Authentication Options

Authentication Support

The NTP standard specifies an extension which provides cryptographic authentication of received NTP packets. This is implemented in xntpd using the DES or MD5 algorithms to compute a digital signature, or message digest. The specification allows any one of possibly 4 billion keys, numbered with 32-bit key identifiers, to be used to authenticate an association. The servers involved in an association must agree on the key and key identifier used to authenticate their messages.

Keys and related information are specified in a key file, which should be exchanged and stored using secure procedures beyond the scope of the protocol. There are three classes of keys involved in the current implementation. One class is used for ordinary NTP associations, another for the ntpq utility program and the third for the xntpdc utility program.

Authentication Commands

keys keyfile

Specifies the file name containing the encryption keys and key identifiers used by xntpd, ntpq and xntpdc when operating in authenticated mode. The format of this file is described later in this document.

trustedkey key [ ... ]

Specifies the encryption key identifiers which are trusted for the purposes of authenticating peers suitable for synchronization. The authentication procedures require that both the local and remote servers share the same key and key identifier for this purpose, although different keys can be used with different servers. The key arguments are 32-bit unsigned integers. Note that NTP key 0 is fixed and globally known. If meaningful authentication is to be performed the 0 key should not be trusted.

requestkey key

Specifies the key identifier to use with the xntpdc program, which uses a proprietary protocol specific to this implementation of xntpd. This program is useful to diagnose and repair problems that affect xntpd operation. The key argument to this command is a 32-bit unsigned integer. If no requestkey command is included in the configuration file, or if the keys don't match, such requests will be ignored.

controlkey key

Specifies the key identifier to use with the ntpq program, which uses the standard protocol defined in RFC-1305. This program is useful to diagnose and repair problems that affect xntpd operation. The key argument to this command is a 32-bit unsigned integer. If no requestkey command is included in the configuration file, or if the keys don't match, such requests will be ignored.

authdelay delay

Specifies the amount of time it takes to encrypt an NTP authentication field on the local computer. This value is used to correct transmit timestamps when the authentication is used on outgoing packets. The value usually lies somewhere in the range 0.0001 seconds to 0.003 seconds, though it is very dependent on the CPU speed of the host computer. The exact value can be computed using the authspeed program included with the distribution.

Authentication Key File Format

In the case of DES, the keys are 56 bits long with, depending on type, a parity check on each byte. In the case of MD5, the keys are 64 bits (8 bytes). xntpd reads its keys from a file specified using the -k command line option or the keys statement in the configuration file. While key number 0 is fixed by the NTP standard (as 56 zero bits) and may not be changed, one or more of the keys numbered 1 through 15 may be arbitrarily set in the keys file.

The key file uses the same comment conventions as the configuration file. Key entries use a fixed format of the form

keyno type key

where keyno is a positive integer, type is a single character which defines the key format, and key is the key itself.

The key may be given in one of three different formats, controlled by the type character. The three key types, and corresponding formats, are listed following.

S

The key is a 64-bit hexadecimal number in the format specified in the DES specification; that is, the high order seven bits of each octet are used to form the 56-bit key while the low order bit of each octet is given a value such that odd parity is maintained for the octet. Leading zeroes must be specified (i.e., the key must be exactly 16 hex digits long) and odd parity must be maintained. Hence a zero key, in standard format, would be given as 0101010101010101.

N

The key is a 64-bit hexadecimal number in the format specified in the NTP standard. This is the same as the DES format, except the bits in each octet have been rotated one bit right so that the parity bit is now the high order bit of the octet. Leading zeroes must be specified and odd parity must be maintained. A zero key in NTP format would be specified as 8080808080808080.

A

The key is a 1-to-8 character ASCII string. A key is formed from this by using the low order 7 bits of each ASCII character in the string, with zeroes added on the right when necessary to form a full width 56-bit key, in the same way that encryption keys are formed from Unix passwords.

M

The key is a 1-to-8 character ASCII string, using the MD5 authentication scheme. Note that both the keys and the authentication schemes (DES or MD5) must be identical between a set of peers sharing the same key number.

Note that the keys used by the ntpq and xntpdc programs are checked against passwords requested by the programs and entered by hand, so it is generally appropriate to specify these keys in ASCII format.