frost/secp256k1-zkp
3
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

ecmult_gen: Simplify ecmult_gen context after making table static

Browse Source 
This is a backwards-compatible API change: Before this commit, a context
initialized for signing was required to call functions that rely on
ecmult_gen. After this commit, this is no longer necessary because the
static ecmult_gen table is always present. In practice this means that
the corresponding functions will just work instead of calling the
illegal callback when given a context which is not (officially)
initialized for signing.

This is in line with 6815761, which made the analogous change with
respect to ecmult and contexts initialized for signing. But as opposed
to 681571, which removed the ecmult context entirely, we cannot remove
the ecmult_gen context entirely because it is still used for random
blinding. Moreover, since the secp256k1_context_no_precomp context is
const and cannot meaningfully support random blinding, we refrain (for
now) from changing its API, i.e., the illegal callback will still be
called when trying to use ecmult_gen operations with the static
secp256k1_context_no_precomp context.
This commit is contained in:
Tim Ruffing
2021-11-09 10:17:02 +01:00
parent e43ba02cfc
commit 3b0c2185ea
8 changed files with 109 additions and 101 deletions
Download Patch File Download Diff File Expand all files Collapse all files

37
src/ecmult_gen_impl.h
View File

@@ -14,35 +14,34 @@
#include "hash_impl.h"
#include "ecmult_static_context.h"
static const size_t SECP256K1_ECMULT_GEN_CONTEXT_PREALLOCATED_SIZE = 0;
static void secp256k1_ecmult_gen_context_init(secp256k1_ecmult_gen_context *ctx) {
ctx->prec = NULL;
}
static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context *ctx, void **prealloc) {
if (ctx->prec != NULL) {
return;
}
(void)prealloc;
ctx->prec = (secp256k1_ge_storage (*)[ECMULT_GEN_PREC_N][ECMULT_GEN_PREC_G])secp256k1_ecmult_static_context;
static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context *ctx) {
secp256k1_ecmult_gen_blind(ctx, NULL);
ctx->built = 1;
}
static int secp256k1_ecmult_gen_context_is_built(const secp256k1_ecmult_gen_context* ctx) {
return ctx->prec != NULL;
}
static void secp256k1_ecmult_gen_context_finalize_memcpy(secp256k1_ecmult_gen_context *dst, const secp256k1_ecmult_gen_context *src) {
(void)dst, (void)src;
return ctx->built;
}
static void secp256k1_ecmult_gen_context_clear(secp256k1_ecmult_gen_context *ctx) {
ctx->built = 0;
secp256k1_scalar_clear(&ctx->blind);
secp256k1_gej_clear(&ctx->initial);
ctx->prec = NULL;
}
/* For accelerating the computation of a*G:
* To harden against timing attacks, use the following mechanism:
* * Break up the multiplicand into groups of PREC_B bits, called n_0, n_1, n_2, ..., n_(PREC_N-1).
* * Compute sum(n_i * (PREC_G)^i * G + U_i, i=0 ... PREC_N-1), where:
* * U_i = U * 2^i, for i=0 ... PREC_N-2
* * U_i = U * (1-2^(PREC_N-1)), for i=PREC_N-1
* where U is a point with no known corresponding scalar. Note that sum(U_i, i=0 ... PREC_N-1) = 0.
* For each i, and each of the PREC_G possible values of n_i, (n_i * (PREC_G)^i * G + U_i) is
* precomputed (call it prec(i, n_i)). The formula now becomes sum(prec(i, n_i), i=0 ... PREC_N-1).
* None of the resulting prec group elements have a known scalar, and neither do any of
* the intermediate sums while computing a*G.
* The prec values are stored in secp256k1_ecmult_gen_prec_table[j][i] = (PREC_G)^j * i * G + U_i.
*/
static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp256k1_gej *r, const secp256k1_scalar *gn) {
secp256k1_ge add;
secp256k1_ge_storage adds;
@@ -67,7 +66,7 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
* by Dag Arne Osvik, Adi Shamir, and Eran Tromer
* (https://www.tau.ac.il/~tromer/papers/cache.pdf)
*/
secp256k1_ge_storage_cmov(&adds, &(*ctx->prec)[j][i], i == bits);
secp256k1_ge_storage_cmov(&adds, &secp256k1_ecmult_gen_prec_table[j][i], i == bits);
}
secp256k1_ge_from_storage(&add, &adds);
secp256k1_gej_add_ge(r, r, &add);