MODULI
Section: File Formats (5)
Page Index
BSD mandoc
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-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.
- 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-keygen1
moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02).
Subsequent
ssh-keygen1
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
Fa /etc/ssh/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