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

	ctx = EVP_PKEY_CTX_new(pkey);
	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.



1.0.2k				  2017-01-26		    EVP_PKEY_derive(3)