moduli(5) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | STANDARDS | COLOPHON

MODULI(5)                  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
       -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 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; 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 (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
       cipher.  sshd(8) then randomly selects a modulus from /etc/moduli
       that best meets the size requirement.

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

       This page is part of the openssh (Portable OpenSSH) project.
       Information about the project can be found at
       http://www.openssh.com/portable.html.  If you have a bug report
       for this manual page, see ⟨http://www.openssh.com/report.html⟩.
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GNU                          April 16, 2022                    MODULI(5)