frost trusted dealer: add example file

This commit adds an example file to demonstrate how to use the module.

.gitignore

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

Makefile.am

@@ -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

examples/frost.c

@@ -0,0 +1,224 @@
+/***********************************************************************
+ * 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_frost_share share;
+    secp256k1_frost_secnonce secnonce;
+};
+
+struct signer {
+    secp256k1_pubkey pubshare;
+    secp256k1_frost_pubnonce pubnonce;
+    secp256k1_frost_session session;
+    secp256k1_frost_partial_sig partial_sig;
+};
+
+/* Create shares and coefficient commitments */
+int create_shares(const secp256k1_context* ctx, struct signer_secrets *signer_secrets, struct signer *signers, secp256k1_xonly_pubkey *pk) {
+    int i;
+    secp256k1_frost_share shares[N_SIGNERS];
+    secp256k1_pubkey pubshares[N_SIGNERS];
+    unsigned char seed[32];
+
+    if (!fill_random(seed, sizeof(seed))) {
+        return 0;
+    }
+
+    if (!secp256k1_frost_shares_trusted_gen(ctx, shares, pubshares, pk, seed, THRESHOLD, N_SIGNERS)) {
+        return 0;
+    }
+
+    for (i = 0; i < N_SIGNERS; i++) {
+        signer_secrets[i].share = shares[i];
+        signers[i].pubshare = pubshares[i];
+    }
+
+    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 *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, 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, 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 *pk, unsigned char *sig64, const secp256k1_frost_tweak_cache *cache) {
+    int i;
+    size_t signer_id = 0;
+    int signers[THRESHOLD];
+    int is_signer[N_SIGNERS];
+    const secp256k1_frost_pubnonce *pubnonces[THRESHOLD];
+    size_t 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].share, msg32, pk, NULL)) {
+            return 0;
+        }
+        is_signer[i] = 0; /* Initialize is_signer */
+    }
+    /* Select a random subset of signers */
+    for (i = 0; i < THRESHOLD; i++) {
+        size_t 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_id + 1;
+    }
+    /* 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, pk, signer_id + 1, 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].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;
+    struct signer_secrets signer_secrets[N_SIGNERS];
+    struct signer signers[N_SIGNERS];
+    secp256k1_xonly_pubkey 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 shares.........");
+    if (!create_shares(ctx, signer_secrets, signers, &pk)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+    printf("Tweaking................");
+    /* Optionally tweak the key */
+    if (!tweak(ctx, &pk, &cache)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+    printf("Signing message.........");
+    if (!sign(ctx, signer_secrets, signers, msg, &pk, sig, &cache)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+    printf("Verifying signature.....");
+    if (!secp256k1_schnorrsig_verify(ctx, sig, msg, 32, &pk)) {
+        printf("FAILED\n");
+        return 1;
+    }
+    printf("ok\n");
+    secp256k1_context_destroy(ctx);
+    return 0;
+}

include/secp256k1_frost.h

@@ -15,7 +15,8 @@ extern "C" {
  * This module implements a variant of Flexible Round-Optimized Schnorr
  * Threshold Signatures (FROST) by Chelsea Komlo and Ian Goldberg
  * (https://crysp.uwaterloo.ca/software/frost/). Signatures are compatible with
- * BIP-340 ("Schnorr").
+ * BIP-340 ("Schnorr"). There's an example C source file in the module's
+ * directory (examples/frost.c) that demonstrates how it can be used.
  *
  * The module also supports BIP-341 ("Taproot") and BIP-32 ("ordinary") public
  * key tweaking, and adaptor signatures.