initial just the nostr from the c

examples/schnorr.c

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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 <stdlib.h>
+#include <assert.h>
+#include <string.h>
+
+#include <secp256k1.h>
+#include <secp256k1_extrakeys.h>
+#include <secp256k1_schnorrsig.h>
+
+#include "examples_util.h"
+
+int main(void) {
+    unsigned char msg[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!'};
+    unsigned char msg_hash[32];
+    unsigned char tag[] = {'m', 'y', '_', 'f', 'a', 'n', 'c', 'y', '_', 'p', 'r', 'o', 't', 'o', 'c', 'o', 'l'};
+    unsigned char seckey[32];
+    unsigned char randomize[32];
+    unsigned char auxiliary_rand[32];
+    unsigned char serialized_pubkey[32];
+    unsigned char signature[64];
+    int is_signature_valid, is_signature_valid2;
+    int return_val;
+    secp256k1_xonly_pubkey pubkey;
+    secp256k1_keypair keypair;
+    /* Before we can call actual API functions, we need to create a "context". */
+    secp256k1_context* ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
+    if (!fill_random(randomize, sizeof(randomize))) {
+        printf("Failed to generate randomness\n");
+        return EXIT_FAILURE;
+    }
+    /* 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 (!fill_random(seckey, sizeof(seckey))) {
+        printf("Failed to generate randomness\n");
+        return EXIT_FAILURE;
+    }
+    /* Try to create a keypair with a valid context. This only fails if the
+     * secret key is zero or out of range (greater than secp256k1's order). Note
+     * that the probability of this occurring is negligible with a properly
+     * functioning random number generator. */
+    if (!secp256k1_keypair_create(ctx, &keypair, seckey)) {
+        printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
+        return EXIT_FAILURE;
+    }
+
+    /* Extract the X-only public key from the keypair. We pass NULL for
+     * `pk_parity` as the parity isn't needed for signing or verification.
+     * `secp256k1_keypair_xonly_pub` supports returning the parity for
+     * other use cases such as tests or verifying Taproot tweaks.
+     * This should never fail with a valid context and public key. */
+    return_val = secp256k1_keypair_xonly_pub(ctx, &pubkey, NULL, &keypair);
+    assert(return_val);
+
+    /* Serialize the public key. Should always return 1 for a valid public key. */
+    return_val = secp256k1_xonly_pubkey_serialize(ctx, serialized_pubkey, &pubkey);
+    assert(return_val);
+
+    /*** Signing ***/
+
+    /* Instead of signing (possibly very long) messages directly, we sign a
+     * 32-byte hash of the message in this example.
+     *
+     * We use secp256k1_tagged_sha256 to create this hash. This function expects
+     * a context-specific "tag", which restricts the context in which the signed
+     * messages should be considered valid. For example, if protocol A mandates
+     * to use the tag "my_fancy_protocol" and protocol B mandates to use the tag
+     * "my_boring_protocol", then signed messages from protocol A will never be
+     * valid in protocol B (and vice versa), even if keys are reused across
+     * protocols. This implements "domain separation", which is considered good
+     * practice. It avoids attacks in which users are tricked into signing a
+     * message that has intended consequences in the intended context (e.g.,
+     * protocol A) but would have unintended consequences if it were valid in
+     * some other context (e.g., protocol B). */
+    return_val = secp256k1_tagged_sha256(ctx, msg_hash, tag, sizeof(tag), msg, sizeof(msg));
+    assert(return_val);
+
+    /* Generate 32 bytes of randomness to use with BIP-340 schnorr signing. */
+    if (!fill_random(auxiliary_rand, sizeof(auxiliary_rand))) {
+        printf("Failed to generate randomness\n");
+        return EXIT_FAILURE;
+    }
+
+    /* Generate a Schnorr signature.
+     *
+     * We use the secp256k1_schnorrsig_sign32 function that provides a simple
+     * interface for signing 32-byte messages (which in our case is a hash of
+     * the actual message). BIP-340 recommends passing 32 bytes of randomness
+     * to the signing function to improve security against side-channel attacks.
+     * Signing with a valid context, a 32-byte message, a verified keypair, and
+     * any 32 bytes of auxiliary random data should never fail. */
+    return_val = secp256k1_schnorrsig_sign32(ctx, signature, msg_hash, &keypair, auxiliary_rand);
+    assert(return_val);
+
+    /*** Verification ***/
+
+    /* Deserialize the public key. This will return 0 if the public key can't
+     * be parsed correctly */
+    if (!secp256k1_xonly_pubkey_parse(ctx, &pubkey, serialized_pubkey)) {
+        printf("Failed parsing the public key\n");
+        return EXIT_FAILURE;
+    }
+
+    /* Compute the tagged hash on the received messages using the same tag as the signer. */
+    return_val = secp256k1_tagged_sha256(ctx, msg_hash, tag, sizeof(tag), msg, sizeof(msg));
+    assert(return_val);
+
+    /* Verify a signature. This will return 1 if it's valid and 0 if it's not. */
+    is_signature_valid = secp256k1_schnorrsig_verify(ctx, signature, msg_hash, 32, &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(serialized_pubkey, sizeof(serialized_pubkey));
+    printf("Signature: ");
+    print_hex(signature, sizeof(signature));
+
+    /* This will clear everything from the context and free the memory */
+    secp256k1_context_destroy(ctx);
+
+    /* Bonus example: if all we need is signature verification (and no key
+       generation or signing), we don't need to use a context created via
+       secp256k1_context_create(). We can simply use the static (i.e., global)
+       context secp256k1_context_static. See its description in
+       include/secp256k1.h for details. */
+    is_signature_valid2 = secp256k1_schnorrsig_verify(secp256k1_context_static,
+                                                      signature, msg_hash, 32, &pubkey);
+    assert(is_signature_valid2 == is_signature_valid);
+
+    /* 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.
+     *
+     * Here we are preventing these writes from being optimized out, as any good compiler
+     * will remove any writes that aren't used. */
+    secure_erase(seckey, sizeof(seckey));
+    return EXIT_SUCCESS;
+}