ecdh.c
/*
* Copyright 2023-2024 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
* https://www.openssl.org/source/license.html
*/
#include <stdio.h>
#include <string.h>
#include <openssl/core_names.h>
#include <openssl/evp.h>
#include <openssl/err.h>
/*
* This is a demonstration of key exchange using ECDH.
*
* EC key exchange requires 2 parties (peers) to first agree on shared group
* parameters (the EC curve name). Each peer then generates a public/private
* key pair using the shared curve name. Each peer then gives their public key
* to the other peer. A peer can then derive the same shared secret using their
* private key and the other peers public key.
*/
/* Object used to store information for a single Peer */
typedef struct peer_data_st {
const char *name; /* name of peer */
const char *curvename; /* The shared curve name */
EVP_PKEY *priv; /* private keypair */
EVP_PKEY *pub; /* public key to send to other peer */
unsigned char *secret; /* allocated shared secret buffer */
size_t secretlen;
} PEER_DATA;
/*
* The public key needs to be given to the other peer
* The following code extracts the public key data from the private key
* and then builds an EVP_KEY public key.
*/
static int get_peer_public_key(PEER_DATA *peer, OSSL_LIB_CTX *libctx)
{
int ret = 0;
EVP_PKEY_CTX *ctx;
OSSL_PARAM params[3];
unsigned char pubkeydata[256];
size_t pubkeylen;
/* Get the EC encoded public key data from the peers private key */
if (!EVP_PKEY_get_octet_string_param(peer->priv, OSSL_PKEY_PARAM_PUB_KEY,
pubkeydata, sizeof(pubkeydata),
&pubkeylen))
return 0;
/* Create a EC public key from the public key data */
ctx = EVP_PKEY_CTX_new_from_name(libctx, "EC", NULL);
if (ctx == NULL)
return 0;
params[0] = OSSL_PARAM_construct_utf8_string(OSSL_PKEY_PARAM_GROUP_NAME,
(char *)peer->curvename, 0);
params[1] = OSSL_PARAM_construct_octet_string(OSSL_PKEY_PARAM_PUB_KEY,
pubkeydata, pubkeylen);
params[2] = OSSL_PARAM_construct_end();
ret = EVP_PKEY_fromdata_init(ctx) > 0
&& (EVP_PKEY_fromdata(ctx, &peer->pub, EVP_PKEY_PUBLIC_KEY,
params) > 0);
EVP_PKEY_CTX_free(ctx);
return ret;
}
static int create_peer(PEER_DATA *peer, OSSL_LIB_CTX *libctx)
{
int ret = 0;
EVP_PKEY_CTX *ctx = NULL;
OSSL_PARAM params[2];
params[0] = OSSL_PARAM_construct_utf8_string(OSSL_PKEY_PARAM_GROUP_NAME,
(char *)peer->curvename, 0);
params[1] = OSSL_PARAM_construct_end();
ctx = EVP_PKEY_CTX_new_from_name(libctx, "EC", NULL);
if (ctx == NULL)
return 0;
if (EVP_PKEY_keygen_init(ctx) <= 0
|| !EVP_PKEY_CTX_set_params(ctx, params)
|| EVP_PKEY_generate(ctx, &peer->priv) <= 0
|| !get_peer_public_key(peer, libctx)) {
EVP_PKEY_free(peer->priv);
peer->priv = NULL;
goto err;
}
ret = 1;
err:
EVP_PKEY_CTX_free(ctx);
return ret;
}
static void destroy_peer(PEER_DATA *peer)
{
EVP_PKEY_free(peer->priv);
EVP_PKEY_free(peer->pub);
}
static int generate_secret(PEER_DATA *peerA, EVP_PKEY *peerBpub,
OSSL_LIB_CTX *libctx)
{
unsigned char *secret = NULL;
size_t secretlen = 0;
EVP_PKEY_CTX *derivectx;
/* Create an EVP_PKEY_CTX that contains peerA's private key */
derivectx = EVP_PKEY_CTX_new_from_pkey(libctx, peerA->priv, NULL);
if (derivectx == NULL)
return 0;
if (EVP_PKEY_derive_init(derivectx) <= 0)
goto cleanup;
/* Set up peerB's public key */
if (EVP_PKEY_derive_set_peer(derivectx, peerBpub) <= 0)
goto cleanup;
/*
* For backwards compatibility purposes the OpenSSL ECDH provider supports
* optionally using a X963KDF to expand the secret data. This can be done
* with code similar to the following.
*
* OSSL_PARAM params[5];
* size_t outlen = 128;
* unsigned char ukm[] = { 1, 2, 3, 4 };
* params[0] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE,
* "X963KDF", 0);
* params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST,
* "SHA256", 0);
* params[2] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN,
* &outlen);
* params[3] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM,
* ukm, sizeof(ukm));
* params[4] = OSSL_PARAM_construct_end();
* if (!EVP_PKEY_CTX_set_params(derivectx, params))
* goto cleanup;
*
* Note: After the secret is generated below, the peer could alternatively
* pass the secret to a KDF to derive additional key data from the secret.
* See demos/kdf/hkdf.c for an example (where ikm is the secret key)
*/
/* Calculate the size of the secret and allocate space */
if (EVP_PKEY_derive(derivectx, NULL, &secretlen) <= 0)
goto cleanup;
secret = (unsigned char *)OPENSSL_malloc(secretlen);
if (secret == NULL)
goto cleanup;
/*
* Derive the shared secret. In this example 32 bytes are generated.
* For EC curves the secret size is related to the degree of the curve
* which is 256 bits for P-256.
*/
if (EVP_PKEY_derive(derivectx, secret, &secretlen) <= 0)
goto cleanup;
peerA->secret = secret;
peerA->secretlen = secretlen;
printf("Shared secret (%s):\n", peerA->name);
BIO_dump_indent_fp(stdout, peerA->secret, peerA->secretlen, 2);
putchar('\n');
return 1;
cleanup:
OPENSSL_free(secret);
EVP_PKEY_CTX_free(derivectx);
return 0;
}
int main(void)
{
int ret = EXIT_FAILURE;
/* Initialise the 2 peers that will share a secret */
PEER_DATA peer1 = {"peer 1", "P-256"};
PEER_DATA peer2 = {"peer 2", "P-256"};
/*
* Setting libctx to NULL uses the default library context
* Use OSSL_LIB_CTX_new() to create a non default library context
*/
OSSL_LIB_CTX *libctx = NULL;
/* Each peer creates a (Ephemeral) keypair */
if (!create_peer(&peer1, libctx)
|| !create_peer(&peer2, libctx)) {
fprintf(stderr, "Create peer failed\n");
goto cleanup;
}
/*
* Each peer uses its private key and the other peers public key to
* derive a shared secret
*/
if (!generate_secret(&peer1, peer2.pub, libctx)
|| !generate_secret(&peer2, peer1.pub, libctx)) {
fprintf(stderr, "Generate secrets failed\n");
goto cleanup;
}
/* For illustrative purposes demonstrate that the derived secrets are equal */
if (peer1.secretlen != peer2.secretlen
|| CRYPTO_memcmp(peer1.secret, peer2.secret, peer1.secretlen) != 0) {
fprintf(stderr, "Derived secrets do not match\n");
goto cleanup;
} else {
fprintf(stdout, "Derived secrets match\n");
}
ret = EXIT_SUCCESS;
cleanup:
if (ret != EXIT_SUCCESS)
ERR_print_errors_fp(stderr);
destroy_peer(&peer2);
destroy_peer(&peer1);
return ret;
}
