x25519.c
/*
* Copyright 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
*/
#include <stdio.h>
#include <string.h>
#include <openssl/core_names.h>
#include <openssl/evp.h>
/*
* This is a demonstration of key exchange using X25519.
*
* The variables beginning `peer1_` / `peer2_` are data which would normally be
* accessible to that peer.
*
* Ordinarily you would use random keys, which are demonstrated
* below when use_kat=0. A known answer test is demonstrated
* when use_kat=1.
*/
/* A property query used for selecting the X25519 implementation. */
static const char *propq = NULL;
static const unsigned char peer1_privk_data[32] = {
0x80, 0x5b, 0x30, 0x20, 0x25, 0x4a, 0x70, 0x2c,
0xad, 0xa9, 0x8d, 0x7d, 0x47, 0xf8, 0x1b, 0x20,
0x89, 0xd2, 0xf9, 0x14, 0xac, 0x92, 0x27, 0xf2,
0x10, 0x7e, 0xdb, 0x21, 0xbd, 0x73, 0x73, 0x5d
};
static const unsigned char peer2_privk_data[32] = {
0xf8, 0x84, 0x19, 0x69, 0x79, 0x13, 0x0d, 0xbd,
0xb1, 0x76, 0xd7, 0x0e, 0x7e, 0x0f, 0xb6, 0xf4,
0x8c, 0x4a, 0x8c, 0x5f, 0xd8, 0x15, 0x09, 0x0a,
0x71, 0x78, 0x74, 0x92, 0x0f, 0x85, 0xc8, 0x43
};
static const unsigned char expected_result[32] = {
0x19, 0x71, 0x26, 0x12, 0x74, 0xb5, 0xb1, 0xce,
0x77, 0xd0, 0x79, 0x24, 0xb6, 0x0a, 0x5c, 0x72,
0x0c, 0xa6, 0x56, 0xc0, 0x11, 0xeb, 0x43, 0x11,
0x94, 0x3b, 0x01, 0x45, 0xca, 0x19, 0xfe, 0x09
};
typedef struct peer_data_st {
const char *name; /* name of peer */
EVP_PKEY *privk; /* privk generated for peer */
unsigned char pubk_data[32]; /* generated pubk to send to other peer */
unsigned char *secret; /* allocated shared secret buffer */
size_t secret_len;
} PEER_DATA;
/*
* Prepare for X25519 key exchange. The public key to be sent to the remote peer
* is put in pubk_data, which should be a 32-byte buffer. Returns 1 on success.
*/
static int keyexch_x25519_before(
OSSL_LIB_CTX *libctx,
const unsigned char *kat_privk_data,
PEER_DATA *local_peer)
{
int rv = 0;
size_t pubk_data_len = 0;
/* Generate or load X25519 key for the peer */
if (kat_privk_data != NULL)
local_peer->privk =
EVP_PKEY_new_raw_private_key_ex(libctx, "X25519", propq,
kat_privk_data,
sizeof(peer1_privk_data));
else
local_peer->privk = EVP_PKEY_Q_keygen(libctx, propq, "X25519");
if (local_peer->privk == NULL) {
fprintf(stderr, "Could not load or generate private key\n");
goto end;
}
/* Get public key corresponding to the private key */
if (EVP_PKEY_get_octet_string_param(local_peer->privk,
OSSL_PKEY_PARAM_PUB_KEY,
local_peer->pubk_data,
sizeof(local_peer->pubk_data),
&pubk_data_len) == 0) {
fprintf(stderr, "EVP_PKEY_get_octet_string_param() failed\n");
goto end;
}
/* X25519 public keys are always 32 bytes */
if (pubk_data_len != 32) {
fprintf(stderr, "EVP_PKEY_get_octet_string_param() "
"yielded wrong length\n");
goto end;
}
rv = 1;
end:
if (rv == 0) {
EVP_PKEY_free(local_peer->privk);
local_peer->privk = NULL;
}
return rv;
}
/*
* Complete X25519 key exchange. remote_peer_pubk_data should be the 32 byte
* public key value received from the remote peer. On success, returns 1 and the
* secret is pointed to by *secret. The caller must free it.
*/
static int keyexch_x25519_after(
OSSL_LIB_CTX *libctx,
int use_kat,
PEER_DATA *local_peer,
const unsigned char *remote_peer_pubk_data)
{
int rv = 0;
EVP_PKEY *remote_peer_pubk = NULL;
EVP_PKEY_CTX *ctx = NULL;
local_peer->secret = NULL;
/* Load public key for remote peer. */
remote_peer_pubk =
EVP_PKEY_new_raw_public_key_ex(libctx, "X25519", propq,
remote_peer_pubk_data, 32);
if (remote_peer_pubk == NULL) {
fprintf(stderr, "EVP_PKEY_new_raw_public_key_ex() failed\n");
goto end;
}
/* Create key exchange context. */
ctx = EVP_PKEY_CTX_new_from_pkey(libctx, local_peer->privk, propq);
if (ctx == NULL) {
fprintf(stderr, "EVP_PKEY_CTX_new_from_pkey() failed\n");
goto end;
}
/* Initialize derivation process. */
if (EVP_PKEY_derive_init(ctx) == 0) {
fprintf(stderr, "EVP_PKEY_derive_init() failed\n");
goto end;
}
/* Configure each peer with the other peer's public key. */
if (EVP_PKEY_derive_set_peer(ctx, remote_peer_pubk) == 0) {
fprintf(stderr, "EVP_PKEY_derive_set_peer() failed\n");
goto end;
}
/* Determine the secret length. */
if (EVP_PKEY_derive(ctx, NULL, &local_peer->secret_len) == 0) {
fprintf(stderr, "EVP_PKEY_derive() failed\n");
goto end;
}
/*
* We are using X25519, so the secret generated will always be 32 bytes.
* However for exposition, the code below demonstrates a generic
* implementation for arbitrary lengths.
*/
if (local_peer->secret_len != 32) { /* unreachable */
fprintf(stderr, "Secret is always 32 bytes for X25519\n");
goto end;
}
/* Allocate memory for shared secrets. */
local_peer->secret = OPENSSL_malloc(local_peer->secret_len);
if (local_peer->secret == NULL) {
fprintf(stderr, "Could not allocate memory for secret\n");
goto end;
}
/* Derive the shared secret. */
if (EVP_PKEY_derive(ctx, local_peer->secret,
&local_peer->secret_len) == 0) {
fprintf(stderr, "EVP_PKEY_derive() failed\n");
goto end;
}
printf("Shared secret (%s):\n", local_peer->name);
BIO_dump_indent_fp(stdout, local_peer->secret, local_peer->secret_len, 2);
putchar('\n');
rv = 1;
end:
EVP_PKEY_CTX_free(ctx);
EVP_PKEY_free(remote_peer_pubk);
if (rv == 0) {
OPENSSL_clear_free(local_peer->secret, local_peer->secret_len);
local_peer->secret = NULL;
}
return rv;
}
static int keyexch_x25519(int use_kat)
{
int rv = 0;
OSSL_LIB_CTX *libctx = NULL;
PEER_DATA peer1 = {"peer 1"}, peer2 = {"peer 2"};
/*
* Each peer generates its private key and sends its public key
* to the other peer. The private key is stored locally for
* later use.
*/
if (keyexch_x25519_before(libctx, use_kat ? peer1_privk_data : NULL,
&peer1) == 0)
return 0;
if (keyexch_x25519_before(libctx, use_kat ? peer2_privk_data : NULL,
&peer2) == 0)
return 0;
/*
* Each peer uses the other peer's public key to perform key exchange.
* After this succeeds, each peer has the same secret in its
* PEER_DATA.
*/
if (keyexch_x25519_after(libctx, use_kat, &peer1, peer2.pubk_data) == 0)
return 0;
if (keyexch_x25519_after(libctx, use_kat, &peer2, peer1.pubk_data) == 0)
return 0;
/*
* Here we demonstrate the secrets are equal for exposition purposes.
*
* Although in practice you will generally not need to compare secrets
* produced through key exchange, if you do compare cryptographic secrets,
* always do so using a constant-time function such as CRYPTO_memcmp, never
* using memcmp(3).
*/
if (CRYPTO_memcmp(peer1.secret, peer2.secret, peer1.secret_len) != 0) {
fprintf(stderr, "Negotiated secrets do not match\n");
goto end;
}
/* If we are doing the KAT, the secret should equal our reference result. */
if (use_kat && CRYPTO_memcmp(peer1.secret, expected_result,
peer1.secret_len) != 0) {
fprintf(stderr, "Did not get expected result\n");
goto end;
}
rv = 1;
end:
/* The secrets are sensitive, so ensure they are erased before freeing. */
OPENSSL_clear_free(peer1.secret, peer1.secret_len);
OPENSSL_clear_free(peer2.secret, peer2.secret_len);
EVP_PKEY_free(peer1.privk);
EVP_PKEY_free(peer2.privk);
OSSL_LIB_CTX_free(libctx);
return rv;
}
int main(int argc, char **argv)
{
/* Test X25519 key exchange with known result. */
printf("Key exchange using known answer (deterministic):\n");
if (keyexch_x25519(1) == 0)
return 1;
/* Test X25519 key exchange with random keys. */
printf("Key exchange using random keys:\n");
if (keyexch_x25519(0) == 0)
return 1;
return 0;
}
