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.\" ========================================================================
.\"
.IX Title "DH_GENERATE_KEY 3"
.TH DH_GENERATE_KEY 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
DH_generate_key, DH_compute_key, DH_compute_key_padded \- perform Diffie\-Hellman key exchange
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/dh.h>
\&
\& int DH_generate_key(DH *dh);
\&
\& int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh);
\&
\& int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fIDH_generate_key()\fR performs the first step of a Diffie-Hellman key
exchange by generating private and public \s-1DH\s0 values. By calling
\&\fIDH_compute_key()\fR or \fIDH_compute_key_padded()\fR, these are combined with
the other party's public value to compute the shared key.
.PP
\&\fIDH_generate_key()\fR expects \fBdh\fR to contain the shared parameters
\&\fBdh\->p\fR and \fBdh\->g\fR. It generates a random private \s-1DH\s0 value
unless \fBdh\->priv_key\fR is already set, and computes the
corresponding public value \fBdh\->pub_key\fR, which can then be
published.
.PP
\&\fIDH_compute_key()\fR computes the shared secret from the private \s-1DH\s0 value
in \fBdh\fR and the other party's public value in \fBpub_key\fR and stores
it in \fBkey\fR. \fBkey\fR must point to \fBDH_size(dh)\fR bytes of memory.
The padding style is \s-1RFC 5246 \s0(8.1.2) that strips leading zero bytes.
It is not constant time due to the leading zero bytes being stripped.
The return value should be considered public.
.PP
\&\fIDH_compute_key_padded()\fR is similar but stores a fixed number of bytes.
The padding style is \s-1NIST SP 800\-56A \s0(C.1) that retains leading zero bytes.
It is constant time due to the leading zero bytes being retained.
The return value should be considered public.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIDH_generate_key()\fR returns 1 on success, 0 otherwise.
.PP
\&\fIDH_compute_key()\fR returns the size of the shared secret on success, \-1
on error.
.PP
\&\fIDH_compute_key_padded()\fR returns \fBDH_size(dh)\fR on success, \-1 on error.
.PP
The error codes can be obtained by \fIERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIDH_new\fR\|(3), \fIERR_get_error\fR\|(3), \fIRAND_bytes\fR\|(3), \fIDH_size\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIDH_compute_key_padded()\fR was added in OpenSSL 1.0.2.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2021 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.