EVP_KEYEXCH-DH - DH Key Exchange algorithm support
Key exchange support for the
DH key type.
- "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).
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.
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 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>.