NAME

       EVP_PKEY_derive_init, EVP_PKEY_derive_init_ex, EVP_PKEY_derive_set_peer_ex,
       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_init_ex(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
        int EVP_PKEY_derive_set_peer_ex(EVP_PKEY_CTX *ctx, EVP_PKEY *peer,
                                        int validate_peer);
        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 implicit
       fetches.

       EVP_PKEY_derive_init_ex() is the same as EVP_PKEY_derive_init() but additionally sets the
       passed parameters params on the context before returning.

       EVP_PKEY_derive_set_peer_ex() sets the peer key: this will normally be a public key. The
       validate_peer will validate the public key if this value is non zero.

       EVP_PKEY_derive_set_peer() is similiar to EVP_PKEY_derive_set_peer_ex() with validate_peer
       set to 1.

       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

       The EVP_PKEY_derive_init(), EVP_PKEY_derive_set_peer() and EVP_PKEY_derive() functions
       were originally added in OpenSSL 1.0.0.

       The EVP_PKEY_derive_init_ex() and EVP_PKEY_derive_set_peer_ex() functions were added in
       OpenSSL 3.0.

COPYRIGHT

       Copyright 2006-2021 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>.