.\"	$OpenBSD: DH_generate_parameters.3,v 1.7 2017/06/10 13:10:52 schwarze Exp $
.\"	OpenSSL 05ea606a May 20 20:52:46 2016 -0400
.\"
.\" This file was written by Ulf Moeller <ulf@openssl.org>.
.\" Copyright (c) 2000, 2016 The OpenSSL Project.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in
.\"    the documentation and/or other materials provided with the
.\"    distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\"    software must display the following acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\"    endorse or promote products derived from this software without
.\"    prior written permission. For written permission, please contact
.\"    openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\"    nor may "OpenSSL" appear in their names without prior written
.\"    permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\"    acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: June 10 2017 $
.Dt DH_GENERATE_PARAMETERS 3
.Os
.Sh NAME
.Nm DH_generate_parameters_ex ,
.Nm DH_check ,
.Nm DH_generate_parameters
.Nd generate and check Diffie-Hellman parameters
.Sh SYNOPSIS
.In openssl/dh.h
.Ft int
.Fo DH_generate_parameters_ex
.Fa "DH *dh"
.Fa "int prime_len"
.Fa "int generator"
.Fa "BN_GENCB *cb"
.Fc
.Ft int
.Fo DH_check
.Fa "DH *dh"
.Fa "int *codes"
.Fc
.Pp
Deprecated:
.Pp
.Ft DH *
.Fo DH_generate_parameters
.Fa "int prime_len"
.Fa "int generator"
.Fa "void (*callback)(int, int, void *)"
.Fa "void *cb_arg"
.Fc
.Sh DESCRIPTION
.Fn DH_generate_parameters_ex
generates Diffie-Hellman parameters that can be shared among a group of
users, and stores them in the provided
.Vt DH
structure.
.Pp
.Fa prime_len
is the length in bits of the safe prime to be generated.
.Fa generator
is a small number > 1, typically 2 or 5.
.Pp
A callback function may be used to provide feedback about the progress
of the key generation.
If
.Fa cb
is not
.Dv NULL ,
it will be called as described in
.Xr BN_generate_prime 3
while a random prime number is generated, and when a prime has been
found,
.Fn BN_GENCB_call cb 3 0
is called; see
.Xr BN_GENCB_call 3 .
.Pp
.Fn DH_check
validates Diffie-Hellman parameters.
If no problems are found,
.Pf * Ar codes
is set to zero.
Otherwise, one or more of the following bits are set:
.Bl -tag -width Ds
.It Dv DH_CHECK_P_NOT_PRIME
The parameter
.Fa dh->p
is not prime.
.It Dv DH_CHECK_P_NOT_SAFE_PRIME
The parameter
.Fa dh->p
is not a safe prime.
.It Dv DH_UNABLE_TO_CHECK_GENERATOR
The generator
.Fa dh->g
cannot be checked for suitability: it is neither 2 nor 5.
.It Dv DH_NOT_SUITABLE_GENERATOR
The generator
.Fa dh->g
is not suitable.
.El
.Sh RETURN VALUES
.Fn DH_generate_parameters_ex
and
.Fn DH_check
return 1 if the check could be performed, or 0 otherwise.
.Pp
.Fn DH_generate_parameters
(deprecated) returns a pointer to the
.Vt DH
structure, or
.Dv NULL
if the parameter generation fails.
.Pp
The error codes can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr DH_new 3 ,
.Xr ERR_get_error 3 ,
.Xr RAND_bytes 3
.Sh HISTORY
.Fn DH_check
is available in all versions of SSLeay and OpenSSL.
The
.Fa cb_arg
argument to
.Fn DH_generate_parameters
was added in SSLeay 0.9.0.
.Pp
In versions before OpenSSL 0.9.5,
.Dv DH_CHECK_P_NOT_STRONG_PRIME
is used instead of
.Dv DH_CHECK_P_NOT_SAFE_PRIME .
.Sh CAVEATS
.Fn DH_generate_parameters_ex
and
.Fn DH_generate_parameters
may run for several hours before finding a suitable prime.
.Pp
The parameters generated by
.Fn DH_generate_parameters_ex
and
.Fn DH_generate_parameters
are not to be used in signature schemes.
.Sh BUGS
If
.Fa generator
is not 2 or 5,
.Fa dh->g Ns = Ns Fa generator
is not a usable generator.