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.
