EVP_KEYEXCH-ECDH(7SSL) OpenSSL EVP_KEYEXCH-ECDH(7SSL)
NAME
EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support
DESCRIPTION
Key exchange support for the ECDH key type.
ECDH Key Exchange parameters
"ecdh-cofactor-mode" (OSSL_EXCHANGE_PARAM_EC_ECDH_COFACTOR_MODE)
<integer>
Sets or gets the ECDH mode of operation for the associated key
exchange ctx.
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.
"kdf-type" (OSSL_EXCHANGE_PARAM_KDF_TYPE) <UTF8 string>
"kdf-digest" (OSSL_EXCHANGE_PARAM_KDF_DIGEST) <UTF8 string>
"kdf-digest-props" (OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS) <UTF8 string>
"kdf-outlen" (OSSL_EXCHANGE_PARAM_KDF_OUTLEN) <unsigned integer>
"kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet string>
The OpenSSL FIPS provider also supports the following parameters:
"fips-indicator" (OSSL_EXCHANGE_PARAM_FIPS_APPROVED_INDICATOR) <integer>
"key-check" (OSSL_EXCHANGE_PARAM_FIPS_KEY_CHECK) <integer>
"digest-check" (OSSL_EXCHANGE_PARAM_FIPS_DIGEST_CHECK) <integer>
See "Common Key Exchange parameters" in provider-keyexch(7).
"ecdh-cofactor-check" (OSSL_EXCHANGE_PARAM_FIPS_ECDH_COFACTOR_CHECK)
<integer>
If required this parameter should before OSSL_FUNC_keyexch_derive().
The default value of 1 causes an error during the
OSSL_FUNC_keyexch_derive if the EC curve has a cofactor that is not
1, and the cofactor is not used. Setting this to 0 will ignore the
error and set the approved "fips-indicator" to 0. This option
breaks FIPS compliance if it causes the approved "fips-indicator" to
return 0.
EXAMPLES
Examples of key agreement can be found in demos/keyexch.
Keys for the host and peer must be generated as shown in "Examples" in
EVP_PKEY-EC(7) using the same curve name.
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);
}
SEE ALSO
EVP_PKEY-EC(7) EVP_PKEY(3), provider-keyexch(7), provider-keymgmt(7),
OSSL_PROVIDER-default(7), OSSL_PROVIDER-FIPS(7),
COPYRIGHT
Copyright 2020-2024 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>.
3.5.4 2025-09-30 EVP_KEYEXCH-ECDH(7SSL)
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