.\"	$OpenBSD: DH_generate_key.3,v 1.9 2018/03/27 17:35:50 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
.\"
.\" This file was written by Ulf Moeller <ulf@openssl.org>.
.\" Copyright (c) 2000 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: March 27 2018 $
.Dt DH_GENERATE_KEY 3
.Os
.Sh NAME
.Nm DH_generate_key ,
.Nm DH_compute_key
.Nd perform Diffie-Hellman key exchange
.Sh SYNOPSIS
.In openssl/dh.h
.Ft int
.Fo DH_generate_key
.Fa "DH *dh"
.Fc
.Ft int
.Fo DH_compute_key
.Fa "unsigned char *key"
.Fa "BIGNUM *pub_key"
.Fa "DH *dh"
.Fc
.Sh DESCRIPTION
.Fn DH_generate_key
performs the first step of a Diffie-Hellman key exchange by generating
private and public DH values.
By calling
.Fn DH_compute_key ,
these are combined with the other party's public value to compute the
shared key.
.Pp
.Fn DH_generate_key
expects
.Fa dh
to contain the shared parameters
.Sy dh->p
and
.Sy dh->g .
It generates a random private DH value unless
.Sy dh->priv_key
is already set, and computes the corresponding public value
.Sy dh->pub_key ,
which can then be published.
.Pp
.Fn DH_compute_key
computes the shared secret from the private DH value in
.Fa dh
and the other party's public value in
.Fa pub_key
and stores it in
.Fa key .
.Fa key
must point to
.Fn DH_size dh
bytes of memory.
.Sh RETURN VALUES
.Fn DH_generate_key
returns 1 on success, or 0 otherwise.
.Pp
.Fn DH_compute_key
returns the size of the shared secret on success, or -1 on error.
.Pp
The error codes can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr DH_get0_key 3 ,
.Xr DH_new 3 ,
.Xr DH_size 3 ,
.Xr ERR_get_error 3 ,
.Xr RAND_bytes 3
.Sh HISTORY
.Fn DH_generate_key
and
.Fn DH_compute_key
first appeared in SSLeay 0.5.1 and have been available since
.Ox 2.4 .