ntp-92/man/ntp-keygen.8

.Dd October 13, 2003
.Dt NTP_KEYGEN 8
.Os
.Sh NAME
.Nm ntp-keygen
.Nd generate public and private keys
.Sh SYNOPSIS
.Nm
.Op Fl deGgHIMnPT
.Op Fl c Ar RSA-MD2 | RSA-MD5 | RSA-SHA | RSA-SHA1 | RSA-MDC2 | RSA-RIPEMD160 | DSA-SHA | DSA-SHA1
.Op Fl i Ar name
.Op Fl p Ar password
.Op Fl S {RSA | DSA}
.Op Fl s Ar name
.Op Fl v Ar keys
.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 -p password option
specifies the write password and -q password option the read password for
previously encrypted files. The ntp-keygen 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 ntpd configuration command crypto pw 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 ntpkey_ and end with the postfix
_hostname.filestamp, where hostname is the owner name, usually the string
returned by the Unix gethostname() routine, and 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 rm ntpkey* command or all files
generated at a specific time can be removed by a rm *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 /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, ntpd extracts the filestamp from the file itself.
This allows clients to verify that the file and generation times are
always current. The ntp-keygen 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 ntp-keygen program is logged in directly as
root. The recommended procedure is change to the keys directory, usually
/ust/local/etc, then run the program. When run for the first time, or if
all 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 su command to
assume root may not work properly, since by default the OpenSSL library
looks for the random seed file .rnd in the user home directory. However,
there should be only one .rnd, most conveniently in the root directory, so
it is convenient to define the $RANDFILE environment variable used by the
OpenSSL library as the path to /.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 /etc using the 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.
.Ss "Trusted Hosts and Groups"
Each cryptographic configuration involves selection of a signature scheme
and identification scheme, called a cryptotype, as explained in the
Authentication Options page. 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 Automatic NTP
Configuration Options page.
.Pp
On each trusted host as root, change to the keys directory. To insure a
fresh fileset, remove all ntpkey files. Then run ntp-keygen -T to generate
keys and a trusted certificate. On all other hosts do the same, but leave
off the -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 ntp-keygen with the -S type
option, where type is either RSA or 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 ntp-keygen with the -c
scheme option and selected scheme as needed. If ntp-keygen 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
ntp-keygen with the same flags as before to generate new certificates
using existing keys. However, if the host or sign key is changed, ntpd
should be restarted. When ntpd 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 Identification Schemes page. 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 ntp-keygen -P -p password to generate the host key file
ntpkey_RSAkey_alice.filestamp and trusted private certificate file
ntpkey_RSA-MD5_cert_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 ntpkey_host_bob to the
host key file and soft link ntpkey_cert_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
ntp-keygen -T -I -p password to produce her parameter file
ntpkey_IFFpar_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 ntpkey_iff_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 ntp-keygen -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 ntpkey_iff_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
ntp-keygen -T -G -p password to produce her parameter file
ntpkey_GQpar_alice.filestamp, which includes both server and client keys.
Copy this file to all group hosts and install a soft link from the generic
ntpkey_gq_alice to this file. In addition, on each host bob install a soft
link from generic ntpkey_gq_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 ntp-keygen -V n -p password, where n is the number of revokable keys
(typically 5) to produce the parameter file ntpkeys_MVpar_trish.filestamp
and client key files ntpkeys_MVkeyd_trish.filestamp where d is the key
number (0 < d < n). Copy the parameter file to alice and install a soft
link from the generic ntpkey_mv_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 ntpkey_mvkey_bob to the client key file.
As the MV scheme is independent of keys and certificates, these files can
be refreshed as needed.
.Sh OPTIONS
.Bl -tag -width Ds
.It Fl c Ar RSA-MD2 | RSA-MD5 | RSA-SHA | RSA-SHA1 | RSA-MDC2 | RSA-RIPEMD160 | DSA-SHA | DSA-SHA1
Select certificate message digest/signature encryption scheme.
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 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 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 password and
the DES-CBC algorithm.
.It Fl q
Set the password for reading files to password.
.It Fl S Ar RSA | DSA
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 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 ntp-keygen 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 .rnd, which must be available when starting the NTP daemon
or the ntp-keygen program. The NTP daemon will first look for the file
using the path specified by the randfile subcommand of the crypto
configuration command. If not specified in this way, or when starting the
ntp-keygen program, the OpenSSL library will look for the file using the
path specified by the RANDFILE environment variable in the user home
directory, whether root or some other user. If the RANDFILE environment
variable is not present, the library will look for the .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 ntp-keygen program and ntpd 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 the herd the keys are entered one per line in the
format
.Dl keyno type key
where keyno is a positive integer in the range 1-65,535, type is the
string MD5 defining the key format and 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 '#' character.
.Pp
Note that the keys used by the ntpq and ntpdc 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 ntp-keygen program generates a MD5 symmetric keys file
ntpkey_MD5key_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 ntp.keys, so ntp-keygen
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 ntpq and ntpdc utilities.
.Sh SEE ALSO
.Xr ntpdc 8 ,
.Xr ntpq 8
.Sh BUGS
It can take quite a while to generate the RSA public/private key
pair and Diffie-Hellman parameters, from a few seconds on a modern
workstation to several minutes on older machines.