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=head1 NAME

EVP_PKEY_derive_init, EVP_PKEY_derive_init_ex, EVP_PKEY_derive_set_peer,
EVP_PKEY_derive - derive public key algorithm shared secret

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_PKEY_derive_init_ex(EVP_PKEY_CTX *ctx, EVP_KEYEXCH *exchange);
 int EVP_PKEY_derive_init(EVP_PKEY_CTX *ctx);
 int EVP_PKEY_derive_set_peer(EVP_PKEY_CTX *ctx, EVP_PKEY *peer);
 int EVP_PKEY_derive(EVP_PKEY_CTX *ctx, unsigned char *key, size_t *keylen);

=head1 DESCRIPTION

The EVP_PKEY_derive_init_ex() function initializes a public key algorithm
context for shared secret derivation using the key exchange algorithm
B<exchange>.
The key exchange algorithm B<exchange> should be fetched using a call to
L<EVP_KEYEXCH_fetch(3)>.
The EVP_PKEY object associated with B<ctx> must be compatible with that
algorithm.
B<exchange> may be NULL in which case the EVP_KEYEXCH algorithm is fetched
implicitly based on the type of EVP_PKEY associated with B<ctx>.
See L<EVP_KEYEXCH_fetch(3)> for more information about implict fetches.

The EVP_PKEY_derive_init() function is the same as EVP_PKEY_derive() except that
the EVP_KEYEXCH algorithm is always implicitly fetched.

The EVP_PKEY_derive_set_peer() function sets the peer key: this will normally
be a public key.

The EVP_PKEY_derive() derives a shared secret using B<ctx>.
If B<key> is B<NULL> then the maximum size of the output buffer is written to
the B<keylen> parameter. If B<key> is not B<NULL> then before the call the
B<keylen> parameter should contain the length of the B<key> buffer, if the call
is successful the shared secret is written to B<key> and the amount of data
written to B<keylen>.

=head1 NOTES

After the call to EVP_PKEY_derive_init() or EVP_PKEY_derive_init_ex() algorithm
specific control operations can be performed to set any appropriate parameters
for the operation.

The function EVP_PKEY_derive() can be called more than once on the same
context if several operations are performed using the same parameters.

=head1 RETURN VALUES

EVP_PKEY_derive_init_ex(), EVP_PKEY_derive_init() and EVP_PKEY_derive() return 1
for success and 0 or a negative value for failure.
In particular a return value of -2 indicates the operation is not supported by
the public key algorithm.

=head1 EXAMPLE

Derive shared secret (for example DH or EC keys):

 #include <openssl/evp.h>
 #include <openssl/rsa.h>

 EVP_PKEY_CTX *ctx;
 ENGINE *eng;
 unsigned char *skey;
 size_t skeylen;
 EVP_PKEY *pkey, *peerkey;
 /* NB: assumes pkey, eng, peerkey have been already set up */

 ctx = EVP_PKEY_CTX_new(pkey, eng);
 if (!ctx)
     /* Error occurred */
 if (EVP_PKEY_derive_init(ctx) <= 0)
     /* Error */
 if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0)
     /* Error */

 /* Determine buffer length */
 if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0)
     /* Error */

 skey = OPENSSL_malloc(skeylen);

 if (!skey)
     /* malloc failure */

 if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
     /* Error */

 /* Shared secret is skey bytes written to buffer skey */

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_encrypt(3)>,
L<EVP_PKEY_decrypt(3)>,
L<EVP_PKEY_sign(3)>,
L<EVP_PKEY_verify(3)>,
L<EVP_PKEY_verify_recover(3)>,
L<EVP_KEYEXCH_fetch(3)>

=head1 HISTORY

These functions were added in OpenSSL 1.0.0. The EVP_PKEY_derive_init_ex()
function was added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2006-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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