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
generator The recommended generator for use with this
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.
This page is part of the openssh (Portable OpenSSH) project.
Information about the project can be found at
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BSD September 26, 2012 BSD