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 de-
gree 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, con-
taining 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) pro-
duces 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 repre-
sented 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 out-
put to sufficiently key the selected symmetric 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)
Generated by dwww version 1.16 on Tue Dec 16 11:16:23 CET 2025.