# moduli - Man Page

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 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; 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) 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 ā 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.

## See Also

## Standards

M. Friedl, N. Provos, and W. Simpson, *Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol*, RFC 4419, March 2006.

## Referenced By

gsisshd(8), gsisshd_config(5), gsissh-keygen(1), sshd(8), sshd_config(5), ssh-keygen(1).