EVP_KEYEXCH-ECDH(7)

NAME
EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support

DESCRIPTION
Key exchange support for the ECDH key type.

This algorithm supports ECDH key agreement for general elliptic curves. Specialised support for the X25519 and X448 curves is documented in EVP_KEYEXCH-X25519(7) and EVP_KEYEXCH-X448(7).

As specified in RFC 8998 <https://www.rfc-editor.org/rfc/rfc8998.html>, when curveSM2 is the negotiated key agreement group in TLS, ECDH shared secret derivation is used instead of the normal key agreement protocol (KAP) specified in the "GB/T 32918.3-2016" standard for the SM2 elliptic curve. This SM2 key agreement variant is also sometimes called "ECDH over SM2". When a curveSM2 key is generated or imported, the EC group name parameter to specify remains "SM2". The resulting curveSM2 key can only be used for "ECDH over SM2" key agreement. "ECDH over SM2" is TLS-specific, it should not be used in other protocols, absent a specification of its use in the protocol in question.

The curveSM2MLKEM768 hybrid post-quantum key exchange algorithm uses "ECDH over SM2" for its "classical" component.

  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),

HISTORY
Support for "ECDH over SM2" via curveSM2 was added in OpenSSL 4.0.

COPYRIGHT
Copyright 2020-2026 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>.