=pod

=head1 NAME

EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support

=head1 DESCRIPTION

Key exchange support for the B<ECDH> key type.

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

As specified in L<RFC 8998|https://www.rfc-editor.org/rfc/rfc8998.html>, when
B<curveSM2> is the negotiated key agreement group in TLS, B<ECDH> shared secret
derivation is used instead of the normal key agreement protocol (KAP) specified
in the C<GB/T 32918.3-2016> standard for the B<SM2> elliptic curve.
This B<SM2> key agreement variant is also sometimes called C<ECDH over SM2>.
When a B<curveSM2> key is generated or imported, the EC group name parameter to
specify remains C<SM2>.
The resulting B<curveSM2> key can only be used for C<ECDH over SM2> key
agreement.
C<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 B<curveSM2MLKEM768> hybrid post-quantum key exchange algorithm
uses C<ECDH over SM2> for its C<classical> component.

=head2 ECDH Key Exchange parameters

=over 4

=item "ecdh-cofactor-mode" (B<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 L<provider-keymgmt(7)> for the related
B<OSSL_PKEY_PARAM_USE_COFACTOR_ECDH> parameter that can be set on a
per-key basis.

=item "kdf-type" (B<OSSL_EXCHANGE_PARAM_KDF_TYPE>) <UTF8 string>

=item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string>

=item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string>

=item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer>

=item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string>

=back

The OpenSSL FIPS provider also supports the following parameters:

=over 4

=item "fips-indicator" (B<OSSL_EXCHANGE_PARAM_FIPS_APPROVED_INDICATOR>) <integer>

=item "key-check" (B<OSSL_EXCHANGE_PARAM_FIPS_KEY_CHECK>) <integer>

=item "digest-check" (B<OSSL_EXCHANGE_PARAM_FIPS_DIGEST_CHECK>) <integer>

See L<provider-keyexch(7)/Common Key Exchange parameters>.

=item "ecdh-cofactor-check" (B<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.

=back

=head1 EXAMPLES

Examples of key agreement can be found in demos/keyexch.

Keys for the host and peer must be generated as shown in
L<EVP_PKEY-EC(7)/Examples> using the same curve name.

The code to generate a shared secret for the normal case is identical to
L<EVP_KEYEXCH-DH(7)/Examples>.

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);
    }

=head1 SEE ALSO

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

=head1 HISTORY

Support for C<ECDH over SM2> via B<curveSM2> was added in OpenSSL 4.0.

=head1 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
L<https://www.openssl.org/source/license.html>.

=cut
