gsimoduli (5) Linux Manual Page
moduli – Diffie-Hellman moduli
Description
The /etc/gsissh/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-keygen1 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 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-keygen1 are Sophie Germain primes (type 4). Further primality testing with ssh-keygen1 produces safe prime moduli (type 2) that are ready for use in sshd(8). Other types are not used by OpenSSH.
The ssh-keygen1 moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02). Subsequent ssh-keygen1 primality tests are Miller-Rabin tests (flag 0x04).
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/gsissh/moduli that best meets the size requirement.
See Also
ssh-keygen1, sshd(8)
Standards
- M. Friedl N. Provos W. Simpson March 2006 RFC 4419 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 2006