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.

The file consists of newline-separated records, one per modulus,
containing 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; (p+1)*2 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 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 randomly selects a modulus from
/etc/ssh/moduli
that best meets the size requirement.