MODULI(5) FILE FORMATS MODULI(5)
NAME
moduli - Diffie-Hellman moduli
DESCRIPTION
The /etc/ssh/moduli file contains prime numbers and genera-
tors 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 use-
ful. A second primalitytesting 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.
Last change: September 26 2012 1
MODULI(5) FILE FORMATS MODULI(5)
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 tri-
als 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 ran-
domly selects a modulus from Fa /etc/ssh/moduli that
best meets the size requirement.
SEE ALSO
ssh-keygen(1), sshd(8)
STANDARDS
W. Simpson, N. Provos and M. Friedl, Diffie-Hellman Group
Exchange for the Secure Shell (SSH) Transport Layer Proto-
col, RFC 4419, 2006.
Last change: September 26 2012 2
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