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frost trusted dealer: add example file
This commit adds an example file to demonstrate how to use the module.
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‎.gitignore

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@@ -66,6 +66,7 @@ libsecp256k1.pc
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contrib/gh-pr-create.sh
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musig_example
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frost_example
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### CMake
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/CMakeUserPresets.json

‎Makefile.am

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@@ -195,6 +195,17 @@ musig_example_LDFLAGS += -lbcrypt
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endif
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TESTS += musig_example
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endif
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if ENABLE_MODULE_FROST
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noinst_PROGRAMS += frost_example
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frost_example_SOURCES = examples/frost.c
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frost_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
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frost_example_LDADD = libsecp256k1.la
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frost_example_LDFLAGS = -static
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if BUILD_WINDOWS
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frost_example_LDFLAGS += -lbcrypt
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endif
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TESTS += frost_example
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endif
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endif
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### Precomputed tables

‎examples/frost.c

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/***********************************************************************
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* Copyright (c) 2021-2023 Jesse Posner *
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* Distributed under the MIT software license, see the accompanying *
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* file COPYING or https://www.opensource.org/licenses/mit-license.php.*
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***********************************************************************/
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/**
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* This file demonstrates how to use the FROST module to create a threshold
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* signature. Additionally, see the documentation in include/secp256k1_frost.h.
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*/
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#include <stdio.h>
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#include <assert.h>
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#include <string.h>
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#include <secp256k1.h>
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#include <secp256k1_schnorrsig.h>
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#include <secp256k1_frost.h>
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#include "examples_util.h"
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/* Number of public keys involved in creating the aggregate signature */
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#define N_SIGNERS 5
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/* Threshold required in creating the aggregate signature */
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#define THRESHOLD 3
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struct signer_secrets {
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secp256k1_frost_share share;
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secp256k1_frost_secnonce secnonce;
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};
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struct signer {
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secp256k1_pubkey pubshare;
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secp256k1_frost_pubnonce pubnonce;
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secp256k1_frost_session session;
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secp256k1_frost_partial_sig partial_sig;
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};
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/* Create shares and coefficient commitments */
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int create_shares(const secp256k1_context* ctx, struct signer_secrets *signer_secrets, struct signer *signers, secp256k1_xonly_pubkey *pk) {
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int i;
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secp256k1_frost_share shares[N_SIGNERS];
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secp256k1_pubkey pubshares[N_SIGNERS];
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unsigned char seed[32];
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if (!fill_random(seed, sizeof(seed))) {
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return 0;
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}
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if (!secp256k1_frost_shares_trusted_gen(ctx, shares, pubshares, pk, seed, THRESHOLD, N_SIGNERS)) {
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return 0;
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}
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for (i = 0; i < N_SIGNERS; i++) {
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signer_secrets[i].share = shares[i];
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signers[i].pubshare = pubshares[i];
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}
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return 1;
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}
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/* Tweak the pubkey corresponding to the provided tweak cache, update the cache
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* and return the tweaked aggregate pk. */
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int tweak(const secp256k1_context* ctx, secp256k1_xonly_pubkey *pk, secp256k1_frost_tweak_cache *cache) {
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secp256k1_pubkey output_pk;
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unsigned char ordinary_tweak[32] = "this could be a BIP32 tweak....";
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unsigned char xonly_tweak[32] = "this could be a taproot tweak..";
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if (!secp256k1_frost_pubkey_tweak(ctx, cache, pk)) {
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return 0;
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}
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/* Ordinary tweaking which, for example, allows deriving multiple child
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* public keys from a single aggregate key using BIP32 */
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if (!secp256k1_frost_pubkey_ec_tweak_add(ctx, NULL, cache, ordinary_tweak)) {
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return 0;
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}
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/* If one is not interested in signing, the same output_pk can be obtained
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* by calling `secp256k1_frost_pubkey_get` right after key aggregation to
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* get the full pubkey and then call `secp256k1_ec_pubkey_tweak_add`. */
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/* Xonly tweaking which, for example, allows creating taproot commitments */
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if (!secp256k1_frost_pubkey_xonly_tweak_add(ctx, &output_pk, cache, xonly_tweak)) {
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return 0;
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}
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/* Note that if we wouldn't care about signing, we can arrive at the same
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* output_pk by providing the untweaked public key to
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* `secp256k1_xonly_pubkey_tweak_add` (after converting it to an xonly pubkey
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* if necessary with `secp256k1_xonly_pubkey_from_pubkey`). */
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/* Now we convert the output_pk to an xonly pubkey to allow to later verify
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* the Schnorr signature against it. For this purpose we can ignore the
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* `pk_parity` output argument; we would need it if we would have to open
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* the taproot commitment. */
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if (!secp256k1_xonly_pubkey_from_pubkey(ctx, pk, NULL, &output_pk)) {
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return 0;
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}
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return 1;
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}
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/* Sign a message hash with the given threshold and aggregate shares and store
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* the result in sig */
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int sign(const secp256k1_context* ctx, struct signer_secrets *signer_secrets, struct signer *signer, const unsigned char* msg32, secp256k1_xonly_pubkey *pk, unsigned char *sig64, const secp256k1_frost_tweak_cache *cache) {
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int i;
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size_t signer_id = 0;
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int signers[THRESHOLD];
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int is_signer[N_SIGNERS];
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const secp256k1_frost_pubnonce *pubnonces[THRESHOLD];
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size_t ids[THRESHOLD];
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const secp256k1_frost_partial_sig *partial_sigs[THRESHOLD];
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for (i = 0; i < N_SIGNERS; i++) {
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unsigned char session_id[32];
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/* Create random session ID. It is absolutely necessary that the session ID
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* is unique for every call of secp256k1_frost_nonce_gen. Otherwise
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* it's trivial for an attacker to extract the secret key! */
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if (!fill_random(session_id, sizeof(session_id))) {
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return 0;
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}
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/* Initialize session and create secret nonce for signing and public
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* nonce to send to the other signers. */
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if (!secp256k1_frost_nonce_gen(ctx, &signer_secrets[i].secnonce, &signer[i].pubnonce, session_id, &signer_secrets[i].share, msg32, pk, NULL)) {
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return 0;
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}
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is_signer[i] = 0; /* Initialize is_signer */
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}
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/* Select a random subset of signers */
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for (i = 0; i < THRESHOLD; i++) {
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size_t subset_seed;
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while (1) {
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if (!fill_random((unsigned char*)&subset_seed, sizeof(subset_seed))) {
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return 0;
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}
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signer_id = subset_seed % N_SIGNERS;
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/* Check if signer has already been assigned */
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if (!is_signer[signer_id]) {
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is_signer[signer_id] = 1;
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signers[i] = signer_id;
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break;
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}
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}
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/* Mark signer as assigned */
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pubnonces[i] = &signer[signer_id].pubnonce;
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/* pubkeys[i] = &signer[signer_id].pubkey; */
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ids[i] = signer_id + 1;
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}
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/* Signing communication round 1: Exchange nonces */
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for (i = 0; i < THRESHOLD; i++) {
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signer_id = signers[i];
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if (!secp256k1_frost_nonce_process(ctx, &signer[signer_id].session, pubnonces, THRESHOLD, msg32, pk, signer_id + 1, ids, cache, NULL)) {
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return 0;
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}
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/* partial_sign will clear the secnonce by setting it to 0. That's because
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* you must _never_ reuse the secnonce (or use the same session_id to
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* create a secnonce). If you do, you effectively reuse the nonce and
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* leak the secret key. */
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if (!secp256k1_frost_partial_sign(ctx, &signer[signer_id].partial_sig, &signer_secrets[signer_id].secnonce, &signer_secrets[signer_id].share, &signer[signer_id].session, cache)) {
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return 0;
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}
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partial_sigs[i] = &signer[signer_id].partial_sig;
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}
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/* Communication round 2: A production system would exchange
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* partial signatures here before moving on. */
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for (i = 0; i < THRESHOLD; i++) {
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signer_id = signers[i];
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/* To check whether signing was successful, it suffices to either verify
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* the aggregate signature with the aggregate public key using
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* secp256k1_schnorrsig_verify, or verify all partial signatures of all
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* signers individually. Verifying the aggregate signature is cheaper but
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* verifying the individual partial signatures has the advantage that it
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* can be used to determine which of the partial signatures are invalid
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* (if any), i.e., which of the partial signatures cause the aggregate
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* signature to be invalid and thus the protocol run to fail. It's also
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* fine to first verify the aggregate sig, and only verify the individual
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* sigs if it does not work.
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*/
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if (!secp256k1_frost_partial_sig_verify(ctx, &signer[signer_id].partial_sig, &signer[signer_id].pubnonce, &signer[signer_id].pubshare, &signer[signer_id].session, cache)) {
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return 0;
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}
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}
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return secp256k1_frost_partial_sig_agg(ctx, sig64, &signer[signer_id].session, partial_sigs, THRESHOLD);
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}
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int main(void) {
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secp256k1_context* ctx;
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struct signer_secrets signer_secrets[N_SIGNERS];
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struct signer signers[N_SIGNERS];
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secp256k1_xonly_pubkey pk;
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secp256k1_frost_tweak_cache cache;
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unsigned char msg[32] = "this_could_be_the_hash_of_a_msg!";
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unsigned char sig[64];
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/* Create a context for signing and verification */
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ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
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printf("Creating shares.........");
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if (!create_shares(ctx, signer_secrets, signers, &pk)) {
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printf("FAILED\n");
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return 1;
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}
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printf("ok\n");
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printf("Tweaking................");
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/* Optionally tweak the key */
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if (!tweak(ctx, &pk, &cache)) {
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printf("FAILED\n");
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return 1;
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}
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printf("ok\n");
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printf("Signing message.........");
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if (!sign(ctx, signer_secrets, signers, msg, &pk, sig, &cache)) {
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printf("FAILED\n");
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return 1;
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}
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printf("ok\n");
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printf("Verifying signature.....");
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if (!secp256k1_schnorrsig_verify(ctx, sig, msg, 32, &pk)) {
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printf("FAILED\n");
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return 1;
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}
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printf("ok\n");
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secp256k1_context_destroy(ctx);
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return 0;
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}

‎include/secp256k1_frost.h

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* This module implements a variant of Flexible Round-Optimized Schnorr
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* Threshold Signatures (FROST) by Chelsea Komlo and Ian Goldberg
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* (https://crysp.uwaterloo.ca/software/frost/). Signatures are compatible with
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* BIP-340 ("Schnorr").
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* BIP-340 ("Schnorr"). There's an example C source file in the module's
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* directory (examples/frost.c) that demonstrates how it can be used.
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*
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* The module also supports BIP-341 ("Taproot") and BIP-32 ("ordinary") public
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* key tweaking, and adaptor signatures.

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