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 17 #include "examples_util.h"
18 19 int main(void) {
20 /* Instead of signing the message directly, we must sign a 32-byte hash.
21 * Here the message is "Hello, world!" and the hash function was SHA-256.
22 * An actual implementation should just call SHA-256, but this example
23 * hardcodes the output to avoid depending on an additional library.
24 * See https://bitcoin.stackexchange.com/questions/81115/if-someone-wanted-to-pretend-to-be-satoshi-by-posting-a-fake-signature-to-defrau/81116#81116 */
25 unsigned char msg_hash[32] = {
26 0x31, 0x5F, 0x5B, 0xDB, 0x76, 0xD0, 0x78, 0xC4,
27 0x3B, 0x8A, 0xC0, 0x06, 0x4E, 0x4A, 0x01, 0x64,
28 0x61, 0x2B, 0x1F, 0xCE, 0x77, 0xC8, 0x69, 0x34,
29 0x5B, 0xFC, 0x94, 0xC7, 0x58, 0x94, 0xED, 0xD3,
30 };
31 unsigned char seckey[32];
32 unsigned char randomize[32];
33 unsigned char compressed_pubkey[33];
34 unsigned char serialized_signature[64];
35 size_t len;
36 int is_signature_valid, is_signature_valid2;
37 int return_val;
38 secp256k1_pubkey pubkey;
39 secp256k1_ecdsa_signature sig;
40 /* Before we can call actual API functions, we need to create a "context". */
41 secp256k1_context* ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
42 if (!fill_random(randomize, sizeof(randomize))) {
43 printf("Failed to generate randomness\n");
44 return EXIT_FAILURE;
45 }
46 /* Randomizing the context is recommended to protect against side-channel
47 * leakage See `secp256k1_context_randomize` in secp256k1.h for more
48 * information about it. This should never fail. */
49 return_val = secp256k1_context_randomize(ctx, randomize);
50 assert(return_val);
51 52 /*** Key Generation ***/
53 if (!fill_random(seckey, sizeof(seckey))) {
54 printf("Failed to generate randomness\n");
55 return EXIT_FAILURE;
56 }
57 /* If the secret key is zero or out of range (greater than secp256k1's
58 * order), we fail. Note that the probability of this occurring is negligible
59 * with a properly functioning random number generator. */
60 if (!secp256k1_ec_seckey_verify(ctx, seckey)) {
61 printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
62 return EXIT_FAILURE;
63 }
64 65 /* Public key creation using a valid context with a verified secret key should never fail */
66 return_val = secp256k1_ec_pubkey_create(ctx, &pubkey, seckey);
67 assert(return_val);
68 69 /* Serialize the pubkey in a compressed form(33 bytes). Should always return 1. */
70 len = sizeof(compressed_pubkey);
71 return_val = secp256k1_ec_pubkey_serialize(ctx, compressed_pubkey, &len, &pubkey, SECP256K1_EC_COMPRESSED);
72 assert(return_val);
73 /* Should be the same size as the size of the output, because we passed a 33 byte array. */
74 assert(len == sizeof(compressed_pubkey));
75 76 /*** Signing ***/
77 78 /* Generate an ECDSA signature `noncefp` and `ndata` allows you to pass a
79 * custom nonce function, passing `NULL` will use the RFC-6979 safe default.
80 * Signing with a valid context, verified secret key
81 * and the default nonce function should never fail. */
82 return_val = secp256k1_ecdsa_sign(ctx, &sig, msg_hash, seckey, NULL, NULL);
83 assert(return_val);
84 85 /* Serialize the signature in a compact form. Should always return 1
86 * according to the documentation in secp256k1.h. */
87 return_val = secp256k1_ecdsa_signature_serialize_compact(ctx, serialized_signature, &sig);
88 assert(return_val);
89 90 91 /*** Verification ***/
92 93 /* Deserialize the signature. This will return 0 if the signature can't be parsed correctly. */
94 if (!secp256k1_ecdsa_signature_parse_compact(ctx, &sig, serialized_signature)) {
95 printf("Failed parsing the signature\n");
96 return EXIT_FAILURE;
97 }
98 99 /* Deserialize the public key. This will return 0 if the public key can't be parsed correctly. */
100 if (!secp256k1_ec_pubkey_parse(ctx, &pubkey, compressed_pubkey, sizeof(compressed_pubkey))) {
101 printf("Failed parsing the public key\n");
102 return EXIT_FAILURE;
103 }
104 105 /* Verify a signature. This will return 1 if it's valid and 0 if it's not. */
106 is_signature_valid = secp256k1_ecdsa_verify(ctx, &sig, msg_hash, &pubkey);
107 108 printf("Is the signature valid? %s\n", is_signature_valid ? "true" : "false");
109 printf("Secret Key: ");
110 print_hex(seckey, sizeof(seckey));
111 printf("Public Key: ");
112 print_hex(compressed_pubkey, sizeof(compressed_pubkey));
113 printf("Signature: ");
114 print_hex(serialized_signature, sizeof(serialized_signature));
115 116 /* This will clear everything from the context and free the memory */
117 secp256k1_context_destroy(ctx);
118 119 /* Bonus example: if all we need is signature verification (and no key
120 generation or signing), we don't need to use a context created via
121 secp256k1_context_create(). We can simply use the static (i.e., global)
122 context secp256k1_context_static. See its description in
123 include/secp256k1.h for details. */
124 is_signature_valid2 = secp256k1_ecdsa_verify(secp256k1_context_static,
125 &sig, msg_hash, &pubkey);
126 assert(is_signature_valid2 == is_signature_valid);
127 128 /* It's best practice to try to clear secrets from memory after using them.
129 * This is done because some bugs can allow an attacker to leak memory, for
130 * example through "out of bounds" array access (see Heartbleed), or the OS
131 * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
132 *
133 * Here we are preventing these writes from being optimized out, as any good compiler
134 * will remove any writes that aren't used. */
135 secure_erase(seckey, sizeof(seckey));
136 137 return EXIT_SUCCESS;
138 }
139