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 -G,
calculates numbers that are likely to be useful. A second primalitytesting pass, using ssh-keygen -T, provides a high degree of assurance
that the numbers are prime and are safe for use in Diffie-Hellman oper‐
ations by sshd(8). This moduli format is used as the output from each
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
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
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
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
generator The recommended generator for use with this modulus
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/moduli that best meets the size
This page is part of the openssh (Portable OpenSSH) project. Informa‐
tion 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. This page
was obtained from the tarball openssh-7.4p1.tar.gz fetched from
http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/ on 2017-03-13.
If you discover any rendering problems in this HTML version of the
page, or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the information in
this COLOPHON (which is not part of the original manual page), send a
mail to firstname.lastname@example.org
BSD September 26, 2012 BSD