MODULI(5) BSD File Formats Manual MODULI(5)
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-Hellman operations by sshd(8). This moduli
format is used as the output from each pass.
consists of newline-separated records, one per modulus,
containing seven space-separated fields. These fields are as
The time that
the modulus was last processed as YYYYMMDDHHMMSS.
specifying the internal structure of the prime modulus.
Supported types are:
prime; (p-1)/2 is also prime.
2p+1 is also prime.
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
indicating the type of primality tests that the number has
been subjected to represented as a bitmask of the following
number – not prime.
Miller-Rabin primality tests.
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).
indicating the number of primality trials that have been
performed on the modulus.
indicating the size of the prime in bits.
generator for use with this modulus (hexadecimal).
itself in hexadecimal.
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/ssh/moduli that best meets the size
N. Provos , and
W. Simpson ,
Diffie-Hellman Group Exchange for the Secure Shell (SSH)
Transport Layer Protocol ,
RFC 4419 ,
September 26, 2012 BSD