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frost: share generation

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This commit adds share generation, as well as share serialization and
parsing.
This commit is contained in:
Jesse Posner 2024-06-17 18:10:13 -07:00
parent 702dd38c6b
commit 6e2398d018
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GPG Key ID: 49A08EAB3A812D69
5 changed files with 291 additions and 0 deletions
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83
include/secp256k1_frost.h
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@ -1,6 +1,8 @@
#ifndef SECP256K1_FROST_H #ifndef SECP256K1_FROST_H
#define SECP256K1_FROST_H #define SECP256K1_FROST_H
#include "secp256k1_extrakeys.h"
#ifdef __cplusplus #ifdef __cplusplus
extern "C" { extern "C" {
#endif #endif
@ -15,6 +17,87 @@ extern "C" {
* (https://crysp.uwaterloo.ca/software/frost/). * (https://crysp.uwaterloo.ca/software/frost/).
*/ */
/** Opaque data structures
*
* The exact representation of data inside is implementation defined and not
* guaranteed to be portable between different platforms or versions. If you
* need to convert to a format suitable for storage, transmission, or
* comparison, use the corresponding serialization and parsing functions.
*/
/** Opaque data structure that holds a signer's _secret_ share.
*
* Guaranteed to be 36 bytes in size. Serialized and parsed with
* `frost_share_serialize` and `frost_share_parse`.
*/
typedef struct {
unsigned char data[36];
} secp256k1_frost_share;
/** Serialize a FROST share
*
* Returns: 1 when the share could be serialized, 0 otherwise
* Args: ctx: pointer to a context object
* Out: out32: pointer to a 32-byte array to store the serialized share
* In: share: pointer to the share
*/
SECP256K1_API int secp256k1_frost_share_serialize(
const secp256k1_context *ctx,
unsigned char *out32,
const secp256k1_frost_share *share
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
/** Parse a FROST share.
*
* Returns: 1 when the share could be parsed, 0 otherwise.
* Args: ctx: pointer to a context object
* Out: share: pointer to a share object
* In: in32: pointer to the 32-byte share to be parsed
*/
SECP256K1_API int secp256k1_frost_share_parse(
const secp256k1_context *ctx,
secp256k1_frost_share *share,
const unsigned char *in32
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
/** Creates key shares
*
* To generate a key, each participant generates a share for each other
* participant. For example, in the case of 2 particpants, Alice and Bob, they
* each generate 2 shares, distribute 1 share to each other using a secure
* channel, and keep 1 for themselves.
*
* Each participant must transmit shares over secure channels to each other
* participant.
*
* Each call to this function must have a UNIQUE and uniformly RANDOM seed32
* that must that must NOT BE REUSED in subsequent calls to this function and
* must be KEPT SECRET (even from other participants).
*
* Returns: 0 if the arguments are invalid, 1 otherwise
* Args: ctx: pointer to a context object
* Out: shares: pointer to the key shares
* vss_commitment: pointer to the VSS commitment
* pok64: pointer to the proof of knowledge
* In: seed32: 32-byte random seed as explained above. Must be
* unique to this call to secp256k1_frost_shares_gen
* and must be uniformly random.
* threshold: the minimum number of signers required to produce a
* signature
* n_participants: the total number of participants
* ids33: array of 33-byte participant IDs
*/
SECP256K1_API int secp256k1_frost_shares_gen(
const secp256k1_context *ctx,
secp256k1_frost_share *shares,
secp256k1_pubkey *vss_commitment,
unsigned char *pok64,
const unsigned char *seed32,
size_t threshold,
size_t n_participants,
const unsigned char * const* ids33
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(8);
#ifdef __cplusplus #ifdef __cplusplus
} }
#endif #endif

2
src/modules/frost/Makefile.am.include
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@ -1,2 +1,4 @@
include_HEADERS += include/secp256k1_frost.h include_HEADERS += include/secp256k1_frost.h
noinst_HEADERS += src/modules/frost/main_impl.h noinst_HEADERS += src/modules/frost/main_impl.h
noinst_HEADERS += src/modules/frost/keygen.h
noinst_HEADERS += src/modules/frost/keygen_impl.h

13
src/modules/frost/keygen.h Normal file
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@ -0,0 +1,13 @@
/**********************************************************************
* Copyright (c) 2021-2024 Jesse Posner *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or http://www.opensource.org/licenses/mit-license.php.*
**********************************************************************/
#ifndef SECP256K1_MODULE_FROST_KEYGEN_H
#define SECP256K1_MODULE_FROST_KEYGEN_H
#include "../../../include/secp256k1.h"
#include "../../../include/secp256k1_frost.h"
#endif

191
src/modules/frost/keygen_impl.h Normal file
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@ -0,0 +1,191 @@
/**********************************************************************
* Copyright (c) 2021-2024 Jesse Posner *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or http://www.opensource.org/licenses/mit-license.php.*
**********************************************************************/
#ifndef SECP256K1_MODULE_FROST_KEYGEN_IMPL_H
#define SECP256K1_MODULE_FROST_KEYGEN_IMPL_H
#include <string.h>
#include "../../../include/secp256k1.h"
#include "../../../include/secp256k1_extrakeys.h"
#include "../../../include/secp256k1_frost.h"
#include "keygen.h"
#include "../../ecmult.h"
#include "../../field.h"
#include "../../group.h"
#include "../../hash.h"
#include "../../scalar.h"
/* Computes indexhash = tagged_hash(pk) */
static int secp256k1_frost_compute_indexhash(secp256k1_scalar *indexhash, const unsigned char *id33) {
secp256k1_sha256 sha;
unsigned char buf[32];
secp256k1_sha256_initialize_tagged(&sha, (unsigned char*)"FROST/index", sizeof("FROST/index") - 1);
secp256k1_sha256_write(&sha, id33, 33);
secp256k1_sha256_finalize(&sha, buf);
secp256k1_scalar_set_b32(indexhash, buf, NULL);
/* The x-coordinate must not be zero (see
* draft-irtf-cfrg-frost-08#section-4.2.2) */
if (secp256k1_scalar_is_zero(indexhash)) {
return 0;
}
return 1;
}
static const unsigned char secp256k1_frost_share_magic[4] = { 0xa1, 0x6a, 0x42, 0x03 };
static void secp256k1_frost_share_save(secp256k1_frost_share* share, secp256k1_scalar *s) {
memcpy(&share->data[0], secp256k1_frost_share_magic, 4);
secp256k1_scalar_get_b32(&share->data[4], s);
}
static int secp256k1_frost_share_load(const secp256k1_context* ctx, secp256k1_scalar *s, const secp256k1_frost_share* share) {
int overflow;
ARG_CHECK(secp256k1_memcmp_var(&share->data[0], secp256k1_frost_share_magic, 4) == 0);
secp256k1_scalar_set_b32(s, &share->data[4], &overflow);
/* Parsed shares cannot overflow */
VERIFY_CHECK(!overflow);
return 1;
}
int secp256k1_frost_share_serialize(const secp256k1_context* ctx, unsigned char *out32, const secp256k1_frost_share* share) {
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(out32 != NULL);
ARG_CHECK(share != NULL);
memcpy(out32, &share->data[4], 32);
return 1;
}
int secp256k1_frost_share_parse(const secp256k1_context* ctx, secp256k1_frost_share* share, const unsigned char *in32) {
secp256k1_scalar tmp;
int overflow;
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(share != NULL);
ARG_CHECK(in32 != NULL);
secp256k1_scalar_set_b32(&tmp, in32, &overflow);
if (overflow) {
return 0;
}
secp256k1_frost_share_save(share, &tmp);
return 1;
}
static void secp256k1_frost_derive_coeff(secp256k1_scalar *coeff, const unsigned char *polygen32, size_t i) {
secp256k1_sha256 sha;
unsigned char buf[32];
secp256k1_sha256_initialize_tagged(&sha, (unsigned char*)"FROST/coeffgen", sizeof("FROST/coeffgen") - 1);
secp256k1_sha256_write(&sha, polygen32, 32);
secp256k1_write_be64(&buf[0], i);
secp256k1_sha256_write(&sha, buf, 8);
secp256k1_sha256_finalize(&sha, buf);
secp256k1_scalar_set_b32(coeff, buf, NULL);
}
static int secp256k1_frost_vss_gen(const secp256k1_context *ctx, secp256k1_pubkey *vss_commitment, unsigned char *pok64, const unsigned char *polygen32, size_t threshold) {
secp256k1_sha256 sha;
unsigned char buf[32];
secp256k1_keypair keypair;
secp256k1_gej rj;
secp256k1_ge rp;
size_t i;
int ret = 1;
for (i = 0; i < threshold; i++) {
secp256k1_scalar coeff_i;
secp256k1_frost_derive_coeff(&coeff_i, polygen32, i);
/* Compute proof-of-knowledge for constant term */
if (i == threshold - 1) {
secp256k1_scalar_get_b32(buf, &coeff_i);
ret &= secp256k1_keypair_create(ctx, &keypair, buf);
secp256k1_sha256_initialize_tagged(&sha, (unsigned char*)"FROST/KeygenPoK", sizeof("FROST/KeygenPoK") - 1);
secp256k1_sha256_finalize(&sha, buf);
ret &= secp256k1_schnorrsig_sign32(ctx, pok64, buf, &keypair, NULL);
}
/* Compute commitment to each coefficient */
secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &rj, &coeff_i);
secp256k1_ge_set_gej(&rp, &rj);
secp256k1_pubkey_save(&vss_commitment[threshold - i - 1], &rp);
}
return ret;
}
static int secp256k1_frost_share_gen(secp256k1_frost_share *share, const unsigned char *polygen32, size_t threshold, const unsigned char *id33) {
secp256k1_scalar idx;
secp256k1_scalar share_i;
size_t i;
int ret = 1;
/* Derive share */
/* See draft-irtf-cfrg-frost-08#appendix-C.1 */
secp256k1_scalar_set_int(&share_i, 0);
if (!secp256k1_frost_compute_indexhash(&idx, id33)) {
return 0;
}
for (i = 0; i < threshold; i++) {
secp256k1_scalar coeff_i;
secp256k1_frost_derive_coeff(&coeff_i, polygen32, i);
/* Horner's method to evaluate polynomial to derive shares */
secp256k1_scalar_add(&share_i, &share_i, &coeff_i);
if (i < threshold - 1) {
secp256k1_scalar_mul(&share_i, &share_i, &idx);
}
}
secp256k1_frost_share_save(share, &share_i);
return ret;
}
int secp256k1_frost_shares_gen(const secp256k1_context *ctx, secp256k1_frost_share *shares, secp256k1_pubkey *vss_commitment, unsigned char *pok64, const unsigned char *seed32, size_t threshold, size_t n_participants, const unsigned char * const* ids33) {
secp256k1_sha256 sha;
unsigned char polygen[32];
size_t i;
int ret = 1;
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
ARG_CHECK(shares != NULL);
for (i = 0; i < n_participants; i++) {
memset(&shares[i], 0, sizeof(shares[i]));
}
ARG_CHECK(vss_commitment != NULL);
ARG_CHECK(pok64 != NULL);
ARG_CHECK(seed32 != NULL);
ARG_CHECK(ids33 != NULL);
ARG_CHECK(threshold > 1);
ARG_CHECK(n_participants >= threshold);
/* Commit to all inputs */
secp256k1_sha256_initialize(&sha);
secp256k1_sha256_write(&sha, seed32, 32);
secp256k1_write_be64(&polygen[0], threshold);
secp256k1_write_be64(&polygen[8], n_participants);
secp256k1_sha256_write(&sha, polygen, 16);
for (i = 0; i < n_participants; i++) {
secp256k1_sha256_write(&sha, ids33[i], 33);
}
secp256k1_sha256_finalize(&sha, polygen);
ret &= secp256k1_frost_vss_gen(ctx, vss_commitment, pok64, polygen, threshold);
for (i = 0; i < n_participants; i++) {
ret &= secp256k1_frost_share_gen(&shares[i], polygen, threshold, ids33[i]);
}
return ret;
}
#endif

2
src/modules/frost/main_impl.h
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@ -7,4 +7,6 @@
#ifndef SECP256K1_MODULE_FROST_MAIN #ifndef SECP256K1_MODULE_FROST_MAIN
#define SECP256K1_MODULE_FROST_MAIN #define SECP256K1_MODULE_FROST_MAIN
#include "keygen_impl.h"
#endif #endif