Copyright (c) 2008 Damien Miller <djm (at] mindrot.org>
Permission to use, copy, modify, and distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.
THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.Dd $Mdocdate: October 14 2010 $ .Dt MODULI 5 .Os .Sh NAME .Nm moduli .Nd Diffie-Hellman moduli .Sh DESCRIPTION The
a /etc/moduli file contains prime numbers and generators for use by .Xr sshd 8 in the Diffie-Hellman Group Exchange key exchange method.
p New moduli may be generated with .Xr ssh-keygen 1 using a two-step process. An initial .Em candidate generation pass, using c ssh-keygen -G , calculates numbers that are likely to be useful. A second .Em primality testing pass, using c ssh-keygen -T , provides a high degree of assurance that the numbers are prime and are safe for use in Diffie-Hellman operations by .Xr sshd 8 . This .Nm format is used as the output from each pass.
p The file consists of newline-separated records, one per modulus, containing seven space-separated fields. These fields are as follows: l -tag -width Description -offset indent t timestamp The time that the modulus was last processed as YYYYMMDDHHMMSS. t type Decimal number specifying the internal structure of the prime modulus. Supported types are:
p l -tag -width 0x00 -compact t 0 Unknown, not tested. t 2 "Safe" prime; (p-1)/2 is also prime. t 4 Sophie Germain; (p+1)*2 is also prime. .El
p Moduli candidates initially produced by .Xr ssh-keygen 1 are Sophie Germain primes (type 4). Further primality testing with .Xr ssh-keygen 1 produces safe prime moduli (type 2) that are ready for use in .Xr sshd 8 . Other types are not used by OpenSSH. t tests Decimal number indicating the type of primality tests that the number has been subjected to represented as a bitmask of the following values:
p l -tag -width 0x00 -compact t 0x00 Not tested. t 0x01 Composite number \(en not prime. t 0x02 Sieve of Eratosthenes. t 0x04 Probabilistic Miller-Rabin primality tests. .El
p The .Xr ssh-keygen 1 moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02). Subsequent .Xr ssh-keygen 1 primality tests are Miller-Rabin tests (flag 0x04). t trials Decimal number indicating the number of primality trials that have been performed on the modulus. t size Decimal number indicating the size of the prime in bits. t generator The recommended generator for use with this modulus (hexadecimal). t modulus The modulus itself in hexadecimal. .El
p When performing Diffie-Hellman Group Exchange, .Xr sshd 8 first estimates the size of the modulus required to produce enough Diffie-Hellman output to sufficiently key the selected symmetric cipher. .Xr sshd 8 then randomly selects a modulus from .Fa /etc/moduli that best meets the size requirement. .Sh SEE ALSO .Xr ssh-keygen 1 , .Xr sshd 8 .Rs .%R RFC 4419 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" .%D 2006 .Re
