2008-07-23 09:33:08 +00:00
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.\" $OpenBSD: moduli.5,v 1.12 2008/06/26 05:57:54 djm Exp $
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.\"
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.\" Copyright (c) 2008 Damien Miller <djm@mindrot.org>
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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2008-09-29 10:53:05 +00:00
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.Dd June 26 2008
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2008-07-23 09:33:08 +00:00
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.Dt MODULI 5
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.Os
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.Sh NAME
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.Nm moduli
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.Nd Diffie Hellman moduli
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.Sh DESCRIPTION
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The
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.Pa /etc/moduli
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file contains prime numbers and generators for use by
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.Xr sshd 8
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in the Diffie-Hellman Group Exchange key exchange method.
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.Pp
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New moduli may be generated with
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.Xr ssh-keygen 1
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using a two-step process.
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An initial
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.Em candidate generation
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pass, using
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.Ic ssh-keygen -G ,
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calculates numbers that are likely to be useful.
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A second
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.Em primality testing
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pass, using
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.Ic ssh-keygen -T
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provides a high degree of assurance that the numbers are prime and are
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safe for use in Diffie Hellman operations by
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.Xr sshd 8 .
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This
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.Nm
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format is used as the output from each pass.
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.Pp
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The file consists of newline-separated records, one per modulus,
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containing seven space separated fields.
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These fields are as follows:
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.Pp
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.Bl -tag -width Description -offset indent
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.It timestamp
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The time that the modulus was last processed as YYYYMMDDHHMMSS.
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.It type
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Decimal number specifying the internal structure of the prime modulus.
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Supported types are:
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.Pp
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.Bl -tag -width 0x00 -compact
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.It 0
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Unknown, not tested
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.It 2
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"Safe" prime; (p-1)/2 is also prime.
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.It 4
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Sophie Germain; (p+1)*2 is also prime.
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.El
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.Pp
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Moduli candidates initially produced by
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.Xr ssh-keygen 1
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are Sophie Germain primes (type 4).
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Futher primality testing with
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.Xr ssh-keygen 1
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produces safe prime moduli (type 2) that are ready for use in
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.Xr sshd 8 .
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Other types are not used by OpenSSH.
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.It tests
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Decimal number indicating the type of primality tests that the number
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has been subjected to represented as a bitmask of the following values:
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.Pp
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.Bl -tag -width 0x00 -compact
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.It 0x00
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Not tested
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.It 0x01
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Composite number - not prime.
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.It 0x02
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Sieve of Eratosthenes
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.It 0x04
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Probabalistic Miller-Rabin primality tests.
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.El
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.Pp
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The
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.Xr ssh-keygen 1
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moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02).
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Subsequent
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.Xr ssh-keygen 1
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primality tests are Miller-Rabin tests (flag 0x04).
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.It trials
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Decimal number indicating of primaility trials that have been performed
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on the modulus.
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.It size
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Decimal number indicating the size of the prime in bits.
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.It generator
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The recommended generator for use with this modulus (hexadecimal).
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.It modulus
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The modulus itself in hexadecimal.
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.El
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.Pp
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When performing Diffie Hellman Group Exchange,
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.Xr sshd 8
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first estimates the size of the modulus required to produce enough
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Diffie Hellman output to sufficiently key the selected symmetric cipher.
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.Xr sshd 8
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then randomly selects a modulus from
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.Fa /etc/moduli
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that best meets the size requirement.
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.Pp
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.Sh SEE ALSO
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.Xr ssh-keygen 1 ,
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.Xr sshd 8 ,
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.Rs
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.%R RFC 4419
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.%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
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.%D 2006
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.Re
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