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EVP_PKEY_DERIVE(3)		    OpenSSL		    EVP_PKEY_DERIVE(3)

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

SYNOPSIS
	#include <openssl/evp.h>

	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);

DESCRIPTION
       The EVP_PKEY_derive_init() function initializes a public	key algorithm
       context using key pkey for shared secret	derivation.

       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 ctx.	 If key	is
       NULL then the maximum size of the output	buffer is written to the
       keylen parameter. If key	is not NULL then before	the call the keylen
       parameter should	contain	the length of the key buffer, if the call is
       successful the shared secret is written to key and the amount of	data
       written to keylen.

NOTES
       After the call to EVP_PKEY_derive_init()	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.

RETURN VALUES
       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.

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 */

SEE ALSO
       EVP_PKEY_CTX_new(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3),
       EVP_PKEY_sign(3), EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3),

HISTORY
       These functions were first added	to OpenSSL 1.0.0.

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

       Licensed	under the OpenSSL license (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
       <https://www.openssl.org/source/license.html>.

1.1.1a				  2018-11-20		    EVP_PKEY_DERIVE(3)

NAME | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | EXAMPLE | SEE ALSO | HISTORY | COPYRIGHT

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