=pod

=head1 NAME

EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support

=head1 DESCRIPTION

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

=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>

Sets or gets the Key Derivation Function type to apply within the associated key
exchange ctx.

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

Sets or gets the Digest algorithm to be used as part of the Key Derivation Function
associated with the given key exchange ctx.

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

Sets properties to be used upon look up of the implementation for the selected
Digest algorithm for the Key Derivation Function associated with the given key
exchange ctx.

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

Sets or gets the desired size for the output of the chosen Key Derivation Function
associated with the given key exchange ctx.
The length of the "kdf-outlen" parameter should not exceed that of a B<size_t>.

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

Sets the User Key Material to be used as part of the selected Key Derivation
Function associated with the given key exchange ctx.

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

Gets a pointer to the User Key Material to be used as part of the selected
Key Derivation Function associated with the given key exchange ctx. Providers
usually do not need to support this gettable parameter as its sole purpose
is to support functionality of the deprecated EVP_PKEY_CTX_get0_ecdh_kdf_ukm()
function.

=back

=head1 EXAMPLES

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_PKET_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 COPYRIGHT

Copyright 2020-2021 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
