---
name: "moduli"
section: "5"
platform: "linux"
package: "openssh"
version: "10.6p1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/moduli"
---

# moduli(5)

Diffie-Hellman moduli

## NAME

**moduli** – Diffie-Hellman moduli

## DESCRIPTION

The */etc/ssh/moduli* file contains prime numbers and generators for use by [sshd(8)](/sshd/8.md) in the Diffie-Hellman Group Exchange key exchange method.

New moduli may be generated with [ssh-keygen(1)](/ssh-keygen/1.md) 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)](/sshd/8.md). 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)](/ssh-keygen/1.md) are Sophie Germain primes (type 4). Further primality testing with [ssh-keygen(1)](/ssh-keygen/1.md) produces safe prime moduli (type 2) that are ready for use in [sshd(8)](/sshd/8.md). 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)](/ssh-keygen/1.md) moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02). Subsequent [ssh-keygen(1)](/ssh-keygen/1.md) 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)](/sshd/8.md) first estimates the size of the modulus required to produce enough Diffie-Hellman output to sufficiently key the selected symmetric cipher. [sshd(8)](/sshd/8.md) then randomly selects a modulus from */etc/ssh/moduli* that best meets the size requirement.

## SEE ALSO

[ssh-keygen(1)](/ssh-keygen/1.md), [sshd(8)](/sshd/8.md)

## STANDARDS

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