may be postdated.
The POSTDATED flag indicates that a ticket has been
postdated. The application server can check the authtime
field in the ticket to see when the original authentication
occurred. Some services may choose to reject postdated
tickets, or they may only accept them within a certain
period after the original authentication. When the KDC
issues a POSTDATED ticket, it will also be marked as
INVALID, so that the application client must present the
ticket to the KDC to be validated before use.
2.5. Proxiable and proxy tickets
At times it may be necessary for a principal to allow a
service to perform an operation on its behalf. The service
Section 2.5. - 12 - Expires 11 January 1998
Version 5 - Specification Revision 6
must be able to take on the identity of the client, but only
for a particular purpose. A principal can allow a service
to take on the principal's identity for a particular purpose
by granting it a proxy.
The process of granting a proxy using the proxy and
proxiable flags is used to provide credentials for use with
specific services. Though conceptually also a proxy, user's
wishing to delegate their identity for ANY purpose must use
the ticket forwarding mechanism described in the next sec-
tion to forward a ticket granting ticket.
The PROXIABLE flag in a ticket is normally only inter-
preted by the ticket-granting service. It can be ignored by
application servers. When set, this flag tells the ticket-
granting server that it is OK to issue a new ticket (but not
a ticket-granting ticket) with a different network address
based on this ticket. This flag is set if requested by the
client on initial authentication. By default, the client
will request that it be set when requesting a ticket grant-
ing ticket, and reset when requesting any other ticket.
This flag allows a client to pass a proxy to a server
to perform a remote request on its behalf, e.g. a print ser-
vice client can give the print server a proxy to access the
client's files on a particular file server in order to
satisfy a print request.
In order to complicate the use of stolen credentials,
Kerberos tickets are usually valid from only those network
addresses specifically included in the ticket[4]. When
granting a proxy, the client must specify the new network
address from which the proxy is to be used, or indicate that
the proxy is to be issued for use from any address.
The PROXY flag is set in a ticket by the TGS when it
issues a proxy ticket. Application servers may check this
flag and at their option they may require additional authen-
tication from the agent presenting the proxy in order to
provide an audit trail.
2.6. Forwardable tickets
Authentication forwarding is an instance of a proxy
where the service is granted complete use of the client's
identity. An example where it might be used is when a user
logs in to a remote system and wants authentication to work
from that system as if the login were local.
The FORWARDABLE flag in a ticket is normally only
__________________________
[4] Though it is permissible to request or issue tick-
ets with no network addresses specified.
Section 2.6. - 13 - Expires 11 January 1998
Version 5 - Specification Revision 6
interpreted by the ticket-granting service. It can be
ignored by application servers. The FORWARDABLE flag has an
interpretation similar to that of the PROXIABLE flag, except
ticket-granting tickets may also be issued with different
network addresses. This flag is reset by default, but users
may request that it be set by setting the FORWARDABLE option
in the AS request when they request their initial ticket-
granting ticket.
This flag allows for authentication forwarding without
requiring the user to enter a password again. If the flag
is not set, then authentication forwarding is not permitted,
but the same result can still be achieved if the user
engages in the AS exchange specifying the requested network
addresses and supplies a password.
The FORWARDED flag is set by the TGS when a client
presents a ticket with the FORWARDABLE flag set and requests
a forwarded ticket by specifying the FORWARDED KDC option
and supplying a set of addresses for the new ticket. It is
also set in all tickets issued based on tickets with the
FORWARDED flag set. Application servers may choose to pro-
cess FORWARDED tickets differently than non-FORWARDED tick-
ets.
2.7. Other KDC options
There are two additional options which may be set in a
client's request of the KDC. The RENEWABLE-OK option indi-
cates that the client will accept a renewable ticket if a
ticket with the requested life cannot otherwise be provided.
If a ticket with the requested life cannot be provided, then
the KDC may issue a renewable ticket with a renew-till equal
to the the requested endtime. The value of the renew-till
field may still be adjusted by site-determined limits or
limits imposed by the individual principal or server.
The ENC-TKT-IN-SKEY option is honored only by the
ticket-granting service. It indicates that the ticket to be
issued for the end server is to be encrypted in the session
key from the a additional second ticket-granting ticket pro-
vided with the request. See section 3.3.3 for specific
details.
__________________________
[5] The password-changing request must not be honored
unless the requester can provide the old password (the
user's current secret key). Otherwise, it would be
possible for someone to walk up to an unattended ses-
sion and change another user's password.
[6] To authenticate a user logging on to a local sys-
tem, the credentials obtained in the AS exchange may
first be used in a TGS exchange to obtain credentials
Section 3.1. - 14 - Expires 11 January 1998
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3. Message Exchanges
The following sections describe the interactions between
network clients and servers and the messages involved in
those exchanges.
3.1. The Authentication Service Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_AS_REQ 5.4.1
2. Kerberos to client KRB_AS_REP or 5.4.2
KRB_ERROR 5.9.1
The Authentication Service (AS) Exchange between the
client and the Kerberos Authentication Server is initiated
by a client when it wishes to obtain authentication creden-
tials for a given server but currently holds no credentials.
In its basic form, the client's secret key is used for en-
cryption and decryption. This exchange is typically used at
the initiation of a login session to obtain credentials for
a Ticket-Granting Server which will subsequently be used to
obtain credentials for other servers (see section 3.3)
without requiring further use of the client's secret key.
This exchange is also used to request credentials for ser-
vices which must not be mediated through the Ticket-Granting
Service, but rather require a principal's secret key, such
as the password-changing service[5]. This exchange does not
by itself provide any assurance of the the identity of the
user[6].
The exchange consists of two messages: KRB_AS_REQ from
the client to Kerberos, and KRB_AS_REP or KRB_ERROR in
reply. The formats for these messages are described in sec-
tions 5.4.1, 5.4.2, and 5.9.1.
In the request, the client sends (in cleartext) its own
identity and the identity of the server for which it is
requesting credentials. The response, KRB_AS_REP, contains
a ticket for the client to present to the server, and a ses-
sion key that will be shared by the client and the server.
The session key and additional information are encrypted in
the client's secret key. The KRB_AS_REP message contains
information which can be used to detect replays, and to
associate it with the message to which it replies. Various
errors can occur; these are indicated by an error response
(KRB_ERROR) instead of the KRB_AS_REP response. The error
__________________________
for a local server. Those credentials must then be
verified by a local server through successful comple-
tion of the Client/Server exchange.
Section 3.1. - 15 - Expires 11 January 1998
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message is not encrypted. The KRB_ERROR message contains
information which can be used to associate it with the mes-
sage to which it replies. The lack of encryption in the
KRB_ERROR message precludes the ability to detect replays,
fabrications, or modifications of such messages.
Without preautentication, the authentication server
does not know whether the client is actually the principal
named in the request. It simply sends a reply without know-
ing or caring whether they are the same. This is acceptable
because nobody but the principal whose identity was given in
the request will be able to use the reply. Its critical
information is encrypted in that principal's key. The ini-
tial request supports an optional field that can be used to
pass additional information that might be needed for the
initial exchange. This field may be used for pre-
authentication as described in section <<sec preauth>>.
3.1.1. Generation of KRB_AS_REQ message
The client may specify a number of options in the ini-
tial request. Among these options are whether pre-
authentication is to be performed; whether the requested
ticket is to be renewable, proxiable, or forwardable;
whether it should be postdated or allow postdating of
derivative tickets; and whether a renewable ticket will be
accepted in lieu of a non-renewable ticket if the requested
ticket expiration date cannot be satisfied by a non-
renewable ticket (due to configuration constraints; see sec-
tion 4). See section A.1 for pseudocode.
The client prepares the KRB_AS_REQ message and sends it
to the KDC.
3.1.2. Receipt of KRB_AS_REQ message
If all goes well, processing the KRB_AS_REQ message
will result in the creation of a ticket for the client to
present to the server. The format for the ticket is
described in section 5.3.1. The contents of the ticket are
determined as follows.
3.1.3. Generation of KRB_AS_REP message
The authentication server looks up the client and
server principals named in the KRB_AS_REQ in its database,
extracting their respective keys. If required, the server
pre-authenticates the request, and if the pre-authentication
check fails, an error message with the code
KDC_ERR_PREAUTH_FAILED is returned. If the server cannot
accommodate the requested encryption type, an error message
with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it
generates a "random" session key[7].
__________________________
Section 3.1.3. - 16 - Expires 11 January 1998
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If there are multiple encryption keys registered for a
client in the Kerberos database (or if the key registered
supports multiple encryption types; e.g. DES-CBC-CRC and
DES-CBC-MD5), then the etype field from the AS request is
used by the KDC to select the encryption method to be used
for encrypting the response to the client. If there is more
than one supported, strong encryption type in the etype
list, the first valid etype for which an encryption key is
available is used. The encryption method used to respond to
a TGS request is taken from the keytype of the session key
found in the ticket granting ticket.
When the etype field is present in a KDC request,
whether an AS or TGS request, the KDC will attempt to assign
the type of the random session key from the list of methods
in the etype field. The KDC will select the appropriate
type using the list of methods provided together with infor-
mation from the Kerberos database indicating acceptable
encryption methods for the application server. The KDC will
not issue tickets with a weak session key encryption type.
If the requested start time is absent, indicates a time
in the past, or is within the window of acceptable clock
skew for the KDC and the POSTDATE option has not been speci-
fied, then the start time of the ticket is set to the
authentication server's current time. If it indicates a
time in the future beyond the acceptable clock skew, but the
POSTDATED option has not been specified then the error
KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the
requested start time is checked against the policy of the
local realm (the administrator might decide to prohibit cer-
tain types or ranges of postdated tickets), and if accept-
able, the ticket's start time is set as requested and the
INVALID flag is set in the new ticket. The postdated ticket
must be validated before use by presenting it to the KDC
after the start time has been reached.
__________________________
[7] "Random" means that, among other things, it should
be impossible to guess the next session key based on
knowledge of past session keys. This can only be
achieved in a pseudo-random number generator if it is
based on cryptographic principles. It is more desir-
able to use a truly random number generator, such as
one based on measurements of random physical phenomena.
Section 3.1.3. - 17 - Expires 11 January 1998
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The expiration time of the ticket will be set to the minimum
of the following:
+The expiration time (endtime) requested in the KRB_AS_REQ
message.
+The ticket's start time plus the maximum allowable lifetime
associated with the client principal (the authentication
server's database includes a maximum ticket lifetime field
in each principal's record; see section 4).
+The ticket's start time plus the maximum allowable lifetime
associated with the server principal.
+The ticket's start time plus the maximum lifetime set by
the policy of the local realm.
If the requested expiration time minus the start time
(as determined above) is less than a site-determined minimum
lifetime, an error message with code KDC_ERR_NEVER_VALID is
returned. If the requested expiration time for the ticket
exceeds what was determined as above, and if the
"RENEWABLE-OK" option was requested, then the "RENEWABLE"
flag is set in the new ticket, and the renew-till value is
set as if the "RENEWABLE" option were requested (the field
and option names are described fully in section 5.4.1).
If the RENEWABLE option has been requested or if the
RENEWABLE-OK option has been set and a renewable ticket is
to be issued, then the renew-till field is set to the
minimum of:
+Its requested value.
+The start time of the ticket plus the minimum of the two
maximum renewable lifetimes associated with the principals'
database entries.
+The start time of the ticket plus the maximum renewable
lifetime set by the policy of the local realm.
The flags field of the new ticket will have the follow-
ing options set if they have been requested and if the pol-
icy of the local realm allows: FORWARDABLE, MAY-POSTDATE,
POSTDATED, PROXIABLE, RENEWABLE. If the new ticket is post-
dated (the start time is in the future), its INVALID flag
will also be set.
If all of the above succeed, the server formats a
KRB_AS_REP message (see section 5.4.2), copying the
addresses in the request into the caddr of the response,
placing any required pre-authentication data into the padata
of the response, and encrypts the ciphertext part in the
client's key using the requested encryption method, and
Section 3.1.3. - 18 - Expires 11 January 1998
Version 5 - Specification Revision 6
sends it to the client. See section A.2 for pseudocode.
3.1.4. Generation of KRB_ERROR message
Several errors can occur, and the Authentication Server
responds by returning an error message, KRB_ERROR, to the
client, with the error-code and e-text fields set to
appropriate values. The error message contents and details
are described in Section 5.9.1.
3.1.5. Receipt of KRB_AS_REP message
If the reply message type is KRB_AS_REP, then the
client verifies that the cname and crealm fields in the
cleartext portion of the reply match what it requested. If
any padata fields are present, they may be used to derive
the proper secret key to decrypt the message. The client
decrypts the encrypted part of the response using its secret
key, verifies that the nonce in the encrypted part matches
the nonce it supplied in its request (to detect replays).
It also verifies that the sname and srealm in the response
match those in the request (or are otherwise expected
values), and that the host address field is also correct.
It then stores the ticket, session key, start and expiration
times, and other information for later use. The key-
expiration field from the encrypted part of the response may
be checked to notify the user of impending key expiration
(the client program could then suggest remedial action, such
as a password change). See section A.3 for pseudocode.
Proper decryption of the KRB_AS_REP message is not suf-
ficient to verify the identity of the user; the user and an
attacker could cooperate to generate a KRB_AS_REP format
message which decrypts properly but is not from the proper
KDC. If the host wishes to verify the identity of the user,
it must require the user to present application credentials
which can be verified using a securely-stored secret key for
the host. If those credentials can be verified, then the
identity of the user can be assured.
3.1.6. Receipt of KRB_ERROR message
If the reply message type is KRB_ERROR, then the client
interprets it as an error and performs whatever
application-specific tasks are necessary to recover.
3.2. The Client/Server Authentication Exchange
Summary
Message direction Message type Section
Client to Application server KRB_AP_REQ 5.5.1
[optional] Application server to client KRB_AP_REP or 5.5.2
KRB_ERROR 5.9.1
Section 3.2. - 19 - Expires 11 January 1998
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The client/server authentication (CS) exchange is used
by network applications to authenticate the client to the
server and vice versa. The client must have already
acquired credentials for the server using the AS or TGS
exchange.
3.2.1. The KRB_AP_REQ message
The KRB_AP_REQ contains authentication information
which should be part of the first message in an authenti-
cated transaction. It contains a ticket, an authenticator,
and some additional bookkeeping information (see section
5.5.1 for the exact format). The ticket by itself is insuf-
ficient to authenticate a client, since tickets are passed
across the network in cleartext[8], so the authenticator is
used to prevent invalid replay of tickets by proving to the
server that the client knows the session key of the ticket
and thus is entitled to use the ticket. The KRB_AP_REQ mes-
sage is referred to elsewhere as the "authentication
header."
3.2.2. Generation of a KRB_AP_REQ message
When a client wishes to initiate authentication to a
server, it obtains (either through a credentials cache, the
AS exchange, or the TGS exchange) a ticket and session key
for the desired service. The client may re-use any tickets
it holds until they expire. To use a ticket the client con-
structs a new Authenticator from the the system time, its
name, and optionally an application specific checksum, an
initial sequence number to be used in KRB_SAFE or KRB_PRIV
messages, and/or a session subkey to be used in negotiations
for a session key unique to this particular session.
Authenticators may not be re-used and will be rejected if
replayed to a server[9]. If a sequence number is to be
included, it should be randomly chosen so that even after
many messages have been exchanged it is not likely to col-
lide with other sequence numbers in use.
The client may indicate a requirement of mutual
__________________________
[8] Tickets contain both an encrypted and unencrypted
portion, so cleartext here refers to the entire unit,
which can be copied from one message and replayed in
another without any cryptographic skill.
[9] Note that this can make applications based on un-
reliable transports difficult to code correctly. If the
transport might deliver duplicated messages, either a
new authenticator must be generated for each retry, or
the application server must match requests and replies
and replay the first reply in response to a detected
duplicate.
Section 3.2.2. - 20 - Expires 11 January 1998
Version 5 - Specification Revision 6
authentication or the use of a session-key based ticket by
setting the appropriate flag(s) in the ap-options field of
the message.
The Authenticator is encrypted in the session key and
combined with the ticket to form the KRB_AP_REQ message
which is then sent to the end server along with any addi-
tional application-specific information. See section A.9
for pseudocode.
3.2.3. Receipt of KRB_AP_REQ message
Authentication is based on the server's current time of
day (clocks must be loosely synchronized), the authentica-
tor, and the ticket. Several errors are possible. If an
error occurs, the server is expected to reply to the client
with a KRB_ERROR message. This message may be encapsulated
in the application protocol if its "raw" form is not accept-
able to the protocol. The format of error messages is
described in section 5.9.1.
The algorithm for verifying authentication information
is as follows. If the message type is not KRB_AP_REQ, the
server returns the KRB_AP_ERR_MSG_TYPE error. If the key
version indicated by the Ticket in the KRB_AP_REQ is not one
the server can use (e.g., it indicates an old key, and the
server no longer possesses a copy of the old key), the
KRB_AP_ERR_BADKEYVER error is returned. If the USE-
SESSION-KEY flag is set in the ap-options field, it indi-
cates to the server that the ticket is encrypted in the ses-
sion key from the server's ticket-granting ticket rather
than its secret key[10]. Since it is possible for the
server to be registered in multiple realms, with different
keys in each, the srealm field in the unencrypted portion of
the ticket in the KRB_AP_REQ is used to specify which secret
key the server should use to decrypt that ticket. The
KRB_AP_ERR_NOKEY error code is returned if the server
doesn't have the proper key to decipher the ticket.
The ticket is decrypted using the version of the
server's key specified by the ticket. If the decryption
routines detect a modification of the ticket (each encryp-
tion system must provide safeguards to detect modified
ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY
error is returned (chances are good that different keys were
used to encrypt and decrypt).
The authenticator is decrypted using the session key
extracted from the decrypted ticket. If decryption shows it
to have been modified, the KRB_AP_ERR_BAD_INTEGRITY error is
__________________________
[10] This is used for user-to-user authentication as
described in [8].
Section 3.2.3. - 21 - Expires 11 January 1998
Version 5 - Specification Revision 6
returned. The name and realm of the client from the ticket
are compared against the same fields in the authenticator.
If they don't match, the KRB_AP_ERR_BADMATCH error is
returned (they might not match, for example, if the wrong
session key was used to encrypt the authenticator). The
addresses in the ticket (if any) are then searched for an
address matching the operating-system reported address of
the client. If no match is found or the server insists on
ticket addresses but none are present in the ticket, the
KRB_AP_ERR_BADADDR error is returned.
If the local (server) time and the client time in the
authenticator differ by more than the allowable clock skew
(e.g., 5 minutes), the KRB_AP_ERR_SKEW error is returned.
If the server name, along with the client name, time and
microsecond fields from the Authenticator match any
recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
returned[11]. The server must remember any authenticator
presented within the allowable clock skew, so that a replay
attempt is guaranteed to fail. If a server loses track of
any authenticator presented within the allowable clock skew,
it must reject all requests until the clock skew interval
has passed. This assures that any lost or re-played authen-
ticators will fall outside the allowable clock skew and can
no longer be successfully replayed (If this is not done, an
attacker could conceivably record the ticket and authentica-
tor sent over the network to a server, then disable the
client's host, pose as the disabled host, and replay the
ticket and authenticator to subvert the authentication.).
If a sequence number is provided in the authenticator, the
server saves it for later use in processing KRB_SAFE and/or
KRB_PRIV messages. If a subkey is present, the server
either saves it for later use or uses it to help generate
its own choice for a subkey to be returned in a KRB_AP_REP
message.
The server computes the age of the ticket: local
(server) time minus the start time inside the Ticket. If
the start time is later than the current time by more than
the allowable clock skew or if the INVALID flag is set in
the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Oth-
erwise, if the current time is later than end time by more
than the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED
error is returned.
If all these checks succeed without an error, the
__________________________
[11] Note that the rejection here is restricted to au-
thenticators from the same principal to the same
server. Other client principals communicating with the
same server principal should not be have their authen-
ticators rejected if the time and microsecond fields
happen to match some other client's authenticator.
Section 3.2.3. - 22 - Expires 11 January 1998
Version 5 - Specification Revision 6
server is assured that the client possesses the credentials
of the principal named in the ticket and thus, the client
has been authenticated to the server. See section A.10 for
pseudocode.
Passing these checks provides only authentication of
the named principal; it does not imply authorization to use
the named service. Applications must make a separate
authorization decisions based upon the authenticated name of
the user, the requested operation, local acces control
information such as that contained in a .k5login or .k5users
file, and possibly a separate distributed authorization ser-
vice.
3.2.4. Generation of a KRB_AP_REP message
Typically, a client's request will include both the
authentication information and its initial request in the
same message, and the server need not explicitly reply to
the KRB_AP_REQ. However, if mutual authentication (not only
authenticating the client to the server, but also the server
to the client) is being performed, the KRB_AP_REQ message
will have MUTUAL-REQUIRED set in its ap-options field, and a
KRB_AP_REP message is required in response. As with the
error message, this message may be encapsulated in the
application protocol if its "raw" form is not acceptable to
the application's protocol. The timestamp and microsecond
field used in the reply must be the client's timestamp and
microsecond field (as provided in the authenticator)[12].
If a sequence number is to be included, it should be ran-
domly chosen as described above for the authenticator. A
subkey may be included if the server desires to negotiate a
different subkey. The KRB_AP_REP message is encrypted in
the session key extracted from the ticket. See section A.11
for pseudocode.
3.2.5. Receipt of KRB_AP_REP message
If a KRB_AP_REP message is returned, the client uses
the session key from the credentials obtained for the
server[13] to decrypt the message, and verifies that the
__________________________
[12] In the Kerberos version 4 protocol, the timestamp
in the reply was the client's timestamp plus one. This
is not necessary in version 5 because version 5 mes-
sages are formatted in such a way that it is not possi-
ble to create the reply by judicious message surgery
(even in encrypted form) without knowledge of the ap-
propriate encryption keys.
[13] Note that for encrypting the KRB_AP_REP message,
the sub-session key is not used, even if present in the
Authenticator.
Section 3.2.5. - 23 - Expires 11 January 1998
Version 5 - Specification Revision 6
timestamp and microsecond fields match those in the Authen-
ticator it sent to the server. If they match, then the
client is assured that the server is genuine. The sequence
number and subkey (if present) are retained for later use.
See section A.12 for pseudocode.
3.2.6. Using the encryption key
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred,
the client and server share an encryption key which can be
used by the application. The "true session key" to be used
for KRB_PRIV, KRB_SAFE, or other application-specific uses
may be chosen by the application based on the subkeys in the
KRB_AP_REP message and the authenticator[14]. In some
cases, the use of this session key will be implicit in the
protocol; in others the method of use must be chosen from
several alternatives. We leave the protocol negotiations of
how to use the key (e.g. selecting an encryption or check-
sum type) to the application programmer; the Kerberos proto-
col does not constrain the implementation options, but an
example of how this might be done follows.
One way that an application may choose to negotiate a
key to be used for subequent integrity and privacy protec-
tion is for the client to propose a key in the subkey field
of the authenticator. The server can then choose a key
using the proposed key from the client as input, returning
the new subkey in the subkey field of the application reply.
This key could then be used for subsequent communication.
To make this example more concrete, if the encryption method
in use required a 56 bit key, and for whatever reason, one
of the parties was prevented from using a key with more than
40 unknown bits, this method would allow the the party which
is prevented from using more than 40 bits to either propose
(if the client) an initial key with a known quantity for 16
of those bits, or to mask 16 of the bits (if the server)
with the known quantity. The application implementor is
warned, however, that this is only an example, and that an
analysis of the particular crytosystem to be used, and the
reasons for limiting the key length, must be made before
deciding whether it is acceptable to mask bits of the key.
With both the one-way and mutual authentication
exchanges, the peers should take care not to send sensitive
information to each other without proper assurances. In
particular, applications that require privacy or integrity
should use the KRB_AP_REP response from the server to client
__________________________
[14] Implementations of the protocol may wish to pro-
vide routines to choose subkeys based on session keys
and random numbers and to generate a negotiated key to
be returned in the KRB_AP_REP message.
Section 3.2.6. - 24 - Expires 11 January 1998
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to assure both client and server of their peer's identity.
If an application protocol requires privacy of its messages,
it can use the KRB_PRIV message (section 3.5). The KRB_SAFE
message (section 3.4) can be used to assure integrity.
3.3. The Ticket-Granting Service (TGS) Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos
Ticket-Granting Server is initiated by a client when it
wishes to obtain authentication credentials for a given
server (which might be registered in a remote realm), when
it wishes to renew or validate an existing ticket, or when
it wishes to obtain a proxy ticket. In the first case, the
client must already have acquired a ticket for the Ticket-
Granting Service using the AS exchange (the ticket-granting
ticket is usually obtained when a client initially authenti-
cates to the system, such as when a user logs in). The mes-
sage format for the TGS exchange is almost identical to that
for the AS exchange. The primary difference is that encryp-
tion and decryption in the TGS exchange does not take place
under the client's key. Instead, the session key from the
ticket-granting ticket or renewable ticket, or sub-session
key from an Authenticator is used. As is the case for all
application servers, expired tickets are not accepted by the
TGS, so once a renewable or ticket-granting ticket expires,
the client must use a separate exchange to obtain valid
tickets.
The TGS exchange consists of two messages: A request
(KRB_TGS_REQ) from the client to the Kerberos Ticket-
Granting Server, and a reply (KRB_TGS_REP or KRB_ERROR).
The KRB_TGS_REQ message includes information authenticating
the client plus a request for credentials. The authentica-
tion information consists of the authentication header
(KRB_AP_REQ) which includes the client's previously obtained
ticket-granting, renewable, or invalid ticket. In the
ticket-granting ticket and proxy cases, the request may
include one or more of: a list of network addresses, a col-
lection of typed authorization data to be sealed in the
ticket for authorization use by the application server, or
additional tickets (the use of which are described later).
The TGS reply (KRB_TGS_REP) contains the requested creden-
tials, encrypted in the session key from the ticket-granting
ticket or renewable ticket, or if present, in the sub-
session key from the Authenticator (part of the authentica-
tion header). The KRB_ERROR message contains an error code
Section 3.3. - 25 - Expires 11 January 1998
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and text explaining what went wrong. The KRB_ERROR message
is not encrypted. The KRB_TGS_REP message contains informa-
tion which can be used to detect replays, and to associate
it with the message to which it replies. The KRB_ERROR mes-
sage also contains information which can be used to associ-
ate it with the message to which it replies, but the lack of
encryption in the KRB_ERROR message precludes the ability to
detect replays or fabrications of such messages.
3.3.1. Generation of KRB_TGS_REQ message
Before sending a request to the ticket-granting ser-
vice, the client must determine in which realm the applica-
tion server is registered[15]. If the client does not
already possess a ticket-granting ticket for the appropriate
realm, then one must be obtained. This is first attempted
by requesting a ticket-granting ticket for the destination
realm from a Kerberos server for which the client does
posess a ticket-granting ticket (using the KRB_TGS_REQ mes-
sage recursively). The Kerberos server may return a TGT for
the desired realm in which case one can proceed. Alterna-
tively, the Kerberos server may return a TGT for a realm
which is "closer" to the desired realm (further along the
standard hierarchical path), in which case this step must be
repeated with a Kerberos server in the realm specified in
the returned TGT. If neither are returned, then the request
must be retried with a Kerberos server for a realm higher in
the hierarchy. This request will itself require a ticket-
granting ticket for the higher realm which must be obtained
by recursively applying these directions.
Once the client obtains a ticket-granting ticket for
the appropriate realm, it determines which Kerberos servers
serve that realm, and contacts one. The list might be
obtained through a configuration file or network service or
it may be generated from the name of the realm; as long as
the secret keys exchanged by realms are kept secret, only
denial of service results from using a false Kerberos
server.
__________________________
[15] This can be accomplished in several ways. It
might be known beforehand (since the realm is part of
the principal identifier), it might be stored in a
nameserver, or it might be obtained from a configura-
tion file. If the realm to be used is obtained from a
nameserver, there is a danger of being spoofed if the
nameservice providing the realm name is not authenti-
cated. This might result in the use of a realm which
has been compromised, and would result in an attacker's
ability to compromise the authentication of the appli-
cation server to the client.
Section 3.3.1. - 26 - Expires 11 January 1998
Version 5 - Specification Revision 6
As in the AS exchange, the client may specify a number
of options in the KRB_TGS_REQ message. The client prepares
the KRB_TGS_REQ message, providing an authentication header
as an element of the padata field, and including the same
fields as used in the KRB_AS_REQ message along with several
optional fields: the enc-authorization-data field for appli-
cation server use and additional tickets required by some
options.
In preparing the authentication header, the client can
select a sub-session key under which the response from the
Kerberos server will be encrypted[16]. If the sub-session
key is not specified, the session key from the ticket-
granting ticket will be used. If the enc-authorization-data
is present, it must be encrypted in the sub-session key, if
present, from the authenticator portion of the authentica-
tion header, or if not present, using the session key from
the ticket-granting ticket.
Once prepared, the message is sent to a Kerberos server
for the destination realm. See section A.5 for pseudocode.
3.3.2. Receipt of KRB_TGS_REQ message
The KRB_TGS_REQ message is processed in a manner simi-
lar to the KRB_AS_REQ message, but there are many additional
checks to be performed. First, the Kerberos server must
determine which server the accompanying ticket is for and it
must select the appropriate key to decrypt it. For a normal
KRB_TGS_REQ message, it will be for the ticket granting ser-
vice, and the TGS's key will be used. If the TGT was issued
by another realm, then the appropriate inter-realm key must
be used. If the accompanying ticket is not a ticket grant-
ing ticket for the current realm, but is for an application
server in the current realm, the RENEW, VALIDATE, or PROXY
options are specified in the request, and the server for
which a ticket is requested is the server named in the
accompanying ticket, then the KDC will decrypt the ticket in
the authentication header using the key of the server for
which it was issued. If no ticket can be found in the
padata field, the KDC_ERR_PADATA_TYPE_NOSUPP error is
returned.
Once the accompanying ticket has been decrypted, the
user-supplied checksum in the Authenticator must be verified
against the contents of the request, and the message
rejected if the checksums do not match (with an error code
__________________________
[16] If the client selects a sub-session key, care must
be taken to ensure the randomness of the selected sub-
session key. One approach would be to generate a ran-
dom number and XOR it with the session key from the
ticket-granting ticket.
Section 3.3.2. - 27 - Expires 11 January 1998
Version 5 - Specification Revision 6
of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or
not collision-proof (with an error code of
KRB_AP_ERR_INAPP_CKSUM). If the checksum type is not sup-
ported, the KDC_ERR_SUMTYPE_NOSUPP error is returned. If
the authorization-data are present, they are decrypted using
the sub-session key from the Authenticator.
If any of the decryptions indicate failed integrity
checks, the KRB_AP_ERR_BAD_INTEGRITY error is returned.
3.3.3. Generation of KRB_TGS_REP message
The KRB_TGS_REP message shares its format with the
KRB_AS_REP (KRB_KDC_REP), but with its type field set to
KRB_TGS_REP. The detailed specification is in section
5.4.2.
The response will include a ticket for the requested
server. The Kerberos database is queried to retrieve the
record for the requested server (including the key with
which the ticket will be encrypted). If the request is for
a ticket granting ticket for a remote realm, and if no key
is shared with the requested realm, then the Kerberos server
will select the realm "closest" to the requested realm with
which it does share a key, and use that realm instead. This
is the only case where the response from the KDC will be for
a different server than that requested by the client.
By default, the address field, the client's name and
realm, the list of transited realms, the time of initial
authentication, the expiration time, and the authorization
data of the newly-issued ticket will be copied from the
ticket-granting ticket (TGT) or renewable ticket. If the
transited field needs to be updated, but the transited type
is not supported, the KDC_ERR_TRTYPE_NOSUPP error is
returned.
If the request specifies an endtime, then the endtime
of the new ticket is set to the minimum of (a) that request,
(b) the endtime from the TGT, and (c) the starttime of the
TGT plus the minimum of the maximum life for the application
server and the maximum life for the local realm (the maximum
life for the requesting principal was already applied when
the TGT was issued). If the new ticket is to be a renewal,
then the endtime above is replaced by the minimum of (a) the
value of the renew_till field of the ticket and (b) the
starttime for the new ticket plus the life (endtime-
starttime) of the old ticket.
If the FORWARDED option has been requested, then the
resulting ticket will contain the addresses specified by the
client. This option will only be honored if the FORWARDABLE
flag is set in the TGT. The PROXY option is similar; the
resulting ticket will contain the addresses specified by the
Section 3.3.3. - 28 - Expires 11 January 1998
Version 5 - Specification Revision 6
client. It will be honored only if the PROXIABLE flag in
the TGT is set. The PROXY option will not be honored on
requests for additional ticket-granting tickets.
If the requested start time is absent, indicates a time
in the past, or is within the window of acceptable clock
skew for the KDC and the POSTDATE option has not been speci-
fied, then the start time of the ticket is set to the
authentication server's current time. If it indicates a
time in the future beyond the acceptable clock skew, but the
POSTDATED option has not been specified or the MAY-POSTDATE
flag is not set in the TGT, then the error
KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the
ticket-granting ticket has the MAY-POSTDATE flag set, then
the resulting ticket will be postdated and the requested
starttime is checked against the policy of the local realm.
If acceptable, the ticket's start time is set as requested,
and the INVALID flag is set. The postdated ticket must be
validated before use by presenting it to the KDC after the
starttime has been reached. However, in no case may the
starttime, endtime, or renew-till time of a newly-issued
postdated ticket extend beyond the renew-till time of the
ticket-granting ticket.
If the ENC-TKT-IN-SKEY option has been specified and an
additional ticket has been included in the request, the KDC
will decrypt the additional ticket using the key for the
server to which the additional ticket was issued and verify
that it is a ticket-granting ticket. If the name of the
requested server is missing from the request, the name of
the client in the additional ticket will be used. Otherwise
the name of the requested server will be compared to the
name of the client in the additional ticket and if dif-
ferent, the request will be rejected. If the request
succeeds, the session key from the additional ticket will be
used to encrypt the new ticket that is issued instead of
using the key of the server for which the new ticket will be
used[17].
If the name of the server in the ticket that is
presented to the KDC as part of the authentication header is
not that of the ticket-granting server itself, the server is
registered in the realm of the KDC, and the RENEW option is
requested, then the KDC will verify that the RENEWABLE flag
is set in the ticket, that the INVALID flag is not set in
the ticket, and that the renew_till time is still in the
future. If the VALIDATE option is rqeuested, the KDC will
__________________________
[17] This allows easy implementation of user-to-user
authentication [8], which uses ticket-granting ticket
session keys in lieu of secret server keys in situa-
tions where such secret keys could be easily comprom-
ised.
Section 3.3.3. - 29 - Expires 11 January 1998
Version 5 - Specification Revision 6
check that the starttime has passed and the INVALID flag is
set. If the PROXY option is requested, then the KDC will
check that the PROXIABLE flag is set in the ticket. If the
tests succeed, and the ticket passes the hotlist check
described in the next paragraph, the KDC will issue the
appropriate new ticket.
3.3.3.1. Checking for revoked tickets
Whenever a request is made to the ticket-granting
server, the presented ticket(s) is(are) checked against a
hot-list of tickets which have been canceled. This hot-list
might be implemented by storing a range of issue timestamps
for "suspect tickets"; if a presented ticket had an authtime
in that range, it would be rejected. In this way, a stolen
ticket-granting ticket or renewable ticket cannot be used to
gain additional tickets (renewals or otherwise) once the
theft has been reported. Any normal ticket obtained before
it was reported stolen will still be valid (because they
require no interaction with the KDC), but only until their
normal expiration time.
The ciphertext part of the response in the KRB_TGS_REP
message is encrypted in the sub-session key from the Authen-
ticator, if present, or the session key key from the
ticket-granting ticket. It is not encrypted using the
client's secret key. Furthermore, the client's key's
expiration date and the key version number fields are left
out since these values are stored along with the client's
database record, and that record is not needed to satisfy a
request based on a ticket-granting ticket. See section A.6
for pseudocode.
3.3.3.2. Encoding the transited field
If the identity of the server in the TGT that is
presented to the KDC as part of the authentication header is
that of the ticket-granting service, but the TGT was issued
from another realm, the KDC will look up the inter-realm key
shared with that realm and use that key to decrypt the
ticket. If the ticket is valid, then the KDC will honor the
request, subject to the constraints outlined above in the
section describing the AS exchange. The realm part of the
client's identity will be taken from the ticket-granting
ticket. The name of the realm that issued the ticket-
granting ticket will be added to the transited field of the
ticket to be issued. This is accomplished by reading the
transited field from the ticket-granting ticket (which is
treated as an unordered set of realm names), adding the new
realm to the set, then constructing and writing out its
encoded (shorthand) form (this may involve a rearrangement
of the existing encoding).
Section 3.3.3.2. - 30 - Expires 11 January 1998
Version 5 - Specification Revision 6
Note that the ticket-granting service does not add the
name of its own realm. Instead, its responsibility is to
add the name of the previous realm. This prevents a mali-
cious Kerberos server from intentionally leaving out its own
name (it could, however, omit other realms' names).
The names of neither the local realm nor the
principal's realm are to be included in the transited field.
They appear elsewhere in the ticket and both are known to
have taken part in authenticating the principal. Since the
endpoints are not included, both local and single-hop
inter-realm authentication result in a transited field that
is empty.
Because the name of each realm transited is added to
this field, it might potentially be very long. To decrease
the length of this field, its contents are encoded. The
initially supported encoding is optimized for the normal
case of inter-realm communication: a hierarchical arrange-
ment of realms using either domain or X.500 style realm
names. This encoding (called DOMAIN-X500-COMPRESS) is now
described.
Realm names in the transited field are separated by a
",". The ",", "\", trailing "."s, and leading spaces (" ")
are special characters, and if they are part of a realm
name, they must be quoted in the transited field by preced-
ing them with a "\".
A realm name ending with a "." is interpreted as being
prepended to the previous realm. For example, we can encode
traversal of EDU, MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU,
and CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-
points, that they would not be included in this field, and
we would have:
"EDU,MIT.,WASHINGTON.EDU"
A realm name beginning with a "/" is interpreted as being
appended to the previous realm[18]. If it is to stand by
itself, then it should be preceded by a space (" "). For
example, we can encode traversal of /COM/HP/APOLLO, /COM/HP,
/COM, and /COM/DEC as:
"/COM,/HP,/APOLLO, /COM/DEC".
__________________________
[18] For the purpose of appending, the realm preceding
the first listed realm is considered to be the null
realm ("").
Section 3.3.3.2. - 31 - Expires 11 January 1998
Version 5 - Specification Revision 6
Like the example above, if /COM/HP/APOLLO and /COM/DEC are
endpoints, they they would not be included in this field,
and we would have:
"/COM,/HP"
A null subfield preceding or following a "," indicates
that all realms between the previous realm and the next
realm have been traversed[19]. Thus, "," means that all
realms along the path between the client and the server have
been traversed. ",EDU, /COM," means that that all realms
from the client's realm up to EDU (in a domain style hierar-
chy) have been traversed, and that everything from /COM down
to the server's realm in an X.500 style has also been
traversed. This could occur if the EDU realm in one hierar-
chy shares an inter-realm key directly with the /COM realm
in another hierarchy.
3.3.4. Receipt of KRB_TGS_REP message
When the KRB_TGS_REP is received by the client, it is pro-
cessed in the same manner as the KRB_AS_REP processing
described above. The primary difference is that the cipher-
text part of the response must be decrypted using the ses-
sion key from the ticket-granting ticket rather than the
client's secret key. See section A.7 for pseudocode.
3.4. The KRB_SAFE Exchange
The KRB_SAFE message may be used by clients requiring
the ability to detect modifications of messages they
exchange. It achieves this by including a keyed collision-
proof checksum of the user data and some control informa-
tion. The checksum is keyed with an encryption key (usually
the last key negotiated via subkeys, or the session key if
no negotiation has occured).
3.4.1. Generation of a KRB_SAFE message
When an application wishes to send a KRB_SAFE message, it
collects its data and the appropriate control information
and computes a checksum over them. The checksum algorithm
should be a keyed one-way hash function (such as the RSA-
MD5-DES checksum algorithm specified in section 6.4.5, or
the DES MAC), generated using the sub-session key if
present, or the session key. Different algorithms may be
__________________________
[19] For the purpose of interpreting null subfields,
the client's realm is considered to precede those in
the transited field, and the server's realm is con-
sidered to follow them.
Section 3.4.1. - 32 - Expires 11 January 1998
Version 5 - Specification Revision 6
selected by changing the checksum type in the message.
Unkeyed or non-collision-proof checksums are not suitable
for this use.
The control information for the KRB_SAFE message
includes both a timestamp and a sequence number. The
designer of an application using the KRB_SAFE message must
choose at least one of the two mechanisms. This choice
should be based on the needs of the application protocol.
Sequence numbers are useful when all messages sent will
be received by one's peer. Connection state is presently
required to maintain the session key, so maintaining the
next sequence number should not present an additional prob-
lem.
If the application protocol is expected to tolerate
lost messages without them being resent, the use of the
timestamp is the appropriate replay detection mechanism.
Using timestamps is also the appropriate mechanism for
multi-cast protocols where all of one's peers share a common
sub-session key, but some messages will be sent to a subset
of one's peers.
After computing the checksum, the client then transmits
the information and checksum to the recipient in the message
format specified in section 5.6.1.
3.4.2. Receipt of KRB_SAFE message
When an application receives a KRB_SAFE message, it verifies
it as follows. If any error occurs, an error code is
reported for use by the application.
The message is first checked by verifying that the pro-
tocol version and type fields match the current version and
KRB_SAFE, respectively. A mismatch generates a
KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application verifies that the checksum used is a collision-
proof keyed checksum, and if it is not, a
KRB_AP_ERR_INAPP_CKSUM error is generated. The recipient
verifies that the operating system's report of the sender's
address matches the sender's address in the message, and (if
a recipient address is specified or the recipient requires
an address) that one of the recipient's addresses appears as
the recipient's address in the message. A failed match for
either case generates a KRB_AP_ERR_BADADDR error. Then the
timestamp and usec and/or the sequence number fields are
checked. If timestamp and usec are expected and not
present, or they are present but not current, the
KRB_AP_ERR_SKEW error is generated. If the server name,
along with the client name, time and microsecond fields from
the Authenticator match any recently-seen (sent or
received[20] ) such tuples, the KRB_AP_ERR_REPEAT error is
__________________________
[20] This means that a client and server running on the
Version 5 - Specification Revision 6
generated. If an incorrect sequence number is included, or
a sequence number is expected but not present, the
KRB_AP_ERR_BADORDER error is generated. If neither a time-
stamp and usec or a sequence number is present, a
KRB_AP_ERR_MODIFIED error is generated. Finally, the check-
sum is computed over the data and control information, and
if it doesn't match the received checksum, a
KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application is assured
that the message was generated by its peer and was not modi-
fied in transit.
3.5. The KRB_PRIV Exchange
The KRB_PRIV message may be used by clients requiring
confidentiality and the ability to detect modifications of
exchanged messages. It achieves this by encrypting the mes-
sages and adding control information.
3.5.1. Generation of a KRB_PRIV message
When an application wishes to send a KRB_PRIV message, it
collects its data and the appropriate control information
(specified in section 5.7.1) and encrypts them under an
encryption key (usually the last key negotiated via subkeys,
or the session key if no negotiation has occured). As part
of the control information, the client must choose to use
either a timestamp or a sequence number (or both); see the
discussion in section 3.4.1 for guidelines on which to use.
After the user data and control information are encrypted,
the client transmits the ciphertext and some "envelope"
information to the recipient.
3.5.2. Receipt of KRB_PRIV message
When an application receives a KRB_PRIV message, it verifies
it as follows. If any error occurs, an error code is
reported for use by the application.
The message is first checked by verifying that the pro-
tocol version and type fields match the current version and
KRB_PRIV, respectively. A mismatch generates a
KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application then decrypts the ciphertext and processes the
resultant plaintext. If decryption shows the data to have
been modified, a KRB_AP_ERR_BAD_INTEGRITY error is gen-
erated. The recipient verifies that the operating system's
report of the sender's address matches the sender's address
__________________________
same host and communicating with one another using the
KRB_SAFE messages should not share a common replay
cache to detect KRB_SAFE replays.
Section 3.5.2. - 34 - Expires 11 January 1998
Version 5 - Specification Revision 6
in the message, and (if a recipient address is specified or
the recipient requires an address) that one of the
recipient's addresses appears as the recipient's address in
the message. A failed match for either case generates a
KRB_AP_ERR_BADADDR error. Then the timestamp and usec
and/or the sequence number fields are checked. If timestamp
and usec are expected and not present, or they are present
but not current, the KRB_AP_ERR_SKEW error is generated. If
the server name, along with the client name, time and
microsecond fields from the Authenticator match any
recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
generated. If an incorrect sequence number is included, or
a sequence number is expected but not present, the
KRB_AP_ERR_BADORDER error is generated. If neither a time-
stamp and usec or a sequence number is present, a
KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application can assume
the message was generated by its peer, and was securely
transmitted (without intruders able to see the unencrypted
contents).
3.6. The KRB_CRED Exchange
The KRB_CRED message may be used by clients requiring
the ability to send Kerberos credentials from one host to
another. It achieves this by sending the tickets together
with encrypted data containing the session keys and other
information associated with the tickets.
3.6.1. Generation of a KRB_CRED message
When an application wishes to send a KRB_CRED message it
first (using the KRB_TGS exchange) obtains credentials to be
sent to the remote host. It then constructs a KRB_CRED mes-
sage using the ticket or tickets so obtained, placing the
session key needed to use each ticket in the key field of
the corresponding KrbCredInfo sequence of the encrypted part
of the the KRB_CRED message.
Other information associated with each ticket and
obtained during the KRB_TGS exchange is also placed in the
corresponding KrbCredInfo sequence in the encrypted part of
the KRB_CRED message. The current time and, if specifically
required by the application the nonce, s-address, and r-
address fields, are placed in the encrypted part of the
KRB_CRED message which is then encrypted under an encryption
key previosuly exchanged in the KRB_AP exchange (usually the
last key negotiated via subkeys, or the session key if no
negotiation has occured).
3.6.2. Receipt of KRB_CRED message
When an application receives a KRB_CRED message, it verifies
Section 3.6.2. - 35 - Expires 11 January 1998
Version 5 - Specification Revision 6
it. If any error occurs, an error code is reported for use
by the application. The message is verified by checking
that the protocol version and type fields match the current
version and KRB_CRED, respectively. A mismatch generates a
KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application then decrypts the ciphertext and processes the
resultant plaintext. If decryption shows the data to have
been modified, a KRB_AP_ERR_BAD_INTEGRITY error is gen-
erated.
If present or required, the recipient verifies that the
operating system's report of the sender's address matches
the sender's address in the message, and that one of the
recipient's addresses appears as the recipient's address in
the message. A failed match for either case generates a
KRB_AP_ERR_BADADDR error. The timestamp and usec fields
(and the nonce field if required) are checked next. If the
timestamp and usec are not present, or they are present but
not current, the KRB_AP_ERR_SKEW error is generated.
If all the checks succeed, the application stores each
of the new tickets in its ticket cache together with the
session key and other information in the corresponding
KrbCredInfo sequence from the encrypted part of the KRB_CRED
message.
4. The Kerberos Database
The Kerberos server must have access to a database contain-
ing the principal identifiers and secret keys of principals
to be authenticated[21].
4.1. Database contents
A database entry should contain at least the following
fields:
Field Value
name Principal's identif-
ier
key Principal's secret key
p_kvno Principal's key version
max_life Maximum lifetime for Tickets
__________________________
[21] The implementation of the Kerberos server need not
combine the database and the server on the same
machine; it is feasible to store the principal database
in, say, a network name service, as long as the entries
stored therein are protected from disclosure to and
modification by unauthorized parties. However, we
recommend against such strategies, as they can make
system management and threat analysis quite complex.
Section 4.1. - 36 - Expires 11 January 1998
Version 5 - Specification Revision 6
max_renewable_life Maximum total lifetime for renewable Tickets
The name field is an encoding of the principal's identifier.
The key field contains an encryption key. This key is the
principal's secret key. (The key can be encrypted before
storage under a Kerberos "master key" to protect it in case
the database is compromised but the master key is not. In
that case, an extra field must be added to indicate the mas-
ter key version used, see below.) The p_kvno field is the
key version number of the principal's secret key. The
max_life field contains the maximum allowable lifetime (end-
time - starttime) for any Ticket issued for this principal.
The max_renewable_life field contains the maximum allowable
total lifetime for any renewable Ticket issued for this
principal. (See section 3.1 for a description of how these
lifetimes are used in determining the lifetime of a given
Ticket.)
A server may provide KDC service to several realms, as
long as the database representation provides a mechanism to
distinguish between principal records with identifiers which
differ only in the realm name.
When an application server's key changes, if the change
is routine (i.e. not the result of disclosure of the old
key), the old key should be retained by the server until all
tickets that had been issued using that key have expired.
Because of this, it is possible for several keys to be
active for a single principal. Ciphertext encrypted in a
principal's key is always tagged with the version of the key
that was used for encryption, to help the recipient find the
proper key for decryption.
When more than one key is active for a particular prin-
cipal, the principal will have more than one record in the
Kerberos database. The keys and key version numbers will
differ between the records (the rest of the fields may or
may not be the same). Whenever Kerberos issues a ticket, or
responds to a request for initial authentication, the most
recent key (known by the Kerberos server) will be used for
encryption. This is the key with the highest key version
number.
4.2. Additional fields
Project Athena's KDC implementation uses additional fields
in its database:
Field Value
K_kvno Kerberos' key version
expiration Expiration date for entry
attributes Bit field of attributes
mod_date Timestamp of last modification
Section 4.2. - 37 - Expires 11 January 1998
Version 5 - Specification Revision 6
mod_name Modifying principal's identifier
The K_kvno field indicates the key version of the Kerberos
master key under which the principal's secret key is
encrypted.
After an entry's expiration date has passed, the KDC
will return an error to any client attempting to gain tick-
ets as or for the principal. (A database may want to main-
tain two expiration dates: one for the principal, and one
for the principal's current key. This allows password aging
to work independently of the principal's expiration date.
However, due to the limited space in the responses, the KDC
must combine the key expiration and principal expiration
date into a single value called "key_exp", which is used as
a hint to the user to take administrative action.)
The attributes field is a bitfield used to govern the
operations involving the principal. This field might be
useful in conjunction with user registration procedures, for
site-specific policy implementations (Project Athena
currently uses it for their user registration process con-
trolled by the system-wide database service, Moira [9]), to
identify whether a principal can play the role of a client
or server or both, to note whether a server is appropriate
trusted to recieve credentials delegated by a client, or to
identify the "string to key" conversion algorithm used for a
principal's key[22]. Other bits are used to indicate that
certain ticket options should not be allowed in tickets
encrypted under a principal's key (one bit each): Disallow
issuing postdated tickets, disallow issuing forwardable
tickets, disallow issuing tickets based on TGT authentica-
tion, disallow issuing renewable tickets, disallow issuing
proxiable tickets, and disallow issuing tickets for which
the principal is the server.
The mod_date field contains the time of last modifica-
tion of the entry, and the mod_name field contains the name
of the principal which last modified the entry.
4.3. Frequently Changing Fields
Some KDC implementations may wish to maintain the last
time that a request was made by a particular principal.
Information that might be maintained includes the time of
the last request, the time of the last request for a
ticket-granting ticket, the time of the last use of a
ticket-granting ticket, or other times. This information
can then be returned to the user in the last-req field (see
__________________________
[22] See the discussion of the padata field in section
5.4.2 for details on why this can be useful.
Section 4.3. - 38 - Expires 11 January 1998
Version 5 - Specification Revision 6
section 5.2).
Other frequently changing information that can be main-
tained is the latest expiration time for any tickets that
have been issued using each key. This field would be used
to indicate how long old keys must remain valid to allow the
continued use of outstanding tickets.
4.4. Site Constants
The KDC implementation should have the following confi-
gurable constants or options, to allow an administrator to
make and enforce policy decisions:
+ The minimum supported lifetime (used to determine whether
the KDC_ERR_NEVER_VALID error should be returned). This
constant should reflect reasonable expectations of
round-trip time to the KDC, encryption/decryption time,
and processing time by the client and target server, and
it should allow for a minimum "useful" lifetime.
+ The maximum allowable total (renewable) lifetime of a
ticket (renew_till - starttime).
+ The maximum allowable lifetime of a ticket (endtime -
starttime).
+ Whether to allow the issue of tickets with empty address
fields (including the ability to specify that such tick-
ets may only be issued if the request specifies some
authorization_data).
+ Whether proxiable, forwardable, renewable or post-datable
tickets are to be issued.
5. Message Specifications
The following sections describe the exact contents and
encoding of protocol messages and objects. The ASN.1 base
definitions are presented in the first subsection. The
remaining subsections specify the protocol objects (tickets
and authenticators) and messages. Specification of encryp-
tion and checksum techniques, and the fields related to
them, appear in section 6.
5.1. ASN.1 Distinguished Encoding Representation
All uses of ASN.1 in Kerberos shall use the Dis-
tinguished Encoding Representation of the data elements as
described in the X.509 specification, section 8.7 [10].
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5.2. ASN.1 Base Definitions
The following ASN.1 base definitions are used in the
rest of this section. Note that since the underscore char-
acter (_) is not permitted in ASN.1 names, the hyphen (-) is
used in its place for the purposes of ASN.1 names.
Realm ::= GeneralString
PrincipalName ::= SEQUENCE {
name-type[0] INTEGER,
name-string[1] SEQUENCE OF GeneralString
}
Kerberos realms are encoded as GeneralStrings. Realms shall
not contain a character with the code 0 (the ASCII NUL).
Most realms will usually consist of several components
separated by periods (.), in the style of Internet Domain
Names, or separated by slashes (/) in the style of X.500
names. Acceptable forms for realm names are specified in
section 7. A PrincipalName is a typed sequence of com-
ponents consisting of the following sub-fields:
name-type This field specifies the type of name that fol-
lows. Pre-defined values for this field are
specified in section 7.2. The name-type should be
treated as a hint. Ignoring the name type, no two
names can be the same (i.e. at least one of the
components, or the realm, must be different).
This constraint may be eliminated in the future.
name-stringThis field encodes a sequence of components that
form a name, each component encoded as a General-
String. Taken together, a PrincipalName and a
Realm form a principal identifier. Most Princi-
palNames will have only a few components (typi-
cally one or two).
KerberosTime ::= GeneralizedTime
-- Specifying UTC time zone (Z)
The timestamps used in Kerberos are encoded as General-
izedTimes. An encoding shall specify the UTC time zone (Z)
and shall not include any fractional portions of the
seconds. It further shall not include any separators.
Example: The only valid format for UTC time 6 minutes, 27
seconds after 9 pm on 6 November 1985 is 19851106210627Z.
HostAddress ::= SEQUENCE {
addr-type[0] INTEGER,
address[1] OCTET STRING
Section 5.2. - 40 - Expires 11 January 1998
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}
HostAddresses ::= SEQUENCE OF SEQUENCE {
addr-type[0] INTEGER,
address[1] OCTET STRING
}
The host adddress encodings consists of two fields:
addr-type This field specifies the type of address that
follows. Pre-defined values for this field are
specified in section 8.1.
address This field encodes a single address of type addr-
type.
The two forms differ slightly. HostAddress contains exactly
one address; HostAddresses contains a sequence of possibly
many addresses.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type[0] INTEGER,
ad-data[1] OCTET STRING
}
ad-data This field contains authorization data to be
interpreted according to the value of the
corresponding ad-type field.
ad-type This field specifies the format for the ad-data
subfield. All negative values are reserved for
local use. Non-negative values are reserved for
registered use.
APOptions ::= BIT STRING {
reserved(0),
use-session-key(1),
mutual-required(2)
}
TicketFlags ::= BIT STRING {
reserved(0),
forwardable(1),
forwarded(2),
proxiable(3),
proxy(4),
may-postdate(5),
postdated(6),
invalid(7),
renewable(8),
initial(9),
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pre-authent(10),
hw-authent(11),
transited-policy-checked(12),
ok-as-delegate(13)
}
KDCOptions ::= BIT STRING {
reserved(0),
forwardable(1),
forwarded(2),
proxiable(3),
proxy(4),
allow-postdate(5),
postdated(6),
unused7(7),
renewable(8),
unused9(9),
unused10(10),
unused11(11),
unused12(12),
unused13(13),
disable-transited-check(26),
renewable-ok(27),
enc-tkt-in-skey(28),
renew(30),
validate(31)
}
ASN.1 Bit strings have a length and a value. When
used in Kerberos for the APOptions, TicketFlags,
and KDCOptions, the length of the bit string on
generated values should be the smallest multiple
of 32 bits needed to include the highest order bit
that is set (1), but in no case less than 32 bits.
Implementations should accept values of bit
strings of any length and treat the value of flags
cooresponding to bits beyond the end of the bit
string as if the bit were reset (0). Comparisonof
bit strings of different length should treat the
smaller string as if it were padded with zeros
beyond the high order bits to the length of the
longer string[23].
__________________________
[23] Warning for implementations that unpack and repack
data structures during the generation and verification
of embedded checksums: Because any checksums applied to
data structures must be checked against the original
data the length of bit strings must be preserved within
a data structure between the time that a checksum is
generated through transmission to the time that the
checksum is verified.
Section 5.2. - 42 - Expires 11 January 1998
Version 5 - Specification Revision 6
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type[0] INTEGER,
lr-value[1] KerberosTime
}
lr-type This field indicates how the following lr-value
field is to be interpreted. Negative values indi-
cate that the information pertains only to the
responding server. Non-negative values pertain to
all servers for the realm.
If the lr-type field is zero (0), then no informa-
tion is conveyed by the lr-value subfield. If the
absolute value of the lr-type field is one (1),
then the lr-value subfield is the time of last
initial request for a TGT. If it is two (2), then
the lr-value subfield is the time of last initial
request. If it is three (3), then the lr-value
subfield is the time of issue for the newest
ticket-granting ticket used. If it is four (4),
then the lr-value subfield is the time of the last
renewal. If it is five (5), then the lr-value
subfield is the time of last request (of any
type).
lr-value This field contains the time of the last request.
The time must be interpreted according to the con-
tents of the accompanying lr-type subfield.
See section 6 for the definitions of Checksum, Check-
sumType, EncryptedData, EncryptionKey, EncryptionType, and
KeyType.
5.3. Tickets and Authenticators
This section describes the format and encryption param-
eters for tickets and authenticators. When a ticket or
authenticator is included in a protocol message it is
treated as an opaque object.
5.3.1. Tickets
A ticket is a record that helps a client authenticate
to a service. A Ticket contains the following information:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno[0] INTEGER,
realm[1] Realm,
sname[2] PrincipalName,
enc-part[3] EncryptedData
}
Section 5.3.1. - 43 - Expires 11 January 1998
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-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags[0] TicketFlags,
key[1] EncryptionKey,
crealm[2] Realm,
cname[3] PrincipalName,
transited[4] TransitedEncoding,
authtime[5] KerberosTime,
starttime[6] KerberosTime OPTIONAL,
endtime[7] KerberosTime,
renew-till[8] KerberosTime OPTIONAL,
caddr[9] HostAddresses OPTIONAL,
authorization-data[10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type[0] INTEGER, -- must be registered
contents[1] OCTET STRING
}
The encoding of EncTicketPart is encrypted in the key shared
by Kerberos and the end server (the server's secret key).
See section 6 for the format of the ciphertext.
tkt-vno This field specifies the version number for the
ticket format. This document describes version
number 5.
realm This field specifies the realm that issued a
ticket. It also serves to identify the realm part
of the server's principal identifier. Since a
Kerberos server can only issue tickets for servers
within its realm, the two will always be identi-
cal.
sname This field specifies the name part of the server's
identity.
enc-part This field holds the encrypted encoding of the
EncTicketPart sequence.
flags This field indicates which of various options were
used or requested when the ticket was issued. It
is a bit-field, where the selected options are
indicated by the bit being set (1), and the
unselected options and reserved fields being reset
(0). Bit 0 is the most significant bit. The
encoding of the bits is specified in section 5.2.
The flags are described in more detail above in
section 2. The meanings of the flags are:
Section 5.3.1. - 44 - Expires 11 January 1998
Version 5 - Specification Revision 6
Bit(s) Name Description
0 RESERVED
Reserved for future expansion of this
field.
1 FORWARDABLE
The FORWARDABLE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. When set, this
flag tells the ticket-granting server
that it is OK to issue a new ticket-
granting ticket with a different network
address based on the presented ticket.
2 FORWARDED
When set, this flag indicates that the
ticket has either been forwarded or was
issued based on authentication involving
a forwarded ticket-granting ticket.
3 PROXIABLE
The PROXIABLE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. The PROXIABLE
flag has an interpretation identical to
that of the FORWARDABLE flag, except
that the PROXIABLE flag tells the
ticket-granting server that only non-
ticket-granting tickets may be issued
with different network addresses.
4 PROXY
When set, this flag indicates that a
ticket is a proxy.
5 MAY-POSTDATE
The MAY-POSTDATE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. This flag tells
the ticket-granting server that a post-
dated ticket may be issued based on this
ticket-granting ticket.
6 POSTDATED
This flag indicates that this ticket has
been postdated. The end-service can
check the authtime field to see when the
original authentication occurred.
7 INVALID
This flag indicates that a ticket is
invalid, and it must be validated by the
KDC before use. Application servers
must reject tickets which have this flag
set.
Section 5.3.1. - 45 - Expires 11 January 1998
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8 RENEWABLE
The RENEWABLE flag is normally only
interpreted by the TGS, and can usually
be ignored by end servers (some particu-
larly careful servers may wish to disal-
low renewable tickets). A renewable
ticket can be used to obtain a replace-
ment ticket that expires at a later
date.
9 INITIAL
This flag indicates that this ticket was
issued using the AS protocol, and not
issued based on a ticket-granting
ticket.
10 PRE-AUTHENT
This flag indicates that during initial
authentication, the client was authenti-
cated by the KDC before a ticket was
issued. The strength of the pre-
authentication method is not indicated,
but is acceptable to the KDC.
11 HW-AUTHENT
This flag indicates that the protocol
employed for initial authentication
required the use of hardware expected to
be possessed solely by the named client.
The hardware authentication method is
selected by the KDC and the strength of
the method is not indicated.
Section 5.3.1. - 46 - Expires 11 January 1998
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12 TRANSITED This flag indicates that the KDC for the
POLICY-CHECKED realm has checked the transited field
against a realm defined policy for
trusted certifiers. If this flag is
reset (0), then the application server
must check the transited field itself,
and if unable to do so it must reject
the authentication. If the flag is set
(1) then the application server may skip
its own validation of the transited
field, relying on the validation
performed by the KDC. At its option the
application server may still apply its
own validation based on a separate
policy for acceptance.
Section 5.3.1. - 47 - Expires 11 January 1998
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13 OK-AS-DELEGATE This flag indicates that the server (not
the client) specified in the ticket has
been determined by policy of the realm
to be a suitable recipient of
delegation. A client can use the
presence of this flag to help it make a
decision whether to delegate credentials
(either grant a proxy or a forwarded
ticket granting ticket) to this server.
The client is free to ignore the value
of this flag. When setting this flag,
an administrator should consider the
security and placement of the server on
which the service will run, as well as
whether the service requires the use of
delegated credentials.
Section 5.3.1. - 48 - Expires 11 January 1998
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14 ANONYMOUS
This flag indicates that the principal
named in the ticket is a generic princi-
pal for the realm and does not identify
the individual using the ticket. The
purpose of the ticket is only to
securely distribute a session key, and
not to identify the user. Subsequent
requests using the same ticket and ses-
sion may be considered as originating
from the same user, but requests with
the same username but a different ticket
are likely to originate from different
users.
15-31 RESERVED
Reserved for future use.
key This field exists in the ticket and the KDC
response and is used to pass the session key from
Kerberos to the application server and the client.
The field's encoding is described in section 6.2.
crealm This field contains the name of the realm in which
the client is registered and in which initial
authentication took place.
cname This field contains the name part of the client's
principal identifier.
transited This field lists the names of the Kerberos realms
that took part in authenticating the user to whom
this ticket was issued. It does not specify the
order in which the realms were transited. See
section 3.3.3.2 for details on how this field
encodes the traversed realms.
authtime This field indicates the time of initial authenti-
cation for the named principal. It is the time of
issue for the original ticket on which this ticket
is based. It is included in the ticket to provide
additional information to the end service, and to
provide the necessary information for implementa-
tion of a `hot list' service at the KDC. An end
service that is particularly paranoid could refuse
to accept tickets for which the initial authenti-
cation occurred "too far" in the past.
This field is also returned as part of the
response from the KDC. When returned as part of
the response to initial authentication
Section 5.3.1. - 49 - Expires 11 January 1998
Version 5 - Specification Revision 6
(KRB_AS_REP), this is the current time on the Ker-
beros server[24].
starttime This field in the ticket specifies the time after
which the ticket is valid. Together with endtime,
this field specifies the life of the ticket. If
it is absent from the ticket, its value should be
treated as that of the authtime field.
endtime This field contains the time after which the
ticket will not be honored (its expiration time).
Note that individual services may place their own
limits on the life of a ticket and may reject
tickets which have not yet expired. As such, this
is really an upper bound on the expiration time
for the ticket.
renew-tillThis field is only present in tickets that have
the RENEWABLE flag set in the flags field. It
indicates the maximum endtime that may be included
in a renewal. It can be thought of as the abso-
lute expiration time for the ticket, including all
renewals.
caddr This field in a ticket contains zero (if omitted)
or more (if present) host addresses. These are
the addresses from which the ticket can be used.
If there are no addresses, the ticket can be used
from any location. The decision by the KDC to
issue or by the end server to accept zero-address
tickets is a policy decision and is left to the
Kerberos and end-service administrators; they may
refuse to issue or accept such tickets. The sug-
gested and default policy, however, is that such
tickets will only be issued or accepted when addi-
tional information that can be used to restrict
the use of the ticket is included in the
authorization_data field. Such a ticket is a
capability.
Network addresses are included in the ticket to
make it harder for an attacker to use stolen
credentials. Because the session key is not sent
over the network in cleartext, credentials can't
__________________________
[24] It is NOT recommended that this time value be used
to adjust the workstation's clock since the workstation
cannot reliably determine that such a KRB_AS_REP actu-
ally came from the proper KDC in a timely manner.
Section 5.3.1. - 50 - Expires 11 January 1998
Version 5 - Specification Revision 6
be stolen simply by listening to the network; an
attacker has to gain access to the session key
(perhaps through operating system security
breaches or a careless user's unattended session)
to make use of stolen tickets.
It is important to note that the network address
from which a connection is received cannot be
reliably determined. Even if it could be, an
attacker who has compromised the client's worksta-
tion could use the credentials from there.
Including the network addresses only makes it more
difficult, not impossible, for an attacker to walk
off with stolen credentials and then use them from
a "safe" location.
authorization-data
The authorization-data field is used to pass
authorization data from the principal on whose
behalf a ticket was issued to the application ser-
vice. If no authorization data is included, this
field will be left out. Experience has shown that
the name of this field is confusing, and that a
better name for this field would be restrictions.
Unfortunately, it is not possible to change the
name of this field at this time.
This field contains restrictions on any authority
obtained on the bases of authentication using the
ticket. It is possible for any principal in
posession of credentials to add entries to the
authorization data field since these entries
further restrict what can be done with the ticket.
Such additions can be made by specifying the addi-
tional entries when a new ticket is obtained dur-
ing the TGS exchange, or they may be added during
chained delegation using the authorization data
field of the authenticator.
Because entries may be added to this field by the
holder of credentials, it is not allowable for the
presence of an entry in the authorization data
field of a ticket to amplify the priveleges one
would obtain from using a ticket.
The data in this field may be specific to the end
service; the field will contain the names of ser-
vice specific objects, and the rights to those
objects. The format for this field is described
in section 5.2. Although Kerberos is not con-
cerned with the format of the contents of the sub-
fields, it does carry type information (ad-type).
Section 5.3.1. - 51 - Expires 11 January 1998
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By using the authorization_data field, a principal
is able to issue a proxy that is valid for a
specific purpose. For example, a client wishing
to print a file can obtain a file server proxy to
be passed to the print server. By specifying the
name of the file in the authorization_data field,
the file server knows that the print server can
only use the client's rights when accessing the
particular file to be printed.
A separate service providing providing authoriza-
tion or certifying group membership may be built
using the authorization-data field. In this case,
the entity granting authorization (not the author-
ized entity), obtains a ticket in its own name
(e.g. the ticket is issued in the name of a
privelege server), and this entity adds restric-
tions on its own authority and delegates the res-
tricted authority through a proxy to the client.
The client would then present this authorization
credential to the application server separately
from the authentication exchange.
Similarly, if one specifies the authorization-data
field of a proxy and leaves the host addresses
blank, the resulting ticket and session key can be
treated as a capability. See [7] for some sug-
gested uses of this field.
The authorization-data field is optional and does
not have to be included in a ticket.
5.3.2. Authenticators
An authenticator is a record sent with a ticket to a
server to certify the client's knowledge of the encryption
key in the ticket, to help the server detect replays, and to
help choose a "true session key" to use with the particular
session. The encoding is encrypted in the ticket's session
key shared by the client and the server:
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno[0] INTEGER,
crealm[1] Realm,
cname[2] PrincipalName,
cksum[3] Checksum OPTIONAL,
cusec[4] INTEGER,
ctime[5] KerberosTime,
subkey[6] EncryptionKey OPTIONAL,
seq-number[7] INTEGER OPTIONAL,
authorization-data[8] AuthorizationData OPTIONAL
}
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authenticator-vno
This field specifies the version number for the
format of the authenticator. This document speci-
fies version 5.
crealm and cname
These fields are the same as those described for
the ticket in section 5.3.1.
cksum This field contains a checksum of the the applica-
tion data that accompanies the KRB_AP_REQ.
cusec This field contains the microsecond part of the
client's timestamp. Its value (before encryption)
ranges from 0 to 999999. It often appears along
with ctime. The two fields are used together to
specify a reasonably accurate timestamp.
ctime This field contains the current time on the
client's host.
subkey This field contains the client's choice for an
encryption key which is to be used to protect this
specific application session. Unless an applica-
tion specifies otherwise, if this field is left
out the session key from the ticket will be used.
seq-numberThis optional field includes the initial sequence
number to be used by the KRB_PRIV or KRB_SAFE mes-
sages when sequence numbers are used to detect
replays (It may also be used by application
specific messages). When included in the authen-
ticator this field specifies the initial sequence
number for messages from the client to the server.
When included in the AP-REP message, the initial
sequence number is that for messages from the
server to the client. When used in KRB_PRIV or
KRB_SAFE messages, it is incremented by one after
each message is sent.
For sequence numbers to adequately support the
detection of replays they should be non-repeating,
even across connection boundaries. The initial
sequence number should be random and uniformly
distributed across the full space of possible
sequence numbers, so that it cannot be guessed by
an attacker and so that it and the successive
sequence numbers do not repeat other sequences.
Section 5.3.2. - 53 - Expires 11 January 1998
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authorization-data
This field is the same as described for the ticket
in section 5.3.1. It is optional and will only
appear when additional restrictions are to be
placed on the use of a ticket, beyond those car-
ried in the ticket itself.
5.4. Specifications for the AS and TGS exchanges
This section specifies the format of the messages used
in the exchange between the client and the Kerberos server.
The format of possible error messages appears in section
5.9.1.
5.4.1. KRB_KDC_REQ definition
The KRB_KDC_REQ message has no type of its own.
Instead, its type is one of KRB_AS_REQ or KRB_TGS_REQ
depending on whether the request is for an initial ticket or
an additional ticket. In either case, the message is sent
from the client to the Authentication Server to request
credentials for a service.
The message fields are:
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
pvno[1] INTEGER,
msg-type[2] INTEGER,
padata[3] SEQUENCE OF PA-DATA OPTIONAL,
req-body[4] KDC-REQ-BODY
}
PA-DATA ::= SEQUENCE {
padata-type[1] INTEGER,
padata-value[2] OCTET STRING,
-- might be encoded AP-REQ
}
KDC-REQ-BODY ::= SEQUENCE {
kdc-options[0] KDCOptions,
cname[1] PrincipalName OPTIONAL,
-- Used only in AS-REQ
realm[2] Realm, -- Server's realm
-- Also client's in AS-REQ
sname[3] PrincipalName OPTIONAL,
from[4] KerberosTime OPTIONAL,
till[5] KerberosTime OPTIONAL,
rtime[6] KerberosTime OPTIONAL,
nonce[7] INTEGER,
etype[8] SEQUENCE OF INTEGER,
-- EncryptionType,
-- in preference order
Section 5.4.1. - 54 - Expires 11 January 1998
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addresses[9] HostAddresses OPTIONAL,
enc-authorization-data[10] EncryptedData OPTIONAL,
-- Encrypted AuthorizationData
-- encoding
additional-tickets[11] SEQUENCE OF Ticket OPTIONAL
}
The fields in this message are:
pvno This field is included in each message, and speci-
fies the protocol version number. This document
specifies protocol version 5.
msg-type This field indicates the type of a protocol mes-
sage. It will almost always be the same as the
application identifier associated with a message.
It is included to make the identifier more readily
accessible to the application. For the KDC-REQ
message, this type will be KRB_AS_REQ or
KRB_TGS_REQ.
padata The padata (pre-authentication data) field con-
tains a sequence of authentication information
which may be needed before credentials can be
issued or decrypted. In the case of requests for
additional tickets (KRB_TGS_REQ), this field will
include an element with padata-type of PA-TGS-REQ
and data of an authentication header (ticket-
granting ticket and authenticator). The checksum
in the authenticator (which must be collision-
proof) is to be computed over the KDC-REQ-BODY
encoding. In most requests for initial authenti-
cation (KRB_AS_REQ) and most replies (KDC-REP),
the padata field will be left out.
This field may also contain information needed by
certain extensions to the Kerberos protocol. For
example, it might be used to initially verify the
identity of a client before any response is
returned. This is accomplished with a padata
field with padata-type equal to PA-ENC-TIMESTAMP
and padata-value defined as follows:
padata-type ::= PA-ENC-TIMESTAMP
padata-value ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp[0] KerberosTime, -- client's time
pausec[1] INTEGER OPTIONAL
}
with patimestamp containing the client's time and
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pausec containing the microseconds which may be
omitted if a client will not generate more than
one request per second. The ciphertext (padata-
value) consists of the PA-ENC-TS-ENC sequence,
encrypted using the client's secret key.
The padata field can also contain information
needed to help the KDC or the client select the
key needed for generating or decrypting the
response. This form of the padata is useful for
supporting the use of certain token cards with
Kerberos. The details of such extensions are
specified in separate documents. See [11] for
additional uses of this field.
padata-type
The padata-type element of the padata field indi-
cates the way that the padata-value element is to
be interpreted. Negative values of padata-type
are reserved for unregistered use; non-negative
values are used for a registered interpretation of
the element type.
req-body This field is a placeholder delimiting the extent
of the remaining fields. If a checksum is to be
calculated over the request, it is calculated over
an encoding of the KDC-REQ-BODY sequence which is
enclosed within the req-body field.
kdc-options
This field appears in the KRB_AS_REQ and
KRB_TGS_REQ requests to the KDC and indicates the
flags that the client wants set on the tickets as
well as other information that is to modify the
behavior of the KDC. Where appropriate, the name
of an option may be the same as the flag that is
set by that option. Although in most case, the
bit in the options field will be the same as that
in the flags field, this is not guaranteed, so it
is not acceptable to simply copy the options field
to the flags field. There are various checks that
must be made before honoring an option anyway.
The kdc_options field is a bit-field, where the
selected options are indicated by the bit being
set (1), and the unselected options and reserved
fields being reset (0). The encoding of the bits
is specified in section 5.2. The options are
described in more detail above in section 2. The
meanings of the options are:
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Bit(s) Name Description
0 RESERVED
Reserved for future expansion of this
field.
1 FORWARDABLE
The FORWARDABLE option indicates that
the ticket to be issued is to have its
forwardable flag set. It may only be
set on the initial request, or in a sub-
sequent request if the ticket-granting
ticket on which it is based is also for-
wardable.
2 FORWARDED
The FORWARDED option is only specified
in a request to the ticket-granting
server and will only be honored if the
ticket-granting ticket in the request
has its FORWARDABLE bit set. This
option indicates that this is a request
for forwarding. The address(es) of the
host from which the resulting ticket is
to be valid are included in the
addresses field of the request.
3 PROXIABLE
The PROXIABLE option indicates that the
ticket to be issued is to have its prox-
iable flag set. It may only be set on
the initial request, or in a subsequent
request if the ticket-granting ticket on
which it is based is also proxiable.
4 PROXY
The PROXY option indicates that this is
a request for a proxy. This option will
only be honored if the ticket-granting
ticket in the request has its PROXIABLE
bit set. The address(es) of the host
from which the resulting ticket is to be
valid are included in the addresses
field of the request.
5 ALLOW-POSTDATE
The ALLOW-POSTDATE option indicates that
the ticket to be issued is to have its
MAY-POSTDATE flag set. It may only be
set on the initial request, or in a sub-
sequent request if the ticket-granting
ticket on which it is based also has its
MAY-POSTDATE flag set.
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6 POSTDATED
The POSTDATED option indicates that this
is a request for a postdated ticket.
This option will only be honored if the
ticket-granting ticket on which it is
based has its MAY-POSTDATE flag set.
The resulting ticket will also have its
INVALID flag set, and that flag may be
reset by a subsequent request to the KDC
after the starttime in the ticket has
been reached.
7 UNUSED
This option is presently unused.
8 RENEWABLE
The RENEWABLE option indicates that the
ticket to be issued is to have its
RENEWABLE flag set. It may only be set
on the initial request, or when the
ticket-granting ticket on which the
request is based is also renewable. If
this option is requested, then the rtime
field in the request contains the
desired absolute expiration time for the
ticket.
9-13 UNUSED
These options are presently unused.
14 REQUEST-ANONYMOUS
The REQUEST-ANONYMOUS option indicates
that the ticket to be issued is not to
identify the user to which it was
issued. Instead, the principal identif-
ier is to be generic, as specified by
the policy of the realm (e.g. usually
anonymous@realm). The purpose of the
ticket is only to securely distribute a
session key, and not to identify the
user. The ANONYMOUS flag on the ticket
to be returned should be set. If the
local realms policy does not permit
anonymous credentials, the request is to
be rejected.
15-25 RESERVED
Reserved for future use.
26 DISABLE-TRANSITED-CHECK
By default the KDC will check the
transited field of a ticket-granting-
ticket against the policy of the local
realm before it will issue derivative
tickets based on the ticket granting
ticket. If this flag is set in the
request, checking of the transited field
is disabled. Tickets issued without the
performance of this check will be noted
by the reset (0) value of the
TRANSITED-POLICY-CHECKED flag,
indicating to the application server
that the tranisted field must be checked
locally. KDC's are encouraged but not
required to honor the
DISABLE-TRANSITED-CHECK option.
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27 RENEWABLE-OK
The RENEWABLE-OK option indicates that a
renewable ticket will be acceptable if a
ticket with the requested life cannot
otherwise be provided. If a ticket with
the requested life cannot be provided,
then a renewable ticket may be issued
with a renew-till equal to the the
requested endtime. The value of the
renew-till field may still be limited by
local limits, or limits selected by the
individual principal or server.
28 ENC-TKT-IN-SKEY
This option is used only by the ticket-
granting service. The ENC-TKT-IN-SKEY
option indicates that the ticket for the
end server is to be encrypted in the
session key from the additional ticket-
granting ticket provided.
29 RESERVED
Reserved for future use.
30 RENEW
This option is used only by the ticket-
granting service. The RENEW option
indicates that the present request is
for a renewal. The ticket provided is
encrypted in the secret key for the
server on which it is valid. This
option will only be honored if the
ticket to be renewed has its RENEWABLE
flag set and if the time in its renew-
till field has not passed. The ticket
to be renewed is passed in the padata
field as part of the authentication
header.
31 VALIDATE
This option is used only by the ticket-
granting service. The VALIDATE option
indicates that the request is to vali-
date a postdated ticket. It will only
be honored if the ticket presented is
postdated, presently has its INVALID
flag set, and would be otherwise usable
at this time. A ticket cannot be vali-
dated before its starttime. The ticket
presented for validation is encrypted in
the key of the server for which it is
valid and is passed in the padata field
as part of the authentication header.
cname and sname
These fields are the same as those described for
the ticket in section 5.3.1. sname may only be
Section 5.4.1. - 59 - Expires 11 January 1998
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absent when the ENC-TKT-IN-SKEY option is speci-
fied. If absent, the name of the server is taken
from the name of the client in the ticket passed
as additional-tickets.
enc-authorization-data
The enc-authorization-data, if present (and it can
only be present in the TGS_REQ form), is an encod-
ing of the desired authorization-data encrypted
under the sub-session key if present in the
Authenticator, or alternatively from the session
key in the ticket-granting ticket, both from the
padata field in the KRB_AP_REQ.
realm This field specifies the realm part of the
server's principal identifier. In the AS
exchange, this is also the realm part of the
client's principal identifier.
from This field is included in the KRB_AS_REQ and
KRB_TGS_REQ ticket requests when the requested
ticket is to be postdated. It specifies the
desired start time for the requested ticket.
till This field contains the expiration date requested
by the client in a ticket request. It is option
and if omitted the requested ticket is to have the
maximum endtime permitted according to KDC policy
for the parties to the authentication exchange as
limited by expiration date of the ticket granting
ticket or other preauthentication credentials.
rtime This field is the requested renew-till time sent
from a client to the KDC in a ticket request. It
is optional.
nonce This field is part of the KDC request and
response. It it intended to hold a random number
generated by the client. If the same number is
included in the encrypted response from the KDC,
it provides evidence that the response is fresh
and has not been replayed by an attacker. Nonces
must never be re-used. Ideally, it should be gen-
erated randomly, but if the correct time is known,
it may suffice[25].
__________________________
[25] Note, however, that if the time is used as the
Section 5.4.1. - 60 - Expires 11 January 1998
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etype This field specifies the desired encryption algo-
rithm to be used in the response.
addresses This field is included in the initial request for
tickets, and optionally included in requests for
additional tickets from the ticket-granting
server. It specifies the addresses from which the
requested ticket is to be valid. Normally it
includes the addresses for the client's host. If
a proxy is requested, this field will contain
other addresses. The contents of this field are
usually copied by the KDC into the caddr field of
the resulting ticket.
additional-tickets
Additional tickets may be optionally included in a
request to the ticket-granting server. If the
ENC-TKT-IN-SKEY option has been specified, then
the session key from the additional ticket will be
used in place of the server's key to encrypt the
new ticket. If more than one option which
requires additional tickets has been specified,
then the additional tickets are used in the order
specified by the ordering of the options bits (see
kdc-options, above).
The application code will be either ten (10) or twelve
(12) depending on whether the request is for an initial
ticket (AS-REQ) or for an additional ticket (TGS-REQ).
The optional fields (addresses, authorization-data and
additional-tickets) are only included if necessary to per-
form the operation specified in the kdc-options field.
It should be noted that in KRB_TGS_REQ, the protocol
version number appears twice and two different message types
appear: the KRB_TGS_REQ message contains these fields as
does the authentication header (KRB_AP_REQ) that is passed
in the padata field.
5.4.2. KRB_KDC_REP definition
The KRB_KDC_REP message format is used for the reply
from the KDC for either an initial (AS) request or a subse-
quent (TGS) request. There is no message type for
__________________________
nonce, one must make sure that the workstation time is
monotonically increasing. If the time is ever reset
backwards, there is a small, but finite, probability
that a nonce will be reused.
Section 5.4.2. - 61 - Expires 11 January 1998
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KRB_KDC_REP. Instead, the type will be either KRB_AS_REP or
KRB_TGS_REP. The key used to encrypt the ciphertext part of
the reply depends on the message type. For KRB_AS_REP, the
ciphertext is encrypted in the client's secret key, and the
client's key version number is included in the key version
number for the encrypted data. For KRB_TGS_REP, the cipher-
text is encrypted in the sub-session key from the Authenti-
cator, or if absent, the session key from the ticket-
granting ticket used in the request. In that case, no ver-
sion number will be present in the EncryptedData sequence.
The KRB_KDC_REP message contains the following fields:
AS-REP ::= [APPLICATION 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
padata[2] SEQUENCE OF PA-DATA OPTIONAL,
crealm[3] Realm,
cname[4] PrincipalName,
ticket[5] Ticket,
enc-part[6] EncryptedData
}
EncASRepPart ::= [APPLICATION 25[27]] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
key[0] EncryptionKey,
last-req[1] LastReq,
nonce[2] INTEGER,
key-expiration[3] KerberosTime OPTIONAL,
flags[4] TicketFlags,
authtime[5] KerberosTime,
starttime[6] KerberosTime OPTIONAL,
endtime[7] KerberosTime,
renew-till[8] KerberosTime OPTIONAL,
srealm[9] Realm,
sname[10] PrincipalName,
caddr[11] HostAddresses OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is either KRB_AS_REP or KRB_TGS_REP.
__________________________
[27] An application code in the encrypted part of a
message provides an additional check that the message
was decrypted properly.
Section 5.4.2. - 62 - Expires 11 January 1998
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padata This field is described in detail in section
5.4.1. One possible use for this field is to
encode an alternate "mix-in" string to be used
with a string-to-key algorithm (such as is
described in section 6.3.2). This ability is use-
ful to ease transitions if a realm name needs to
change (e.g. when a company is acquired); in such
a case all existing password-derived entries in
the KDC database would be flagged as needing a
special mix-in string until the next password
change.
crealm, cname, srealm and sname
These fields are the same as those described for
the ticket in section 5.3.1.
ticket The newly-issued ticket, from section 5.3.1.
enc-part This field is a place holder for the ciphertext
and related information that forms the encrypted
part of a message. The description of the
encrypted part of the message follows each appear-
ance of this field. The encrypted part is encoded
as described in section 6.1.
key This field is the same as described for the ticket
in section 5.3.1.
last-req This field is returned by the KDC and specifies
the time(s) of the last request by a principal.
Depending on what information is available, this
might be the last time that a request for a
ticket-granting ticket was made, or the last time
that a request based on a ticket-granting ticket
was successful. It also might cover all servers
for a realm, or just the particular server. Some
implementations may display this information to
the user to aid in discovering unauthorized use of
one's identity. It is similar in spirit to the
last login time displayed when logging into
timesharing systems.
nonce This field is described above in section 5.4.1.
key-expiration
The key-expiration field is part of the response
from the KDC and specifies the time that the
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client's secret key is due to expire. The expira-
tion might be the result of password aging or an
account expiration. This field will usually be
left out of the TGS reply since the response to
the TGS request is encrypted in a session key and
no client information need be retrieved from the
KDC database. It is up to the application client
(usually the login program) to take appropriate
action (such as notifying the user) if the expira-
tion time is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
These fields are duplicates of those found in the
encrypted portion of the attached ticket (see sec-
tion 5.3.1), provided so the client may verify
they match the intended request and to assist in
proper ticket caching. If the message is of type
KRB_TGS_REP, the caddr field will only be filled
in if the request was for a proxy or forwarded
ticket, or if the user is substituting a subset of
the addresses from the ticket granting ticket. If
the client-requested addresses are not present or
not used, then the addresses contained in the
ticket will be the same as those included in the
ticket-granting ticket.
5.5. Client/Server (CS) message specifications
This section specifies the format of the messages used
for the authentication of the client to the application
server.
5.5.1. KRB_AP_REQ definition
The KRB_AP_REQ message contains the Kerberos protocol
version number, the message type KRB_AP_REQ, an options
field to indicate any options in use, and the ticket and
authenticator themselves. The KRB_AP_REQ message is often
referred to as the "authentication header".
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
ap-options[2] APOptions,
ticket[3] Ticket,
authenticator[4] EncryptedData
}
APOptions ::= BIT STRING {
reserved(0),
use-session-key(1),
mutual-required(2)
Section 5.5.1. - 64 - Expires 11 January 1998
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}
pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is KRB_AP_REQ.
ap-optionsThis field appears in the application request
(KRB_AP_REQ) and affects the way the request is
processed. It is a bit-field, where the selected
options are indicated by the bit being set (1),
and the unselected options and reserved fields
being reset (0). The encoding of the bits is
specified in section 5.2. The meanings of the
options are:
Bit(s) Name Description
0 RESERVED
Reserved for future expansion of this
field.
1 USE-SESSION-KEY
The USE-SESSION-KEY option indicates
that the ticket the client is presenting
to a server is encrypted in the session
key from the server's ticket-granting
ticket. When this option is not speci-
fied, the ticket is encrypted in the
server's secret key.
2 MUTUAL-REQUIRED
The MUTUAL-REQUIRED option tells the
server that the client requires mutual
authentication, and that it must respond
with a KRB_AP_REP message.
3-31 RESERVED
Reserved for future use.
ticket This field is a ticket authenticating the client
to the server.
authenticator
This contains the authenticator, which includes
the client's choice of a subkey. Its encoding is
described in section 5.3.2.
5.5.2. KRB_AP_REP definition
The KRB_AP_REP message contains the Kerberos protocol
version number, the message type, and an encrypted time-
stamp. The message is sent in in response to an application
request (KRB_AP_REQ) where the mutual authentication option
Section 5.5.2. - 65 - Expires 11 January 1998
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has been selected in the ap-options field.
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
enc-part[2] EncryptedData
}
EncAPRepPart ::= [APPLICATION 27[29]] SEQUENCE {
ctime[0] KerberosTime,
cusec[1] INTEGER,
subkey[2] EncryptionKey OPTIONAL,
seq-number[3] INTEGER OPTIONAL
}
The encoded EncAPRepPart is encrypted in the shared session
key of the ticket. The optional subkey field can be used in
an application-arranged negotiation to choose a per associa-
tion session key.
pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is KRB_AP_REP.
enc-part This field is described above in section 5.4.2.
ctime This field contains the current time on the
client's host.
cusec This field contains the microsecond part of the
client's timestamp.
subkey This field contains an encryption key which is to
be used to protect this specific application ses-
sion. See section 3.2.6 for specifics on how this
field is used to negotiate a key. Unless an
application specifies otherwise, if this field is
left out, the sub-session key from the authentica-
tor, or if also left out, the session key from the
ticket will be used.
__________________________
[29] An application code in the encrypted part of a
message provides an additional check that the message
was decrypted properly.
Section 5.5.2. - 66 - Expires 11 January 1998
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5.5.3. Error message reply
If an error occurs while processing the application
request, the KRB_ERROR message will be sent in response.
See section 5.9.1 for the format of the error message. The
cname and crealm fields may be left out if the server cannot
determine their appropriate values from the corresponding
KRB_AP_REQ message. If the authenticator was decipherable,
the ctime and cusec fields will contain the values from it.
5.6. KRB_SAFE message specification
This section specifies the format of a message that can
be used by either side (client or server) of an application
to send a tamper-proof message to its peer. It presumes
that a session key has previously been exchanged (for exam-
ple, by using the KRB_AP_REQ/KRB_AP_REP messages).
5.6.1. KRB_SAFE definition
The KRB_SAFE message contains user data along with a
collision-proof checksum keyed with the last encryption key
negotiated via subkeys, or the session key if no negotiation
has occured. The message fields are:
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
safe-body[2] KRB-SAFE-BODY,
cksum[3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data[0] OCTET STRING,
timestamp[1] KerberosTime OPTIONAL,
usec[2] INTEGER OPTIONAL,
seq-number[3] INTEGER OPTIONAL,
s-address[4] HostAddress OPTIONAL,
r-address[5] HostAddress OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is KRB_SAFE.
safe-body This field is a placeholder for the body of the
KRB-SAFE message. It is to be encoded separately
and then have the checksum computed over it, for
use in the cksum field.
Section 5.6.1. - 67 - Expires 11 January 1998
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cksum This field contains the checksum of the applica-
tion data. Checksum details are described in sec-
tion 6.4. The checksum is computed over the
encoding of the KRB-SAFE-BODY sequence.
user-data This field is part of the KRB_SAFE and KRB_PRIV
messages and contain the application specific data
that is being passed from the sender to the reci-
pient.
timestamp This field is part of the KRB_SAFE and KRB_PRIV
messages. Its contents are the current time as
known by the sender of the message. By checking
the timestamp, the recipient of the message is
able to make sure that it was recently generated,
and is not a replay.
usec This field is part of the KRB_SAFE and KRB_PRIV
headers. It contains the microsecond part of the
timestamp.
seq-number
This field is described above in section 5.3.2.
s-address This field specifies the address in use by the
sender of the message.
r-address This field specifies the address in use by the
recipient of the message. It may be omitted for
some uses (such as broadcast protocols), but the
recipient may arbitrarily reject such messages.
This field along with s-address can be used to
help detect messages which have been incorrectly
or maliciously delivered to the wrong recipient.
5.7. KRB_PRIV message specification
This section specifies the format of a message that can
be used by either side (client or server) of an application
to securely and privately send a message to its peer. It
presumes that a session key has previously been exchanged
(for example, by using the KRB_AP_REQ/KRB_AP_REP messages).
5.7.1. KRB_PRIV definition
The KRB_PRIV message contains user data encrypted in
the Session Key. The message fields are:
__________________________
[31] An application code in the encrypted part of a
Version 5 - Specification Revision 6
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
enc-part[3] EncryptedData
}
EncKrbPrivPart ::= [APPLICATION 28[31]] SEQUENCE {
user-data[0] OCTET STRING,
timestamp[1] KerberosTime OPTIONAL,
usec[2] INTEGER OPTIONAL,
seq-number[3] INTEGER OPTIONAL,
s-address[4] HostAddress OPTIONAL, -- sender's addr
r-address[5] HostAddress OPTIONAL -- recip's addr
}
pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is KRB_PRIV.
enc-part This field holds an encoding of the EncKrbPrivPart
sequence encrypted under the session key[32].
This encrypted encoding is used for the enc-part
field of the KRB-PRIV message. See section 6 for
the format of the ciphertext.
user-data, timestamp, usec, s-address and r-address
These fields are described above in section 5.6.1.
seq-number
This field is described above in section 5.3.2.
5.8. KRB_CRED message specification
This section specifies the format of a message that can
be used to send Kerberos credentials from one principal to
__________________________
message provides an additional check that the message
was decrypted properly.
[32] If supported by the encryption method in use, an
initialization vector may be passed to the encryption
procedure, in order to achieve proper cipher chaining.
The initialization vector might come from the last
block of the ciphertext from the previous KRB_PRIV mes-
sage, but it is the application's choice whether or not
to use such an initialization vector. If left out, the
default initialization vector for the encryption algo-
rithm will be used.
Section 5.8. - 69 - Expires 11 January 1998
Version 5 - Specification Revision 6
another. It is presented here to encourage a common mechan-
ism to be used by applications when forwarding tickets or
providing proxies to subordinate servers. It presumes that
a session key has already been exchanged perhaps by using
the KRB_AP_REQ/KRB_AP_REP messages.
5.8.1. KRB_CRED definition
The KRB_CRED message contains a sequence of tickets to
be sent and information needed to use the tickets, including
the session key from each. The information needed to use
the tickets is encrypted under an encryption key previously
exchanged or transferred alongside the KRB_CRED message.
The message fields are:
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER, -- KRB_CRED
tickets[2] SEQUENCE OF Ticket,
enc-part[3] EncryptedData
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info[0] SEQUENCE OF KrbCredInfo,
nonce[1] INTEGER OPTIONAL,
timestamp[2] KerberosTime OPTIONAL,
usec[3] INTEGER OPTIONAL,
s-address[4] HostAddress OPTIONAL,
r-address[5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key[0] EncryptionKey,
prealm[1] Realm OPTIONAL,
pname[2] PrincipalName OPTIONAL,
flags[3] TicketFlags OPTIONAL,
authtime[4] KerberosTime OPTIONAL,
starttime[5] KerberosTime OPTIONAL,
endtime[6] KerberosTime OPTIONAL
renew-till[7] KerberosTime OPTIONAL,
srealm[8] Realm OPTIONAL,
sname[9] PrincipalName OPTIONAL,
caddr[10] HostAddresses OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is KRB_CRED.
Section 5.8.1. - 70 - Expires 11 January 1998
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tickets
These are the tickets obtained from the KDC
specifically for use by the intended recipient.
Successive tickets are paired with the correspond-
ing KrbCredInfo sequence from the enc-part of the
KRB-CRED message.
enc-part This field holds an encoding of the EncKrbCredPart
sequence encrypted under the session key shared
between the sender and the intended recipient.
This encrypted encoding is used for the enc-part
field of the KRB-CRED message. See section 6 for
the format of the ciphertext.
nonce If practical, an application may require the
inclusion of a nonce generated by the recipient of
the message. If the same value is included as the
nonce in the message, it provides evidence that
the message is fresh and has not been replayed by
an attacker. A nonce must never be re-used; it
should be generated randomly by the recipient of
the message and provided to the sender of the mes-
sage in an application specific manner.
timestamp and usec
These fields specify the time that the KRB-CRED
message was generated. The time is used to pro-
vide assurance that the message is fresh.
s-address and r-address
These fields are described above in section 5.6.1.
They are used optionally to provide additional
assurance of the integrity of the KRB-CRED mes-
sage.
key This field exists in the corresponding ticket
passed by the KRB-CRED message and is used to pass
the session key from the sender to the intended
recipient. The field's encoding is described in
section 6.2.
The following fields are optional. If present, they
can be associated with the credentials in the remote ticket
file. If left out, then it is assumed that the recipient of
the credentials already knows their value.
prealm and pname
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The name and realm of the delegated principal
identity.
flags, authtime, starttime, endtime, renew-till, srealm,
sname, and caddr
These fields contain the values of the correspond-
ing fields from the ticket found in the ticket
field. Descriptions of the fields are identical
to the descriptions in the KDC-REP message.
5.9. Error message specification
This section specifies the format for the KRB_ERROR
message. The fields included in the message are intended to
return as much information as possible about an error. It
is not expected that all the information required by the
fields will be available for all types of errors. If the
appropriate information is not available when the message is
composed, the corresponding field will be left out of the
message.
Note that since the KRB_ERROR message is not protected
by any encryption, it is quite possible for an intruder to
synthesize or modify such a message. In particular, this
means that the client should not use any fields in this mes-
sage for security-critical purposes, such as setting a sys-
tem clock or generating a fresh authenticator. The message
can be useful, however, for advising a user on the reason
for some failure.
5.9.1. KRB_ERROR definition
The KRB_ERROR message consists of the following fields:
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
ctime[2] KerberosTime OPTIONAL,
cusec[3] INTEGER OPTIONAL,
stime[4] KerberosTime,
susec[5] INTEGER,
error-code[6] INTEGER,
crealm[7] Realm OPTIONAL,
cname[8] PrincipalName OPTIONAL,
realm[9] Realm, -- Correct realm
sname[10] PrincipalName, -- Correct name
e-text[11] GeneralString OPTIONAL,
e-data[12] OCTET STRING OPTIONAL,
e-cksum[13] Checksum OPTIONAL
}
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pvno and msg-type
These fields are described above in section 5.4.1.
msg-type is KRB_ERROR.
ctime This field is described above in section 5.4.1.
cusec This field is described above in section 5.5.2.
stime This field contains the current time on the
server. It is of type KerberosTime.
susec This field contains the microsecond part of the
server's timestamp. Its value ranges from 0 to
999999. It appears along with stime. The two
fields are used in conjunction to specify a rea-
sonably accurate timestamp.
error-codeThis field contains the error code returned by
Kerberos or the server when a request fails. To
interpret the value of this field see the list of
error codes in section 8. Implementations are
encouraged to provide for national language sup-
port in the display of error messages.
crealm, cname, srealm and sname
These fields are described above in section 5.3.1.
e-text This field contains additional text to help
explain the error code associated with the failed
request (for example, it might include a principal
name which was unknown).
e-data This field contains additional data about the
error for use by the application to help it
recover from or handle the error. If the error-
code is KDC_ERR_PREAUTH_REQUIRED, then the e-data
field will contain an encoding of a sequence of
padata fields, each corresponding to an acceptable
pre-authentication method and optionally contain-
ing data for the method:
e-cksum This field contains an optional checksum for the
KRB-ERROR message. The checksum is calculated
over the Kerberos ASN.1 encoding of the KRB-ERROR
Section 5.9.1. - 73 - Expires 11 January 1998
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message with the checksum absent. The checksum is
then added to the KRB-ERROR structure and the mes-
sage is re-encoded. The Checksum should be calcu-
lated using the session key from the ticket grant-
ing ticket or service ticket, where available. If
the error is in response to a TGS or AP request,
the checksum should be calculated uing the the
session key from the client's ticket. If the
error is in response to an AS request, then the
checksum should be calulated using the client's
secret key ONLY if there has been suitable preau-
thentication to prove knowledge of the secret key
by the client[33]. If a checksum can not be com-
puted because the key to be used is not available,
no checksum will be included.
METHOD-DATA ::= SEQUENCE of PA-DATA
If the error-code is KRB_AP_ERR_METHOD, then the
e-data field will contain an encoding of the fol-
lowing sequence:
METHOD-DATA ::= SEQUENCE {
method-type[0] INTEGER,
method-data[1] OCTET STRING OPTIONAL
}
method-type will indicate the required alternate
method; method-data will contain any required
additional information.
6. Encryption and Checksum Specifications
The Kerberos protocols described in this document are
designed to use stream encryption ciphers, which can be
simulated using commonly available block encryption ciphers,
such as the Data Encryption Standard, [12] in conjunction
with block chaining and checksum methods [13]. Encryption
is used to prove the identities of the network entities par-
ticipating in message exchanges. The Key Distribution
Center for each realm is trusted by all principals
registered in that realm to store a secret key in confi-
dence. Proof of knowledge of this secret key is used to
verify the authenticity of a principal.
The KDC uses the principal's secret key (in the AS
__________________________
[33] This prevents an attacker who generates an in-
correct AS request from obtaining verifiable plaintext
for use in an off-line password guessing attack.
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exchange) or a shared session key (in the TGS exchange) to
encrypt responses to ticket requests; the ability to obtain
the secret key or session key implies the knowledge of the
appropriate keys and the identity of the KDC. The ability
of a principal to decrypt the KDC response and present a
Ticket and a properly formed Authenticator (generated with
the session key from the KDC response) to a service verifies
the identity of the principal; likewise the ability of the
service to extract the session key from the Ticket and prove
its knowledge thereof in a response verifies the identity of
the service.
The Kerberos protocols generally assume that the
encryption used is secure from cryptanalysis; however, in
some cases, the order of fields in the encrypted portions of
messages are arranged to minimize the effects of poorly
chosen keys. It is still important to choose good keys. If
keys are derived from user-typed passwords, those passwords
need to be well chosen to make brute force attacks more dif-
ficult. Poorly chosen keys still make easy targets for
intruders.
The following sections specify the encryption and
checksum mechanisms currently defined for Kerberos. The
encodings, chaining, and padding requirements for each are
described. For encryption methods, it is often desirable to
place random information (often referred to as a confounder)
at the start of the message. The requirements for a con-
founder are specified with each encryption mechanism.
Some encryption systems use a block-chaining method to
improve the the security characteristics of the ciphertext.
However, these chaining methods often don't provide an
integrity check upon decryption. Such systems (such as DES
in CBC mode) must be augmented with a checksum of the plain-
text which can be verified at decryption and used to detect
any tampering or damage. Such checksums should be good at
detecting burst errors in the input. If any damage is
detected, the decryption routine is expected to return an
error indicating the failure of an integrity check. Each
encryption type is expected to provide and verify an
appropriate checksum. The specification of each encryption
method sets out its checksum requirements.
Finally, where a key is to be derived from a user's
password, an algorithm for converting the password to a key
of the appropriate type is included. It is desirable for
the string to key function to be one-way, and for the map-
ping to be different in different realms. This is important
because users who are registered in more than one realm will
often use the same password in each, and it is desirable
that an attacker compromising the Kerberos server in one
realm not obtain or derive the user's key in another.
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For an discussion of the integrity characteristics of
the candidate encryption and checksum methods considered for
Kerberos, the the reader is referred to [14].
6.1. Encryption Specifications
The following ASN.1 definition describes all encrypted
messages. The enc-part field which appears in the unen-
crypted part of messages in section 5 is a sequence consist-
ing of an encryption type, an optional key version number,
and the ciphertext.
EncryptedData ::= SEQUENCE {
etype[0] INTEGER, -- EncryptionType
kvno[1] INTEGER OPTIONAL,
cipher[2] OCTET STRING -- ciphertext
}
etype This field identifies which encryption algorithm
was used to encipher the cipher. Detailed specif-
ications for selected encryption types appear
later in this section.
kvno This field contains the version number of the key
under which data is encrypted. It is only present
in messages encrypted under long lasting keys,
such as principals' secret keys.
cipher This field contains the enciphered text, encoded
as an OCTET STRING.
The cipher field is generated by applying the specified
encryption algorithm to data composed of the message and
algorithm-specific inputs. Encryption mechanisms defined
for use with Kerberos must take sufficient measures to
guarantee the integrity of the plaintext, and we recommend
they also take measures to protect against precomputed dic-
tionary attacks. If the encryption algorithm is not itself
capable of doing so, the protections can often be enhanced
by adding a checksum and a confounder.
The suggested format for the data to be encrypted
includes a confounder, a checksum, the encoded plaintext,
and any necessary padding. The msg-seq field contains the
part of the protocol message described in section 5 which is
to be encrypted. The confounder, checksum, and padding are
all untagged and untyped, and their length is exactly suffi-
cient to hold the appropriate item. The type and length is
implicit and specified by the particular encryption type
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being used (etype). The format for the data to be encrypted
is described in the following diagram:
+-----------+----------+-------------+-----+
|confounder | check | msg-seq | pad |
+-----------+----------+-------------+-----+
The format cannot be described in ASN.1, but for those who
prefer an ASN.1-like notation:
CipherText ::= ENCRYPTED SEQUENCE {
confounder[0] UNTAGGED[35] OCTET STRING(conf_length) OPTIONAL,
check[1] UNTAGGED OCTET STRING(checksum_length) OPTIONAL,
msg-seq[2] MsgSequence,
pad UNTAGGED OCTET STRING(pad_length) OPTIONAL
}
One generates a random confounder of the appropriate
length, placing it in confounder; zeroes out check; calcu-
lates the appropriate checksum over confounder, check, and
msg-seq, placing the result in check; adds the necessary
padding; then encrypts using the specified encryption type
and the appropriate key.
Unless otherwise specified, a definition of an encryp-
tion algorithm that specifies a checksum, a length for the
confounder field, or an octet boundary for padding uses this
ciphertext format[36]. Those fields which are not specified
will be omitted.
In the interest of allowing all implementations using a
__________________________
[35] In the above specification, UNTAGGED OCTET
STRING(length) is the notation for an octet string with
its tag and length removed. It is not a valid ASN.1
type. The tag bits and length must be removed from the
confounder since the purpose of the confounder is so
that the message starts with random data, but the tag
and its length are fixed. For other fields, the length
and tag would be redundant if they were included be-
cause they are specified by the encryption type.
[36] The ordering of the fields in the CipherText is
important. Additionally, messages encoded in this for-
mat must include a length as part of the msg-seq field.
This allows the recipient to verify that the message
has not been truncated. Without a length, an attacker
could use a chosen plaintext attack to generate a mes-
sage which could be truncated, while leaving the check-
sum intact. Note that if the msg-seq is an encoding of
an ASN.1 SEQUENCE or OCTET STRING, then the length is
part of that encoding.
Section 6.1. - 77 - Expires 11 January 1998
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particular encryption type to communicate with all others
using that type, the specification of an encryption type
defines any checksum that is needed as part of the encryp-
tion process. If an alternative checksum is to be used, a
new encryption type must be defined.
Some cryptosystems require additional information
beyond the key and the data to be encrypted. For example,
DES, when used in cipher-block-chaining mode, requires an
initialization vector. If required, the description for
each encryption type must specify the source of such addi-
tional information.
6.2. Encryption Keys
The sequence below shows the encoding of an encryption
key:
EncryptionKey ::= SEQUENCE {
keytype[0] INTEGER,
keyvalue[1] OCTET STRING
}
keytype This field specifies the type of encryption key
that follows in the keyvalue field. It will
almost always correspond to the encryption algo-
rithm used to generate the EncryptedData, though
more than one algorithm may use the same type of
key (the mapping is many to one). This might hap-
pen, for example, if the encryption algorithm uses
an alternate checksum algorithm for an integrity
check, or a different chaining mechanism.
keyvalue This field contains the key itself, encoded as an
octet string.
All negative values for the encryption key type are
reserved for local use. All non-negative values are
reserved for officially assigned type fields and interpreta-
tions.
6.3. Encryption Systems
6.3.1. The NULL Encryption System (null)
If no encryption is in use, the encryption system is
said to be the NULL encryption system. In the NULL encryp-
tion system there is no checksum, confounder or padding.
The ciphertext is simply the plaintext. The NULL Key is
used by the null encryption system and is zero octets in
length, with keytype zero (0).
Section 6.3.1. - 78 - Expires 11 January 1998
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6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc)
The des-cbc-crc encryption mode encrypts information
under the Data Encryption Standard [12] using the cipher
block chaining mode [13]. A CRC-32 checksum (described in
ISO 3309 [15]) is applied to the confounder and message
sequence (msg-seq) and placed in the cksum field. DES
blocks are 8 bytes. As a result, the data to be encrypted
(the concatenation of confounder, checksum, and message)
must be padded to an 8 byte boundary before encryption. The
details of the encryption of this data are identical to
those for the des-cbc-md5 encryption mode.
Note that, since the CRC-32 checksum is not collision-
proof, an attacker could use a probabilistic chosen-
plaintext attack to generate a valid message even if a con-
founder is used [14]. The use of collision-proof checksums
is recommended for environments where such attacks represent
a significant threat. The use of the CRC-32 as the checksum
for ticket or authenticator is no longer mandated as an
interoperability requirement for Kerberos Version 5 Specifi-
cation 1 (See section 9.1 for specific details).
6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4)
The des-cbc-md4 encryption mode encrypts information
under the Data Encryption Standard [12] using the cipher
block chaining mode [13]. An MD4 checksum (described in
[16]) is applied to the confounder and message sequence
(msg-seq) and placed in the cksum field. DES blocks are 8
bytes. As a result, the data to be encrypted (the concate-
nation of confounder, checksum, and message) must be padded
to an 8 byte boundary before encryption. The details of the
encryption of this data are identical to those for the des-
cbc-md5 encryption mode.
6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5)
The des-cbc-md5 encryption mode encrypts information
under the Data Encryption Standard [12] using the cipher
block chaining mode [13]. An MD5 checksum (described in
[17].) is applied to the confounder and message sequence
(msg-seq) and placed in the cksum field. DES blocks are 8
bytes. As a result, the data to be encrypted (the concate-
nation of confounder, checksum, and message) must be padded
to an 8 byte boundary before encryption.
Plaintext and DES ciphtertext are encoded as 8-octet
blocks which are concatenated to make the 64-bit inputs for
the DES algorithms. The first octet supplies the 8 most
significant bits (with the octet's MSbit used as the DES
input block's MSbit, etc.), the second octet the next 8
Section 6.3.4. - 79 - Expires 11 January 1998
Version 5 - Specification Revision 6
bits, ..., and the eighth octet supplies the 8 least signi-
ficant bits.
Encryption under DES using cipher block chaining
requires an additional input in the form of an initializa-
tion vector. Unless otherwise specified, zero should be
used as the initialization vector. Kerberos' use of DES
requires an 8-octet confounder.
The DES specifications identify some "weak" and "semi-
weak" keys; those keys shall not be used for encrypting mes-
sages for use in Kerberos. Additionally, because of the way
that keys are derived for the encryption of checksums, keys
shall not be used that yield "weak" or "semi-weak" keys when
eXclusive-ORed with the constant F0F0F0F0F0F0F0F0.
A DES key is 8 octets of data, with keytype one (1).
This consists of 56 bits of key, and 8 parity bits (one per
octet). The key is encoded as a series of 8 octets written
in MSB-first order. The bits within the key are also
encoded in MSB order. For example, if the encryption key is
(B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8)
where B1,B2,...,B56 are the key bits in MSB order, and
P1,P2,...,P8 are the parity bits, the first octet of the key
would be B1,B2,...,B7,P1 (with B1 as the MSbit). [See the
FIPS 81 introduction for reference.]
To generate a DES key from a text string (password),
the text string normally must have the realm and each com-
ponent of the principal's name appended[37], then padded
with ASCII nulls to an 8 byte boundary. This string is then
fan-folded and eXclusive-ORed with itself to form an 8 byte
DES key. The parity is corrected on the key, and it is used
to generate a DES CBC checksum on the initial string (with
the realm and name appended). Next, parity is corrected on
the CBC checksum. If the result matches a "weak" or "semi-
weak" key as described in the DES specification, it is
eXclusive-ORed with the constant 00000000000000F0. Finally,
the result is returned as the key. Pseudocode follows:
string_to_key(string,realm,name) {
odd = 1;
s = string + realm;
for(each component in name) {
s = s + component;
}
tempkey = NULL;
pad(s); /* with nulls to 8 byte boundary */
for(8byteblock in s) {
__________________________
[37] In some cases, it may be necessary to use a dif-
ferent "mix-in" string for compatibility reasons; see
the discussion of padata in section 5.4.2.
Section 6.3.4. - 80 - Expires 11 January 1998
Version 5 - Specification Revision 6
if(odd == 0) {
odd = 1;
reverse(8byteblock)
}
else odd = 0;
tempkey = tempkey XOR 8byteblock;
}
fixparity(tempkey);
key = DES-CBC-check(s,tempkey);
fixparity(key);
if(is_weak_key_key(key))
key = key XOR 0xF0;
return(key);
}
6.3.5. Triple DES EDE in outer CBC mode with an SHA1 check-
sum (des3-cbc-sha1)
The des3-cbc-sha1 encryption encodes information using
three Data Encryption Standard transformations with three
DES keys. The first key is used to perform a DES ECB
encryption on an eight-octet data block using the first DES
key, followed by a DES ECB decryption of the result using
the second DES key, and a DES ECB encryption of the result
using the third DES key. Because DES blocks are 8 bytes,
the data to be encrypted (the concatenation of confounder,
checksum, and message) must first be padded to an 8 byte
boundary before encryption. To support the outer CBC mode,
the input is padded an eight-octet boundary. The first 8
octets of the data to be encrypted (the confounder) is
exclusive-ored with an initialization vector of zero and
then ECB encrypted using triple DES as described above.
Subsequent blocks of 8 octets are exclusive-ored with the
ciphertext produced by the encryption on the previous block
before ECB encryption.
An HMAC-SHA1 checksum (described in [18].) is applied
to the confounder and message sequence (msg-seq) and placed
in the cksum field.
Plaintext are encoded as 8-octet blocks which are con-
catenated to make the 64-bit inputs for the DES algorithms.
The first octet supplies the 8 most significant bits (with
the octet's MSbit used as the DES input block's MSbit,
etc.), the second octet the next 8 bits, ..., and the eighth
octet supplies the 8 least significant bits.
Encryption under Triple DES using cipher block chaining
requires an additional input in the form of an initializa-
tion vector. Unless otherwise specified, zero should be
used as the initialization vector. Kerberos' use of DES
requires an 8-octet confounder.
The DES specifications identify some "weak" and "semi-
Section 6.3.5. - 81 - Expires 11 January 1998
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weak" keys; those keys shall not be used for encrypting mes-
sages for use in Kerberos. Additionally, because of the way
that keys are derived for the encryption of checksums, keys
shall not be used that yield "weak" or "semi-weak" keys when
eXclusive-ORed with the constant F0F0F0F0F0F0F0F0.
A Triple DES key is 24 octets of data, with keytype
seven (7). This consists of 168 bits of key, and 24 parity
bits (one per octet). The key is encoded as a series of 24
octets written in MSB-first order, with the first 8 octets
treated as the first DES key, the second 8 octets as the
second key, and the third 8 octets the third DES key. The
bits within each key are also encoded in MSB order. For
example, if the encryption key is
(B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8)
where B1,B2,...,B56 are the key bits in MSB order, and
P1,P2,...,P8 are the parity bits, the first octet of the key
would be B1,B2,...,B7,P1 (with B1 as the MSbit). [See the
FIPS 81 introduction for reference.]
To generate a DES key from a text string (password),
the text string normally must have the realm and each com-
ponent of the principal's name appended[38],
The input string (with any salt data appended to it) is
n-folded into a 24 octet (192 bit) string. To n-fold a
number X, replicate the input value to a length that is the
least common multiple of n and the length of X. Before each
repetition, the input X is rotated to the right by 13 bit
positions. The successive n-bit chunks are added together
using 1's-complement addition (addition with end-around
carry) to yield a n-bit result. (This transformation was
proposed by Richard Basch)
Each successive set of 8 octets is taken as a DES key,
and its parity is adjusted in the same manner as previously
described. If any of the three sets of 8 octets match a
"weak" or "semi-weak" key as described in the DES specifica-
tion, that chunk is eXclusive-ORed with the constant
00000000000000F0. The resulting DES keys are then used in
sequence to perform a Triple-DES CBC encryption of the n-
folded input string (appended with any salt data), using a
zero initial vector. Parity, weak, and semi-weak keys are
once again corrected and the result is returned as the 24
octet key.
Pseudocode follows:
string_to_key(string,realm,name) {
__________________________
[38] In some cases, it may be necessary to use a dif-
ferent "mix-in" string for compatibility reasons; see
the discussion of padata in section 5.4.2.
Section 6.3.5. - 82 - Expires 11 January 1998
Version 5 - Specification Revision 6
s = string + realm;
for(each component in name) {
s = s + component;
}
tkey[24] = fold(s);
fixparity(tkey);
if(isweak(tkey[0-7])) tkey[0-7] = tkey[0-7] XOR 0xF0;
if(isweak(tkey[8-15])) tkey[8-15] = tkey[8-15] XOR 0xF0;
if(is_weak(tkey[16-23])) tkey[16-23] = tkey[16-23] XOR 0xF0;
key[24] = 3DES-CBC(data=fold(s),key=tkey,iv=0);
fixparity(key);
if(is_weak(key[0-7])) key[0-7] = key[0-7] XOR 0xF0;
if(is_weak(key[8-15])) key[8-15] = key[8-15] XOR 0xF0;
if(is_weak(key[16-23])) key[16-23] = key[16-23] XOR 0xF0;
return(key);
}
6.4. Checksums
The following is the ASN.1 definition used for a check-
sum:
Checksum ::= SEQUENCE {
cksumtype[0] INTEGER,
checksum[1] OCTET STRING
}
cksumtype This field indicates the algorithm used to gen-
erate the accompanying checksum.
checksum This field contains the checksum itself, encoded
as an octet string.
Detailed specification of selected checksum types
appear later in this section. Negative values for the
checksum type are reserved for local use. All non-negative
values are reserved for officially assigned type fields and
interpretations.
Checksums used by Kerberos can be classified by two
properties: whether they are collision-proof, and whether
they are keyed. It is infeasible to find two plaintexts
which generate the same checksum value for a collision-proof
checksum. A key is required to perturb or initialize the
algorithm in a keyed checksum. To prevent message-stream
modification by an active attacker, unkeyed checksums should
only be used when the checksum and message will be subse-
quently encrypted (e.g. the checksums defined as part of the
encryption algorithms covered earlier in this section).
Collision-proof checksums can be made tamper-proof if
the checksum value is encrypted before inclusion in a mes-
sage. In such cases, the composition of the checksum and
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the encryption algorithm must be considered a separate
checksum algorithm (e.g. RSA-MD5 encrypted using DES is a
new checksum algorithm of type RSA-MD5-DES). For most keyed
checksums, as well as for the encrypted forms of unkeyed
collision-proof checksums, Kerberos prepends a confounder
before the checksum is calculated.
6.4.1. The CRC-32 Checksum (crc32)
The CRC-32 checksum calculates a checksum based on a
cyclic redundancy check as described in ISO 3309 [15]. The
resulting checksum is four (4) octets in length. The CRC-32
is neither keyed nor collision-proof. The use of this
checksum is not recommended. An attacker using a proba-
bilistic chosen-plaintext attack as described in [14] might
be able to generate an alternative message that satisfies
the checksum. The use of collision-proof checksums is
recommended for environments where such attacks represent a
significant threat.
6.4.2. The RSA MD4 Checksum (rsa-md4)
The RSA-MD4 checksum calculates a checksum using the
RSA MD4 algorithm [16]. The algorithm takes as input an
input message of arbitrary length and produces as output a
128-bit (16 octet) checksum. RSA-MD4 is believed to be
collision-proof.
6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-
des)
The RSA-MD4-DES checksum calculates a keyed collision-
proof checksum by prepending an 8 octet confounder before
the text, applying the RSA MD4 checksum algorithm, and
encrypting the confounder and the checksum using DES in
cipher-block-chaining (CBC) mode using a variant of the key,
where the variant is computed by eXclusive-ORing the key
with the constant F0F0F0F0F0F0F0F0[39]. The initialization
vector should be zero. The resulting checksum is 24 octets
long (8 octets of which are redundant). This checksum is
tamper-proof and believed to be collision-proof.
The DES specifications identify some "weak keys" and
__________________________
[39] A variant of the key is used to limit the use of a
key to a particular function, separating the functions
of generating a checksum from other encryption per-
formed using the session key. The constant
F0F0F0F0F0F0F0F0 was chosen because it maintains key
parity. The properties of DES precluded the use of the
complement. The same constant is used for similar pur-
pose in the Message Integrity Check in the Privacy
Enhanced Mail standard.
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"semi-weak keys"; those keys shall not be used for generat-
ing RSA-MD4 checksums for use in Kerberos.
The format for the checksum is described in the follow-
ing diagram:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| des-cbc(confounder + rsa-md4(confounder+msg),key=var(key),iv=0) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The format cannot be described in ASN.1, but for those who
prefer an ASN.1-like notation:
rsa-md4-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
confounder[0] UNTAGGED OCTET STRING(8),
check[1] UNTAGGED OCTET STRING(16)
}
6.4.4. The RSA MD5 Checksum (rsa-md5)
The RSA-MD5 checksum calculates a checksum using the
RSA MD5 algorithm. [17]. The algorithm takes as input an
input message of arbitrary length and produces as output a
128-bit (16 octet) checksum. RSA-MD5 is believed to be
collision-proof.
6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-
des)
The RSA-MD5-DES checksum calculates a keyed collision-
proof checksum by prepending an 8 octet confounder before
the text, applying the RSA MD5 checksum algorithm, and
encrypting the confounder and the checksum using DES in
cipher-block-chaining (CBC) mode using a variant of the key,
where the variant is computed by eXclusive-ORing the key
with the constant F0F0F0F0F0F0F0F0. The initialization vec-
tor should be zero. The resulting checksum is 24 octets
long (8 octets of which are redundant). This checksum is
tamper-proof and believed to be collision-proof.
The DES specifications identify some "weak keys" and
"semi-weak keys"; those keys shall not be used for encrypt-
ing RSA-MD5 checksums for use in Kerberos.
The format for the checksum is described in the follow-
ing diagram:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| des-cbc(confounder + rsa-md5(confounder+msg),key=var(key),iv=0) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The format cannot be described in ASN.1, but for those who
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prefer an ASN.1-like notation:
rsa-md5-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
confounder[0] UNTAGGED OCTET STRING(8),
check[1] UNTAGGED OCTET STRING(16)
}
6.4.6. DES cipher-block chained checksum (des-mac)
The DES-MAC checksum is computed by prepending an 8
octet confounder to the plaintext, performing a DES CBC-mode
encryption on the result using the key and an initialization
vector of zero, taking the last block of the ciphertext,
prepending the same confounder and encrypting the pair using
DES in cipher-block-chaining (CBC) mode using a a variant of
the key, where the variant is computed by eXclusive-ORing
the key with the constant F0F0F0F0F0F0F0F0. The initializa-
tion vector should be zero. The resulting checksum is 128
bits (16 octets) long, 64 bits of which are redundant. This
checksum is tamper-proof and collision-proof.
The format for the checksum is described in the follow-
ing diagram:
+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+
| des-cbc(confounder + des-mac(conf+msg,iv=0,key),key=var(key),iv=0) |
+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+
The format cannot be described in ASN.1, but for those who
prefer an ASN.1-like notation:
des-mac-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
confounder[0] UNTAGGED OCTET STRING(8),
check[1] UNTAGGED OCTET STRING(8)
}
The DES specifications identify some "weak" and "semi-
weak" keys; those keys shall not be used for generating
DES-MAC checksums for use in Kerberos, nor shall a key be
used whose variant is "weak" or "semi-weak".
6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative
(rsa-md4-des-k)
The RSA-MD4-DES-K checksum calculates a keyed
collision-proof checksum by applying the RSA MD4 checksum
algorithm and encrypting the results using DES in cipher-
block-chaining (CBC) mode using a DES key as both key and
initialization vector. The resulting checksum is 16 octets
long. This checksum is tamper-proof and believed to be
collision-proof. Note that this checksum type is the old
method for encoding the RSA-MD4-DES checksum and it is no
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longer recommended.
6.4.8. DES cipher-block chained checksum alternative (des-
mac-k)
The DES-MAC-K checksum is computed by performing a DES
CBC-mode encryption of the plaintext, and using the last
block of the ciphertext as the checksum value. It is keyed
with an encryption key and an initialization vector; any
uses which do not specify an additional initialization vec-
tor will use the key as both key and initialization vector.
The resulting checksum is 64 bits (8 octets) long. This
checksum is tamper-proof and collision-proof. Note that
this checksum type is the old method for encoding the DES-
MAC checksum and it is no longer recommended.
The DES specifications identify some "weak keys" and
"semi-weak keys"; those keys shall not be used for generat-
ing DES-MAC checksums for use in Kerberos.
7. Naming Constraints
7.1. Realm Names
Although realm names are encoded as GeneralStrings and
although a realm can technically select any name it chooses,
interoperability across realm boundaries requires agreement
on how realm names are to be assigned, and what information
they imply.
To enforce these conventions, each realm must conform
to the conventions itself, and it must require that any
realms with which inter-realm keys are shared also conform
to the conventions and require the same from its neighbors.
Kerberos realm names are case sensitive. Realm names
that differ only in the case of the characters are not
equivalent. There are presently four styles of realm names:
domain, X500, other, and reserved. Examples of each style
follow:
domain: ATHENA.MIT.EDU (example)
X500: C=US/O=OSF (example)
other: NAMETYPE:rest/of.name=without-restrictions (example)
reserved: reserved, but will not conflict with above
Domain names must look like domain names: they consist of
components separated by periods (.) and they contain neither
colons (:) nor slashes (/). Domain names must be converted
to upper case when used as realm names.
X.500 names contain an equal (=) and cannot contain a
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colon (:) before the equal. The realm names for X.500 names
will be string representations of the names with components
separated by slashes. Leading and trailing slashes will not
be included.
Names that fall into the other category must begin with
a prefix that contains no equal (=) or period (.) and the
prefix must be followed by a colon (:) and the rest of the
name. All prefixes must be assigned before they may be
used. Presently none are assigned.
The reserved category includes strings which do not
fall into the first three categories. All names in this
category are reserved. It is unlikely that names will be
assigned to this category unless there is a very strong
argument for not using the "other" category.
These rules guarantee that there will be no conflicts
between the various name styles. The following additional
constraints apply to the assignment of realm names in the
domain and X.500 categories: the name of a realm for the
domain or X.500 formats must either be used by the organiza-
tion owning (to whom it was assigned) an Internet domain
name or X.500 name, or in the case that no such names are
registered, authority to use a realm name may be derived
from the authority of the parent realm. For example, if
there is no domain name for E40.MIT.EDU, then the adminis-
trator of the MIT.EDU realm can authorize the creation of a
realm with that name.
This is acceptable because the organization to which
the parent is assigned is presumably the organization
authorized to assign names to its children in the X.500 and
domain name systems as well. If the parent assigns a realm
name without also registering it in the domain name or X.500
hierarchy, it is the parent's responsibility to make sure
that there will not in the future exists a name identical to
the realm name of the child unless it is assigned to the
same entity as the realm name.
7.2. Principal Names
As was the case for realm names, conventions are needed
to ensure that all agree on what information is implied by a
principal name. The name-type field that is part of the
principal name indicates the kind of information implied by
the name. The name-type should be treated as a hint.
Ignoring the name type, no two names can be the same (i.e.
at least one of the components, or the realm, must be dif-
ferent). This constraint may be eliminated in the future.
The following name types are defined:
name-type value meaning
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NT-UNKNOWN 0 Name type not known
NT-PRINCIPAL 1 General principal name (e.g. username, or DCE principal)
NT-SRV-INST 2 Service and other unique instance (krbtgt)
NT-SRV-HST 3 Service with host name as instance (telnet, rcommands)
NT-SRV-XHST 4 Service with slash-separated host name components
NT-UID 5 Unique ID
When a name implies no information other than its uniqueness
at a particular time the name type PRINCIPAL should be used.
The principal name type should be used for users, and it
might also be used for a unique server. If the name is a
unique machine generated ID that is guaranteed never to be
reassigned then the name type of UID should be used (note
that it is generally a bad idea to reassign names of any
type since stale entries might remain in access control
lists).
If the first component of a name identifies a service
and the remaining components identify an instance of the
service in a server specified manner, then the name type of
SRV-INST should be used. An example of this name type is
the Kerberos ticket-granting service whose name has a first
component of krbtgt and a second component identifying the
realm for which the ticket is valid.
If instance is a single component following the service
name and the instance identifies the host on which the
server is running, then the name type SRV-HST should be
used. This type is typically used for Internet services
such as telnet and the Berkeley R commands. If the separate
components of the host name appear as successive components
following the name of the service, then the name type SRV-
XHST should be used. This type might be used to identify
servers on hosts with X.500 names where the slash (/) might
otherwise be ambiguous.
A name type of UNKNOWN should be used when the form of
the name is not known. When comparing names, a name of type
UNKNOWN will match principals authenticated with names of
any type. A principal authenticated with a name of type
UNKNOWN, however, will only match other names of type UNK-
NOWN.
Names of any type with an initial component of "krbtgt"
are reserved for the Kerberos ticket granting service. See
section 8.2.3 for the form of such names.
7.2.1. Name of server principals
The principal identifier for a server on a host will
generally be composed of two parts: (1) the realm of the KDC
with which the server is registered, and (2) a two-component
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name of type NT-SRV-HST if the host name is an Internet
domain name or a multi-component name of type NT-SRV-XHST if
the name of the host is of a form such as X.500 that allows
slash (/) separators. The first component of the two- or
multi-component name will identify the service and the
latter components will identify the host. Where the name of
the host is not case sensitive (for example, with Internet
domain names) the name of the host must be lower case. If
specified by the application protocol for services such as
telnet and the Berkeley R commands which run with system
privileges, the first component may be the string "host"
instead of a service specific identifier. When a host has
an official name and one or more aliases, the official name
of the host must be used when constructing the name of the
server principal.
8. Constants and other defined values
8.1. Host address types
All negative values for the host address type are
reserved for local use. All non-negative values are
reserved for officially assigned type fields and interpreta-
tions.
The values of the types for the following addresses are
chosen to match the defined address family constants in the
Berkeley Standard Distributions of Unix. They can be found
in <sys/socket.h> with symbolic names AF_xxx (where xxx is
an abbreviation of the address family name).
Internet addresses
Internet addresses are 32-bit (4-octet) quantities,
encoded in MSB order. The type of internet addresses is two
(2).
CHAOSnet addresses
CHAOSnet addresses are 16-bit (2-octet) quantities,
encoded in MSB order. The type of CHAOSnet addresses is
five (5).
ISO addresses
ISO addresses are variable-length. The type of ISO
addresses is seven (7).
Xerox Network Services (XNS) addresses
XNS addresses are 48-bit (6-octet) quantities, encoded
in MSB order. The type of XNS addresses is six (6).
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AppleTalk Datagram Delivery Protocol (DDP) addresses
AppleTalk DDP addresses consist of an 8-bit node number
and a 16-bit network number. The first octet of the address
is the node number; the remaining two octets encode the net-
work number in MSB order. The type of AppleTalk DDP
addresses is sixteen (16).
DECnet Phase IV addresses
DECnet Phase IV addresses are 16-bit addresses, encoded
in LSB order. The type of DECnet Phase IV addresses is
twelve (12).
8.2. KDC messages
8.2.1. IP transport
When contacting a Kerberos server (KDC) for a
KRB_KDC_REQ request using UDP IP transport, the client shall
send a UDP datagram containing only an encoding of the
request to port 88 (decimal) at the KDC's IP address; the
KDC will respond with a reply datagram containing only an
encoding of the reply message (either a KRB_ERROR or a
KRB_KDC_REP) to the sending port at the sender's IP address.
Kerberos servers supporting IP transport must accept
UDP requests on port 88 (decimal). Servers may also accept
TCP requests on port 88 (decimal). When the KRB_KDC_REQ
message is sent to the KDC by TCP, a new connection will be
established for each authentication exchange and the
KRB_KDC_REP or KRB_ERROR message will be returned to the
client on the TCP stream that was established for the
request. The connection will be broken after the reply has
been received (or upon time-out). Care must be taken in
managing TCP/IP connections with the KDC to prevent denial
of service attacks based on the number of TCP/IP connections
with the KDC that remain open.
8.2.2. OSI transport
During authentication of an OSI client to an OSI
server, the mutual authentication of an OSI server to an OSI
client, the transfer of credentials from an OSI client to an
OSI server, or during exchange of private or integrity
checked messages, Kerberos protocol messages may be treated
as opaque objects and the type of the authentication mechan-
ism will be:
OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1), security(5),
kerberosv5(2)}
Depending on the situation, the opaque object will be an
authentication header (KRB_AP_REQ), an authentication reply
(KRB_AP_REP), a safe message (KRB_SAFE), a private message
Section 8.2.2. - 91 - Expires 11 January 1998
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(KRB_PRIV), or a credentials message (KRB_CRED). The opaque
data contains an application code as specified in the ASN.1
description for each message. The application code may be
used by Kerberos to determine the message type.
8.2.3. Name of the TGS
The principal identifier of the ticket-granting service
shall be composed of three parts: (1) the realm of the KDC
issuing the TGS ticket (2) a two-part name of type NT-SRV-
INST, with the first part "krbtgt" and the second part the
name of the realm which will accept the ticket-granting
ticket. For example, a ticket-granting ticket issued by the
ATHENA.MIT.EDU realm to be used to get tickets from the
ATHENA.MIT.EDU KDC has a principal identifier of
"ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU")
(name). A ticket-granting ticket issued by the
ATHENA.MIT.EDU realm to be used to get tickets from the
MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU"
(realm), ("krbtgt", "MIT.EDU") (name).
8.3. Protocol constants and associated values
The following tables list constants used in the protocol and defines their
meanings.
Encryption type etype value block size minimum pad size confounder size
NULL 0 1 0 0
des-cbc-crc 1 8 4 8
des-cbc-md4 2 8 0 8
des-cbc-md5 3 8 0 8
<reserved> 4
des3-cbc-md5 5 8 0 8
<reserved> 6
des3-cbc-sha1 7 8 0 8
sign-dsa-generate 8 (pkinit)
encrypt-rsa-priv 9 (pkinit)
encrypt-rsa-pub 10 (pkinit)
ENCTYPE_PK_CROSS 48 (reserved for pkcross)
<reserved> 0x8003
Checksum type sumtype value checksum size
CRC32 1 4
rsa-md4 2 16
rsa-md4-des 3 24
des-mac 4 16
des-mac-k 5 8
rsa-md4-des-k 6 16
rsa-md5 7 16
rsa-md5-des 8 24
rsa-md5-des3 9 24
hmac-sha1-des3 10 20 (I had this as 10, is it 12)
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padata type padata-type value
PA-TGS-REQ 1
PA-ENC-TIMESTAMP 2
PA-PW-SALT 3
<reserved> 4
PA-ENC-UNIX-TIME 5
PA-SANDIA-SECUREID 6
PA-SESAME 7
PA-OSF-DCE 8
PA-CYBERSAFE-SECUREID 9
PA-AFS3-SALT 10
PA-ETYPE-INFO 11
SAM-CHALLENGE 12 (sam/otp)
SAM-RESPONSE 13 (sam/otp)
PA-PK-AS-REQ 14 (pkinit)
PA-PK-AS-REP 15 (pkinit)
PA-PK-AS-SIGN 16 (pkinit)
PA-PK-KEY-REQ 17 (pkinit)
PA-PK-KEY-REP 18 (pkinit)
authorization data type ad-type value
reserved values 0-63
OSF-DCE 64
SESAME 65
alternate authentication type method-type value
reserved values 0-63
ATT-CHALLENGE-RESPONSE 64
transited encoding type tr-type value
DOMAIN-X500-COMPRESS 1
reserved values all others
Label Value Meaning or MIT code
pvno 5 current Kerberos protocol version number
message types
KRB_AS_REQ 10 Request for initial authentication
KRB_AS_REP 11 Response to KRB_AS_REQ request
KRB_TGS_REQ 12 Request for authentication based on TGT
KRB_TGS_REP 13 Response to KRB_TGS_REQ request
KRB_AP_REQ 14 application request to server
KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL
KRB_SAFE 20 Safe (checksummed) application message
KRB_PRIV 21 Private (encrypted) application message
KRB_CRED 22 Private (encrypted) message to forward credentials
KRB_ERROR 30 Error response
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name types
KRB_NT_UNKNOWN 0 Name type not known
KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for users
KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt)
KRB_NT_SRV_HST 3 Service with host name as instance (telnet, rcommands)
KRB_NT_SRV_XHST 4 Service with host as remaining components
KRB_NT_UID 5 Unique ID
error codes
KDC_ERR_NONE 0 No error
KDC_ERR_NAME_EXP 1 Client's entry in database has expired
KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired
KDC_ERR_BAD_PVNO 3 Requested protocol version number not supported
KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key
KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key
KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database
KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database
KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database
KDC_ERR_NULL_KEY 9 The client or server has a null key
KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating
KDC_ERR_NEVER_VALID 11 Requested start time is later than end time
KDC_ERR_POLICY 12 KDC policy rejects request
KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option
KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type
KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type
KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type
KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type
KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked
KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked
KDC_ERR_TGT_REVOKED 20 TGT has been revoked
KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later
KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later
KDC_ERR_KEY_EXPIRED 23 Password has expired - change password to reset
KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was invalid
KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authenticationrequired-
KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't match
KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for user2user only
KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path
KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field failed
KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired
KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid
KRB_AP_ERR_REPEAT 34 Request is a replay
KRB_AP_ERR_NOT_US 35 The ticket isn't for us
KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match
KRB_AP_ERR_SKEW 37 Clock skew too great
KRB_AP_ERR_BADADDR 38 Incorrect net address
KRB_AP_ERR_BADVERSION 39 Protocol version mismatch
KRB_AP_ERR_MSG_TYPE 40 Invalid msg type
KRB_AP_ERR_MODIFIED 41 Message stream modified
KRB_AP_ERR_BADORDER 42 Message out of order
KRB_AP_ERR_BADKEYVER 44 Specified version of key is not available
KRB_AP_ERR_NOKEY 45 Service key not available
KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed
KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction
KRB_AP_ERR_METHOD 48 Alternative authentication method required
KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message
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KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in message
KRB_ERR_GENERIC 60 Generic error (description in e-text)
KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation
KDC_ERROR_CLIENT_NOT_TRUSTED 62 (pkinit)
KDC_ERROR_KDC_NOT_TRUSTED 63 (pkinit)
KDC_ERROR_INVALID_SIG 64 (pkinit)
KDC_ERR_KEY_TOO_WEAK 65 (pkinit)
9. Interoperability requirements
Version 5 of the Kerberos protocol supports a myriad of
options. Among these are multiple encryption and checksum
types, alternative encoding schemes for the transited field,
optional mechanisms for pre-authentication, the handling of
tickets with no addresses, options for mutual authentica-
tion, user to user authentication, support for proxies, for-
warding, postdating, and renewing tickets, the format of
realm names, and the handling of authorization data.
In order to ensure the interoperability of realms, it
is necessary to define a minimal configuration which must be
supported by all implementations. This minimal configura-
tion is subject to change as technology does. For example,
if at some later date it is discovered that one of the
required encryption or checksum algorithms is not secure, it
will be replaced.
9.1. Specification 1
This section defines the first specification of these
options. Implementations which are configured in this way
can be said to support Kerberos Version 5 Specification 1
(5.1).
Encryption and checksum methods
The following encryption and checksum mechanisms must be
supported. Implementations may support other mechanisms as
well, but the additional mechanisms may only be used when
communicating with principals known to also support them:
This list is to be determined.
Encryption: DES-CBC-MD5
Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5
__________________________
- This error carries additional information in the e-
data field. The contents of the e-data field for this
message is described in section 5.9.1.
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Realm Names
All implementations must understand hierarchical realms in
both the Internet Domain and the X.500 style. When a ticket
granting ticket for an unknown realm is requested, the KDC
must be able to determine the names of the intermediate
realms between the KDCs realm and the requested realm.
Transited field encoding
DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be
supported. Alternative encodings may be supported, but they
may be used only when that encoding is supported by ALL
intermediate realms.
Pre-authentication methods
The TGS-REQ method must be supported. The TGS-REQ method is
not used on the initial request. The PA-ENC-TIMESTAMP
method must be supported by clients but whether it is
enabled by default may be determined on a realm by realm
basis. If not used in the initial request and the error
KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-
TIMESTAMP as an acceptable method, the client should retry
the initial request using the PA-ENC-TIMESTAMP pre-
authentication method. Servers need not support the PA-
ENC-TIMESTAMP method, but if not supported the server should
ignore the presence of PA-ENC-TIMESTAMP pre-authentication
in a request.
Mutual authentication
Mutual authentication (via the KRB_AP_REP message) must be
supported.
Ticket addresses and flags
All KDC's must pass on tickets that carry no addresses (i.e.
if a TGT contains no addresses, the KDC will return deriva-
tive tickets), but each realm may set its own policy for
issuing such tickets, and each application server will set
its own policy with respect to accepting them.
Proxies and forwarded tickets must be supported. Indi-
vidual realms and application servers can set their own pol-
icy on when such tickets will be accepted.
All implementations must recognize renewable and post-
dated tickets, but need not actually implement them. If
these options are not supported, the starttime and endtime
in the ticket shall specify a ticket's entire useful life.
When a postdated ticket is decoded by a server, all imple-
mentations shall make the presence of the postdated flag
Section 9.1. - 96 - Expires 11 January 1998
Version 5 - Specification Revision 6
visible to the calling server.
User-to-user authentication
Support for user to user authentication (via the ENC-TKT-
IN-SKEY KDC option) must be provided by implementations, but
individual realms may decide as a matter of policy to reject
such requests on a per-principal or realm-wide basis.
Authorization data
Implementations must pass all authorization data subfields
from ticket-granting tickets to any derivative tickets
unless directed to suppress a subfield as part of the defin-
ition of that registered subfield type (it is never
incorrect to pass on a subfield, and no registered subfield
types presently specify suppression at the KDC).
Implementations must make the contents of any authori-
zation data subfields available to the server when a ticket
is used. Implementations are not required to allow clients
to specify the contents of the authorization data fields.
9.2. Recommended KDC values
Following is a list of recommended values for a KDC imple-
mentation, based on the list of suggested configuration con-
stants (see section 4.4).
minimum lifetime 5 minutes
maximum renewable lifetime1 week
maximum ticket lifetime1 day
empty addresses only when suitable restrictions appear
in authorization data
proxiable, etc. Allowed.
Section 9.2. - 97 - Expires 11 January 1998
Version 5 - Specification Revision 6
10. REFERENCES
1. B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti-
cation Service for Computer Networks," IEEE Communica-
tions Magazine, Vol. 32(9), pp. 33-38 (September 1994).
2. S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H.
Saltzer, Section E.2.1: Kerberos Authentication and
Authorization System, M.I.T. Project Athena, Cambridge,
Massachusetts (December 21, 1987).
3. J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker-
beros: An Authentication Service for Open Network Sys-
tems," pp. 191-202 in Usenix Conference Proceedings,
Dallas, Texas (February, 1988).
4. Roger M. Needham and Michael D. Schroeder, "Using
Encryption for Authentication in Large Networks of Com-
puters," Communications of the ACM, Vol. 21(12),
pp. 993-999 (December, 1978).
5. Dorothy E. Denning and Giovanni Maria Sacco, "Time-
stamps in Key Distribution Protocols," Communications
of the ACM, Vol. 24(8), pp. 533-536 (August 1981).
6. John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o,
"The Evolution of the Kerberos Authentication Service,"
in an IEEE Computer Society Text soon to be published
(June 1992).
7. B. Clifford Neuman, "Proxy-Based Authorization and
Accounting for Distributed Systems," in Proceedings of
the 13th International Conference on Distributed Com-
puting Systems, Pittsburgh, PA (May, 1993).
8. Don Davis and Ralph Swick, "Workstation Services and
Kerberos Authentication at Project Athena," Technical
Memorandum TM-424, MIT Laboratory for Computer Science
(February 1990).
9. P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som-
merfeld, and K. Raeburn, Section E.1: Service Manage-
ment System, M.I.T. Project Athena, Cambridge, Mas-
sachusetts (1987).
10. CCITT, Recommendation X.509: The Directory Authentica-
tion Framework, December 1988.
11. J. Pato, Using Pre-Authentication to Avoid Password
Guessing Attacks, Open Software Foundation DCE Request
for Comments 26 (December 1992).
Section 10. - 98 - Expires 11 January 1998
Version 5 - Specification Revision 6
12. National Bureau of Standards, U.S. Department of Com-
merce, "Data Encryption Standard," Federal Information
Processing Standards Publication 46, Washington, DC
(1977).
13. National Bureau of Standards, U.S. Department of Com-
merce, "DES Modes of Operation," Federal Information
Processing Standards Publication 81, Springfield, VA
(December 1980).
14. Stuart G. Stubblebine and Virgil D. Gligor, "On Message
Integrity in Cryptographic Protocols," in Proceedings
of the IEEE Symposium on Research in Security and
Privacy, Oakland, California (May 1992).
15. International Organization for Standardization, "ISO
Information Processing Systems - Data Communication -
High-Level Data Link Control Procedure - Frame Struc-
ture," IS 3309 (October 1984). 3rd Edition.
16. R. Rivest, "The MD4 Message Digest Algorithm," RFC
1320, MIT Laboratory for Computer Science (April
1992).
17. R. Rivest, "The MD5 Message Digest Algorithm," RFC
1321, MIT Laboratory for Computer Science (April
1992).
18. H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication," Working Draft
draft-ietf-ipsec-hmac-md5-01.txt, (August 1996).
Section 10. - 99 - Expires 11 January 1998
Version 5 - Specification Revision 6
A. Pseudo-code for protocol processing
This appendix provides pseudo-code describing how the
messages are to be constructed and interpreted by clients
and servers.
A.1. KRB_AS_REQ generation
request.pvno := protocol version; /* pvno = 5 */
request.msg-type := message type; /* type = KRB_AS_REQ */
if(pa_enc_timestamp_required) then
request.padata.padata-type = PA-ENC-TIMESTAMP;
get system_time;
padata-body.patimestamp,pausec = system_time;
encrypt padata-body into request.padata.padata-value
using client.key; /* derived from password */
endif
body.kdc-options := users's preferences;
body.cname := user's name;
body.realm := user's realm;
body.sname := service's name; /* usually "krbtgt", "localrealm" */
if (body.kdc-options.POSTDATED is set) then
body.from := requested starting time;
else
omit body.from;
endif
body.till := requested end time;
if (body.kdc-options.RENEWABLE is set) then
body.rtime := requested final renewal time;
endif
body.nonce := random_nonce();
body.etype := requested etypes;
if (user supplied addresses) then
body.addresses := user's addresses;
else
omit body.addresses;
endif
omit body.enc-authorization-data;
request.req-body := body;
kerberos := lookup(name of local kerberos server (or servers));
send(packet,kerberos);
wait(for response);
if (timed_out) then
retry or use alternate server;
endif
A.2. KRB_AS_REQ verification and KRB_AS_REP generation
decode message into req;
client := lookup(req.cname,req.realm);
server := lookup(req.sname,req.realm);
Section A.2. - 100 - Expires 11 January 1998
Version 5 - Specification Revision 6
get system_time;
kdc_time := system_time.seconds;
if (!client) then
/* no client in Database */
error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN);
endif
if (!server) then
/* no server in Database */
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
endif
if(client.pa_enc_timestamp_required and
pa_enc_timestamp not present) then
error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP));
endif
if(pa_enc_timestamp present) then
decrypt req.padata-value into decrypted_enc_timestamp
using client.key;
using auth_hdr.authenticator.subkey;
if (decrypt_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
if(decrypted_enc_timestamp is not within allowable skew) then
error_out(KDC_ERR_PREAUTH_FAILED);
endif
if(decrypted_enc_timestamp and usec is replay)
error_out(KDC_ERR_PREAUTH_FAILED);
endif
add decrypted_enc_timestamp and usec to replay cache;
endif
use_etype := first supported etype in req.etypes;
if (no support for req.etypes) then
error_out(KDC_ERR_ETYPE_NOSUPP);
endif
new_tkt.vno := ticket version; /* = 5 */
new_tkt.sname := req.sname;
new_tkt.srealm := req.srealm;
reset all flags in new_tkt.flags;
/* It should be noted that local policy may affect the */
/* processing of any of these flags. For example, some */
/* realms may refuse to issue renewable tickets */
if (req.kdc-options.FORWARDABLE is set) then
set new_tkt.flags.FORWARDABLE;
endif
if (req.kdc-options.PROXIABLE is set) then
set new_tkt.flags.PROXIABLE;
endif
Section A.2. - 101 - Expires 11 January 1998
Version 5 - Specification Revision 6
if (req.kdc-options.ALLOW-POSTDATE is set) then
set new_tkt.flags.MAY-POSTDATE;
endif
if ((req.kdc-options.RENEW is set) or
(req.kdc-options.VALIDATE is set) or
(req.kdc-options.PROXY is set) or
(req.kdc-options.FORWARDED is set) or
(req.kdc-options.ENC-TKT-IN-SKEY is set)) then
error_out(KDC_ERR_BADOPTION);
endif
new_tkt.session := random_session_key();
new_tkt.cname := req.cname;
new_tkt.crealm := req.crealm;
new_tkt.transited := empty_transited_field();
new_tkt.authtime := kdc_time;
if (req.kdc-options.POSTDATED is set) then
if (against_postdate_policy(req.from)) then
error_out(KDC_ERR_POLICY);
endif
set new_tkt.flags.POSTDATED;
set new_tkt.flags.INVALID;
new_tkt.starttime := req.from;
else
omit new_tkt.starttime; /* treated as authtime when omitted */
endif
if (req.till = 0) then
till := infinity;
else
till := req.till;
endif
new_tkt.endtime := min(till,
new_tkt.starttime+client.max_life,
new_tkt.starttime+server.max_life,
new_tkt.starttime+max_life_for_realm);
if ((req.kdc-options.RENEWABLE-OK is set) and
(new_tkt.endtime < req.till)) then
/* we set the RENEWABLE option for later processing */
set req.kdc-options.RENEWABLE;
req.rtime := req.till;
endif
if (req.rtime = 0) then
rtime := infinity;
else
rtime := req.rtime;
endif
if (req.kdc-options.RENEWABLE is set) then
set new_tkt.flags.RENEWABLE;
Section A.2. - 102 - Expires 11 January 1998
Version 5 - Specification Revision 6
new_tkt.renew-till := min(rtime,
new_tkt.starttime+client.max_rlife,
new_tkt.starttime+server.max_rlife,
new_tkt.starttime+max_rlife_for_realm);
else
omit new_tkt.renew-till; /* only present if RENEWABLE */
endif
if (req.addresses) then
new_tkt.caddr := req.addresses;
else
omit new_tkt.caddr;
endif
new_tkt.authorization_data := empty_authorization_data();
encode to-be-encrypted part of ticket into OCTET STRING;
new_tkt.enc-part := encrypt OCTET STRING
using etype_for_key(server.key), server.key, server.p_kvno;
/* Start processing the response */
resp.pvno := 5;
resp.msg-type := KRB_AS_REP;
resp.cname := req.cname;
resp.crealm := req.realm;
resp.ticket := new_tkt;
resp.key := new_tkt.session;
resp.last-req := fetch_last_request_info(client);
resp.nonce := req.nonce;
resp.key-expiration := client.expiration;
resp.flags := new_tkt.flags;
resp.authtime := new_tkt.authtime;
resp.starttime := new_tkt.starttime;
resp.endtime := new_tkt.endtime;
if (new_tkt.flags.RENEWABLE) then
resp.renew-till := new_tkt.renew-till;
endif
resp.realm := new_tkt.realm;
resp.sname := new_tkt.sname;
resp.caddr := new_tkt.caddr;
encode body of reply into OCTET STRING;
resp.enc-part := encrypt OCTET STRING
using use_etype, client.key, client.p_kvno;
send(resp);
Section A.2. - 103 - Expires 11 January 1998
Version 5 - Specification Revision 6
A.3. KRB_AS_REP verification
decode response into resp;
if (resp.msg-type = KRB_ERROR) then
if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)) then
set pa_enc_timestamp_required;
goto KRB_AS_REQ;
endif
process_error(resp);
return;
endif
/* On error, discard the response, and zero the session key */
/* from the response immediately */
key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype,
resp.padata);
unencrypted part of resp := decode of decrypt of resp.enc-part
using resp.enc-part.etype and key;
zero(key);
if (common_as_rep_tgs_rep_checks fail) then
destroy resp.key;
return error;
endif
if near(resp.princ_exp) then
print(warning message);
endif
save_for_later(ticket,session,client,server,times,flags);
A.4. KRB_AS_REP and KRB_TGS_REP common checks
if (decryption_error() or
(req.cname != resp.cname) or
(req.realm != resp.crealm) or
(req.sname != resp.sname) or
(req.realm != resp.realm) or
(req.nonce != resp.nonce) or
(req.addresses != resp.caddr)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
/* make sure no flags are set that shouldn't be, and that all that */
/* should be are set */
if (!check_flags_for_compatability(req.kdc-options,resp.flags)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.from = 0) and
(resp.starttime is not within allowable skew)) then
destroy resp.key;
return KRB_AP_ERR_SKEW;
Section A.4. - 104 - Expires 11 January 1998
Version 5 - Specification Revision 6
endif
if ((req.from != 0) and (req.from != resp.starttime)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.till != 0) and (resp.endtime > req.till)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.kdc-options.RENEWABLE is set) and
(req.rtime != 0) and (resp.renew-till > req.rtime)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.kdc-options.RENEWABLE-OK is set) and
(resp.flags.RENEWABLE) and
(req.till != 0) and
(resp.renew-till > req.till)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
A.5. KRB_TGS_REQ generation
/* Note that make_application_request might have to recursivly */
/* call this routine to get the appropriate ticket-granting ticket */
request.pvno := protocol version; /* pvno = 5 */
request.msg-type := message type; /* type = KRB_TGS_REQ */
body.kdc-options := users's preferences;
/* If the TGT is not for the realm of the end-server */
/* then the sname will be for a TGT for the end-realm */
/* and the realm of the requested ticket (body.realm) */
/* will be that of the TGS to which the TGT we are */
/* sending applies */
body.sname := service's name;
body.realm := service's realm;
if (body.kdc-options.POSTDATED is set) then
body.from := requested starting time;
else
omit body.from;
endif
body.till := requested end time;
if (body.kdc-options.RENEWABLE is set) then
body.rtime := requested final renewal time;
endif
body.nonce := random_nonce();
body.etype := requested etypes;
if (user supplied addresses) then
body.addresses := user's addresses;
else
omit body.addresses;
Section A.5. - 105 - Expires 11 January 1998
Version 5 - Specification Revision 6
endif
body.enc-authorization-data := user-supplied data;
if (body.kdc-options.ENC-TKT-IN-SKEY) then
body.additional-tickets_ticket := second TGT;
endif
request.req-body := body;
check := generate_checksum (req.body,checksumtype);
request.padata[0].padata-type := PA-TGS-REQ;
request.padata[0].padata-value := create a KRB_AP_REQ using
the TGT and checksum
/* add in any other padata as required/supplied */
kerberos := lookup(name of local kerberose server (or servers));
send(packet,kerberos);
wait(for response);
if (timed_out) then
retry or use alternate server;
endif
A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation
/* note that reading the application request requires first
determining the server for which a ticket was issued, and choosing the
correct key for decryption. The name of the server appears in the
plaintext part of the ticket. */
if (no KRB_AP_REQ in req.padata) then
error_out(KDC_ERR_PADATA_TYPE_NOSUPP);
endif
verify KRB_AP_REQ in req.padata;
/* Note that the realm in which the Kerberos server is operating is
determined by the instance from the ticket-granting ticket. The realm
in the ticket-granting ticket is the realm under which the ticket
granting ticket was issued. It is possible for a single Kerberos
server to support more than one realm. */
auth_hdr := KRB_AP_REQ;
tgt := auth_hdr.ticket;
if (tgt.sname is not a TGT for local realm and is not req.sname) then
error_out(KRB_AP_ERR_NOT_US);
realm := realm_tgt_is_for(tgt);
decode remainder of request;
if (auth_hdr.authenticator.cksum is missing) then
error_out(KRB_AP_ERR_INAPP_CKSUM);
endif
Section A.6. - 106 - Expires 11 January 1998
Version 5 - Specification Revision 6
if (auth_hdr.authenticator.cksum type is not supported) then
error_out(KDC_ERR_SUMTYPE_NOSUPP);
endif
if (auth_hdr.authenticator.cksum is not both collision-proof and keyed) then
error_out(KRB_AP_ERR_INAPP_CKSUM);
endif
set computed_checksum := checksum(req);
if (computed_checksum != auth_hdr.authenticatory.cksum) then
error_out(KRB_AP_ERR_MODIFIED);
endif
server := lookup(req.sname,realm);
if (!server) then
if (is_foreign_tgt_name(server)) then
server := best_intermediate_tgs(server);
else
/* no server in Database */
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
endif
endif
session := generate_random_session_key();
use_etype := first supported etype in req.etypes;
if (no support for req.etypes) then
error_out(KDC_ERR_ETYPE_NOSUPP);
endif
new_tkt.vno := ticket version; /* = 5 */
new_tkt.sname := req.sname;
new_tkt.srealm := realm;
reset all flags in new_tkt.flags;
/* It should be noted that local policy may affect the */
/* processing of any of these flags. For example, some */
/* realms may refuse to issue renewable tickets */
new_tkt.caddr := tgt.caddr;
resp.caddr := NULL; /* We only include this if they change */
if (req.kdc-options.FORWARDABLE is set) then
if (tgt.flags.FORWARDABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.FORWARDABLE;
endif
if (req.kdc-options.FORWARDED is set) then
if (tgt.flags.FORWARDABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.FORWARDED;
Section A.6. - 107 - Expires 11 January 1998
Version 5 - Specification Revision 6
new_tkt.caddr := req.addresses;
resp.caddr := req.addresses;
endif
if (tgt.flags.FORWARDED is set) then
set new_tkt.flags.FORWARDED;
endif
if (req.kdc-options.PROXIABLE is set) then
if (tgt.flags.PROXIABLE is reset)
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.PROXIABLE;
endif
if (req.kdc-options.PROXY is set) then
if (tgt.flags.PROXIABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.PROXY;
new_tkt.caddr := req.addresses;
resp.caddr := req.addresses;
endif
if (req.kdc-options.ALLOW-POSTDATE is set) then
if (tgt.flags.MAY-POSTDATE is reset)
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.MAY-POSTDATE;
endif
if (req.kdc-options.POSTDATED is set) then
if (tgt.flags.MAY-POSTDATE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.POSTDATED;
set new_tkt.flags.INVALID;
if (against_postdate_policy(req.from)) then
error_out(KDC_ERR_POLICY);
endif
new_tkt.starttime := req.from;
endif
if (req.kdc-options.VALIDATE is set) then
if (tgt.flags.INVALID is reset) then
error_out(KDC_ERR_POLICY);
endif
if (tgt.starttime > kdc_time) then
error_out(KRB_AP_ERR_NYV);
endif
if (check_hot_list(tgt)) then
error_out(KRB_AP_ERR_REPEAT);
endif
tkt := tgt;
reset new_tkt.flags.INVALID;
endif
Section A.6. - 108 - Expires 11 January 1998
Version 5 - Specification Revision 6
if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW,
and those already processed) is set) then
error_out(KDC_ERR_BADOPTION);
endif
new_tkt.authtime := tgt.authtime;
if (req.kdc-options.RENEW is set) then
/* Note that if the endtime has already passed, the ticket would */
/* have been rejected in the initial authentication stage, so */
/* there is no need to check again here */
if (tgt.flags.RENEWABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
if (tgt.renew-till >= kdc_time) then
error_out(KRB_AP_ERR_TKT_EXPIRED);
endif
tkt := tgt;
new_tkt.starttime := kdc_time;
old_life := tgt.endttime - tgt.starttime;
new_tkt.endtime := min(tgt.renew-till,
new_tkt.starttime + old_life);
else
new_tkt.starttime := kdc_time;
if (req.till = 0) then
till := infinity;
else
till := req.till;
endif
new_tkt.endtime := min(till,
new_tkt.starttime+client.max_life,
new_tkt.starttime+server.max_life,
new_tkt.starttime+max_life_for_realm,
tgt.endtime);
if ((req.kdc-options.RENEWABLE-OK is set) and
(new_tkt.endtime < req.till) and
(tgt.flags.RENEWABLE is set) then
/* we set the RENEWABLE option for later processing */
set req.kdc-options.RENEWABLE;
req.rtime := min(req.till, tgt.renew-till);
endif
endif
if (req.rtime = 0) then
rtime := infinity;
else
rtime := req.rtime;
endif
if ((req.kdc-options.RENEWABLE is set) and
(tgt.flags.RENEWABLE is set)) then
set new_tkt.flags.RENEWABLE;
new_tkt.renew-till := min(rtime,
Section A.6. - 109 - Expires 11 January 1998
Version 5 - Specification Revision 6
new_tkt.starttime+client.max_rlife,
new_tkt.starttime+server.max_rlife,
new_tkt.starttime+max_rlife_for_realm,
tgt.renew-till);
else
new_tkt.renew-till := OMIT; /* leave the renew-till field out */
endif
if (req.enc-authorization-data is present) then
decrypt req.enc-authorization-data into decrypted_authorization_data
using auth_hdr.authenticator.subkey;
if (decrypt_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
endif
new_tkt.authorization_data := req.auth_hdr.ticket.authorization_data +
decrypted_authorization_data;
new_tkt.key := session;
new_tkt.crealm := tgt.crealm;
new_tkt.cname := req.auth_hdr.ticket.cname;
if (realm_tgt_is_for(tgt) := tgt.realm) then
/* tgt issued by local realm */
new_tkt.transited := tgt.transited;
else
/* was issued for this realm by some other realm */
if (tgt.transited.tr-type not supported) then
error_out(KDC_ERR_TRTYPE_NOSUPP);
endif
new_tkt.transited := compress_transited(tgt.transited + tgt.realm)
endif
encode encrypted part of new_tkt into OCTET STRING;
if (req.kdc-options.ENC-TKT-IN-SKEY is set) then
if (server not specified) then
server = req.second_ticket.client;
endif
if ((req.second_ticket is not a TGT) or
(req.second_ticket.client != server)) then
error_out(KDC_ERR_POLICY);
endif
new_tkt.enc-part := encrypt OCTET STRING using
using etype_for_key(second-ticket.key), second-ticket.key;
else
new_tkt.enc-part := encrypt OCTET STRING
using etype_for_key(server.key), server.key, server.p_kvno;
endif
resp.pvno := 5;
resp.msg-type := KRB_TGS_REP;
resp.crealm := tgt.crealm;
resp.cname := tgt.cname;
Section A.6. - 110 - Expires 11 January 1998
Version 5 - Specification Revision 6
resp.ticket := new_tkt;
resp.key := session;
resp.nonce := req.nonce;
resp.last-req := fetch_last_request_info(client);
resp.flags := new_tkt.flags;
resp.authtime := new_tkt.authtime;
resp.starttime := new_tkt.starttime;
resp.endtime := new_tkt.endtime;
omit resp.key-expiration;
resp.sname := new_tkt.sname;
resp.realm := new_tkt.realm;
if (new_tkt.flags.RENEWABLE) then
resp.renew-till := new_tkt.renew-till;
endif
encode body of reply into OCTET STRING;
if (req.padata.authenticator.subkey)
resp.enc-part := encrypt OCTET STRING using use_etype,
req.padata.authenticator.subkey;
else resp.enc-part := encrypt OCTET STRING using use_etype, tgt.key;
send(resp);
A.7. KRB_TGS_REP verification
decode response into resp;
if (resp.msg-type = KRB_ERROR) then
process_error(resp);
return;
endif
/* On error, discard the response, and zero the session key from
the response immediately */
if (req.padata.authenticator.subkey)
unencrypted part of resp := decode of decrypt of resp.enc-part
using resp.enc-part.etype and subkey;
else unencrypted part of resp := decode of decrypt of resp.enc-part
using resp.enc-part.etype and tgt's session key;
if (common_as_rep_tgs_rep_checks fail) then
destroy resp.key;
return error;
endif
check authorization_data as necessary;
save_for_later(ticket,session,client,server,times,flags);
Section A.7. - 111 - Expires 11 January 1998
Version 5 - Specification Revision 6
A.8. Authenticator generation
body.authenticator-vno := authenticator vno; /* = 5 */
body.cname, body.crealm := client name;
if (supplying checksum) then
body.cksum := checksum;
endif
get system_time;
body.ctime, body.cusec := system_time;
if (selecting sub-session key) then
select sub-session key;
body.subkey := sub-session key;
endif
if (using sequence numbers) then
select initial sequence number;
body.seq-number := initial sequence;
endif
A.9. KRB_AP_REQ generation
obtain ticket and session_key from cache;
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_AP_REQ */
if (desired(MUTUAL_AUTHENTICATION)) then
set packet.ap-options.MUTUAL-REQUIRED;
else
reset packet.ap-options.MUTUAL-REQUIRED;
endif
if (using session key for ticket) then
set packet.ap-options.USE-SESSION-KEY;
else
reset packet.ap-options.USE-SESSION-KEY;
endif
packet.ticket := ticket; /* ticket */
generate authenticator;
encode authenticator into OCTET STRING;
encrypt OCTET STRING into packet.authenticator using session_key;
A.10. KRB_AP_REQ verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_AP_REQ) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
if (packet.ticket.tkt_vno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.ap_options.USE-SESSION-KEY is set) then
retrieve session key from ticket-granting ticket for
packet.ticket.{sname,srealm,enc-part.etype};
Section A.10. - 112 - Expires 11 January 1998
Version 5 - Specification Revision 6
else
retrieve service key for
packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno};
endif
if (no_key_available) then
if (cannot_find_specified_skvno) then
error_out(KRB_AP_ERR_BADKEYVER);
else
error_out(KRB_AP_ERR_NOKEY);
endif
endif
decrypt packet.ticket.enc-part into decr_ticket using retrieved key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
decrypt packet.authenticator into decr_authenticator
using decr_ticket.key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if (decr_authenticator.{cname,crealm} !=
decr_ticket.{cname,crealm}) then
error_out(KRB_AP_ERR_BADMATCH);
endif
if (decr_ticket.caddr is present) then
if (sender_address(packet) is not in decr_ticket.caddr) then
error_out(KRB_AP_ERR_BADADDR);
endif
elseif (application requires addresses) then
error_out(KRB_AP_ERR_BADADDR);
endif
if (not in_clock_skew(decr_authenticator.ctime,
decr_authenticator.cusec)) then
error_out(KRB_AP_ERR_SKEW);
endif
if (repeated(decr_authenticator.{ctime,cusec,cname,crealm})) then
error_out(KRB_AP_ERR_REPEAT);
endif
save_identifier(decr_authenticator.{ctime,cusec,cname,crealm});
get system_time;
if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or
(decr_ticket.flags.INVALID is set)) then
/* it hasn't yet become valid */
error_out(KRB_AP_ERR_TKT_NYV);
endif
if (system_time-decr_ticket.endtime > CLOCK_SKEW) then
error_out(KRB_AP_ERR_TKT_EXPIRED);
endif
/* caller must check decr_ticket.flags for any pertinent details */
return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);
A.11. KRB_AP_REP generation
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_AP_REP */
Section A.11. - 113 - Expires 11 January 1998
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body.ctime := packet.ctime;
body.cusec := packet.cusec;
if (selecting sub-session key) then
select sub-session key;
body.subkey := sub-session key;
endif
if (using sequence numbers) then
select initial sequence number;
body.seq-number := initial sequence;
endif
encode body into OCTET STRING;
select encryption type;
encrypt OCTET STRING into packet.enc-part;
A.12. KRB_AP_REP verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_AP_REP) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
cleartext := decrypt(packet.enc-part) using ticket's session key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if (cleartext.ctime != authenticator.ctime) then
error_out(KRB_AP_ERR_MUT_FAIL);
endif
if (cleartext.cusec != authenticator.cusec) then
error_out(KRB_AP_ERR_MUT_FAIL);
endif
if (cleartext.subkey is present) then
save cleartext.subkey for future use;
endif
if (cleartext.seq-number is present) then
save cleartext.seq-number for future verifications;
endif
return(AUTHENTICATION_SUCCEEDED);
A.13. KRB_SAFE generation
collect user data in buffer;
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_SAFE */
body.user-data := buffer; /* DATA */
if (using timestamp) then
get system_time;
body.timestamp, body.usec := system_time;
Section A.13. - 114 - Expires 11 January 1998
Version 5 - Specification Revision 6
endif
if (using sequence numbers) then
body.seq-number := sequence number;
endif
body.s-address := sender host addresses;
if (only one recipient) then
body.r-address := recipient host address;
endif
checksum.cksumtype := checksum type;
compute checksum over body;
checksum.checksum := checksum value; /* checksum.checksum */
packet.cksum := checksum;
packet.safe-body := body;
A.14. KRB_SAFE verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_SAFE) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
if (packet.checksum.cksumtype is not both collision-proof and keyed) then
error_out(KRB_AP_ERR_INAPP_CKSUM);
endif
if (safe_priv_common_checks_ok(packet)) then
set computed_checksum := checksum(packet.body);
if (computed_checksum != packet.checksum) then
error_out(KRB_AP_ERR_MODIFIED);
endif
return (packet, PACKET_IS_GENUINE);
else
return common_checks_error;
endif
A.15. KRB_SAFE and KRB_PRIV common checks
if (packet.s-address != O/S_sender(packet)) then
/* O/S report of sender not who claims to have sent it */
error_out(KRB_AP_ERR_BADADDR);
endif
if ((packet.r-address is present) and
(packet.r-address != local_host_address)) then
/* was not sent to proper place */
error_out(KRB_AP_ERR_BADADDR);
endif
if (((packet.timestamp is present) and
(not in_clock_skew(packet.timestamp,packet.usec))) or
(packet.timestamp is not present and timestamp expected)) then
error_out(KRB_AP_ERR_SKEW);
endif
if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
error_out(KRB_AP_ERR_REPEAT);
endif
Section A.15. - 115 - Expires 11 January 1998
Version 5 - Specification Revision 6
if (((packet.seq-number is present) and
((not in_sequence(packet.seq-number)))) or
(packet.seq-number is not present and sequence expected)) then
error_out(KRB_AP_ERR_BADORDER);
endif
if (packet.timestamp not present and packet.seq-number not present) then
error_out(KRB_AP_ERR_MODIFIED);
endif
save_identifier(packet.{timestamp,usec,s-address},
sender_principal(packet));
return PACKET_IS_OK;
A.16. KRB_PRIV generation
collect user data in buffer;
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_PRIV */
packet.enc-part.etype := encryption type;
body.user-data := buffer;
if (using timestamp) then
get system_time;
body.timestamp, body.usec := system_time;
endif
if (using sequence numbers) then
body.seq-number := sequence number;
endif
body.s-address := sender host addresses;
if (only one recipient) then
body.r-address := recipient host address;
endif
encode body into OCTET STRING;
select encryption type;
encrypt OCTET STRING into packet.enc-part.cipher;
A.17. KRB_PRIV verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_PRIV) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
cleartext := decrypt(packet.enc-part) using negotiated key;
if (decryption_error()) then
Section A.17. - 116 - Expires 11 January 1998
Version 5 - Specification Revision 6
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if (safe_priv_common_checks_ok(cleartext)) then
return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED);
else
return common_checks_error;
endif
A.18. KRB_CRED generation
invoke KRB_TGS; /* obtain tickets to be provided to peer */
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_CRED */
for (tickets[n] in tickets to be forwarded) do
packet.tickets[n] = tickets[n].ticket;
done
packet.enc-part.etype := encryption type;
for (ticket[n] in tickets to be forwarded) do
body.ticket-info[n].key = tickets[n].session;
body.ticket-info[n].prealm = tickets[n].crealm;
body.ticket-info[n].pname = tickets[n].cname;
body.ticket-info[n].flags = tickets[n].flags;
body.ticket-info[n].authtime = tickets[n].authtime;
body.ticket-info[n].starttime = tickets[n].starttime;
body.ticket-info[n].endtime = tickets[n].endtime;
body.ticket-info[n].renew-till = tickets[n].renew-till;
body.ticket-info[n].srealm = tickets[n].srealm;
body.ticket-info[n].sname = tickets[n].sname;
body.ticket-info[n].caddr = tickets[n].caddr;
done
get system_time;
body.timestamp, body.usec := system_time;
if (using nonce) then
body.nonce := nonce;
endif
if (using s-address) then
body.s-address := sender host addresses;
endif
if (limited recipients) then
body.r-address := recipient host address;
endif
encode body into OCTET STRING;
select encryption type;
encrypt OCTET STRING into packet.enc-part.cipher
Section A.18. - 117 - Expires 11 January 1998
Version 5 - Specification Revision 6
using negotiated encryption key;
A.19. KRB_CRED verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_CRED) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
cleartext := decrypt(packet.enc-part) using negotiated key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if ((packet.r-address is present or required) and
(packet.s-address != O/S_sender(packet)) then
/* O/S report of sender not who claims to have sent it */
error_out(KRB_AP_ERR_BADADDR);
endif
if ((packet.r-address is present) and
(packet.r-address != local_host_address)) then
/* was not sent to proper place */
error_out(KRB_AP_ERR_BADADDR);
endif
if (not in_clock_skew(packet.timestamp,packet.usec)) then
error_out(KRB_AP_ERR_SKEW);
endif
if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
error_out(KRB_AP_ERR_REPEAT);
endif
if (packet.nonce is required or present) and
(packet.nonce != expected-nonce) then
error_out(KRB_AP_ERR_MODIFIED);
endif
for (ticket[n] in tickets that were forwarded) do
save_for_later(ticket[n],key[n],principal[n],
server[n],times[n],flags[n]);
return
A.20. KRB_ERROR generation
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_ERROR */
get system_time;
packet.stime, packet.susec := system_time;
packet.realm, packet.sname := server name;
if (client time available) then
Section A.20. - 118 - Expires 11 January 1998
Version 5 - Specification Revision 6
packet.ctime, packet.cusec := client_time;
endif
packet.error-code := error code;
if (client name available) then
packet.cname, packet.crealm := client name;
endif
if (error text available) then
packet.e-text := error text;
endif
if (error data available) then
packet.e-data := error data;
endif
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- cxx - Expires 11 January 1998
Table of Contents
Overview .............................................. 2
Background ............................................ 2
1. Introduction ....................................... 3
1.1. Cross-Realm Operation ............................ 5
1.2. Authorization .................................... 6
1.3. Environmental assumptions ........................ 7
1.4. Glossary of terms ................................ 8
2. Ticket flag uses and requests ...................... 10
2.1. Initial and pre-authenticated tickets ............ 10
2.2. Invalid tickets .................................. 11
2.3. Renewable tickets ................................ 11
2.4. Postdated tickets ................................ 12
2.5. Proxiable and proxy tickets ...................... 12
2.6. Forwardable tickets .............................. 13
2.7. Other KDC options ................................ 14
3. Message Exchanges .................................. 14
3.1. The Authentication Service Exchange .............. 14
3.1.1. Generation of KRB_AS_REQ message ............... 16
3.1.2. Receipt of KRB_AS_REQ message .................. 16
3.1.3. Generation of KRB_AS_REP message ............... 16
3.1.4. Generation of KRB_ERROR message ................ 19
3.1.5. Receipt of KRB_AS_REP message .................. 19
3.1.6. Receipt of KRB_ERROR message ................... 19
3.2. The Client/Server Authentication Exchange ........ 19
3.2.1. The KRB_AP_REQ message ......................... 20
- i - Expires 11 January 1998
Version 5 - Specification Revision 6
3.2.2. Generation of a KRB_AP_REQ message ............. 20
3.2.3. Receipt of KRB_AP_REQ message .................. 21
3.2.4. Generation of a KRB_AP_REP message ............. 23
3.2.5. Receipt of KRB_AP_REP message .................. 23
3.2.6. Using the encryption key ....................... 24
3.3. The Ticket-Granting Service (TGS) Exchange ....... 25
3.3.1. Generation of KRB_TGS_REQ message .............. 26
3.3.2. Receipt of KRB_TGS_REQ message ................. 27
3.3.3. Generation of KRB_TGS_REP message .............. 28
3.3.3.1. Checking for revoked tickets ................. 30
3.3.3.2. Encoding the transited field ................. 30
3.3.4. Receipt of KRB_TGS_REP message ................. 32
3.4. The KRB_SAFE Exchange ............................ 32
3.4.1. Generation of a KRB_SAFE message ............... 32
3.4.2. Receipt of KRB_SAFE message .................... 33
3.5. The KRB_PRIV Exchange ............................ 34
3.5.1. Generation of a KRB_PRIV message ............... 34
3.5.2. Receipt of KRB_PRIV message .................... 34
3.6. The KRB_CRED Exchange ............................ 35
3.6.1. Generation of a KRB_CRED message ............... 35
3.6.2. Receipt of KRB_CRED message .................... 35
4. The Kerberos Database .............................. 36
4.1. Database contents ................................ 36
4.2. Additional fields ................................ 37
4.3. Frequently Changing Fields ....................... 38
4.4. Site Constants ................................... 39
5. Message Specifications ............................. 39
- ii - Expires 11 January 1998
Version 5 - Specification Revision 6
5.1. ASN.1 Distinguished Encoding Representation ...... 39
5.2. ASN.1 Base Definitions ........................... 40
5.3. Tickets and Authenticators ....................... 43
5.3.1. Tickets ........................................ 43
5.3.2. Authenticators ................................. 52
5.4. Specifications for the AS and TGS exchanges ...... 54
5.4.1. KRB_KDC_REQ definition ......................... 54
5.4.2. KRB_KDC_REP definition ......................... 61
5.5. Client/Server (CS) message specifications ........ 64
5.5.1. KRB_AP_REQ definition .......................... 64
5.5.2. KRB_AP_REP definition .......................... 65
5.5.3. Error message reply ............................ 67
5.6. KRB_SAFE message specification ................... 67
5.6.1. KRB_SAFE definition ............................ 67
5.7. KRB_PRIV message specification ................... 68
5.7.1. KRB_PRIV definition ............................ 68
5.8. KRB_CRED message specification ................... 69
5.8.1. KRB_CRED definition ............................ 70
5.9. Error message specification ...................... 72
5.9.1. KRB_ERROR definition ........................... 72
6. Encryption and Checksum Specifications ............. 74
6.1. Encryption Specifications ........................ 76
6.2. Encryption Keys .................................. 78
6.3. Encryption Systems ............................... 78
6.3.1. The NULL Encryption System (null) .............. 78
6.3.2. DES in CBC mode with a CRC-32 checksum (des-
cbc-crc) .............................................. 79
6.3.3. DES in CBC mode with an MD4 checksum (des-
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Version 5 - Specification Revision 6
cbc-md4) .............................................. 79
6.3.4. DES in CBC mode with an MD5 checksum (des-
cbc-md5) .............................................. 79
6.3.5. Triple DES EDE in outer CBC mode with an SHA1
checksum (des3-cbc-sha1) .............................. 81
6.4. Checksums ........................................ 83
6.4.1. The CRC-32 Checksum (crc32) .................... 84
6.4.2. The RSA MD4 Checksum (rsa-md4) ................. 84
6.4.3. RSA MD4 Cryptographic Checksum Using DES
(rsa-md4-des) ......................................... 84
6.4.4. The RSA MD5 Checksum (rsa-md5) ................. 85
6.4.5. RSA MD5 Cryptographic Checksum Using DES
(rsa-md5-des) ......................................... 85
6.4.6. DES cipher-block chained checksum (des-mac)
6.4.7. RSA MD4 Cryptographic Checksum Using DES
alternative (rsa-md4-des-k) ........................... 86
6.4.8. DES cipher-block chained checksum alternative
(des-mac-k) ........................................... 87
7. Naming Constraints ................................. 87
7.1. Realm Names ...................................... 87
7.2. Principal Names .................................. 88
7.2.1. Name of server principals ...................... 89
8. Constants and other defined values ................. 90
8.1. Host address types ............................... 90
8.2. KDC messages ..................................... 91
8.2.1. IP transport ................................... 91
8.2.2. OSI transport .................................. 91
8.2.3. Name of the TGS ................................ 92
8.3. Protocol constants and associated values ......... 92
9. Interoperability requirements ...................... 95
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Version 5 - Specification Revision 6
9.1. Specification 1 .................................. 95
9.2. Recommended KDC values ........................... 97
10. REFERENCES ........................................ 98
A. Pseudo-code for protocol processing ................ 100
A.1. KRB_AS_REQ generation ............................ 100
A.2. KRB_AS_REQ verification and KRB_AS_REP genera-
tion .................................................. 100
A.3. KRB_AS_REP verification .......................... 104
A.4. KRB_AS_REP and KRB_TGS_REP common checks ......... 104
A.5. KRB_TGS_REQ generation ........................... 105
A.6. KRB_TGS_REQ verification and KRB_TGS_REP gen-
eration ............................................... 106
A.7. KRB_TGS_REP verification ......................... 111
A.8. Authenticator generation ......................... 112
A.9. KRB_AP_REQ generation ............................ 112
A.10. KRB_AP_REQ verification ......................... 112
A.11. KRB_AP_REP generation ........................... 113
A.12. KRB_AP_REP verification ......................... 114
A.13. KRB_SAFE generation ............................. 114
A.14. KRB_SAFE verification ........................... 115
A.15. KRB_SAFE and KRB_PRIV common checks ............. 115
A.16. KRB_PRIV generation ............................. 116
A.17. KRB_PRIV verification ........................... 116
A.18. KRB_CRED generation ............................. 117
A.19. KRB_CRED verification ........................... 118
A.20. KRB_ERROR generation ............................ 118
- v - Expires 11 January 1998
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