From fee7d4bf9e4ea316ea4ff3151bbe52bec1f0745c Mon Sep 17 00:00:00 2001 From: Elichai Turkel Date: Thu, 30 Apr 2020 13:37:52 +0300 Subject: [PATCH] Add an ECDSA signing and verifying example Co-authored-by: Jonas Nick --- examples/ecdsa.c | 137 ++++++++++++++++++++++++++++++++++++++++++++++ examples/random.h | 73 ++++++++++++++++++++++++ 2 files changed, 210 insertions(+) create mode 100644 examples/ecdsa.c create mode 100644 examples/random.h diff --git a/examples/ecdsa.c b/examples/ecdsa.c new file mode 100644 index 000000000..434c856ba --- /dev/null +++ b/examples/ecdsa.c @@ -0,0 +1,137 @@ +/************************************************************************* + * 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 +#include +#include + +#include + +#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; +} diff --git a/examples/random.h b/examples/random.h new file mode 100644 index 000000000..439226f09 --- /dev/null +++ b/examples/random.h @@ -0,0 +1,73 @@ +/************************************************************************* + * Copyright (c) 2020-2021 Elichai Turkel * + * Distributed under the CC0 software license, see the accompanying file * + * EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 * + *************************************************************************/ + +/* + * This file is an attempt at collecting best practice methods for obtaining randomness with different operating systems. + * It may be out-of-date. Consult the documentation of the operating system before considering to use the methods below. + * + * Platform randomness sources: + * Linux -> `getrandom(2)`(`sys/random.h`), if not available `/dev/urandom` should be used. http://man7.org/linux/man-pages/man2/getrandom.2.html, https://linux.die.net/man/4/urandom + * macOS -> `getentropy(2)`(`sys/random.h`), if not available `/dev/urandom` should be used. https://www.unix.com/man-page/mojave/2/getentropy, https://opensource.apple.com/source/xnu/xnu-517.12.7/bsd/man/man4/random.4.auto.html + * FreeBSD -> `getrandom(2)`(`sys/random.h`), if not available `kern.arandom` should be used. https://www.freebsd.org/cgi/man.cgi?query=getrandom, https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4 + * OpenBSD -> `getentropy(2)`(`unistd.h`), if not available `/dev/urandom` should be used. https://man.openbsd.org/getentropy, https://man.openbsd.org/urandom + * Windows -> `BCryptGenRandom`(`bcrypt.h`). https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom + */ + +#if defined(_WIN32) +#include +#include +#include +#elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) +#include +#elif defined(__OpenBSD__) +#include +#else +#error "Couldn't identify the OS" +#endif + +#include +#include +#include + + +/* Returns 1 on success, and 0 on failure. */ +static int fill_random(unsigned char* data, size_t size) { +#if defined(_WIN32) + NTSTATUS res = BCryptGenRandom(NULL, data, size, BCRYPT_USE_SYSTEM_PREFERRED_RNG); + if (res != STATUS_SUCCESS || size > ULONG_MAX) { + return 0; + } else { + return 1; + } +#elif defined(__linux__) || defined(__FreeBSD__) + /* If `getrandom(2)` is not available you should fallback to /dev/urandom */ + ssize_t res = getrandom(data, size, 0); + if (res < 0 || (size_t)res != size ) { + return 0; + } else { + return 1; + } +#elif defined(__APPLE__) || defined(__OpenBSD__) + /* If `getentropy(2)` is not available you should fallback to either + * `SecRandomCopyBytes` or /dev/urandom */ + int res = getentropy(data, size); + if (res == 0) { + return 1; + } else { + return 0; + } +#endif + return 0; +} + +static void print_hex(unsigned char* data, size_t size) { + size_t i; + printf("0x"); + for (i = 0; i < size; i++) { + printf("%02x", data[i]); + } + printf("\n"); +}