279 lines
8.8 KiB
C
279 lines
8.8 KiB
C
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/*
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* Copyright 2022-2023 The OpenSSL Project Authors. All Rights Reserved.
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*
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* Licensed under the Apache License 2.0 (the "License"). You may not use
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* this file except in compliance with the License. You can obtain a copy
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* in the file LICENSE in the source distribution or at
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* https://www.openssl.org/source/license.html
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*/
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#include <stdio.h>
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#include <string.h>
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#include <openssl/core_names.h>
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#include <openssl/evp.h>
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/*
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* This is a demonstration of key exchange using X25519.
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*
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* The variables beginning `peer1_` / `peer2_` are data which would normally be
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* accessible to that peer.
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*
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* Ordinarily you would use random keys, which are demonstrated
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* below when use_kat=0. A known answer test is demonstrated
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* when use_kat=1.
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*/
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/* A property query used for selecting the X25519 implementation. */
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static const char *propq = NULL;
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static const unsigned char peer1_privk_data[32] = {
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0x80, 0x5b, 0x30, 0x20, 0x25, 0x4a, 0x70, 0x2c,
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0xad, 0xa9, 0x8d, 0x7d, 0x47, 0xf8, 0x1b, 0x20,
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0x89, 0xd2, 0xf9, 0x14, 0xac, 0x92, 0x27, 0xf2,
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0x10, 0x7e, 0xdb, 0x21, 0xbd, 0x73, 0x73, 0x5d
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};
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static const unsigned char peer2_privk_data[32] = {
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0xf8, 0x84, 0x19, 0x69, 0x79, 0x13, 0x0d, 0xbd,
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0xb1, 0x76, 0xd7, 0x0e, 0x7e, 0x0f, 0xb6, 0xf4,
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0x8c, 0x4a, 0x8c, 0x5f, 0xd8, 0x15, 0x09, 0x0a,
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0x71, 0x78, 0x74, 0x92, 0x0f, 0x85, 0xc8, 0x43
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};
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static const unsigned char expected_result[32] = {
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0x19, 0x71, 0x26, 0x12, 0x74, 0xb5, 0xb1, 0xce,
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0x77, 0xd0, 0x79, 0x24, 0xb6, 0x0a, 0x5c, 0x72,
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0x0c, 0xa6, 0x56, 0xc0, 0x11, 0xeb, 0x43, 0x11,
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0x94, 0x3b, 0x01, 0x45, 0xca, 0x19, 0xfe, 0x09
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};
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typedef struct peer_data_st {
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const char *name; /* name of peer */
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EVP_PKEY *privk; /* privk generated for peer */
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unsigned char pubk_data[32]; /* generated pubk to send to other peer */
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unsigned char *secret; /* allocated shared secret buffer */
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size_t secret_len;
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} PEER_DATA;
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/*
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* Prepare for X25519 key exchange. The public key to be sent to the remote peer
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* is put in pubk_data, which should be a 32-byte buffer. Returns 1 on success.
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*/
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static int keyexch_x25519_before(
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OSSL_LIB_CTX *libctx,
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const unsigned char *kat_privk_data,
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PEER_DATA *local_peer)
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{
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int ret = 0;
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size_t pubk_data_len = 0;
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/* Generate or load X25519 key for the peer */
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if (kat_privk_data != NULL)
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local_peer->privk =
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EVP_PKEY_new_raw_private_key_ex(libctx, "X25519", propq,
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kat_privk_data,
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sizeof(peer1_privk_data));
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else
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local_peer->privk = EVP_PKEY_Q_keygen(libctx, propq, "X25519");
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if (local_peer->privk == NULL) {
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fprintf(stderr, "Could not load or generate private key\n");
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goto end;
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}
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/* Get public key corresponding to the private key */
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if (EVP_PKEY_get_octet_string_param(local_peer->privk,
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OSSL_PKEY_PARAM_PUB_KEY,
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local_peer->pubk_data,
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sizeof(local_peer->pubk_data),
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&pubk_data_len) == 0) {
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fprintf(stderr, "EVP_PKEY_get_octet_string_param() failed\n");
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goto end;
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}
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/* X25519 public keys are always 32 bytes */
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if (pubk_data_len != 32) {
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fprintf(stderr, "EVP_PKEY_get_octet_string_param() "
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"yielded wrong length\n");
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goto end;
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}
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ret = 1;
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end:
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if (ret == 0) {
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EVP_PKEY_free(local_peer->privk);
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local_peer->privk = NULL;
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}
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return ret;
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}
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/*
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* Complete X25519 key exchange. remote_peer_pubk_data should be the 32 byte
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* public key value received from the remote peer. On success, returns 1 and the
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* secret is pointed to by *secret. The caller must free it.
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*/
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static int keyexch_x25519_after(
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OSSL_LIB_CTX *libctx,
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int use_kat,
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PEER_DATA *local_peer,
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const unsigned char *remote_peer_pubk_data)
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{
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int ret = 0;
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EVP_PKEY *remote_peer_pubk = NULL;
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EVP_PKEY_CTX *ctx = NULL;
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local_peer->secret = NULL;
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/* Load public key for remote peer. */
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remote_peer_pubk =
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EVP_PKEY_new_raw_public_key_ex(libctx, "X25519", propq,
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remote_peer_pubk_data, 32);
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if (remote_peer_pubk == NULL) {
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fprintf(stderr, "EVP_PKEY_new_raw_public_key_ex() failed\n");
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goto end;
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}
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/* Create key exchange context. */
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ctx = EVP_PKEY_CTX_new_from_pkey(libctx, local_peer->privk, propq);
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if (ctx == NULL) {
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fprintf(stderr, "EVP_PKEY_CTX_new_from_pkey() failed\n");
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goto end;
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}
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/* Initialize derivation process. */
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if (EVP_PKEY_derive_init(ctx) == 0) {
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fprintf(stderr, "EVP_PKEY_derive_init() failed\n");
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goto end;
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}
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/* Configure each peer with the other peer's public key. */
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if (EVP_PKEY_derive_set_peer(ctx, remote_peer_pubk) == 0) {
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fprintf(stderr, "EVP_PKEY_derive_set_peer() failed\n");
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goto end;
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}
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/* Determine the secret length. */
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if (EVP_PKEY_derive(ctx, NULL, &local_peer->secret_len) == 0) {
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fprintf(stderr, "EVP_PKEY_derive() failed\n");
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goto end;
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}
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/*
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* We are using X25519, so the secret generated will always be 32 bytes.
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* However for exposition, the code below demonstrates a generic
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* implementation for arbitrary lengths.
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*/
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if (local_peer->secret_len != 32) { /* unreachable */
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fprintf(stderr, "Secret is always 32 bytes for X25519\n");
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goto end;
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}
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/* Allocate memory for shared secrets. */
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local_peer->secret = OPENSSL_malloc(local_peer->secret_len);
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if (local_peer->secret == NULL) {
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fprintf(stderr, "Could not allocate memory for secret\n");
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goto end;
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}
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/* Derive the shared secret. */
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if (EVP_PKEY_derive(ctx, local_peer->secret,
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&local_peer->secret_len) == 0) {
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fprintf(stderr, "EVP_PKEY_derive() failed\n");
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goto end;
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}
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printf("Shared secret (%s):\n", local_peer->name);
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BIO_dump_indent_fp(stdout, local_peer->secret, local_peer->secret_len, 2);
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putchar('\n');
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ret = 1;
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end:
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EVP_PKEY_CTX_free(ctx);
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EVP_PKEY_free(remote_peer_pubk);
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if (ret == 0) {
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OPENSSL_clear_free(local_peer->secret, local_peer->secret_len);
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local_peer->secret = NULL;
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}
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return ret;
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}
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static int keyexch_x25519(int use_kat)
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{
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int ret = 0;
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OSSL_LIB_CTX *libctx = NULL;
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PEER_DATA peer1 = {"peer 1"}, peer2 = {"peer 2"};
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/*
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* Each peer generates its private key and sends its public key
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* to the other peer. The private key is stored locally for
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* later use.
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*/
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if (keyexch_x25519_before(libctx, use_kat ? peer1_privk_data : NULL,
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&peer1) == 0)
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return 0;
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if (keyexch_x25519_before(libctx, use_kat ? peer2_privk_data : NULL,
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&peer2) == 0)
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return 0;
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/*
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* Each peer uses the other peer's public key to perform key exchange.
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* After this succeeds, each peer has the same secret in its
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* PEER_DATA.
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*/
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if (keyexch_x25519_after(libctx, use_kat, &peer1, peer2.pubk_data) == 0)
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return 0;
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if (keyexch_x25519_after(libctx, use_kat, &peer2, peer1.pubk_data) == 0)
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return 0;
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/*
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* Here we demonstrate the secrets are equal for exposition purposes.
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*
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* Although in practice you will generally not need to compare secrets
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* produced through key exchange, if you do compare cryptographic secrets,
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* always do so using a constant-time function such as CRYPTO_memcmp, never
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* using memcmp(3).
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*/
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if (CRYPTO_memcmp(peer1.secret, peer2.secret, peer1.secret_len) != 0) {
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fprintf(stderr, "Negotiated secrets do not match\n");
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goto end;
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}
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/* If we are doing the KAT, the secret should equal our reference result. */
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if (use_kat && CRYPTO_memcmp(peer1.secret, expected_result,
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peer1.secret_len) != 0) {
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fprintf(stderr, "Did not get expected result\n");
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goto end;
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}
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ret = 1;
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end:
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/* The secrets are sensitive, so ensure they are erased before freeing. */
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OPENSSL_clear_free(peer1.secret, peer1.secret_len);
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OPENSSL_clear_free(peer2.secret, peer2.secret_len);
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EVP_PKEY_free(peer1.privk);
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EVP_PKEY_free(peer2.privk);
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OSSL_LIB_CTX_free(libctx);
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return ret;
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}
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int main(int argc, char **argv)
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{
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/* Test X25519 key exchange with known result. */
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printf("Key exchange using known answer (deterministic):\n");
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if (keyexch_x25519(1) == 0)
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return EXIT_FAILURE;
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/* Test X25519 key exchange with random keys. */
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printf("Key exchange using random keys:\n");
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if (keyexch_x25519(0) == 0)
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return EXIT_FAILURE;
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return EXIT_SUCCESS;
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}
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