xref: /freebsd/crypto/openssh/moduli.5 (revision 031beb4e239bfce798af17f5fe8dba8bcaf13d99)
1.\"	$OpenBSD: moduli.5,v 1.19 2022/04/16 04:30:10 dtucker Exp $
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3.\" Copyright (c) 2008 Damien Miller <djm@mindrot.org>
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16.Dd $Mdocdate: April 16 2022 $
17.Dt MODULI 5
18.Os
19.Sh NAME
20.Nm moduli
21.Nd Diffie-Hellman moduli
22.Sh DESCRIPTION
23The
24.Pa /etc/moduli
25file contains prime numbers and generators for use by
26.Xr sshd 8
27in the Diffie-Hellman Group Exchange key exchange method.
28.Pp
29New moduli may be generated with
30.Xr ssh-keygen 1
31using a two-step process.
32An initial
33.Em candidate generation
34pass, using
35.Ic ssh-keygen -M generate ,
36calculates numbers that are likely to be useful.
37A second
38.Em primality testing
39pass, using
40.Ic ssh-keygen -M screen ,
41provides a high degree of assurance that the numbers are prime and are
42safe for use in Diffie-Hellman operations by
43.Xr sshd 8 .
44This
45.Nm
46format is used as the output from each pass.
47.Pp
48The file consists of newline-separated records, one per modulus,
49containing seven space-separated fields.
50These fields are as follows:
51.Bl -tag -width Description -offset indent
52.It timestamp
53The time that the modulus was last processed as YYYYMMDDHHMMSS.
54.It type
55Decimal number specifying the internal structure of the prime modulus.
56Supported types are:
57.Pp
58.Bl -tag -width 0x00 -compact
59.It 0
60Unknown, not tested.
61.It 2
62"Safe" prime; (p-1)/2 is also prime.
63.It 4
64Sophie Germain; 2p+1 is also prime.
65.El
66.Pp
67Moduli candidates initially produced by
68.Xr ssh-keygen 1
69are Sophie Germain primes (type 4).
70Further primality testing with
71.Xr ssh-keygen 1
72produces safe prime moduli (type 2) that are ready for use in
73.Xr sshd 8 .
74Other types are not used by OpenSSH.
75.It tests
76Decimal number indicating the type of primality tests that the number
77has been subjected to represented as a bitmask of the following values:
78.Pp
79.Bl -tag -width 0x00 -compact
80.It 0x00
81Not tested.
82.It 0x01
83Composite number \(en not prime.
84.It 0x02
85Sieve of Eratosthenes.
86.It 0x04
87Probabilistic Miller-Rabin primality tests.
88.El
89.Pp
90The
91.Xr ssh-keygen 1
92moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02).
93Subsequent
94.Xr ssh-keygen 1
95primality tests are Miller-Rabin tests (flag 0x04).
96.It trials
97Decimal number indicating the number of primality trials
98that have been performed on the modulus.
99.It size
100Decimal number indicating the size of the prime in bits.
101.It generator
102The recommended generator for use with this modulus (hexadecimal).
103.It modulus
104The modulus itself in hexadecimal.
105.El
106.Pp
107When performing Diffie-Hellman Group Exchange,
108.Xr sshd 8
109first estimates the size of the modulus required to produce enough
110Diffie-Hellman output to sufficiently key the selected symmetric cipher.
111.Xr sshd 8
112then randomly selects a modulus from
113.Fa /etc/moduli
114that best meets the size requirement.
115.Sh SEE ALSO
116.Xr ssh-keygen 1 ,
117.Xr sshd 8
118.Sh STANDARDS
119.Rs
120.%A M. Friedl
121.%A N. Provos
122.%A W. Simpson
123.%D March 2006
124.%R RFC 4419
125.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
126.Re
127