ntp-keygen(8) ntp-keygen(8)
NAME
ntp-keygen - generate public and private keys
SYNOPSIS
ntp-keygen [ -deGgHIMPT ] [ -c [RSA-MD2 | RSA-MD5 | RSA-SHA | RSA-SHA1
| RSA-MDC2 | RSA-RIPEMD160 | DSA-SHA | DSA-SHA1 ] ] [ -i name ] [ -m
modulus ] [ -p password ] [ -q password ] [ -S [ RSA | DSA ] ] [ -s
name ] [ -v nkeys ] [ -V nkeys ]
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 soft-
ware 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 identi-
fication algorithms compatible with the Internet standard security
infrastructure.
By default, files are not encrypted by ntp-keygen. 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.
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.
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. File names begin with the prefix ntpkey_ and end with the
postfix _hostname.filestamp, where hostname is 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 guar-
antees uniqueness and simplifies maintenance procedures, since all
files can be quickly removed by a rm ntpkey* command or all files gen-
erated 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.
All files are installed by default in the keys directory /etc/ntp. 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.
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.
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 gener-
ation times are always current. The ntp-keygen program uses the same
extension for all files generated at one time, so each generation is
distinct and can be readily recognized in monitoring data.
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, usu-
ally /etc/ntp, 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.
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 algo-
rithms. However, the scheme specified in the certificate must be com-
patible 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.
Private/public key files and certificates are compatible with other
OpenSSL applications and very likely other libraries as well. Certifi-
cates 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.
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. How-
ever, 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.
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.
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.
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 cer-
tificates. These hosts will automatically and dynamically build author-
itative certificate trails to one or more trusted hosts. A trusted
group is the set of all hosts that have, directly or indirectly, a cer-
tificate 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.
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 gen-
erate 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 instanti-
ate the certificate trails throughout the subnet, but setting up the
environment is completely automatic.
If it is necessary to use a different sign key or different digest/sig-
nature 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 with-
out these options, it generates a new certificate using the same scheme
and sign key.
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.
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.
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 non-
trusted 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.
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 ntp-
key_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 cer-
tificate 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.
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_IFF-
par_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.
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.
For the GQ scheme proceed as in the TC scheme to generate keys and cer-
tificates 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 ntp-
key_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.
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 ntp-
keys_MVpar_trish.filestamp and client key files ntp-
keys_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.
COMMAND LINE OPTIONS
-c [ 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.
-d Enable debugging. This option displays the cryptographic data
produced in eye-friendly billboards.
-e Write the IFF client keys to the standard output. This is
intended for automatic key distribution by mail.
-G Generate parameters and keys for the GQ identification scheme,
obsoleting any that may exist.
-g Generate keys for the GQ identification scheme using the exist-
ing GQ parameters. If the GQ parameters do not yet exist, cre-
ate them first.
-H Generate new host keys, obsoleting any that may exist.
-I Generate parameters for the IFF identification scheme, obsolet-
ing any that may exist.
-i 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.
-M Generate MD5 keys, obsoleting any that may exist.
-m modulus
Set prime modulus size in bits (256 - 2048). Default size is
512.
-P Generate a private certificate. By default, the program gener-
ates public certificates.
-p password
Encrypt generated files containing private data with password
and the DES-CBC algorithm.
-q password
Set the password for reading files to password.
-S [ 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.
-s name Set the issuer name to name. This is used for the issuer field
in certificates and in the file name for identity files.
-T Generate a trusted certificate. By default, the program gener-
ates a non-trusted certificate.
-V nkeys
Generate parameters and keys for the Mu-Varadharajan (MV) iden-
tification scheme.
RANDOM SEED FILE
All cryptographically sound key generation schemes must have means to
randomize the entropy seed used to initialize the internal pseudo-ran-
dom 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.
It is important to understand that entropy must be evolved for each
generation, for otherwise the random number sequence would be pre-
dictable. 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.
The entropy seed used by the OpenSSL library is contained in a file,
usually called .rnd, which must be available when starting the NTP dae-
mon 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.
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 begin-
ning with # are considered comments and ignored by the ntp-keygen pro-
gram 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.
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 alphanu-
meric strings. Following hte heard the keys are entered one per line in
the format
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 charac-
ter is chosen from the 93 printable characters in the range 0x21
through 0x7f excluding space and the ’#’ character.
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.
The ntp-keygen program generates a MD5 symmetric keys file ntp-
key_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. Sub-
sequently, 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.
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.
SEE ALSO
ntpd(8), ntp_auth(5)
Primary source of documentation: /usr/share/doc/ntp-*
This file was automatically generated from HTML source.
ntp-keygen(8)
