Add an ECDSA signing and verifying example

Co-authored-by: Jonas Nick <jonasd.nick@gmail.com>

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 <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;
+}

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 <windows.h>
+#include <ntstatus.h>
+#include <bcrypt.h>
+#elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__)
+#include <sys/random.h>
+#elif defined(__OpenBSD__)
+#include <unistd.h>
+#else
+#error "Couldn't identify the OS"
+#endif
+
+#include <stddef.h>
+#include <limits.h>
+#include <stdio.h>
+
+
+/* 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");
+}