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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 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 Hell-
     man 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 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; (p+1)*2 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 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 (hexadecimal).

           modulus      The modulus itself in hexadecimal.

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

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

     Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, 2006.

BSD                           September 19, 2017                           BSD