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frost trusted dealer: add example file

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This commit adds an example file to demonstrate how to use the module.
This commit is contained in:
Jesse Posner 2024-07-16 00:12:57 -07:00
parent 123ae6a62d
commit 97c472da92
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GPG Key ID: 49A08EAB3A812D69
3 changed files with 306 additions and 0 deletions
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1
.gitignore vendored
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@ -66,6 +66,7 @@ libsecp256k1.pc
contrib/gh-pr-create.sh
musig_example
frost_example
### CMake
/CMakeUserPresets.json

11
Makefile.am
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@ -195,6 +195,17 @@ musig_example_LDFLAGS += -lbcrypt
endif
TESTS += musig_example
endif
if ENABLE_MODULE_FROST
noinst_PROGRAMS += frost_example
frost_example_SOURCES = examples/frost.c
frost_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATIC
frost_example_LDADD = libsecp256k1.la
frost_example_LDFLAGS = -static
if BUILD_WINDOWS
frost_example_LDFLAGS += -lbcrypt
endif
TESTS += frost_example
endif
endif
### Precomputed tables

294
examples/frost.c Normal file
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@ -0,0 +1,294 @@
/***********************************************************************
* Copyright (c) 2021-2023 Jesse Posner *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or https://www.opensource.org/licenses/mit-license.php.*
***********************************************************************/
/**
* This file demonstrates how to use the FROST module to create a threshold
* signature. Additionally, see the documentation in include/secp256k1_frost.h.
*/
#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <secp256k1.h>
#include <secp256k1_schnorrsig.h>
#include <secp256k1_frost.h>
#include "examples_util.h"
/* Number of public keys involved in creating the aggregate signature */
#define N_SIGNERS 5
/* Threshold required in creating the aggregate signature */
#define THRESHOLD 3
struct signer_secrets {
secp256k1_keypair keypair;
secp256k1_frost_share agg_share;
secp256k1_frost_secnonce secnonce;
unsigned char seed[32];
};
struct signer {
secp256k1_pubkey pubshare;
secp256k1_frost_pubnonce pubnonce;
secp256k1_frost_session session;
secp256k1_frost_partial_sig partial_sig;
secp256k1_pubkey vss_commitment[THRESHOLD];
unsigned char vss_hash[32];
unsigned char pok[64];
unsigned char id[33];
};
/* Create a key pair and store it in seckey and pubkey */
int create_keypair_and_seed(const secp256k1_context* ctx, struct signer_secrets *signer_secrets, struct signer *signer) {
unsigned char seckey[32];
secp256k1_pubkey pubkey_tmp;
size_t size = 33;
while (1) {
if (!fill_random(seckey, sizeof(seckey))) {
printf("Failed to generate randomness\n");
return 1;
}
if (secp256k1_keypair_create(ctx, &signer_secrets->keypair, seckey)) {
break;
}
}
if (!secp256k1_keypair_pub(ctx, &pubkey_tmp, &signer_secrets->keypair)) {
return 0;
}
if (!secp256k1_ec_pubkey_serialize(ctx, signer->id, &size, &pubkey_tmp, SECP256K1_EC_COMPRESSED)) {
return 0;
}
if (!fill_random(signer_secrets->seed, sizeof(signer_secrets->seed))) {
return 0;
}
return 1;
}
/* Create shares and coefficient commitments */
int create_shares(const secp256k1_context* ctx, struct signer_secrets *signer_secrets, struct signer *signer, secp256k1_xonly_pubkey *agg_pk) {
int i, j;
secp256k1_frost_share shares[N_SIGNERS][N_SIGNERS];
const secp256k1_pubkey *vss_commitments[N_SIGNERS];
const unsigned char *ids[N_SIGNERS];
for (i = 0; i < N_SIGNERS; i++) {
vss_commitments[i] = signer[i].vss_commitment;
ids[i] = signer[i].id;
}
for (i = 0; i < N_SIGNERS; i++) {
/* Generate a polynomial share for the participants */
if (!secp256k1_frost_shares_gen(ctx, shares[i], signer[i].vss_commitment, signer[i].pok, signer_secrets[i].seed, THRESHOLD, N_SIGNERS, ids)) {
return 0;
}
}
/* KeyGen communication round 1: exchange shares and coefficient
* commitments */
for (i = 0; i < N_SIGNERS; i++) {
const secp256k1_frost_share *assigned_shares[N_SIGNERS];
/* Each participant receives a share from each participant (including
* themselves) corresponding to their index. */
for (j = 0; j < N_SIGNERS; j++) {
assigned_shares[j] = &shares[j][i];
}
/* Each participant aggregates the shares they received. */
if (!secp256k1_frost_share_agg(ctx, &signer_secrets[i].agg_share, agg_pk, assigned_shares, vss_commitments, N_SIGNERS, THRESHOLD, signer[i].id)) {
return 0;
}
for (j = 0; j < N_SIGNERS; j++) {
/* Each participant verifies their shares. share_agg calls this
* internally, so it is only neccessary to call this function if
* share_agg returns an error, to determine which participant(s)
* submitted faulty data. */
if (!secp256k1_frost_share_verify(ctx, THRESHOLD, signer[i].id, assigned_shares[j], &vss_commitments[j])) {
return 0;
}
/* Each participant generates public verification shares that are
* used for verifying partial signatures. */
if (!secp256k1_frost_compute_pubshare(ctx, &signer[j].pubshare, THRESHOLD, signer[j].id, vss_commitments, N_SIGNERS)) {
return 0;
}
}
}
return 1;
}
/* Tweak the pubkey corresponding to the provided tweak cache, update the cache
* and return the tweaked aggregate pk. */
int tweak(const secp256k1_context* ctx, secp256k1_xonly_pubkey *agg_pk, secp256k1_frost_tweak_cache *cache) {
secp256k1_pubkey output_pk;
unsigned char ordinary_tweak[32] = "this could be a BIP32 tweak....";
unsigned char xonly_tweak[32] = "this could be a taproot tweak..";
if (!secp256k1_frost_pubkey_tweak(ctx, cache, agg_pk)) {
return 0;
}
/* Ordinary tweaking which, for example, allows deriving multiple child
* public keys from a single aggregate key using BIP32 */
if (!secp256k1_frost_pubkey_ec_tweak_add(ctx, NULL, cache, ordinary_tweak)) {
return 0;
}
/* If one is not interested in signing, the same output_pk can be obtained
* by calling `secp256k1_frost_pubkey_get` right after key aggregation to
* get the full pubkey and then call `secp256k1_ec_pubkey_tweak_add`. */
/* Xonly tweaking which, for example, allows creating taproot commitments */
if (!secp256k1_frost_pubkey_xonly_tweak_add(ctx, &output_pk, cache, xonly_tweak)) {
return 0;
}
/* Note that if we wouldn't care about signing, we can arrive at the same
* output_pk by providing the untweaked public key to
* `secp256k1_xonly_pubkey_tweak_add` (after converting it to an xonly pubkey
* if necessary with `secp256k1_xonly_pubkey_from_pubkey`). */
/* Now we convert the output_pk to an xonly pubkey to allow to later verify
* the Schnorr signature against it. For this purpose we can ignore the
* `pk_parity` output argument; we would need it if we would have to open
* the taproot commitment. */
if (!secp256k1_xonly_pubkey_from_pubkey(ctx, agg_pk, NULL, &output_pk)) {
return 0;
}
return 1;
}
/* Sign a message hash with the given threshold and aggregate shares and store
* the result in sig */
int sign(const secp256k1_context* ctx, struct signer_secrets *signer_secrets, struct signer *signer, const unsigned char* msg32, secp256k1_xonly_pubkey *agg_pk, unsigned char *sig64, const secp256k1_frost_tweak_cache *cache) {
int i;
int signer_id = 0;
int signers[THRESHOLD];
int is_signer[N_SIGNERS];
const secp256k1_frost_pubnonce *pubnonces[THRESHOLD];
const unsigned char *ids[THRESHOLD];
const secp256k1_frost_partial_sig *partial_sigs[THRESHOLD];
for (i = 0; i < N_SIGNERS; i++) {
unsigned char session_id[32];
/* Create random session ID. It is absolutely necessary that the session ID
* is unique for every call of secp256k1_frost_nonce_gen. Otherwise
* it's trivial for an attacker to extract the secret key! */
if (!fill_random(session_id, sizeof(session_id))) {
return 0;
}
/* Initialize session and create secret nonce for signing and public
* nonce to send to the other signers. */
if (!secp256k1_frost_nonce_gen(ctx, &signer_secrets[i].secnonce, &signer[i].pubnonce, session_id, &signer_secrets[i].agg_share, msg32, agg_pk, NULL)) {
return 0;
}
is_signer[i] = 0; /* Initialize is_signer */
}
/* Select a random subset of signers */
for (i = 0; i < THRESHOLD; i++) {
unsigned int subset_seed;
while (1) {
if (!fill_random((unsigned char*)&subset_seed, sizeof(subset_seed))) {
return 0;
}
signer_id = subset_seed % N_SIGNERS;
/* Check if signer has already been assigned */
if (!is_signer[signer_id]) {
is_signer[signer_id] = 1;
signers[i] = signer_id;
break;
}
}
/* Mark signer as assigned */
pubnonces[i] = &signer[signer_id].pubnonce;
/* pubkeys[i] = &signer[signer_id].pubkey; */
ids[i] = signer[signer_id].id;
}
/* Signing communication round 1: Exchange nonces */
for (i = 0; i < THRESHOLD; i++) {
signer_id = signers[i];
if (!secp256k1_frost_nonce_process(ctx, &signer[signer_id].session, pubnonces, THRESHOLD, msg32, agg_pk, signer[signer_id].id, ids, cache, NULL)) {
return 0;
}
/* partial_sign will clear the secnonce by setting it to 0. That's because
* you must _never_ reuse the secnonce (or use the same session_id to
* create a secnonce). If you do, you effectively reuse the nonce and
* leak the secret key. */
if (!secp256k1_frost_partial_sign(ctx, &signer[signer_id].partial_sig, &signer_secrets[signer_id].secnonce, &signer_secrets[signer_id].agg_share, &signer[signer_id].session, cache)) {
return 0;
}
partial_sigs[i] = &signer[signer_id].partial_sig;
}
/* Communication round 2: A production system would exchange
* partial signatures here before moving on. */
for (i = 0; i < THRESHOLD; i++) {
signer_id = signers[i];
/* To check whether signing was successful, it suffices to either verify
* the aggregate signature with the aggregate public key using
* secp256k1_schnorrsig_verify, or verify all partial signatures of all
* signers individually. Verifying the aggregate signature is cheaper but
* verifying the individual partial signatures has the advantage that it
* can be used to determine which of the partial signatures are invalid
* (if any), i.e., which of the partial signatures cause the aggregate
* signature to be invalid and thus the protocol run to fail. It's also
* fine to first verify the aggregate sig, and only verify the individual
* sigs if it does not work.
*/
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)) {
return 0;
}
}
return secp256k1_frost_partial_sig_agg(ctx, sig64, &signer[signer_id].session, partial_sigs, THRESHOLD);
}
int main(void) {
secp256k1_context* ctx;
int i;
struct signer_secrets signer_secrets[N_SIGNERS];
struct signer signers[N_SIGNERS];
secp256k1_xonly_pubkey agg_pk;
secp256k1_frost_tweak_cache cache;
unsigned char msg[32] = "this_could_be_the_hash_of_a_msg!";
unsigned char sig[64];
/* Create a context for signing and verification */
ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
printf("Creating key pairs......");
for (i = 0; i < N_SIGNERS; i++) {
if (!create_keypair_and_seed(ctx, &signer_secrets[i], &signers[i])) {
printf("FAILED\n");
return 1;
}
}
printf("ok\n");
printf("Creating shares.........");
if (!create_shares(ctx, signer_secrets, signers, &agg_pk)) {
printf("FAILED\n");
return 1;
}
printf("ok\n");
printf("Tweaking................");
/* Optionally tweak the aggregate key */
if (!tweak(ctx, &agg_pk, &cache)) {
printf("FAILED\n");
return 1;
}
printf("ok\n");
printf("Signing message.........");
if (!sign(ctx, signer_secrets, signers, msg, &agg_pk, sig, &cache)) {
printf("FAILED\n");
return 1;
}
printf("ok\n");
printf("Verifying signature.....");
if (!secp256k1_schnorrsig_verify(ctx, sig, msg, 32, &agg_pk)) {
printf("FAILED\n");
return 1;
}
printf("ok\n");
secp256k1_context_destroy(ctx);
return 0;
}