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.