# 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 -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 YYYYM
MDDHHMMSS.
- 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). Futher 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 Probabalistic 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 of primaility 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.

## SEE ALSO

- Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
Protocol, RFC 4419, 2006.