moduli(5)

1MODULI(5)                   BSD File Formats Manual                  MODULI(5)
2

NAME

4     moduli — Diffie Hellman moduli
5

DESCRIPTION

7     The /etc/ssh/moduli file contains prime numbers and generators for use by
8     sshd(8) in the Diffie-Hellman Group Exchange key exchange method.
9
10     New moduli may be generated with ssh-keygen(1) using a two-step process.
11     An initial candidate generation pass, using ssh-keygen -G, calculates
12     numbers that are likely to be useful.  A second primality testing pass,
13     using ssh-keygen -T provides a high degree of assurance that the numbers
14     are prime and are safe for use in Diffie Hellman operations by sshd(8).
15     This moduli format is used as the output from each pass.
16
17     The file consists of newline-separated records, one per modulus, contain‐
18     ing seven space separated fields.  These fields are as follows:
19
20           timestamp    The time that the modulus was last processed as YYYYM‐
21                        MDDHHMMSS.
22
23           type         Decimal number specifying the internal structure of
24                        the prime modulus.  Supported types are:
25
26                        0     Unknown, not tested
27                        2     "Safe" prime; (p-1)/2 is also prime.
28                        4     Sophie Germain; (p+1)*2 is also prime.
29
30                        Moduli candidates initially produced by ssh-keygen(1)
31                        are Sophie Germain primes (type 4).  Further primality
32                        testing with ssh-keygen(1) produces safe prime moduli
33                        (type 2) that are ready for use in sshd(8).  Other
34                        types are not used by OpenSSH.
35
36           tests        Decimal number indicating the type of primality tests
37                        that the number has been subjected to represented as a
38                        bitmask of the following values:
39
40                        0x00  Not tested
41                        0x01  Composite number - not prime.
42                        0x02  Sieve of Eratosthenes
43                        0x04  Probabilistic Miller-Rabin primality tests.
44
45                        The ssh-keygen(1) moduli candidate generation uses the
46                        Sieve of Eratosthenes (flag 0x02).  Subsequent
47                        ssh-keygen(1) primality tests are Miller-Rabin tests
48                        (flag 0x04).
49
50           trials       Decimal number indicating of primality trials that
51                        have been performed on the modulus.
52
53           size         Decimal number indicating the size of the prime in
54                        bits.
55
56           generator    The recommended generator for use with this modulus
57                        (hexadecimal).
58
59           modulus      The modulus itself in hexadecimal.
60
61     When performing Diffie Hellman Group Exchange, sshd(8) first estimates
62     the size of the modulus required to produce enough Diffie Hellman output
63     to sufficiently key the selected symmetric cipher.  sshd(8) then randomly
64     selects a modulus from /etc/ssh/moduli that best meets the size require‐
65     ment.
66

NAME

DESCRIPTION

SEE ALSO