MODULI(5) BSD 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; 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 – 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**

ssh-keygen(1), sshd(8)

**STANDARDS**

M. Friedl

,

N. Provos , and

W. Simpson , *
Diffie-Hellman Group Exchange for the Secure Shell (SSH)
Transport Layer Protocol* ,

RFC 4419 ,

March 2006

2006 .

BSD September 26, 2012 BSD