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 /usr/local/etc, then run the program.

To test and gain experience with Autokey concepts, log in as root and change to the keys directory, usually /usr/local/etc. When run for the first time, or if all files with names beginning with ntpkey* have been removed, use the ntp-keygen command without arguments to generate a default RSA host key and matching RSA-MD5 certificate file with expiration date one year hence, 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 without options, the program uses the existing keys and parameters and generates a new certificate file with new expiration date one year hence, and soft 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 AES128CMAC, MD2, MD5, MDC2, SHA, SHA1 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.

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.

Running the program as other than root and using the Unix su(1) 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.

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 ntpd(8) configuration file 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.

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.

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 filestamp file name extensions, the files for an NTP 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(8) follows it to the file name to extract the filestamp. If a link is not present, ntpd(8) 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 filestamp extension for all files generated at one time, so each generation is distinct and can be readily recognized in monitoring data.

Run the command on as many hosts as necessary. Designate one of them as the trusted host (TH) using ntp-keygen with the -T option and configure it to synchronize from reliable Internet servers. Then configure the other hosts to synchronize to the TH directly or indirectly. A certificate trail is created when Autokey asks the immediately ascendant host towards the TH to sign its certificate, which is then provided to the immediately descendant host on request. All group hosts should have acyclic certificate trails ending on the TH.

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. A different sign key can be assigned using the -S option and this can be either RSA or DSA type. By default, the signature message digest type is MD5, but any combination of sign key type and message digest type supported by the OpenSSL library can be specified using the -c option.

The rules say cryptographic media should be generated with proventic filestamps, which means the host should already be synchronized before this program is run. This of course creates a chicken-and-egg problem when the host is started for the first time. Accordingly, the host time should be set by some other means, such as eyeball-and-wristwatch, at least so that the certificate lifetime is within the current year. After that and when the host is synchronized to a proventic source, the certificate should be re-generated.

Additional information on trusted groups and identity schemes is on the “Autokey Public-Key Authentication” page.

File names begin with the prefix ntpkey_ and end with the suffix _hostname. filestamp, where hostname is the owner name, usually the string returned by the Unix hostname(1) command, 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.

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 section of 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 Automatic NTP Configuration Options section of ntp.conf(5).

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.

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 frequent 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, and soft link.

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, and soft links. However, if the host or sign key is changed, ntpd(8) should be restarted. When 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.

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 schemes described below. 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 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.

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_ RSA key_alice. filestamp and trusted private certificate file ntpkey_ RSA-MD5 _ cert_alice. filestamp, and soft links. 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, and recreating the soft links.

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.

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 email program. Copy or email 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 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.

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 does not 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 clients. 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¶

-b --imbits= modulus

Set the number of bits in the identity modulus for generating identity keys to modulus bits. The number of bits in the identity modulus defaults to 256, but can be set to values from 256 to 2048 (32 to 256 octets). Use the larger moduli with caution, as this can consume considerable computing resources and increases the size of authenticated packets.

-c --certificate= scheme

Select certificate signature encryption/message digest scheme. The scheme can be one of the following: RSA-MD2, RSA-MD5, RSA-MDC2, RSA-SHA, RSA-SHA1, RSA-RIPEMD160, DSA-SHA, or DSA-SHA1. Note that RSA schemes must be used with an RSA sign key and DSA schemes must be used with a DSA sign key. The default without this option is RSA-MD5. If compatibility with FIPS 140-2 is required, either the DSA-SHA or DSA-SHA1 scheme must be used.

-C --cipher= cipher

Select the OpenSSL cipher to encrypt the files containing private keys. The default without this option is three-key triple DES in CBC mode, des-ede3-cbc. The openssl -h command provided with OpenSSL displays available ciphers.

-d --debug-level

Increase debugging verbosity level. This option displays the cryptographic data produced in eye-friendly billboards.

-D --set-debug-level= level

Set the debugging verbosity to level. This option displays the cryptographic data produced in eye-friendly billboards.

-e --id-key

Write the IFF or GQ public parameters from the IFFkey or GQkey client keys file previously specified as unencrypted data to the standard output stream stdout. This is intended for automatic key distribution by email.

-G --gq-params

Generate a new encrypted GQ parameters and key file for the Guillou-Quisquater (GQ) identity scheme. This option is mutually exclusive with the -I and -V options.

-H --host-key

Generate a new encrypted RSA public/private host key file.

-I --iffkey

Generate a new encrypted IFF key file for the Schnorr (IFF) identity scheme. This option is mutually exclusive with the -G and Fl V options.

-i --ident= group

Set the optional Autokey group name to group. This is used in the identity scheme parameter file names of IFF, GQ, and MV client parameters files. In that role, the default is the host name if no group is provided. The group name, if specified using -i or -s following an ‘@’ character, is also used in certificate subject and issuer names in the form host @ group and should match the group specified via crypto ident or server ident in the ntpd configuration file.

-l --lifetime= days

Set the lifetime for certificate expiration to days. The default lifetime is one year (365 days).

-m --modulus= bits

Set the number of bits in the prime modulus for generating files to bits. The modulus defaults to 512, but can be set from 256 to 2048 (32 to 256 octets). Use the larger moduli with caution, as this can consume considerable computing resources and increases the size of authenticated packets.

-M --md5key

Generate a new symmetric keys file containing 10 MD5 keys, and if OpenSSL is available, 10 SHA keys. An MD5 key is a string of 20 random printable ASCII characters, while a SHA key is a string of 40 random hex digits. The file can be edited using a text editor to change the key type or key content. This option is mutually exclusive with all other options.

-p --password= passwd

Set the password for reading and writing encrypted files to passwd. These include the host, sign and identify key files. By default, the password is the string returned by the Unix hostname command.

-P --pvt-cert

Generate a new private certificate used by the PC identity scheme. By default, the program generates public certificates. Note: the PC identity scheme is not recommended for new installations.

-q --export-passwd= passwd

Set the password for writing encrypted IFF, GQ and MV identity files redirected to stdout to passwd. In effect, these files are decrypted with the -p password, then encrypted with the -q password. By default, the password is the string returned by the Unix hostname command.

-s --subject-key= file ... [host] [@ group]

Specify the Autokey host name, where host is the optional host name and group is the optional group name. The host name, and if provided, group name are used in host @ group form as certificate subject and issuer. Specifying -s -@ group is allowed, and results in leaving the host name unchanged, as with -i group. The group name, or if no group is provided, the host name are also used in the file names of IFF, GQ, and MV identity scheme client parameter files. If host is not specified, the default host name is the string returned by the Unix hostname command.

-S --sign-key= [RSA | DSA]

Generate a new encrypted public/private sign key file of the specified type. By default, the sign key is the host key and has the same type. If compatibility with FIPS 140-2 is required, the sign key type must be DSA.

-T --trusted-cert

Generate a trusted certificate. By default, the program generates a non-trusted certificate.

-V --mv-params nkeys

Generate nkeys encrypted server keys and parameters for the Mu-Varadharajan (MV) identity scheme. This option is mutually exclusive with the -I and -G options. Note: support for this option should be considered a work in progress.

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.

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.

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. Since both the ntp-keygen program and ntpd(8) daemon must run as root, the logical place to put this file is in /.rnd or /root/.rnd. 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 file formats begin with two nonencrypted lines. The first line contains the file name, including the generated host name and filestamp, in the format ntpkey_key _ name. filestamp, where key is the key or parameter type, name is the host or group name and filestamp is the filestamp (NTP seconds) when the file was created. By convention, key names in generated file names include both upper and lower case characters, while key names in generated link names include only lower case characters. The filestamp is not used in generated link names. The second line contains the datestamp in conventional Unix date format. Lines beginning with ‘#’ are considered comments and ignored by the ntp-keygen program and ntpd(8) daemon.

The remainder of the file contains cryptographic data, encoded first using ASN.1 rules, then encrypted if necessary, and finally written in PEM-encoded printable ASCII text, preceded and followed by MIME content identifier lines.

The format of the symmetric keys file, ordinarily named ntp.keys, is somewhat different than the other files in the interest of backward compatibility. Ordinarily, the file is generated by this program, but it can be constructed and edited using an ordinary text editor.

# ntpkey_MD5key_bk.ntp.org.3595864945
# Thu Dec 12 19:22:25 2013
1  MD5 L";Nw<`.I<f4U0)247"i  # MD5 key
2  MD5 &>l0%XXK9O'51VwV<xq~  # MD5 key
3  MD5 lb4zLW~d^!K:]RsD'qb6  # MD5 key
4  MD5 Yue:tL[+vR)M`n~bY,'?  # MD5 key
5  MD5 B;fx'Kgr/&4ZTbL6=RxA  # MD5 key
6  MD5 4eYwa`o}3i@@V@..R9!l  # MD5 key
7  MD5 `A.([h+;wTQ|xfi%Sn_!  # MD5 key
8  MD5 45:V,r4]l6y^JH6"Sh?F  # MD5 key
9  MD5 3-5vcn*6l29DS?Xdsg)*  # MD5 key
10 MD5 2late4Me              # MD5 key
11 SHA1 a27872d3030a9025b8446c751b4551a7629af65c  # SHA1 key
12 SHA1 21bc3b4865dbb9e920902abdccb3e04ff97a5e74  # SHA1 key
13 SHA1 2b7736fe24fef5ba85ae11594132ab5d6f6daba9  # SHA1 key
14 SHA  a5332809c8878dd3a5b918819108a111509aeceb  # SHA  key
15 MD2  2fe16c88c760ff2f16d4267e36c1aa6c926e6964  # MD2  key
16 MD4  b2691811dc19cfc0e2f9bcacd74213f29812183d  # MD4  key
17 MD5  e4d6735b8bdad58ec5ffcb087300a17f7fef1f7c  # MD5  key
18 MDC2 a8d5e2315c025bf3a79174c87fbd10477de2eabc  # MDC2 key
19 RIPEMD160 77ca332cafb30e3cafb174dcd5b80ded7ba9b3d2  # RIPEMD160 key
20 AES128CMAC f92ff73eee86c1e7dc638d6489a04e4e555af878  # AES128CMAC key

Figure 1 shows a typical symmetric keys file used by the reference implementation. Following the header the keys are entered one per line in the format

Note that the keys used by the ntpq(1) and ntpdc(1) 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 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(1) and ntpdc(1) utilities.