d4af9e693f
I have worked hard to reduce diffs against the vendor branch. One notable change in that respect is that we no longer prefer DSA over RSA - the reasons for doing so went away years ago. This may cause some surprises, as ssh will warn about unknown host keys even for hosts whose keys haven't changed. MFC after: 6 weeks
125 lines
3.5 KiB
Groff
125 lines
3.5 KiB
Groff
.\" $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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.Dd $Mdocdate: June 26 2008 $
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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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