man > EVP_KEYEXCH-DH(7)

EVP_KEYEXCH-DH(7SSL)                           OpenSSL                          EVP_KEYEXCH-DH(7SSL)

NAME
       EVP_KEYEXCH-DH - DH Key Exchange algorithm support

DESCRIPTION
       Key exchange support for the DH key type.

   DH key exchange parameters
       "pad" (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" (OSSL_KDF_NAME_X942KDF_ASN1).

       "kdf-type" (OSSL_EXCHANGE_PARAM_KDF_TYPE) <UTF8 string>
           See "Common Key Exchange parameters" in provider-keyexch(7).

       "kdf-digest" (OSSL_EXCHANGE_PARAM_KDF_DIGEST) <UTF8 string>
           See "Common Key Exchange parameters" in provider-keyexch(7).

       "kdf-digest-props" (OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS) <UTF8 string>
           See "Common Key Exchange parameters" in provider-keyexch(7).

       "kdf-outlen" (OSSL_EXCHANGE_PARAM_KDF_OUTLEN) <unsigned integer>
           See "Common Key Exchange parameters" in provider-keyexch(7).

       "kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet string>
           See "Common Key Exchange parameters" in provider-keyexch(7).

       "cekalg" (OSSL_KDF_PARAM_CEK_ALG) <octet string ptr>
           See "KDF Parameters" in provider-kdf(7).

EXAMPLES
       The examples assume a host and peer both generate keys using the same named group (or  domain
       parameters).  See "Examples" in EVP_PKEY-DH(7).  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.

SEE ALSO
       EVP_PKEY-DH(7),  EVP_PKEY-FFC(7),  EVP_PKEY(3),   provider-keyexch(7),   provider-keymgmt(7),
       OSSL_PROVIDER-default(7), OSSL_PROVIDER-FIPS(7),

COPYRIGHT
       Copyright 2020-2022 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>.

3.0.13                                       2026-07-29                         EVP_KEYEXCH-DH(7SSL)