In the context of an Elliptic Curve Diffie-Hellman key exchange, this parameter can be used to select between the plain Diffie-Hellman (DH) or Cofactor Diffie-Hellman (CDH) variants of the key exchange algorithm.
When setting, the value should be 1, 0 or -1, respectively forcing cofactor mode on, off, or resetting it to the default for the private key associated with the given key exchange ctx.
When getting, the value should be either 1 or 0, respectively signaling if the cofactor mode is on or off.
See also provider-keymgmt(7) for the related OSSL_PKEY_PARAM_USE_COFACTOR_ECDH parameter that can be set on a per-key basis.
The code to generate a shared secret for the normal case is identical to ``Examples'' in EVP_KEYEXCH-DH(7).
To derive a shared secret on the host using the host's key and the peer's public key but also using X963KDF with a user key material:
/* It is assumed that the host_key, peer_pub_key and ukm are set up */
void derive_secret(EVP_PKEY *host_key, EVP_PKEY *peer_key,
unsigned char *ukm, size_t ukm_len)
{
unsigned char secret[64];
size_t out_len = sizeof(secret);
size_t secret_len = out_len;
unsigned int pad = 1;
OSSL_PARAM params[6];
EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);
EVP_PKEY_derive_init(dctx);
params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE,
"X963KDF", 0);
params[2] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST,
"SHA1", 0);
params[3] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN,
&out_len);
params[4] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM,
ukm, ukm_len);
params[5] = OSSL_PARAM_construct_end();
EVP_PKEY_CTX_set_params(dctx, params);
EVP_PKEY_derive_set_peer(dctx, peer_pub_key);
EVP_PKEY_derive(dctx, secret, &secret_len);
...
OPENSSL_clear_free(secret, secret_len);
EVP_PKEY_CTX_free(dctx);
}
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>.