282 lines
12 KiB
Plaintext
282 lines
12 KiB
Plaintext
CAT Working Group K. Raeburn
|
||
Internet-draft MIT
|
||
Category: July 14, 2000
|
||
Updates: RFC 1964
|
||
Document: draft-raeburn-cat-gssapi-krb5-3des-00.txt
|
||
|
||
Triple-DES Support for the Kerberos 5 GSSAPI Mechanism
|
||
|
||
Status of this Memo
|
||
|
||
This document is an Internet-Draft and is in full conformance with
|
||
all provisions of Section 10 of RFC2026 [RFC2026]. 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."
|
||
|
||
The list of current Internet-Drafts can be accessed at
|
||
http://www.ietf.org/ietf/1id-abstracts.txt
|
||
|
||
The list of Internet-Draft Shadow Directories can be accessed at
|
||
http://www.ietf.org/shadow.html.
|
||
|
||
1. Abstract
|
||
|
||
The MIT Kerberos 5 release version 1.2 includes support for
|
||
triple-DES with key derivation [KrbRev]. Recent work by the EFF
|
||
[EFF] has demonstrated the vulnerability of single-DES mechanisms
|
||
to brute-force attacks by sufficiently motivated and well-funded
|
||
parties.
|
||
|
||
The GSSAPI Kerberos 5 mechanism definition [GSSAPI-KRB5]
|
||
specifically enumerates encryption and checksum types,
|
||
independently of how such schemes may be used in Kerberos. In the
|
||
long run, a new Kerberos-based mechanism, which does not require
|
||
separately enumerating for the GSSAPI mechanism each of the
|
||
encryption types defined by Kerberos, appears to be a better
|
||
approach. Efforts to produce such a specification are under way.
|
||
|
||
In the interest of providing increased security in the interim,
|
||
however, MIT is proposing adding support for triple-DES to the
|
||
existing mechanism, as described here.
|
||
|
||
2. Conventions Used in this Document
|
||
|
||
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
|
||
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
|
||
this document are to be interpreted as described in RFC 2119.
|
||
|
||
3. New Algorithm Identifiers
|
||
|
||
One new sealing algorithm is defined, for use in WRAP tokens:
|
||
|
||
02 00 - DES3-KD
|
||
|
||
This algorithm uses triple-DES with key derivation, with a usage
|
||
value KG_USAGE_SEAL. Padding is still to 8-byte multiples, and the
|
||
IV for encrypting application data is zero.
|
||
|
||
One new signing algorithm is defined, for use in MIC, Wrap, and
|
||
Delete tokens:
|
||
|
||
04 00 - HMAC SHA1 DES3-KD
|
||
|
||
This algorithm generates an HMAC using SHA-1 and a derived DES3 key
|
||
with usage KG_USAGE_SIGN, as (ought to be described) in [KrbRev].
|
||
|
||
[XXX: The current [KrbRev] description refers to expired I-Ds from
|
||
Marc Horowitz. The text in [KrbRev] may be inadequate to produce
|
||
an interoperable implementation.]
|
||
|
||
The checksum size for this algorithm is 20 octets. See section 5.3
|
||
below for the use of checksum lengths of other than eight bytes.
|
||
|
||
4. Key Derivation
|
||
|
||
For purposes of key derivation, we add three new usage values to the
|
||
list defined in [KrbRev]; one for signing messages, one for
|
||
sealing messages, and one for encrypting sequence numbers:
|
||
|
||
#define KG_USAGE_SEAL 22
|
||
#define KG_USAGE_SIGN 23
|
||
#define KG_USAGE_SEQ 24
|
||
|
||
5. Adjustments to Previous Definitions
|
||
|
||
5.1. Quality of Protection
|
||
|
||
The GSSAPI specification [GSSAPI] says that a zero QOP value
|
||
indicates the "default". The original specification for the
|
||
Kerberos 5 mechanism says that a zero QOP value (or a QOP value
|
||
with the appropriate bits clear) means DES encryption.
|
||
|
||
Rather than continue to force the use of plain DES when the
|
||
application doesn't use mechanism-specific QOP values, the better
|
||
choice appears to be to redefine the DES QOP value as some non-zero
|
||
value, and define a triple-DES value as well. Then a zero value
|
||
continues to imply the default, which would be triple-DES
|
||
protection when given a triple-DES session key.
|
||
|
||
Our values are:
|
||
|
||
GSS_KRB5_INTEG_C_QOP_HMAC_SHA1 0x0004
|
||
/* SHA-1 checksum encrypted with key derivation */
|
||
|
||
GSS_KRB5_CONF_C_QOP_DES 0x0100
|
||
/* plain DES encryption */
|
||
GSS_KRB5_CONF_C_QOP_DES3_KD 0x0200
|
||
/* triple-DES with key derivation */
|
||
|
||
Rather than open the question of whether to specify means for
|
||
deriving a key of one type given a key of another type, and the
|
||
security implications of whether to generate a long key from a
|
||
shorter one, our implementation will simply return an error if the
|
||
QOP value specified does not correspond to the session key type.
|
||
|
||
[Implementation note: MIT's code does not implement QoP, and
|
||
returns an error for any non-zero QoP value.]
|
||
|
||
5.2. MIC Sequence Number Encryption
|
||
|
||
The sequence numbers are encrypted in the context key (as defined
|
||
in [GSSAPI-KRB5] -- this will be either the Kerberos session key or
|
||
asubkey provided by the context initiator), using whatever
|
||
encryption system is designated by the type of that context key.
|
||
The IV is formed from the first N bytes of the SGN_CKSUM field,
|
||
where N is the number of bytes needed for the IV. (With all
|
||
algorithms described here and in [GSSAPI-KRB5], the checksum is at
|
||
least as large as the IV.)
|
||
|
||
5.3. Message Layout
|
||
|
||
Both MIC and Wrap tokens, as defined in [GSSAPI-KRB5], contain an
|
||
checksum field SGN_CKSUM. In [GSSAPI-KRB5], this field was
|
||
specified as being 8 bytes long. We now change this size to be
|
||
"defined by the checksum algorithm", and retroactively amend the
|
||
descriptions of all the checksum algorithms described in
|
||
[GSSAPI-KRB5] to explicitly specify 8-byte output. Application
|
||
data continues to immediately follow the checksum field in the Wrap
|
||
token.
|
||
|
||
The revised message descriptions are thus:
|
||
|
||
MIC:
|
||
|
||
Byte no Name Description
|
||
0..1 TOK_ID Identification field.
|
||
2..3 SGN_ALG Integrity algorithm indicator.
|
||
4..7 Filler Contains ff ff ff ff
|
||
8..15 SND_SEQ Sequence number field.
|
||
16..s+15 SGN_CKSUM Checksum of "to-be-signed data",
|
||
calculated according to algorithm
|
||
specified in SGN_ALG field.
|
||
|
||
Wrap:
|
||
|
||
Byte no Name Description
|
||
0..1 TOK_ID Identification field.
|
||
Tokens emitted by GSS_Wrap() contain
|
||
the hex value 02 01 in this field.
|
||
2..3 SGN_ALG Checksum algorithm indicator.
|
||
4..5 SEAL_ALG Sealing algorithm indicator.
|
||
6..7 Filler Contains ff ff
|
||
8..15 SND_SEQ Encrypted sequence number field.
|
||
16..s+15 SGN_CKSUM Checksum of plaintext padded data,
|
||
calculated according to algorithm
|
||
specified in SGN_ALG field.
|
||
s+16..last Data encrypted or plaintext padded data
|
||
|
||
Where "s" indicates the size of the checksum.
|
||
|
||
As indicated above in section 2, we define the HMAC SHA1 DES3-KD
|
||
checksum algorithm to produce a 20-byte output, so encrypted data
|
||
begins at byte 36.
|
||
|
||
6. Backwards Compatibility Considerations
|
||
|
||
The context initiator SHOULD request of the KDC credentials using
|
||
session-key cryptosystem types supported by that implementation; if
|
||
the only types returned by the KDC are not supported by the
|
||
mechanism implementation, it MUST indicate a failure. This may
|
||
seem obvious, but early implementations of both Kerberos and the
|
||
GSSAPI Kerberos mechanism supported only DES keys, so the
|
||
cryptosystem compatibility question was easy to overlook.
|
||
|
||
Under the current mechanism, no negotiation of algorithm types
|
||
occurs, so server-side (acceptor) implementations cannot request
|
||
that clients not use algorithm types not understood by the server.
|
||
However, administration of the server's Kerberos data has to be
|
||
done in communication with the KDC, and it is from the KDC that the
|
||
client will request credentials. The KDC could therefore be tasked
|
||
with limiting session keys for a given service to types actually
|
||
supported by the Kerberos and GSSAPI software on the server.
|
||
|
||
This does have a drawback for cases where a service principal name
|
||
is used both for GSSAPI-based and non-GSSAPI-based communication,
|
||
if the GSSAPI implementation does not understand triple-DES but the
|
||
Kerberos implementation does. It means that triple-DES session
|
||
keys cannot be issued for that service principal, which keeps the
|
||
protection of non-GSSAPI services weaker than necessary. However,
|
||
in the most recent MIT releases thus far, while triple-DES support
|
||
has been present, it has required additional work to enable, so it
|
||
is not likely to be in use for many services.
|
||
|
||
It would also be possible to have clients attempt to get single-DES
|
||
session keys before trying to get triple-DES session keys, and have
|
||
the KDC refuse to issue the single-DES keys only for the most
|
||
critical of services, for which single-DES protection is considered
|
||
inadequate. However, that would eliminate the possibility of
|
||
connecting with the more secure cryptosystem to any service that
|
||
can be accessed with the weaker cryptosystem.
|
||
|
||
We have chosen to go with the former approach, putting the burden
|
||
on the KDC administration and gaining the best protection possible
|
||
for GSSAPI services, possibly at the cost of protection of
|
||
non-GSSAPI Kerberos services running earlier versions of the
|
||
software.
|
||
|
||
6. Security Considerations
|
||
|
||
Various tradeoffs arise regarding the mixing of new and old
|
||
software, or GSSAPI-based and non-GSSAPI Kerberos authentication.
|
||
They are discussed in section 5.
|
||
|
||
7. References
|
||
|
||
[EFF] Electronic Frontier Foundation, "Cracking DES: Secrets of
|
||
Encryption Research, Wiretap Politics, and Chip Design", O'Reilly &
|
||
Associates, Inc., May, 1998.
|
||
|
||
[GSSAPI] Linn, J., "Generic Security Service Application Program
|
||
Interface Version 2, Update 1", RFC 2743, January, 2000.
|
||
|
||
[GSSAPI-KRB5] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
|
||
RFC 1964, June, 1996.
|
||
|
||
[KrbRev] Neuman, C., Kohl, J., Ts'o, T., "The Kerberos Network
|
||
Authentication Service (V5)",
|
||
draft-ietf-cat-kerberos-revisions-05.txt, March 10, 2000.
|
||
|
||
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
|
||
3", RFC 2026, October, 1996.
|
||
|
||
8. Author's Address
|
||
|
||
Kenneth Raeburn
|
||
Massachusetts Institute of Technology
|
||
77 Massachusetts Avenue
|
||
Cambridge, MA 02139
|
||
|
||
9. Full Copyright Statement
|
||
|
||
Copyright (C) The Internet Society (2000). All Rights Reserved.
|
||
|
||
This document and translations of it may be copied and furnished to
|
||
others, and derivative works that comment on or otherwise explain it
|
||
or assist in its implementation may be prepared, copied, published
|
||
and distributed, in whole or in part, without restriction of any
|
||
kind, provided that the above copyright notice and this paragraph
|
||
are included on all such copies and derivative works. However, this
|
||
document itself may not be modified in any way, such as by removing
|
||
the copyright notice or references to the Internet Society or other
|
||
Internet organizations, except as needed for the purpose of
|
||
developing Internet standards in which case the procedures for
|
||
copyrights defined in the Internet Standards process must be
|
||
followed, or as required to translate it into languages other than
|
||
English.
|
||
|
||
The limited permissions granted above are perpetual and will not be
|
||
revoked by the Internet Society or its successors or assigns.
|
||
|
||
This document and the information contained herein is provided on an
|
||
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
|
||
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
|
||
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
|
||
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
|
||
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
|