=pod

=head1 NAME

EVP_KEYEXCH-DH
- DH Key Exchange algorithm support

=head1 DESCRIPTION

Key exchange support for the B<DH> and B<DHX> key types.

Please note that although both key types support the same key exchange
operations, they cannot be used together in a single key exchange. It
is not possible to use a private key of the B<DH> type in key exchange
with the public key of B<DHX> type and vice versa.

=head2 DH and DHX key exchange parameters

=over 4

=item "pad" (B<OSSL_EXCHANGE_PARAM_PAD>) <unsigned integer>

Sets the padding mode for the associated key exchange ctx.
Setting a value of 1 will turn padding on.
Setting a value of 0 will turn padding off.
If padding is off then the derived shared secret may be smaller than the
largest possible secret size.
If padding is on then the derived shared secret will have its first bytes
filled with zeros where necessary to make the shared secret the same size as
the largest possible secret size.
The padding mode parameter is ignored (and padding implicitly enabled) when
the KDF type is set to "X942KDF-ASN1" (B<OSSL_KDF_NAME_X942KDF_ASN1>).

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

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

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

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

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

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

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

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

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

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

=item "cekalg" (B<OSSL_KDF_PARAM_CEK_ALG>) <octet string ptr>

See L<provider-kdf(7)/KDF Parameters>.

=back

=head1 EXAMPLES

The examples assume a host and peer both generate keys using the same
named group (or domain parameters). See L<EVP_PKEY-DH(7)/Examples>.
Both the host and peer transfer their public key to each other.

To convert the peer's generated key pair to a public key in DER format in order
to transfer to the host:

    EVP_PKEY *peer_key; /* It is assumed this contains the peers generated key */
    unsigned char *peer_pub_der = NULL;
    int peer_pub_der_len;

    peer_pub_der_len = i2d_PUBKEY(peer_key, &peer_pub_der);
    ...
    OPENSSL_free(peer_pub_der);

To convert the received peer's public key from DER format on the host:

    const unsigned char *pd = peer_pub_der;
    EVP_PKEY *peer_pub_key = d2i_PUBKEY(NULL, &pd, peer_pub_der_len);
    ...
    EVP_PKEY_free(peer_pub_key);

To derive a shared secret on the host using the host's key and the peer's public
key:

    /* It is assumed that the host_key and peer_pub_key are set up */
    void derive_secret(EVP_KEY *host_key, EVP_PKEY *peer_pub_key)
    {
        unsigned int pad = 1;
        OSSL_PARAM params[2];
        unsigned char *secret = NULL;
        size_t secret_len = 0;
        EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);

        EVP_PKEY_derive_init(dctx);

        /* Optionally set the padding */
        params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
        params[1] = OSSL_PARAM_construct_end();
        EVP_PKEY_CTX_set_params(dctx, params);

        EVP_PKEY_derive_set_peer(dctx, peer_pub_key);

        /* Get the size by passing NULL as the buffer */
        EVP_PKEY_derive(dctx, NULL, &secret_len);
        secret = OPENSSL_zalloc(secret_len);

        EVP_PKEY_derive(dctx, secret, &secret_len);
        ...
        OPENSSL_clear_free(secret, secret_len);
        EVP_PKEY_CTX_free(dctx);
    }

Very similar code can be used by the peer to derive the same shared secret
using the host's public key and the peer's generated key pair.

=head1 SEE ALSO

L<EVP_PKEY-DH(7)>,
L<EVP_PKEY-FFC(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 2020-2024 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
L<https://www.openssl.org/source/license.html>.

=cut
