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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 -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 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¶

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

June 26, 2008 | Linux 5.14.0-427.18.1.el9_4.x86_64 |