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_extrakeys.h>
17 #include <secp256k1_schnorrsig.h>
18 19 #include "examples_util.h"
20 21 int main(void) {
22 unsigned char msg[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!'};
23 unsigned char msg_hash[32];
24 unsigned char tag[] = {'m', 'y', '_', 'f', 'a', 'n', 'c', 'y', '_', 'p', 'r', 'o', 't', 'o', 'c', 'o', 'l'};
25 unsigned char seckey[32];
26 unsigned char randomize[32];
27 unsigned char auxiliary_rand[32];
28 unsigned char serialized_pubkey[32];
29 unsigned char signature[64];
30 int is_signature_valid, is_signature_valid2;
31 int return_val;
32 secp256k1_xonly_pubkey pubkey;
33 secp256k1_keypair keypair;
34 /* Before we can call actual API functions, we need to create a "context". */
35 secp256k1_context* ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
36 if (!fill_random(randomize, sizeof(randomize))) {
37 printf("Failed to generate randomness\n");
38 return EXIT_FAILURE;
39 }
40 /* Randomizing the context is recommended to protect against side-channel
41 * leakage See `secp256k1_context_randomize` in secp256k1.h for more
42 * information about it. This should never fail. */
43 return_val = secp256k1_context_randomize(ctx, randomize);
44 assert(return_val);
45 46 /*** Key Generation ***/
47 if (!fill_random(seckey, sizeof(seckey))) {
48 printf("Failed to generate randomness\n");
49 return EXIT_FAILURE;
50 }
51 /* Try to create a keypair with a valid context. This only fails if the
52 * secret key is zero or out of range (greater than secp256k1's order). Note
53 * that the probability of this occurring is negligible with a properly
54 * functioning random number generator. */
55 if (!secp256k1_keypair_create(ctx, &keypair, seckey)) {
56 printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
57 return EXIT_FAILURE;
58 }
59 60 /* Extract the X-only public key from the keypair. We pass NULL for
61 * `pk_parity` as the parity isn't needed for signing or verification.
62 * `secp256k1_keypair_xonly_pub` supports returning the parity for
63 * other use cases such as tests or verifying Taproot tweaks.
64 * This should never fail with a valid context and public key. */
65 return_val = secp256k1_keypair_xonly_pub(ctx, &pubkey, NULL, &keypair);
66 assert(return_val);
67 68 /* Serialize the public key. Should always return 1 for a valid public key. */
69 return_val = secp256k1_xonly_pubkey_serialize(ctx, serialized_pubkey, &pubkey);
70 assert(return_val);
71 72 /*** Signing ***/
73 74 /* Instead of signing (possibly very long) messages directly, we sign a
75 * 32-byte hash of the message in this example.
76 *
77 * We use secp256k1_tagged_sha256 to create this hash. This function expects
78 * a context-specific "tag", which restricts the context in which the signed
79 * messages should be considered valid. For example, if protocol A mandates
80 * to use the tag "my_fancy_protocol" and protocol B mandates to use the tag
81 * "my_boring_protocol", then signed messages from protocol A will never be
82 * valid in protocol B (and vice versa), even if keys are reused across
83 * protocols. This implements "domain separation", which is considered good
84 * practice. It avoids attacks in which users are tricked into signing a
85 * message that has intended consequences in the intended context (e.g.,
86 * protocol A) but would have unintended consequences if it were valid in
87 * some other context (e.g., protocol B). */
88 return_val = secp256k1_tagged_sha256(ctx, msg_hash, tag, sizeof(tag), msg, sizeof(msg));
89 assert(return_val);
90 91 /* Generate 32 bytes of randomness to use with BIP-340 schnorr signing. */
92 if (!fill_random(auxiliary_rand, sizeof(auxiliary_rand))) {
93 printf("Failed to generate randomness\n");
94 return EXIT_FAILURE;
95 }
96 97 /* Generate a Schnorr signature.
98 *
99 * We use the secp256k1_schnorrsig_sign32 function that provides a simple
100 * interface for signing 32-byte messages (which in our case is a hash of
101 * the actual message). BIP-340 recommends passing 32 bytes of randomness
102 * to the signing function to improve security against side-channel attacks.
103 * Signing with a valid context, a 32-byte message, a verified keypair, and
104 * any 32 bytes of auxiliary random data should never fail. */
105 return_val = secp256k1_schnorrsig_sign32(ctx, signature, msg_hash, &keypair, auxiliary_rand);
106 assert(return_val);
107 108 /*** Verification ***/
109 110 /* Deserialize the public key. This will return 0 if the public key can't
111 * be parsed correctly */
112 if (!secp256k1_xonly_pubkey_parse(ctx, &pubkey, serialized_pubkey)) {
113 printf("Failed parsing the public key\n");
114 return EXIT_FAILURE;
115 }
116 117 /* Compute the tagged hash on the received messages using the same tag as the signer. */
118 return_val = secp256k1_tagged_sha256(ctx, msg_hash, tag, sizeof(tag), msg, sizeof(msg));
119 assert(return_val);
120 121 /* Verify a signature. This will return 1 if it's valid and 0 if it's not. */
122 is_signature_valid = secp256k1_schnorrsig_verify(ctx, signature, msg_hash, 32, &pubkey);
123 124 125 printf("Is the signature valid? %s\n", is_signature_valid ? "true" : "false");
126 printf("Secret Key: ");
127 print_hex(seckey, sizeof(seckey));
128 printf("Public Key: ");
129 print_hex(serialized_pubkey, sizeof(serialized_pubkey));
130 printf("Signature: ");
131 print_hex(signature, sizeof(signature));
132 133 /* This will clear everything from the context and free the memory */
134 secp256k1_context_destroy(ctx);
135 136 /* Bonus example: if all we need is signature verification (and no key
137 generation or signing), we don't need to use a context created via
138 secp256k1_context_create(). We can simply use the static (i.e., global)
139 context secp256k1_context_static. See its description in
140 include/secp256k1.h for details. */
141 is_signature_valid2 = secp256k1_schnorrsig_verify(secp256k1_context_static,
142 signature, msg_hash, 32, &pubkey);
143 assert(is_signature_valid2 == is_signature_valid);
144 145 /* It's best practice to try to clear secrets from memory after using them.
146 * This is done because some bugs can allow an attacker to leak memory, for
147 * example through "out of bounds" array access (see Heartbleed), or the OS
148 * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
149 *
150 * Here we are preventing these writes from being optimized out, as any good compiler
151 * will remove any writes that aren't used. */
152 secure_erase(seckey, sizeof(seckey));
153 return EXIT_SUCCESS;
154 }
155