moduli(5) — Linux manual page


MODULI(5)                BSD 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

     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-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

           timestamp    The time that the modulus was last processed as

           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

                        0x00  Not tested.
                        0x01  Composite number – not prime.
                        0x02  Sieve of Eratosthenes.
                        0x04  Probabilistic Miller-Rabin primality

                        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

           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 2006.

COLOPHON         top

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BSD                        September 26, 2012                        BSD