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


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>.

3.5.1                             2025-07-01               EVP_KEYEXCH-ECDH(7)