MODULI(5)                  BSD File Formats Manual                 MODULI(5)

NAME         top

     moduli — Diffie-Hellman moduli

DESCRIPTION         top

     The /etc/moduli file contains prime numbers and generators for use by
     sshd(8) in the Diffie-Hellman Group Exchange key exchange method.

     New moduli may be generated with ssh-keygen(1) using a two-step
     process.  An initial candidate generation pass, using ssh-keygen -G,
     calculates numbers that are likely to be useful.  A second primality
     testing pass, using ssh-keygen -T, provides a high degree of assurance
     that the numbers are prime and are safe for use in Diffie-Hellman oper‐
     ations by sshd(8).  This moduli format is used as the output from each

     The file consists of newline-separated records, one per modulus, con‐
     taining seven space-separated fields.  These fields are as follows:

           timestamp    The time that the modulus was last processed as

           type         Decimal number specifying the internal structure of
                        the prime modulus.  Supported types are:

                        0     Unknown, not tested.
                        2     "Safe" prime; (p-1)/2 is also prime.
                        4     Sophie Germain; 2p+1 is also prime.

                        Moduli candidates initially produced by
                        ssh-keygen(1) are Sophie Germain primes (type 4).
                        Further primality testing with ssh-keygen(1) pro‐
                        duces safe prime moduli (type 2) that are ready for
                        use in sshd(8).  Other types are not used by

           tests        Decimal number indicating the type of primality
                        tests that the number has been subjected to repre‐
                        sented as a bitmask of the following values:

                        0x00  Not tested.
                        0x01  Composite number – not prime.
                        0x02  Sieve of Eratosthenes.
                        0x04  Probabilistic Miller-Rabin primality tests.

                        The ssh-keygen(1) moduli candidate generation uses
                        the Sieve of Eratosthenes (flag 0x02).  Subsequent
                        ssh-keygen(1) primality tests are Miller-Rabin tests
                        (flag 0x04).

           trials       Decimal number indicating the number of primality
                        trials that have been performed on the modulus.

           size         Decimal number indicating the size of the prime in

           generator    The recommended generator for use with this modulus

           modulus      The modulus itself in hexadecimal.

     When performing Diffie-Hellman Group Exchange, sshd(8) first estimates
     the size of the modulus required to produce enough Diffie-Hellman out‐
     put to sufficiently key the selected symmetric cipher.  sshd(8) then
     randomly selects a modulus from /etc/moduli that best meets the size

SEE ALSO         top

     ssh-keygen(1), sshd(8)

STANDARDS         top

     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
     the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006

COLOPHON         top

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BSD                          September 26, 2012                          BSD