[openssh.git] / moduli.5

.\"	$OpenBSD: moduli.5,v 1.12 2008/06/26 05:57:54 djm Exp $
.\"
.\" Copyright (c) 2008 Damien Miller <djm@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 $Mdocdate$
.Dt MODULI 5
.Os
.Sh NAME
.Nm moduli
.Nd Diffie Hellman moduli
.Sh DESCRIPTION
The
.Pa /etc/moduli
file contains prime numbers and generators for use by 
.Xr sshd 8
in the Diffie-Hellman Group Exchange key exchange method.
.Pp
New moduli may be generated with
.Xr ssh-keygen 1
using a two-step process.
An initial
.Em candidate generation
pass, using 
.Ic ssh-keygen -G ,
calculates numbers that are likely to be useful.
A second
.Em primality testing
pass, using
.Ic 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.
.Pp
The file consists of newline-separated records, one per modulus,
containing seven space separated fields.
These fields are as follows:
.Pp
.Bl -tag -width Description -offset indent
.It timestamp
The time that the modulus was last processed as YYYYMMDDHHMMSS.
.It type
Decimal number specifying the internal structure of the prime modulus.
Supported types are:
.Pp
.Bl -tag -width 0x00 -compact
.It 0
Unknown, not tested
.It 2
"Safe" prime; (p-1)/2 is also prime.
.It 4
Sophie Germain; (p+1)*2 is also prime.
.El
.Pp
Moduli candidates initially produced by
.Xr ssh-keygen 1
are Sophie Germain primes (type 4).
Futher 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.
.It tests
Decimal number indicating the type of primality tests that the number
has been subjected to represented as a bitmask of the following values:
.Pp
.Bl -tag -width 0x00 -compact
.It 0x00
Not tested
.It 0x01
Composite number - not prime.
.It 0x02
Sieve of Eratosthenes
.It 0x04
Probabalistic Miller-Rabin primality tests.
.El
.Pp
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).
.It trials
Decimal number indicating of primaility trials that have been performed
on the modulus.
.It size
Decimal number indicating the size of the prime in bits.
.It generator
The recommended generator for use with this modulus (hexadecimal).
.It modulus
The modulus itself in hexadecimal.
.El
.Pp
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.
.Pp
.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
Commit	Line	Data
bd6b3feb	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 $Mdocdate$
	17	.Dt MODULI 5
	18	.Os
	19	.Sh NAME
	20	.Nm moduli
	21	.Nd Diffie Hellman moduli
	22	.Sh DESCRIPTION
	23	The
	24	.Pa /etc/moduli
	25	file contains prime numbers and generators for use by
	26	.Xr sshd 8
	27	in the Diffie-Hellman Group Exchange key exchange method.
	28	.Pp
	29	New moduli may be generated with
	30	.Xr ssh-keygen 1
	31	using a two-step process.
	32	An initial
	33	.Em candidate generation
	34	pass, using
	35	.Ic ssh-keygen -G ,
	36	calculates numbers that are likely to be useful.
	37	A second
	38	.Em primality testing
	39	pass, using
	40	.Ic ssh-keygen -T
	41	provides a high degree of assurance that the numbers are prime and are
	42	safe for use in Diffie Hellman operations by
	43	.Xr sshd 8 .
	44	This
	45	.Nm
	46	format is used as the output from each pass.
	47	.Pp
	48	The file consists of newline-separated records, one per modulus,
	49	containing seven space separated fields.
	50	These fields are as follows:
	51	.Pp
	52	.Bl -tag -width Description -offset indent
	53	.It timestamp
	54	The time that the modulus was last processed as YYYYMMDDHHMMSS.
	55	.It type
	56	Decimal number specifying the internal structure of the prime modulus.
	57	Supported types are:
	58	.Pp
	59	.Bl -tag -width 0x00 -compact
	60	.It 0
	61	Unknown, not tested
	62	.It 2
	63	"Safe" prime; (p-1)/2 is also prime.
	64	.It 4
65	Sophie Germain; (p+1)*2 is also prime.
66	.El
67	.Pp
68	Moduli candidates initially produced by
69	.Xr ssh-keygen 1
70	are Sophie Germain primes (type 4).
71	Futher primality testing with
72	.Xr ssh-keygen 1
73	produces safe prime moduli (type 2) that are ready for use in
74	.Xr sshd 8 .
75	Other types are not used by OpenSSH.
76	.It tests
77	Decimal number indicating the type of primality tests that the number
78	has been subjected to represented as a bitmask of the following values:
79	.Pp
80	.Bl -tag -width 0x00 -compact
81	.It 0x00
82	Not tested
83	.It 0x01
84	Composite number - not prime.
85	.It 0x02
86	Sieve of Eratosthenes
87	.It 0x04
88	Probabalistic Miller-Rabin primality tests.
89	.El
90	.Pp
91	The
92	.Xr ssh-keygen 1
93	moduli candidate generation uses the Sieve of Eratosthenes (flag 0x02).
94	Subsequent
95	.Xr ssh-keygen 1
96	primality tests are Miller-Rabin tests (flag 0x04).
97	.It trials
98	Decimal number indicating of primaility trials that have been performed
99	on the modulus.
100	.It size
101	Decimal number indicating the size of the prime in bits.
102	.It generator
103	The recommended generator for use with this modulus (hexadecimal).
104	.It modulus
105	The modulus itself in hexadecimal.
106	.El
107	.Pp
108	When performing Diffie Hellman Group Exchange,
109	.Xr sshd 8
110	first estimates the size of the modulus required to produce enough
111	Diffie Hellman output to sufficiently key the selected symmetric cipher.
112	.Xr sshd 8
113	then randomly selects a modulus from
114	.Fa /etc/moduli
115	that 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