Inside Bitcoin

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
21int 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