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  }