moduli(5) - man - phpMan

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

NAME
     moduli — Diffie-Hellman moduli

DESCRIPTION
     The /etc/ssh/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 sec‐
     ond 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 operations by sshd(8).  This moduli
     format is used as the output from each pass.

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

           timestamp    The time that the modulus was last processed as YYYYMMDDHHMMSS.

           type         Decimal number specifying the internal structure of the prime modulus.  Sup‐
                        ported 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) produces safe
                        prime moduli (type 2) that are ready for use in sshd(8).  Other types are
                        not used by OpenSSH.

           tests        Decimal number indicating the type of primality tests that the number has
                        been subjected to represented 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 per‐
                        formed on the modulus.

           size         Decimal number indicating the size of the prime in bits.

           generator    The recommended generator for use with this modulus (hexadecimal).

           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 output to sufficiently key the selected symmetric ci‐
     pher.  sshd(8) then randomly selects a modulus from /etc/ssh/moduli that best meets the size
     requirement.

SEE ALSO
     ssh-keygen(1), sshd(8)

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

BSD                           September 26, 2012                           BSD