frost: share aggregation
This commit adds share aggregation and verification, as well as computation of public verification shares.
This commit is contained in:
Jesse Posner
parent
2336b02fad
commit
197fb7efb9
@ -98,6 +98,89 @@ SECP256K1_API int secp256k1_frost_shares_gen(
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const unsigned char * const* ids33
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const unsigned char * const* ids33
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) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(8);
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) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(8);
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/** Aggregates shares
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*
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* As part of the key generation protocol, each participant receives a share
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* from each participant, including a share they "receive" from themselves.
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* This function verifies those shares against their VSS commitments,
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* aggregates the shares, and then aggregates the commitments to each
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* participant's first polynomial coefficient to derive the aggregate public
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* key.
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*
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* If this function returns an error, `secp256k1_frost_share_verify` can be
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* called on each share to determine which participants submitted faulty
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* shares.
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*
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* Returns: 0 if the arguments are invalid, 1 otherwise (which does NOT mean
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* the resulting signature verifies).
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* Args: ctx: pointer to a context object
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* Out: agg_share: the aggregated share
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* agg_pk: the aggregated x-only public key
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* In: shares: all key generation shares for the partcipant's index
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* vss_commitments: coefficient commitments of all participants ordered by
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* the x-only pubkeys of the participants
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* n_shares: the total number of shares
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* threshold: the minimum number of shares required to produce a
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* signature
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* id33: the 33-byte ID of the participant whose shares are being
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* aggregated
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*/
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SECP256K1_API int secp256k1_frost_share_agg(
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const secp256k1_context *ctx,
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secp256k1_frost_share *agg_share,
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secp256k1_xonly_pubkey *agg_pk,
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const secp256k1_frost_share * const *shares,
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const secp256k1_pubkey * const *vss_commitments,
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size_t n_shares,
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size_t threshold,
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const unsigned char *id33
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) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(8);
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/** Verifies a share received during a key generation session
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*
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* The signature is verified against the VSS commitment received with the
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* share. This is only useful for purposes of determining which share(s) are
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* invalid if share_agg returns an error.
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*
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* Returns: 0 if the arguments are invalid or the share does not verify, 1
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* otherwise
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* Args ctx: pointer to a context object
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* In: threshold: the minimum number of signers required to produce a
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* signature
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* id33: the 33-byte participant ID of the share recipient
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* share: pointer to a key generation share
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* vss_commitment: the VSS commitment associated with the share
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*/
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SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_frost_share_verify(
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const secp256k1_context *ctx,
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size_t threshold,
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const unsigned char *id33,
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const secp256k1_frost_share *share,
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const secp256k1_pubkey * const *vss_commitment
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) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
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/** Computes a public verification share used for verifying partial signatures
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*
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* Returns: 0 if the arguments are invalid, 1 otherwise
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* Args: ctx: pointer to a context object
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* Out: pubshare: pointer to a struct to store the public verification
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* share
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* In: threshold: the minimum number of signers required to produce a
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* signature
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* id33: the 33-byte participant ID of the participant whose
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* partial signature will be verified with the pubshare
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* vss_commitments: coefficient commitments of all participants
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* n_participants: the total number of participants
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*/
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SECP256K1_API int secp256k1_frost_compute_pubshare(
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const secp256k1_context *ctx,
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secp256k1_pubkey *pubshare,
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size_t threshold,
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const unsigned char *id33,
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const secp256k1_pubkey * const *vss_commitments,
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size_t n_participants
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) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
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#ifdef __cplusplus
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#ifdef __cplusplus
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}
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}
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#endif
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#endif
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@ -188,4 +188,228 @@ int secp256k1_frost_shares_gen(const secp256k1_context *ctx, secp256k1_frost_sha
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return ret;
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return ret;
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}
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}
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typedef struct {
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const secp256k1_context *ctx;
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secp256k1_scalar idx;
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secp256k1_scalar idxn;
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const secp256k1_pubkey * const* vss_commitment;
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} secp256k1_frost_verify_share_ecmult_data;
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typedef struct {
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const secp256k1_context *ctx;
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secp256k1_scalar idx;
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secp256k1_scalar idxn;
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const secp256k1_pubkey * const* vss_commitments;
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size_t threshold;
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} secp256k1_frost_compute_pubshare_ecmult_data;
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typedef struct {
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const secp256k1_context *ctx;
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const secp256k1_pubkey * const* pks;
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size_t threshold;
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} secp256k1_frost_pubkey_combine_ecmult_data;
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static int secp256k1_frost_verify_share_ecmult_callback(secp256k1_scalar *sc, secp256k1_ge *pt, size_t idx, void *data) {
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secp256k1_frost_verify_share_ecmult_data *ctx = (secp256k1_frost_verify_share_ecmult_data *) data;
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if (!secp256k1_pubkey_load(ctx->ctx, pt, *(ctx->vss_commitment)+idx)) {
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return 0;
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}
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*sc = ctx->idxn;
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secp256k1_scalar_mul(&ctx->idxn, &ctx->idxn, &ctx->idx);
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return 1;
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}
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static int secp256k1_frost_compute_pubshare_ecmult_callback(secp256k1_scalar *sc, secp256k1_ge *pt, size_t idx, void *data) {
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secp256k1_frost_compute_pubshare_ecmult_data *ctx = (secp256k1_frost_compute_pubshare_ecmult_data *) data;
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if (!secp256k1_pubkey_load(ctx->ctx, pt, &ctx->vss_commitments[idx/ctx->threshold][idx % ctx->threshold])) {
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return 0;
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}
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if (idx != 0 && idx % ctx->threshold == 0) {
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secp256k1_scalar_set_int(&ctx->idxn, 1);
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}
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*sc = ctx->idxn;
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secp256k1_scalar_mul(&ctx->idxn, &ctx->idxn, &ctx->idx);
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return 1;
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}
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static int secp256k1_frost_pubkey_combine_callback(secp256k1_scalar *sc, secp256k1_ge *pt, size_t idx, void *data) {
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secp256k1_frost_pubkey_combine_ecmult_data *ctx = (secp256k1_frost_pubkey_combine_ecmult_data *) data;
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secp256k1_scalar_set_int(sc, 1);
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/* the public key is the first index of each set of coefficients */
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return secp256k1_pubkey_load(ctx->ctx, pt, &ctx->pks[idx][0]);
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}
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/* See draft-irtf-cfrg-frost-08#appendix-C.2 */
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static int secp256k1_frost_vss_verify_internal(const secp256k1_context* ctx, size_t threshold, const unsigned char *id33, const secp256k1_scalar *share, const secp256k1_pubkey * const* vss_commitment) {
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secp256k1_scalar share_neg;
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secp256k1_gej tmpj, snj;
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secp256k1_ge sng;
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secp256k1_frost_verify_share_ecmult_data verify_share_ecmult_data;
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ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
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/* Use an EC multi-multiplication to verify the following equation:
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* 0 = - share_i*G + idx^0*vss_commitment[0]
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* + ...
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* + idx^(threshold - 1)*vss_commitment[threshold - 1]*/
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verify_share_ecmult_data.ctx = ctx;
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verify_share_ecmult_data.vss_commitment = vss_commitment;
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/* Evaluate the public polynomial at the idx */
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if (!secp256k1_frost_compute_indexhash(&verify_share_ecmult_data.idx, id33)) {
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return 0;
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}
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secp256k1_scalar_set_int(&verify_share_ecmult_data.idxn, 1);
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/* TODO: add scratch */
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if (!secp256k1_ecmult_multi_var(&ctx->error_callback, NULL, &tmpj, NULL, secp256k1_frost_verify_share_ecmult_callback, (void *) &verify_share_ecmult_data, threshold)) {
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return 0;
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}
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secp256k1_scalar_negate(&share_neg, share);
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secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &snj, &share_neg);
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secp256k1_ge_set_gej(&sng, &snj);
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secp256k1_gej_add_ge(&tmpj, &tmpj, &sng);
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return secp256k1_gej_is_infinity(&tmpj);
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}
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/* See draft-irtf-cfrg-frost-08#appendix-C.2 */
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int secp256k1_frost_share_verify(const secp256k1_context* ctx, size_t threshold, const unsigned char *id33, const secp256k1_frost_share *share, const secp256k1_pubkey * const* vss_commitment) {
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secp256k1_scalar share_i;
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VERIFY_CHECK(ctx != NULL);
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ARG_CHECK(id33 != NULL);
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ARG_CHECK(share != NULL);
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ARG_CHECK(vss_commitment != NULL);
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ARG_CHECK(threshold > 1);
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if (!secp256k1_frost_share_load(ctx, &share_i, share)) {
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return 0;
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}
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return secp256k1_frost_vss_verify_internal(ctx, threshold, id33, &share_i, vss_commitment);
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}
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int secp256k1_frost_compute_pubshare(const secp256k1_context* ctx, secp256k1_pubkey *pubshare, size_t threshold, const unsigned char *id33, const secp256k1_pubkey * const* vss_commitments, size_t n_participants) {
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secp256k1_gej pkj;
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secp256k1_ge pkp, tmp;
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secp256k1_frost_compute_pubshare_ecmult_data compute_pubshare_ecmult_data;
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secp256k1_frost_pubkey_combine_ecmult_data pubkey_combine_ecmult_data;
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VERIFY_CHECK(ctx != NULL);
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ARG_CHECK(pubshare != NULL);
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memset(pubshare, 0, sizeof(*pubshare));
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ARG_CHECK(id33 != NULL);
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ARG_CHECK(vss_commitments != NULL);
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ARG_CHECK(n_participants > 1);
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ARG_CHECK(threshold > 1);
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if (threshold > n_participants) {
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return 0;
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}
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/* Use an EC multi-multiplication to compute the following equation:
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* agg_share_i*G = (
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* idx^0*vss_commitment[0][0] + ...
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* + idx^(t - 1)*vss_commitment[0][t - 1]
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* ) + ...
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* + (
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* idx^0*vss_commitment[n - 1][0] + ...
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* + idx^(t - 1)*vss_commitment[n - 1][t - 1]
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* )*/
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compute_pubshare_ecmult_data.ctx = ctx;
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compute_pubshare_ecmult_data.vss_commitments = vss_commitments;
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compute_pubshare_ecmult_data.threshold = threshold;
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/* Evaluate the public polynomial at the idx */
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if (!secp256k1_frost_compute_indexhash(&compute_pubshare_ecmult_data.idx, id33)) {
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return 0;
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}
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secp256k1_scalar_set_int(&compute_pubshare_ecmult_data.idxn, 1);
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/* TODO: add scratch */
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if (!secp256k1_ecmult_multi_var(&ctx->error_callback, NULL, &pkj, NULL, secp256k1_frost_compute_pubshare_ecmult_callback, (void *) &compute_pubshare_ecmult_data, n_participants*threshold)) {
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return 0;
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}
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secp256k1_ge_set_gej(&tmp, &pkj);
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/* Combine pubkeys */
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pubkey_combine_ecmult_data.ctx = ctx;
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pubkey_combine_ecmult_data.pks = vss_commitments;
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pubkey_combine_ecmult_data.threshold = threshold;
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/* TODO: add scratch */
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if (!secp256k1_ecmult_multi_var(&ctx->error_callback, NULL, &pkj, NULL, secp256k1_frost_pubkey_combine_callback, (void *) &pubkey_combine_ecmult_data, n_participants)) {
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return 0;
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}
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secp256k1_ge_set_gej(&pkp, &pkj);
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secp256k1_fe_normalize_var(&pkp.y);
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if (secp256k1_fe_is_odd(&pkp.y)) {
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secp256k1_ge_neg(&tmp, &tmp);
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}
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secp256k1_pubkey_save(pubshare, &tmp);
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return 1;
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}
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int secp256k1_frost_share_agg(const secp256k1_context* ctx, secp256k1_frost_share *agg_share, secp256k1_xonly_pubkey *agg_pk, const secp256k1_frost_share * const* shares, const secp256k1_pubkey * const* vss_commitments, size_t n_shares, size_t threshold, const unsigned char *id33) {
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secp256k1_frost_pubkey_combine_ecmult_data pubkey_combine_ecmult_data;
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secp256k1_gej pkj;
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secp256k1_ge pkp;
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int pk_parity;
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secp256k1_scalar acc;
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size_t i;
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int ret = 1;
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VERIFY_CHECK(ctx != NULL);
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ARG_CHECK(agg_share != NULL);
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memset(agg_share, 0, sizeof(*agg_share));
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ARG_CHECK(agg_pk != NULL);
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memset(agg_pk, 0, sizeof(*agg_pk));
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ARG_CHECK(shares != NULL);
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ARG_CHECK(vss_commitments != NULL);
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ARG_CHECK(id33 != NULL);
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ARG_CHECK(n_shares > 1);
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ARG_CHECK(threshold > 1);
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if (threshold > n_shares) {
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return 0;
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}
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secp256k1_scalar_clear(&acc);
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for (i = 0; i < n_shares; i++) {
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secp256k1_scalar share_i;
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if (!secp256k1_frost_share_load(ctx, &share_i, shares[i])) {
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return 0;
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}
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/* Verify share against commitments */
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ret &= secp256k1_frost_vss_verify_internal(ctx, threshold, id33, &share_i, &vss_commitments[i]);
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secp256k1_scalar_add(&acc, &acc, &share_i);
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}
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/* Combine pubkeys */
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pubkey_combine_ecmult_data.ctx = ctx;
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pubkey_combine_ecmult_data.pks = vss_commitments;
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pubkey_combine_ecmult_data.threshold = threshold;
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/* TODO: add scratch */
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if (!secp256k1_ecmult_multi_var(&ctx->error_callback, NULL, &pkj, NULL, secp256k1_frost_pubkey_combine_callback, (void *) &pubkey_combine_ecmult_data, n_shares)) {
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return 0;
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}
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secp256k1_ge_set_gej(&pkp, &pkj);
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secp256k1_fe_normalize_var(&pkp.y);
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pk_parity = secp256k1_extrakeys_ge_even_y(&pkp);
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||||||
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secp256k1_xonly_pubkey_save(agg_pk, &pkp);
|
||||||
|
|
||||||
|
/* Invert the aggregate share if the combined pubkey has an odd Y coordinate. */
|
||||||
|
if (pk_parity == 1) {
|
||||||
|
secp256k1_scalar_negate(&acc, &acc);
|
||||||
|
}
|
||||||
|
secp256k1_frost_share_save(agg_share, &acc);
|
||||||
|
|
||||||
|
return ret;
|
||||||
|
}
|
||||||
|
|
||||||
#endif
|
#endif
|
||||||
|
|||||||
Loading…
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Reference in New Issue
Block a user