.\" $OpenBSD: BN_get_rfc3526_prime_8192.3,v 1.1 2023/07/20 16:26:40 tb Exp $
.\" checked up to: OpenSSL DH_get_1024_160 99d63d46 Oct 26 13:56:48 2016 -0400
.\"
.\" Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.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: July 20 2023 $
.Dt BN_GET_RFC3526_PRIME_8192 3
.Os
.Sh NAME
.Nm BN_get_rfc2409_prime_768 ,
.Nm BN_get_rfc2409_prime_1024 ,
.Nm BN_get_rfc3526_prime_1536 ,
.Nm BN_get_rfc3526_prime_2048 ,
.Nm BN_get_rfc3526_prime_3072 ,
.Nm BN_get_rfc3526_prime_4096 ,
.Nm BN_get_rfc3526_prime_6144 ,
.Nm BN_get_rfc3526_prime_8192
.Nd standard moduli for Diffie-Hellman key exchange
.Sh SYNOPSIS
.In openssl/bn.h
.Ft BIGNUM *
.Fn BN_get_rfc2409_prime_768 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc2409_prime_1024 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc3526_prime_1536 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc3526_prime_2048 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc3526_prime_3072 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc3526_prime_4096 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc3526_prime_6144 "BIGNUM *bn"
.Ft BIGNUM *
.Fn BN_get_rfc3526_prime_8192 "BIGNUM *bn"
.Sh DESCRIPTION
Each of these functions returns one specific constant Sophie Germain
prime number
.Fa p .
.Pp
If
.Fa bn
is
.Dv NULL ,
a new
.Vt BIGNUM
object is created and returned.
Otherwise, the number is stored in
.Pf * Fa bn
and
.Fa bn
is returned.
.Pp
All these numbers are of the form
.Pp
.EQ
p = 2 sup s - 2 sup left ( s - 64 right ) - 1 + 2 sup 64 *
left { left [ 2 sup left ( s - 130 right ) pi right ] + offset right }
delim $$
.EN
.Pp
where
.Ar s
is the size of the binary representation of the number in bits
and appears at the end of the function names.
As long as the offset is sufficiently small, the above form assures
that the top and bottom 64 bits of each number are all 1.
.Pp
The offsets are defined in the standards as follows:
.Bl -column "8192 = 2 * 2^12" "4743158" -offset indent
.It size Ar s Ta Ar offset
.It Ta
.It \ 768 = 3 * 2^8  Ta  149686
.It 1024 = 2 * 2^9  Ta  129093
.It 1536 = 3 * 2^9  Ta  741804
.It 2048 = 2 * 2^10 Ta  124476
.It 3072 = 3 * 2^10 Ta 1690314
.It 4096 = 2 * 2^11 Ta  240904
.It 6144 = 3 * 2^11 Ta  929484
.It 8192 = 2 * 2^12 Ta 4743158
.El
.Pp
For each of these prime numbers, the finite group of natural numbers
smaller than
.Fa p ,
where the group operation is defined as multiplication modulo
.Fa p ,
is used for Diffie-Hellman key exchange.
The first two of these groups are called the First Oakley Group and
the Second Oakley Group.
Obviously, all these groups are cyclic groups of order
.Fa p ,
respectively, and the numbers returned by these functions are not
secrets.
.Sh RETURN VALUES
If memory allocation fails, these functions return
.Dv NULL .
That can happen even if
.Fa bn
is not
.Dv NULL .
.Sh SEE ALSO
.Xr BN_mod_exp 3 ,
.Xr BN_new 3 ,
.Xr BN_set_flags 3 ,
.Xr DH_new 3
.Sh STANDARDS
RFC 2409, "The Internet Key Exchange (IKE)", defines the Oakley Groups.
.Pp
RFC 2412, "The OAKLEY Key Determination Protocol", contains additional
information about these numbers.
.Pp
RFC 3526, "More Modular Exponential (MODP) Diffie-Hellman groups
for Internet Key Exchange (IKE)", defines the other six numbers.
.Sh HISTORY
.Fn BN_get_rfc2409_prime_768 ,
.Fn BN_get_rfc2409_prime_1024 ,
.Fn BN_get_rfc3526_prime_1536 ,
.Fn BN_get_rfc3526_prime_2048 ,
.Fn BN_get_rfc3526_prime_3072 ,
.Fn BN_get_rfc3526_prime_4096 ,
.Fn BN_get_rfc3526_prime_6144 ,
and
.Fn BN_get_rfc3526_prime_8192
first appeared in OpenSSL 1.1.0 and have been available since
.Ox 6.3 .
The same functions without
.Sy BN_
prefix first appeared in OpenSSL 0.9.8a and
.Ox 4.5 ;
they were removed in
.Ox 7.4 .
.Sh CAVEATS
As all the memory needed for storing the numbers is dynamically
allocated, the
.Dv BN_FLG_STATIC_DATA
flag is not set on the returned
.Vt BIGNUM
objects.
So be careful to not change the returned numbers.