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

NAME

4     moduli — Diffie-Hellman moduli
5

DESCRIPTION

7     The /etc/gsissh/moduli file contains prime numbers and generators for use
8     by 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 -M generate, cal‐
12     culates numbers that are likely to be useful.  A second primality testing
13     pass, using ssh-keygen -M screen, provides a high degree of assurance
14     that the numbers are prime and are safe for use in Diffie-Hellman opera‐
15     tions by sshd(8).  This moduli format is used as the output from each
16     pass.
17
18     The file consists of newline-separated records, one per modulus, contain‐
19     ing seven space-separated fields.  These fields are as follows:
20
21           timestamp    The time that the modulus was last processed as YYYYM‐
22                        MDDHHMMSS.
23
24           type         Decimal number specifying the internal structure of
25                        the prime modulus.  Supported types are:
26
27                        0     Unknown, not tested.
28                        2     "Safe" prime; (p-1)/2 is also prime.
29                        4     Sophie Germain; 2p+1 is also prime.
30
31                        Moduli candidates initially produced by ssh-keygen(1)
32                        are Sophie Germain primes (type 4).  Further primality
33                        testing with ssh-keygen(1) produces safe prime moduli
34                        (type 2) that are ready for use in sshd(8).  Other
35                        types are not used by OpenSSH.
36
37           tests        Decimal number indicating the type of primality tests
38                        that the number has been subjected to represented as a
39                        bitmask of the following values:
40
41                        0x00  Not tested.
42                        0x01  Composite number – not prime.
43                        0x02  Sieve of Eratosthenes.
44                        0x04  Probabilistic Miller-Rabin primality tests.
45
46                        The ssh-keygen(1) moduli candidate generation uses the
47                        Sieve of Eratosthenes (flag 0x02).  Subsequent
48                        ssh-keygen(1) primality tests are Miller-Rabin tests
49                        (flag 0x04).
50
51           trials       Decimal number indicating the number of primality tri‐
52                        als that have been performed on the modulus.
53
54           size         Decimal number indicating the size of the prime in
55                        bits.
56
57           generator    The recommended generator for use with this modulus
58                        (hexadecimal).
59
60           modulus      The modulus itself in hexadecimal.
61
62     When performing Diffie-Hellman Group Exchange, sshd(8) first estimates
63     the size of the modulus required to produce enough Diffie-Hellman output
64     to sufficiently key the selected symmetric cipher.  sshd(8) then randomly
65     selects a modulus from /etc/gsissh/moduli that best meets the size re‐
66     quirement.
67

SEE ALSO

69     ssh-keygen(1), sshd(8)
70

STANDARDS

72     M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
73     the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
74
75BSD                             April 16, 2022                             BSD