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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .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" \&\fBDH_generate_key()\fR performs the first step of a Diffie-Hellman key exchange by generating private and public \s-1DH\s0 values. By calling \&\fBDH_compute_key()\fR or \fBDH_compute_key_padded()\fR, these are combined with the other party's public value to compute the shared key. .PP \&\fBDH_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 \&\fBDH_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 \&\fBDH_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" \&\fBDH_generate_key()\fR returns 1 on success, 0 otherwise. .PP \&\fBDH_compute_key()\fR returns the size of the shared secret on success, \-1 on error. .PP \&\fBDH_compute_key_padded()\fR returns \fBDH_size(dh)\fR on success, \-1 on error. .PP The error codes can be obtained by \fBERR_get_error\fR\|(3). .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBDH_new\fR\|(3), \fBERR_get_error\fR\|(3), \fBRAND_bytes\fR\|(3), \fBDH_size\fR\|(3) .SH "HISTORY" .IX Header "HISTORY" \&\fBDH_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>.