1.\" $OpenBSD: moduli.5,v 1.17 2012/09/26 17:34:38 jmc Exp $ 2.\" $FreeBSD$ 3.\" 4.\" Copyright (c) 2008 Damien Miller <djm@mindrot.org> 5.\" 6.\" Permission to use, copy, modify, and distribute this software for any 7.\" purpose with or without fee is hereby granted, provided that the above 8.\" copyright notice and this permission notice appear in all copies. 9.\" 10.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 11.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 12.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 13.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 14.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 15.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 16.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 17.Dd September 26, 2012 18.Dt MODULI 5 19.Os 20.Sh NAME 21.Nm moduli 22.Nd Diffie-Hellman moduli 23.Sh DESCRIPTION 24The 25.Pa /etc/moduli 26file contains prime numbers and generators for use by 27.Xr sshd 8 28in the Diffie-Hellman Group Exchange key exchange method. 29.Pp 30New moduli may be generated with 31.Xr ssh-keygen 1 32using a two-step process. 33An initial 34.Em candidate generation 35pass, using 36.Ic ssh-keygen -G , 37calculates numbers that are likely to be useful. 38A second 39.Em primality testing 40pass, using 41.Ic ssh-keygen -T , 42provides a high degree of assurance that the numbers are prime and are 43safe for use in Diffie-Hellman operations by 44.Xr sshd 8 . 45This 46.Nm 47format is used as the output from each pass. 48.Pp 49The file consists of newline-separated records, one per modulus, 50containing seven space-separated fields. 51These fields are as follows: 52.Bl -tag -width Description -offset indent 53.It timestamp 54The time that the modulus was last processed as YYYYMMDDHHMMSS. 55.It type 56Decimal number specifying the internal structure of the prime modulus. 57Supported types are: 58.Pp 59.Bl -tag -width 0x00 -compact 60.It 0 61Unknown, not tested. 62.It 2 63"Safe" prime; (p-1)/2 is also prime. 64.It 4 65Sophie Germain; 2p+1 is also prime. 66.El 67.Pp 68Moduli candidates initially produced by 69.Xr ssh-keygen 1 70are Sophie Germain primes (type 4). 71Further primality testing with 72.Xr ssh-keygen 1 73produces safe prime moduli (type 2) that are ready for use in 74.Xr sshd 8 . 75Other types are not used by OpenSSH. 76.It tests 77Decimal number indicating the type of primality tests that the number 78has been subjected to represented as a bitmask of the following values: 79.Pp 80.Bl -tag -width 0x00 -compact 81.It 0x00 82Not tested. 83.It 0x01 84Composite number \(en not prime. 85.It 0x02 86Sieve of Eratosthenes. 87.It 0x04 88Probabilistic Miller-Rabin primality tests. 89.El 90.Pp 91The 92.Xr ssh-keygen 1 93moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02). 94Subsequent 95.Xr ssh-keygen 1 96primality tests are Miller-Rabin tests (flag 0x04). 97.It trials 98Decimal number indicating the number of primality trials 99that have been performed on the modulus. 100.It size 101Decimal number indicating the size of the prime in bits. 102.It generator 103The recommended generator for use with this modulus (hexadecimal). 104.It modulus 105The modulus itself in hexadecimal. 106.El 107.Pp 108When performing Diffie-Hellman Group Exchange, 109.Xr sshd 8 110first estimates the size of the modulus required to produce enough 111Diffie-Hellman output to sufficiently key the selected symmetric cipher. 112.Xr sshd 8 113then randomly selects a modulus from 114.Fa /etc/moduli 115that best meets the size requirement. 116.Sh SEE ALSO 117.Xr ssh-keygen 1 , 118.Xr sshd 8 119.Sh STANDARDS 120.Rs 121.%A M. Friedl 122.%A N. Provos 123.%A W. Simpson 124.%D March 2006 125.%R RFC 4419 126.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol 127.%D 2006 128.Re 129