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.\" Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: January 31 2017 $
.Dt GET_RFC3526_PRIME_8192 3
.Os
.Sh NAME
.Nm get_rfc2409_prime_768 ,
.Nm get_rfc2409_prime_1024 ,
.Nm get_rfc3526_prime_1536 ,
.Nm get_rfc3526_prime_2048 ,
.Nm get_rfc3526_prime_3072 ,
.Nm get_rfc3526_prime_4096 ,
.Nm get_rfc3526_prime_6144 ,
.Nm get_rfc3526_prime_8192
.Nd standard moduli for Diffie-Hellmann key exchange
.Sh SYNOPSIS
.In openssl/bn.h
.Ft BIGNUM *
.Fn get_rfc2409_prime_768 "BIGNUM *bn"
.Ft BIGNUM *
.Fn get_rfc2409_prime_1024 "BIGNUM *bn"
.Ft BIGNUM *
.Fn get_rfc3526_prime_1536 "BIGNUM *bn"
.Ft BIGNUM *
.Fn get_rfc3526_prime_2048 "BIGNUM *bn"
.Ft BIGNUM *
.Fn get_rfc3526_prime_3072 "BIGNUM *bn"
.Ft BIGNUM *
.Fn get_rfc3526_prime_4096 "BIGNUM *bn"
.Ft BIGNUM *
.Fn get_rfc3526_prime_6144 "BIGNUM *bn"
.Ft BIGNUM *
.Fn 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 16n 8n -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-Hellmann key exchange.
The first two of these groups are called the First Oakley Group and
the Second Oakley Group.
Obiviously, 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 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.