man > MODULI(5)

MODULI(5)                                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 -M generate, calculates numbers that are likely
       to be useful.  A second primality testing pass, using ssh-keygen -M screen, provides  a  high
       degree  of assurance that the numbers are prime and are safe for use in Diffie-Hellman opera‐
       tions 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.
                          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  Eratos‐
                          thenes  (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 (hexadecimal).

             modulus      The modulus itself in hexadecimal.

       When  performing Diffie-Hellman Group Exchange, sshd(8) first estimates the size of the modu‐
       lus required to produce enough Diffie-Hellman output to sufficiently key the selected symmet‐
       ric 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)

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

Debian                                     April 16, 2022                                  MODULI(5)