/* * 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 #include #include #include /* * 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 ret = 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; } ret = 1; end: if (ret == 0) { EVP_PKEY_free(local_peer->privk); local_peer->privk = NULL; } return ret; } /* * 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 ret = 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'); ret = 1; end: EVP_PKEY_CTX_free(ctx); EVP_PKEY_free(remote_peer_pubk); if (ret == 0) { OPENSSL_clear_free(local_peer->secret, local_peer->secret_len); local_peer->secret = NULL; } return ret; } static int keyexch_x25519(int use_kat) { int ret = 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; } ret = 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 ret; } 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 EXIT_FAILURE; /* Test X25519 key exchange with random keys. */ printf("Key exchange using random keys:\n"); if (keyexch_x25519(0) == 0) return EXIT_FAILURE; return EXIT_SUCCESS; }