1 /*************************************************************************
2 * Written in 2020-2022 by Elichai Turkel *
3 * To the extent possible under law, the author(s) have dedicated all *
4 * copyright and related and neighboring rights to the software in this *
5 * file to the public domain worldwide. This software is distributed *
6 * without any warranty. For the CC0 Public Domain Dedication, see *
7 * EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
8 *************************************************************************/
9 10 #include <stdio.h>
11 #include <stdlib.h>
12 #include <assert.h>
13 #include <string.h>
14 15 #include <secp256k1.h>
16 #include <secp256k1_ecdh.h>
17 18 #include "examples_util.h"
19 20 int main(void) {
21 unsigned char seckey1[32];
22 unsigned char seckey2[32];
23 unsigned char compressed_pubkey1[33];
24 unsigned char compressed_pubkey2[33];
25 unsigned char shared_secret1[32];
26 unsigned char shared_secret2[32];
27 unsigned char randomize[32];
28 int return_val;
29 size_t len;
30 secp256k1_pubkey pubkey1;
31 secp256k1_pubkey pubkey2;
32 33 /* Before we can call actual API functions, we need to create a "context". */
34 secp256k1_context* ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
35 if (!fill_random(randomize, sizeof(randomize))) {
36 printf("Failed to generate randomness\n");
37 return EXIT_FAILURE;
38 }
39 /* Randomizing the context is recommended to protect against side-channel
40 * leakage See `secp256k1_context_randomize` in secp256k1.h for more
41 * information about it. This should never fail. */
42 return_val = secp256k1_context_randomize(ctx, randomize);
43 assert(return_val);
44 45 /*** Key Generation ***/
46 if (!fill_random(seckey1, sizeof(seckey1)) || !fill_random(seckey2, sizeof(seckey2))) {
47 printf("Failed to generate randomness\n");
48 return EXIT_FAILURE;
49 }
50 /* If the secret key is zero or out of range (greater than secp256k1's
51 * order), we fail. Note that the probability of this occurring is negligible
52 * with a properly functioning random number generator. */
53 if (!secp256k1_ec_seckey_verify(ctx, seckey1) || !secp256k1_ec_seckey_verify(ctx, seckey2)) {
54 printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
55 return EXIT_FAILURE;
56 }
57 58 /* Public key creation using a valid context with a verified secret key should never fail */
59 return_val = secp256k1_ec_pubkey_create(ctx, &pubkey1, seckey1);
60 assert(return_val);
61 return_val = secp256k1_ec_pubkey_create(ctx, &pubkey2, seckey2);
62 assert(return_val);
63 64 /* Serialize pubkey1 in a compressed form (33 bytes), should always return 1 */
65 len = sizeof(compressed_pubkey1);
66 return_val = secp256k1_ec_pubkey_serialize(ctx, compressed_pubkey1, &len, &pubkey1, SECP256K1_EC_COMPRESSED);
67 assert(return_val);
68 /* Should be the same size as the size of the output, because we passed a 33 byte array. */
69 assert(len == sizeof(compressed_pubkey1));
70 71 /* Serialize pubkey2 in a compressed form (33 bytes) */
72 len = sizeof(compressed_pubkey2);
73 return_val = secp256k1_ec_pubkey_serialize(ctx, compressed_pubkey2, &len, &pubkey2, SECP256K1_EC_COMPRESSED);
74 assert(return_val);
75 /* Should be the same size as the size of the output, because we passed a 33 byte array. */
76 assert(len == sizeof(compressed_pubkey2));
77 78 /*** Creating the shared secret ***/
79 80 /* Perform ECDH with seckey1 and pubkey2. Should never fail with a verified
81 * seckey and valid pubkey */
82 return_val = secp256k1_ecdh(ctx, shared_secret1, &pubkey2, seckey1, NULL, NULL);
83 assert(return_val);
84 85 /* Perform ECDH with seckey2 and pubkey1. Should never fail with a verified
86 * seckey and valid pubkey */
87 return_val = secp256k1_ecdh(ctx, shared_secret2, &pubkey1, seckey2, NULL, NULL);
88 assert(return_val);
89 90 /* Both parties should end up with the same shared secret */
91 return_val = memcmp(shared_secret1, shared_secret2, sizeof(shared_secret1));
92 assert(return_val == 0);
93 94 printf("Secret Key1: ");
95 print_hex(seckey1, sizeof(seckey1));
96 printf("Compressed Pubkey1: ");
97 print_hex(compressed_pubkey1, sizeof(compressed_pubkey1));
98 printf("\nSecret Key2: ");
99 print_hex(seckey2, sizeof(seckey2));
100 printf("Compressed Pubkey2: ");
101 print_hex(compressed_pubkey2, sizeof(compressed_pubkey2));
102 printf("\nShared Secret: ");
103 print_hex(shared_secret1, sizeof(shared_secret1));
104 105 /* This will clear everything from the context and free the memory */
106 secp256k1_context_destroy(ctx);
107 108 /* It's best practice to try to clear secrets from memory after using them.
109 * This is done because some bugs can allow an attacker to leak memory, for
110 * example through "out of bounds" array access (see Heartbleed), or the OS
111 * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
112 *
113 * Here we are preventing these writes from being optimized out, as any good compiler
114 * will remove any writes that aren't used. */
115 secure_erase(seckey1, sizeof(seckey1));
116 secure_erase(seckey2, sizeof(seckey2));
117 secure_erase(shared_secret1, sizeof(shared_secret1));
118 secure_erase(shared_secret2, sizeof(shared_secret2));
119 120 return EXIT_SUCCESS;
121 }
122