1.\" $OpenBSD: moduli.5,v 1.12 2008/06/26 05:57:54 djm Exp $ 2.\" 3.\" Copyright (c) 2008 Damien Miller <djm@mindrot.org> 4.\" 5.\" Permission to use, copy, modify, and distribute this software for any 6.\" purpose with or without fee is hereby granted, provided that the above 7.\" copyright notice and this permission notice appear in all copies. 8.\" 9.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 10.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 11.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 12.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 13.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 14.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 15.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 16.Dd June 26, 2008 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 -G , 36calculates numbers that are likely to be useful. 37A second 38.Em primality testing 39pass, using 40.Ic ssh-keygen -T 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.Pp 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; (p+1)*2 is also prime. 66.El 67.Pp 68Moduli candidates initially produced by 69.Xr ssh-keygen 1 70are Sophie Germain primes (type 4). 71Futher 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 - not prime. 85.It 0x02 86Sieve of Eratosthenes 87.It 0x04 88Probabalistic 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 of primaility trials that have been performed 99on 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.Pp 117.Sh SEE ALSO 118.Xr ssh-keygen 1 , 119.Xr sshd 8 , 120.Rs 121.%R RFC 4419 122.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" 123.%D 2006 124.Re 125