EVP_KEYEXCH-DH(7)

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>
    See "Common Key Exchange parameters" in provider-keyexch(7).

"kdf-ukm" (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.

"kdf-ukm" (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_dh_kdf_ukm() function.

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 2020-2021 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 <https://www.openssl.org/source/license.html>.