MODULI(5)MODULI(5)NAMEmoduli - Diffie-Hellman moduliDESCRIPTION
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 candidategeneration pass, using ssh-keygen -G,
calculates numbers that are likely to be useful. A second primali‐
tytesting pass, using ssh-keygen -T, provides a high degree of assur‐
ance 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, con‐
taining 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 (hexadeci‐
mal).
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 Fa
/etc/ssh/moduli that best meets the size requirement.
SEE ALSOssh-keygen(1), sshd(8)STANDARDS
W. Simpson, N. Provos and M. Friedl, Diffie-Hellman Group Exchange for
the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, 2006.
September 26 2012 MODULI(5)
