Section: File Formats (5)
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:
- The time that the modulus was last processed as YYYYMMDDHHMMSS.:
- Decimal number specifying the internal structure of the prime modulus. Supported types are:
- Unknown, not tested.:
- "Safe" prime; (p-1)/2 is also prime.:
- 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.
- Decimal number indicating the type of primality tests that the number has been subjected to represented as a bitmask of the following values:
- Not tested.:
- Composite number - not prime.:
- Sieve of Eratosthenes.:
- Probabilistic Miller-Rabin primality tests.:
The ssh-keygen1 moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02). Subsequent ssh-keygen1 primality tests are Miller-Rabin tests (flag 0x04).
- Decimal number indicating the number of primality trials that have been performed on the modulus.:
- Decimal number indicating the size of the prime in bits.:
- The recommended generator for use with this modulus (hexadecimal).:
- 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.
- M. Friedl N. Provos W. Simpson March 2006 RFC 4419 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 2006