diff --git a/examples/schnorr.c b/examples/schnorr.c new file mode 100644 index 0000000..a3533b6 --- /dev/null +++ b/examples/schnorr.c @@ -0,0 +1,136 @@ +/************************************************************************* + * 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 +#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. */ + 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 auxiliary_rand[32]; + unsigned char serialized_pubkey[32]; + unsigned char signature[64]; + int is_signature_valid; + int return_val; + secp256k1_xonly_pubkey pubkey; + secp256k1_keypair keypair; + /* The specification in secp256k1_extrakeys.h states that `secp256k1_keypair_create` + * needs a context object initialized for signing. And in secp256k1_schnorrsig.h + * they state that `secp256k1_schnorrsig_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; + } + /* Try to create a keypair with a valid context, it should only fail if + * the secret key is zero or out of range. */ + if (secp256k1_keypair_create(ctx, &keypair, seckey)) { + break; + } + } + + /* 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 ***/ + + /* 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 1; + } + + /* Generate a Schnorr signature `noncefp` and `ndata` allows you to pass a + * custom nonce function, passing `NULL` will use the BIP-340 safe default. + * BIP-340 recommends passing 32 bytes of randomness to the nonce function to + * improve security against side-channel attacks. Signing with a valid + * context, verified keypair and the default nonce function should never + * fail. */ + return_val = secp256k1_schnorrsig_sign(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 1; + } + + /* 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); + + /* 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; +}