MODULI
Section: File Formats (5)
Page Index
BSD mandoc
NAME
moduli
 DiffieHellman moduli
DESCRIPTION
The
/etc/ssh/moduli
file contains prime numbers and generators for use by
sshd(8)
in the DiffieHellman Group Exchange key exchange method.
New moduli may be generated with
sshkeygen1
using a twostep process.
An initial
candidate generation
pass, using
sshkeygen G
calculates numbers that are likely to be useful.
A second
primality testing
pass, using
sshkeygen T
provides a high degree of assurance that the numbers are prime and are
safe for use in DiffieHellman operations by
sshd(8).
This
format is used as the output from each pass.
The file consists of newlineseparated records, one per modulus,
containing seven spaceseparated 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; (p1)/2 is also prime.
 4

Sophie Germain; 2p+1 is also prime.
Moduli candidates initially produced by
sshkeygen1
are Sophie Germain primes (type 4).
Further primality testing with
sshkeygen1
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 MillerRabin primality tests.
The
sshkeygen1
moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02).
Subsequent
sshkeygen1
primality tests are MillerRabin 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 DiffieHellman Group Exchange,
sshd(8)
first estimates the size of the modulus required to produce enough
DiffieHellman output to sufficiently key the selected symmetric cipher.
sshd(8)
then randomly selects a modulus from
Fa /etc/ssh/moduli
that best meets the size requirement.
SEE ALSO
sshkeygen1,
sshd(8)
STANDARDS

M. Friedl
N. Provos
W. Simpson
March 2006
RFC 4419
DiffieHellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
2006