EVP_KEYEXCH-ECDH(7ssl) OpenSSL EVP_KEYEXCH-ECDH(7ssl)

EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support

Key exchange support for the ECDH key type.

"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 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);
}

EVP_PKEY-EC(7) EVP_PKEY(3), provider-keyexch(7), provider-keymgmt(7), OSSL_PROVIDER-default(7), OSSL_PROVIDER-FIPS(7),

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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.

2024-10-23 3.4.0