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/*************************************************************************
* Written in 2020-2022 by Elichai Turkel *
* To the extent possible under law, the author(s) have dedicated all *
* copyright and related and neighboring rights to the software in this *
* file to the public domain worldwide. This software is distributed *
* without any warranty. For the CC0 Public Domain Dedication, see *
* EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
*************************************************************************/
#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <secp256k1.h>
#include "random.h"
int main(void) {
/* Instead of signing the message directly, we must sign a 32-byte hash.
* Here the message is "Hello, world!" and the hash function was SHA-256.
* An actual implementation should just call SHA-256, but this example
* hardcodes the output to avoid depending on an additional library.
* See https://bitcoin.stackexchange.com/questions/81115/if-someone-wanted-to-pretend-to-be-satoshi-by-posting-a-fake-signature-to-defrau/81116#81116 */
unsigned char msg_hash[32] = {
0x31, 0x5F, 0x5B, 0xDB, 0x76, 0xD0, 0x78, 0xC4,
0x3B, 0x8A, 0xC0, 0x06, 0x4E, 0x4A, 0x01, 0x64,
0x61, 0x2B, 0x1F, 0xCE, 0x77, 0xC8, 0x69, 0x34,
0x5B, 0xFC, 0x94, 0xC7, 0x58, 0x94, 0xED, 0xD3,
};
unsigned char seckey[32];
unsigned char randomize[32];
unsigned char compressed_pubkey[33];
unsigned char serialized_signature[64];
size_t len;
int is_signature_valid;
int return_val;
secp256k1_pubkey pubkey;
secp256k1_ecdsa_signature sig;
/* The specification in secp256k1.h states that `secp256k1_ec_pubkey_create` needs
* a context object initialized for signing and `secp256k1_ecdsa_verify` needs
* a context initialized for verification, which is why we create a context
* for both signing and verification with the SECP256K1_CONTEXT_SIGN and
* SECP256K1_CONTEXT_VERIFY flags. */
secp256k1_context* ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_VERIFY);
if (!fill_random(randomize, sizeof(randomize))) {
printf("Failed to generate randomness\n");
return 1;
}
/* Randomizing the context is recommended to protect against side-channel
* leakage See `secp256k1_context_randomize` in secp256k1.h for more
* information about it. This should never fail. */
return_val = secp256k1_context_randomize(ctx, randomize);
assert(return_val);
/*** Key Generation ***/
/* If the secret key is zero or out of range (bigger than secp256k1's
* order), we try to sample a new key. Note that the probability of this
* happening is negligible. */
while (1) {
if (!fill_random(seckey, sizeof(seckey))) {
printf("Failed to generate randomness\n");
return 1;
}
if (secp256k1_ec_seckey_verify(ctx, seckey)) {
break;
}
}
/* Public key creation using a valid context with a verified secret key should never fail */
return_val = secp256k1_ec_pubkey_create(ctx, &pubkey, seckey);
assert(return_val);
/* Serialize the pubkey in a compressed form(33 bytes). Should always return 1. */
len = sizeof(compressed_pubkey);
return_val = secp256k1_ec_pubkey_serialize(ctx, compressed_pubkey, &len, &pubkey, SECP256K1_EC_COMPRESSED);
assert(return_val);
/* Should be the same size as the size of the output, because we passed a 33 byte array. */
assert(len == sizeof(compressed_pubkey));
/*** Signing ***/
/* Generate an ECDSA signature `noncefp` and `ndata` allows you to pass a
* custom nonce function, passing `NULL` will use the RFC-6979 safe default.
* Signing with a valid context, verified secret key
* and the default nonce function should never fail. */
return_val = secp256k1_ecdsa_sign(ctx, &sig, msg_hash, seckey, NULL, NULL);
assert(return_val);
/* Serialize the signature in a compact form. Should always return 1
* according to the documentation in secp256k1.h. */
return_val = secp256k1_ecdsa_signature_serialize_compact(ctx, serialized_signature, &sig);
assert(return_val);
/*** Verification ***/
/* Deserialize the signature. This will return 0 if the signature can't be parsed correctly. */
if (!secp256k1_ecdsa_signature_parse_compact(ctx, &sig, serialized_signature)) {
printf("Failed parsing the signature\n");
return 1;
}
/* Deserialize the public key. This will return 0 if the public key can't be parsed correctly. */
if (!secp256k1_ec_pubkey_parse(ctx, &pubkey, compressed_pubkey, sizeof(compressed_pubkey))) {
printf("Failed parsing the public key\n");
return 1;
}
/* Verify a signature. This will return 1 if it's valid and 0 if it's not. */
is_signature_valid = secp256k1_ecdsa_verify(ctx, &sig, msg_hash, &pubkey);
printf("Is the signature valid? %s\n", is_signature_valid ? "true" : "false");
printf("Secret Key: ");
print_hex(seckey, sizeof(seckey));
printf("Public Key: ");
print_hex(compressed_pubkey, sizeof(compressed_pubkey));
printf("Signature: ");
print_hex(serialized_signature, sizeof(serialized_signature));
/* This will clear everything from the context and free the memory */
secp256k1_context_destroy(ctx);
/* It's best practice to try to clear secrets from memory after using them.
* This is done because some bugs can allow an attacker to leak memory, for
* example through "out of bounds" array access (see Heartbleed), Or the OS
* swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
*
* TODO: Prevent these writes from being optimized out, as any good compiler
* will remove any writes that aren't used. */
memset(seckey, 0, sizeof(seckey));
return 0;
}
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