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, cal-
culates numbers that are likely to be useful. A second primality
testing pass, using ssh-keygen -M screen, provides a high degree of as-
surance that the numbers are prime and are safe for use in Diffie-Hell-
man 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 primal-
ity 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 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 output
to sufficiently key the selected symmetric cipher. sshd(8) then ran-
domly 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)
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