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+=pod
+
+=head1 NAME
+
+EVP_KEYEXCH-DH
+- DH Key Exchange algorithm support
+
+=head1 DESCRIPTION
+
+Key exchange support for the B<DH> key type.
+
+=head2 DH key exchange parameters
+
+=over 4
+
+=item "pad" (B<OSSL_EXCHANGE_PARAM_PAD>) <unsigned integer>
+
+See L<provider-keyexch(7)/Common Key Exchange 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 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