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
EVP_PKEY_derive_init() initializes a public key algorithm context ctx for shared secret derivation using the algorithm given when the context was created using EVP_PKEY_CTX_new(3) or variants thereof. The algorithm is used to fetch a EVP_KEYEXCH method implicitly, see "Implicit fetch" in provider(7) for more information about implict fetches.

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

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.

EXAMPLES
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), EVP_KEYEXCH_fetch(3)

HISTORY
These functions were added in OpenSSL 1.0.0.

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 <https://www.openssl.org/source/license.html>.