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 similar 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 2006-2023 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>.