- NAME
- SYNOPSIS
- DESCRIPTION
- NOTES
- RETURN VALUES
- EXAMPLES
- SEE ALSO
- HISTORY
- COPYRIGHT

# 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 provider(7)/Implicit fetch 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 2006-2022 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.