freebsd-dev/usr.sbin/ntp/doc/ntp-keygen.8

.\"
.\" $FreeBSD$
.\"
.Dd May 17, 2006
.Dt NTP-KEYGEN 8
.Os
.Sh NAME
.Nm ntp-keygen
.Nd key generation program for ntpd
.Sh SYNOPSIS
.Nm
.Op Fl deGgHIMnPT
.Op Fl c Ar scheme
.Op Fl i Ar name
.Op Fl p Ar password
.Op Fl S Op Cm RSA | DSA
.Op Fl s Ar name
.Op Fl v Ar nkeys
.Sh DESCRIPTION
This program generates cryptographic data files used by the NTPv4
authentication and identification schemes.
It generates MD5 key files used in symmetric key cryptography.
In addition, if the OpenSSL software library has been installed,
it generates keys, certificate and identity files used in public key
cryptography.
These files are used for cookie encryption,
digital signature and challenge/response identification algorithms
compatible with the Internet standard security infrastructure.
.Pp
All files are in PEM-encoded printable ASCII format,
so they can be embedded as MIME attachments in mail to other sites
and certificate authorities.
By default, files are not encrypted.
The
.Fl p Ar password
option specifies the write password and
.Fl q Ar password
option the read password for previously encrypted files.
The
.Nm
program prompts for the password if it reads an encrypted file
and the password is missing or incorrect.
If an encrypted file is read successfully and
no write password is specified, the read password is used
as the write password by default.
.Pp
The
.Xr ntpd 8
configuration command
.Ic crypto pw Ar password
specifies the read password for previously encrypted files.
The daemon expires on the spot if the password is missing
or incorrect.
For convenience, if a file has been previously encrypted,
the default read password is the name of the host running
the program.
If the previous write password is specified as the host name,
these files can be read by that host with no explicit password.
.Pp
File names begin with the prefix
.Cm ntpkey_
and end with the postfix
.Ar _hostname.filestamp ,
where
.Ar hostname
is the owner name, usually the string returned
by the Unix gethostname() routine, and
.Ar filestamp
is the NTP seconds when the file was generated, in decimal digits.
This both guarantees uniqueness and simplifies maintenance
procedures, since all files can be quickly removed
by a
.Ic rm ntpkey\&*
command or all files generated
at a specific time can be removed by a
.Ic rm
.Ar \&*filestamp
command.
To further reduce the risk of misconfiguration,
the first two lines of a file contain the file name
and generation date and time as comments.
.Pp
All files are installed by default in the keys directory
.Pa /usr/local/etc ,
which is normally in a shared filesystem
in NFS-mounted networks.
The actual location of the keys directory
and each file can be overridden by configuration commands,
but this is not recommended.
Normally, the files for each host are generated by that host
and used only by that host, although exceptions exist
as noted later on this page.
.Pp
Normally, files containing private values,
including the host key, sign key and identification parameters,
are permitted root read/write-only;
while others containing public values are permitted world readable.
Alternatively, files containing private values can be encrypted
and these files permitted world readable,
which simplifies maintenance in shared file systems.
Since uniqueness is insured by the hostname and
file name extensions, the files for a NFS server and
dependent clients can all be installed in the same shared directory.
.Pp
The recommended practice is to keep the file name extensions
when installing a file and to install a soft link
from the generic names specified elsewhere on this page
to the generated files.
This allows new file generations to be activated simply
by changing the link.
If a link is present, ntpd follows it to the file name
to extract the filestamp.
If a link is not present,
.Xr ntpd 8
extracts the filestamp from the file itself.
This allows clients to verify that the file and generation times
are always current.
The
.Nm
program uses the same timestamp extension for all files generated
at one time, so each generation is distinct and can be readily
recognized in monitoring data.
.Ss Running the program
The safest way to run the
.Nm
program is logged in directly as root.
The recommended procedure is change to the keys directory,
usually
.Pa /ust/local/etc ,
then run the program.
When run for the first time,
or if all
.Cm ntpkey
files have been removed,
the program generates a RSA host key file and matching RSA-MD5 certificate file,
which is all that is necessary in many cases.
The program also generates soft links from the generic names
to the respective files.
If run again, the program uses the same host key file,
but generates a new certificate file and link.
.Pp
The host key is used to encrypt the cookie when required and so must be RSA type.
By default, the host key is also the sign key used to encrypt signatures.
When necessary, a different sign key can be specified and this can be
either RSA or DSA type.
By default, the message digest type is MD5, but any combination
of sign key type and message digest type supported by the OpenSSL library
can be specified, including those using the MD2, MD5, SHA, SHA1, MDC2
and RIPE160 message digest algorithms.
However, the scheme specified in the certificate must be compatible
with the sign key.
Certificates using any digest algorithm are compatible with RSA sign keys;
however, only SHA and SHA1 certificates are compatible with DSA sign keys.
.Pp
Private/public key files and certificates are compatible with
other OpenSSL applications and very likely other libraries as well.
Certificates or certificate requests derived from them should be compatible
with extant industry practice, although some users might find
the interpretation of X509v3 extension fields somewhat liberal.
However, the identification parameter files, although encoded
as the other files, are probably not compatible with anything other than Autokey.
.Pp
Running the program as other than root and using the Unix
.Ic su
command
to assume root may not work properly, since by default the OpenSSL library
looks for the random seed file
.Cm .rnd
in the user home directory.
However, there should be only one
.Cm .rnd ,
most conveniently
in the root directory, so it is convenient to define the
.Cm $RANDFILE
environment variable used by the OpenSSL library as the path to
.Cm /.rnd .
.Pp
Installing the keys as root might not work in NFS-mounted
shared file systems, as NFS clients may not be able to write
to the shared keys directory, even as root.
In this case, NFS clients can specify the files in another
directory such as
.Pa /etc
using the
.Ic keysdir
command.
There is no need for one client to read the keys and certificates
of other clients or servers, as these data are obtained automatically
by the Autokey protocol.
.Pp
Ordinarily, cryptographic files are generated by the host that uses them,
but it is possible for a trusted agent (TA) to generate these files
for other hosts; however, in such cases files should always be encrypted.
The subject name and trusted name default to the hostname
of the host generating the files, but can be changed by command line options.
It is convenient to designate the owner name and trusted name
as the subject and issuer fields, respectively, of the certificate.
The owner name is also used for the host and sign key files,
while the trusted name is used for the identity files.
.Pp
.Ss Trusted Hosts and Groups
Each cryptographic configuration involves selection of a signature scheme
and identification scheme, called a cryptotype,
as explained in the
.Sx Authentication Options
section of
.Xr ntp.conf 5 .
The default cryptotype uses RSA encryption, MD5 message digest
and TC identification.
First, configure a NTP subnet including one or more low-stratum
trusted hosts from which all other hosts derive synchronization
directly or indirectly.
Trusted hosts have trusted certificates;
all other hosts have nontrusted certificates.
These hosts will automatically and dynamically build authoritative
certificate trails to one or more trusted hosts.
A trusted group is the set of all hosts that have, directly or indirectly,
a certificate trail ending at a trusted host.
The trail is defined by static configuration file entries
or dynamic means described on the
.Sx Automatic NTP Configuration Options
section of
.Xr ntp.conf 5 .
.Pp
On each trusted host as root, change to the keys directory.
To insure a fresh fileset, remove all
.Cm ntpkey
files.
Then run
.Nm
.Fl T
to generate keys and a trusted certificate.
On all other hosts do the same, but leave off the
.Fl T
flag to generate keys and nontrusted certificates.
When complete, start the NTP daemons beginning at the lowest stratum
and working up the tree.
It may take some time for Autokey to instantiate the certificate trails
throughout the subnet, but setting up the environment is completely automatic.
.Pp
If it is necessary to use a different sign key or different digest/signature
scheme than the default, run
.Nm
with the
.Fl S Ar type
option, where
.Ar type
is either
.Cm RSA
or
.Cm DSA .
The most often need to do this is when a DSA-signed certificate is used.
If it is necessary to use a different certificate scheme than the default,
run
.Nm
with the
.Fl c Ar scheme
option and selected
.Ar scheme
as needed.
If
.Nm
is run again without these options, it generates a new certificate
using the same scheme and sign key.
.Pp
After setting up the environment it is advisable to update certificates
from time to time, if only to extend the validity interval.
Simply run
.Nm
with the same flags as before to generate new certificates
using existing keys.
However, if the host or sign key is changed,
.Xr ntpd 8
should be restarted.
When
.Xr ntpd 8
is restarted, it loads any new files and restarts the protocol.
Other dependent hosts will continue as usual until signatures are refreshed,
at which time the protocol is restarted.
.Ss Identity Schemes
As mentioned on the Autonomous Authentication page,
the default TC identity scheme is vulnerable to a middleman attack.
However, there are more secure identity schemes available,
including PC, IFF, GQ and MV described on the
.Qq Identification Schemes
page
(maybe available at
.Li http://www.eecis.udel.edu/%7emills/keygen.html ) .
These schemes are based on a TA, one or more trusted hosts
and some number of nontrusted hosts.
Trusted hosts prove identity using values provided by the TA,
while the remaining hosts prove identity using values provided
by a trusted host and certificate trails that end on that host.
The name of a trusted host is also the name of its sugroup
and also the subject and issuer name on its trusted certificate.
The TA is not necessarily a trusted host in this sense, but often is.
.Pp
In some schemes there are separate keys for servers and clients.
A server can also be a client of another server,
but a client can never be a server for another client.
In general, trusted hosts and nontrusted hosts that operate
as both server and client have parameter files that contain
both server and client keys.
Hosts that operate
only as clients have key files that contain only client keys.
.Pp
The PC scheme supports only one trusted host in the group.
On trusted host alice run
.Nm
.Fl P
.Fl p Ar password
to generate the host key file
.Pa ntpkey_RSAkey_ Ns Ar alice.filestamp
and trusted private certificate file
.Pa ntpkey_RSA-MD5_cert_ Ns Ar alice.filestamp .
Copy both files to all group hosts;
they replace the files which would be generated in other schemes.
On each host bob install a soft link from the generic name
.Pa ntpkey_host_ Ns Ar bob
to the host key file and soft link
.Pa ntpkey_cert_ Ns Ar bob
to the private certificate file.
Note the generic links are on bob, but point to files generated
by trusted host alice.
In this scheme it is not possible to refresh
either the keys or certificates without copying them
to all other hosts in the group.
.Pp
For the IFF scheme proceed as in the TC scheme to generate keys
and certificates for all group hosts, then for every trusted host in the group,
generate the IFF parameter file.
On trusted host alice run
.Nm
.Fl T
.Fl I
.Fl p Ar password
to produce her parameter file
.Pa ntpkey_IFFpar_ Ns Ar alice.filestamp ,
which includes both server and client keys.
Copy this file to all group hosts that operate as both servers
and clients and install a soft link from the generic
.Pa ntpkey_iff_ Ns Ar alice
to this file.
If there are no hosts restricted to operate only as clients,
there is nothing further to do.
As the IFF scheme is independent
of keys and certificates, these files can be refreshed as needed.
.Pp
If a rogue client has the parameter file, it could masquerade
as a legitimate server and present a middleman threat.
To eliminate this threat, the client keys can be extracted
from the parameter file and distributed to all restricted clients.
After generating the parameter file, on alice run
.Nm
.Fl e
and pipe the output to a file or mail program.
Copy or mail this file to all restricted clients.
On these clients install a soft link from the generic
.Pa ntpkey_iff_ Ns Ar alice
to this file.
To further protect the integrity of the keys,
each file can be encrypted with a secret password.
.Pp
For the GQ scheme proceed as in the TC scheme to generate keys
and certificates for all group hosts, then for every trusted host
in the group, generate the IFF parameter file.
On trusted host alice run
.Nm
.Fl T
.Fl G
.Fl p Ar password
to produce her parameter file
.Pa ntpkey_GQpar_ Ns Ar alice.filestamp ,
which includes both server and client keys.
Copy this file to all group hosts and install a soft link
from the generic
.Pa ntpkey_gq_ Ns Ar alice
to this file.
In addition, on each host bob install a soft link
from generic
.Pa ntpkey_gq_ Ns Ar bob
to this file.
As the GQ scheme updates the GQ parameters file and certificate
at the same time, keys and certificates can be regenerated as needed.
.Pp
For the MV scheme, proceed as in the TC scheme to generate keys
and certificates for all group hosts.
For illustration assume trish is the TA, alice one of several trusted hosts
and bob one of her clients.
On TA trish run
.Nm
.Fl V Ar n
.Fl p Ar password ,
where
.Ar n
is the number of revokable keys (typically 5) to produce
the parameter file
.Pa ntpkeys_MVpar_ Ns Ar trish.filestamp
and client key files
.Pa ntpkeys_MVkeyd_ Ns Ar trish.filestamp
where
.Ar d
is the key number (0 \&<
.Ar d
\&<
.Ar n ) .
Copy the parameter file to alice and install a soft link
from the generic
.Pa ntpkey_mv_ Ns Ar alice
to this file.
Copy one of the client key files to alice for later distribution
to her clients.
It doesn't matter which client key file goes to alice,
since they all work the same way.
Alice copies the client key file to all of her cliens.
On client bob install a soft link from generic
.Pa ntpkey_mvkey_ Ns Ar bob
to the client key file.
As the MV scheme is independent of keys and certificates,
these files can be refreshed as needed.
.Ss Command Line Options
.Bl -tag -width indent
.It Fl c Ar scheme
Select certificate message digest/signature encryption scheme.
The
.Ar scheme
can be one of the following:
. Cm RSA-MD2 , RSA-MD5 , RSA-SHA , RSA-SHA1 , RSA-MDC2 , RSA-RIPEMD160 , DSA-SHA ,
or
.Cm DSA-SHA1 .
Note that RSA schemes must be used with a RSA sign key and DSA
schemes must be used with a DSA sign key.
The default without this option is
.Cm RSA-MD5 .
.It Fl d
Enable debugging.
This option displays the cryptographic data produced in eye-friendly billboards.
.It Fl e
Write the IFF client keys to the standard output.
This is intended for automatic key distribution by mail.
.It Fl G
Generate parameters and keys for the GQ identification scheme,
obsoleting any that may exist.
.It Fl g
Generate keys for the GQ identification scheme
using the existing GQ parameters.
If the GQ parameters do not yet exist, create them first.
.It Fl H
Generate new host keys, obsoleting any that may exist.
.It Fl I
Generate parameters for the IFF identification scheme,
obsoleting any that may exist.
.It Fl i Ar name
Set the suject name to
.Ar name .
This is used as the subject field in certificates
and in the file name for host and sign keys.
.It Fl M
Generate MD5 keys, obsoleting any that may exist.
.It Fl P
Generate a private certificate.
By default, the program generates public certificates.
.It Fl p Ar password
Encrypt generated files containing private data with
.Ar password
and the DES-CBC algorithm.
.It Fl q
Set the password for reading files to password.
.It Fl S Oo Cm RSA | DSA Oc
Generate a new sign key of the designated type,
obsoleting any that may exist.
By default, the program uses the host key as the sign key.
.It Fl s Ar name
Set the issuer name to
.Ar name .
This is used for the issuer field in certificates
and in the file name for identity files.
.It Fl T
Generate a trusted certificate.
By default, the program generates a non-trusted certificate.
.It Fl V Ar nkeys
Generate parameters and keys for the Mu-Varadharajan (MV) identification scheme.
.El
.Ss Random Seed File
All cryptographically sound key generation schemes must have means
to randomize the entropy seed used to initialize
the internal pseudo-random number generator used
by the library routines.
The OpenSSL library uses a designated random seed file for this purpose.
The file must be available when starting the NTP daemon and
.Nm
program.
If a site supports OpenSSL or its companion OpenSSH,
it is very likely that means to do this are already available.
.Pp
It is important to understand that entropy must be evolved
for each generation, for otherwise the random number sequence
would be predictable.
Various means dependent on external events, such as keystroke intervals,
can be used to do this and some systems have built-in entropy sources.
Suitable means are described in the OpenSSL software documentation,
but are outside the scope of this page.
.Pp
The entropy seed used by the OpenSSL library is contained in a file,
usually called
.Cm .rnd ,
which must be available when starting the NTP daemon
or the
.Nm
program.
The NTP daemon will first look for the file
using the path specified by the
.Ic randfile
subcommand of the
.Ic crypto
configuration command.
If not specified in this way, or when starting the
.Nm
program,
the OpenSSL library will look for the file using the path specified
by the
.Ev RANDFILE
environment variable in the user home directory,
whether root or some other user.
If the
.Ev RANDFILE
environment variable is not present,
the library will look for the
.Cm .rnd
file in the user home directory.
If the file is not available or cannot be written,
the daemon exits with a message to the system log and the program
exits with a suitable error message.
.Ss Cryptographic Data Files
All other file formats begin with two lines.
The first contains the file name, including the generated host name
and filestamp.
The second contains the datestamp in conventional Unix date format.
Lines beginning with # are considered comments and ignored by the
.Nm
program and
.Xr ntpd 8
daemon.
Cryptographic values are encoded first using ASN.1 rules,
then encrypted if necessary, and finally written PEM-encoded
printable ASCII format preceded and followed by MIME content identifier lines.
.Pp
The format of the symmetric keys file is somewhat different
than the other files in the interest of backward compatibility.
Since DES-CBC is deprecated in NTPv4, the only key format of interest
is MD5 alphanumeric strings.
Following hte heard the keys are
entered one per line in the format
.D1 Ar keyno type key
where
.Ar keyno
is a positive integer in the range 1-65,535,
.Ar type
is the string MD5 defining the key format and
.Ar key
is the key itself,
which is a printable ASCII string 16 characters or less in length.
Each character is chosen from the 93 printable characters
in the range 0x21 through 0x7f excluding space and the
.Ql #
character.
.Pp
Note that the keys used by the
.Xr ntpq 8
and
.Xr ntpdc 8
programs
are checked against passwords requested by the programs
and entered by hand, so it is generally appropriate to specify these keys
in human readable ASCII format.
.Pp
The
.Nm
program generates a MD5 symmetric keys file
.Pa ntpkey_MD5key_ Ns Ar hostname.filestamp .
Since the file contains private shared keys,
it should be visible only to root and distributed by secure means
to other subnet hosts.
The NTP daemon loads the file
.Pa ntp.keys ,
so
.Nm
installs a soft link from this name to the generated file.
Subsequently, similar soft links must be installed by manual
or automated means on the other subnet hosts.
While this file is not used with the Autokey Version 2 protocol,
it is needed to authenticate some remote configuration commands
used by the
.Xr ntpq 8
and
.Xr ntpdc 8
utilities.
.Sh Bugs
It can take quite a while to generate some cryptographic values,
from one to several minutes with modern architectures
such as UltraSPARC and up to tens of minutes to an hour
with older architectures such as SPARC IPC.