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  candidategeneration  pass, using ssh-keygen -G,
       calculates numbers that are likely to be  useful.   A  second  primali-
       tytesting  pass,  using ssh-keygen -T, provides a high degree of assur-
       ance 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,  con-
       taining seven space-separated 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.  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) 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 performed on the modulus.

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

       generator
              The  recommended  generator for use with this modulus (hexadeci-
              mal).

       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
              cipher.   sshd(8)  then  randomly  selects  a  modulus  from  Fa
              /etc/ssh/moduli that best meets the size requirement.

SEE ALSO

       ssh-keygen(1), sshd(8)

STANDARDS

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

                              September 26 2012                      MODULI(5)