211 lines
8.2 KiB
C
211 lines
8.2 KiB
C
/**********************************************************************
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* Copyright (c) 2013, 2014, 2015 Pieter Wuille, Gregory Maxwell *
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* Distributed under the MIT software license, see the accompanying *
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* file COPYING or http://www.opensource.org/licenses/mit-license.php.*
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**********************************************************************/
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#ifndef SECP256K1_ECMULT_GEN_IMPL_H
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#define SECP256K1_ECMULT_GEN_IMPL_H
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#include "scalar.h"
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#include "group.h"
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#include "ecmult_gen.h"
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#include "hash_impl.h"
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#ifdef USE_ECMULT_STATIC_PRECOMPUTATION
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#include "ecmult_static_context.h"
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#endif
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static void secp256k1_ecmult_gen_context_init(secp256k1_ecmult_gen_context *ctx) {
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ctx->prec = NULL;
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}
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static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context *ctx, const secp256k1_callback* cb) {
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#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
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secp256k1_ge prec[1024];
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secp256k1_gej gj;
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secp256k1_gej nums_gej;
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int i, j;
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#endif
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if (ctx->prec != NULL) {
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return;
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}
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#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
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ctx->prec = (secp256k1_ge_storage (*)[64][16])checked_malloc(cb, sizeof(*ctx->prec));
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/* get the generator */
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secp256k1_gej_set_ge(&gj, &secp256k1_ge_const_g);
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/* Construct a group element with no known corresponding scalar (nothing up my sleeve). */
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{
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static const unsigned char nums_b32[33] = "The scalar for this x is unknown";
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secp256k1_fe nums_x;
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secp256k1_ge nums_ge;
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int r;
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r = secp256k1_fe_set_b32(&nums_x, nums_b32);
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(void)r;
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VERIFY_CHECK(r);
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r = secp256k1_ge_set_xo_var(&nums_ge, &nums_x, 0);
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(void)r;
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VERIFY_CHECK(r);
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secp256k1_gej_set_ge(&nums_gej, &nums_ge);
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/* Add G to make the bits in x uniformly distributed. */
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secp256k1_gej_add_ge_var(&nums_gej, &nums_gej, &secp256k1_ge_const_g, NULL);
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}
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/* compute prec. */
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{
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secp256k1_gej precj[1024]; /* Jacobian versions of prec. */
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secp256k1_gej gbase;
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secp256k1_gej numsbase;
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gbase = gj; /* 16^j * G */
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numsbase = nums_gej; /* 2^j * nums. */
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for (j = 0; j < 64; j++) {
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/* Set precj[j*16 .. j*16+15] to (numsbase, numsbase + gbase, ..., numsbase + 15*gbase). */
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precj[j*16] = numsbase;
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for (i = 1; i < 16; i++) {
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secp256k1_gej_add_var(&precj[j*16 + i], &precj[j*16 + i - 1], &gbase, NULL);
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}
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/* Multiply gbase by 16. */
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for (i = 0; i < 4; i++) {
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secp256k1_gej_double_var(&gbase, &gbase, NULL);
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}
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/* Multiply numbase by 2. */
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secp256k1_gej_double_var(&numsbase, &numsbase, NULL);
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if (j == 62) {
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/* In the last iteration, numsbase is (1 - 2^j) * nums instead. */
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secp256k1_gej_neg(&numsbase, &numsbase);
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secp256k1_gej_add_var(&numsbase, &numsbase, &nums_gej, NULL);
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}
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}
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secp256k1_ge_set_all_gej_var(prec, precj, 1024, cb);
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}
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for (j = 0; j < 64; j++) {
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for (i = 0; i < 16; i++) {
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secp256k1_ge_to_storage(&(*ctx->prec)[j][i], &prec[j*16 + i]);
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}
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}
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#else
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(void)cb;
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ctx->prec = (secp256k1_ge_storage (*)[64][16])secp256k1_ecmult_static_context;
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#endif
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secp256k1_ecmult_gen_blind(ctx, NULL);
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}
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static int secp256k1_ecmult_gen_context_is_built(const secp256k1_ecmult_gen_context* ctx) {
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return ctx->prec != NULL;
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}
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static void secp256k1_ecmult_gen_context_clone(secp256k1_ecmult_gen_context *dst,
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const secp256k1_ecmult_gen_context *src, const secp256k1_callback* cb) {
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if (src->prec == NULL) {
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dst->prec = NULL;
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} else {
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#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
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dst->prec = (secp256k1_ge_storage (*)[64][16])checked_malloc(cb, sizeof(*dst->prec));
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memcpy(dst->prec, src->prec, sizeof(*dst->prec));
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#else
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(void)cb;
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dst->prec = src->prec;
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#endif
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dst->initial = src->initial;
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dst->blind = src->blind;
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}
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}
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static void secp256k1_ecmult_gen_context_clear(secp256k1_ecmult_gen_context *ctx) {
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#ifndef USE_ECMULT_STATIC_PRECOMPUTATION
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free(ctx->prec);
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#endif
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secp256k1_scalar_clear(&ctx->blind);
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secp256k1_gej_clear(&ctx->initial);
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ctx->prec = NULL;
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}
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static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp256k1_gej *r, const secp256k1_scalar *gn) {
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secp256k1_ge add;
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secp256k1_ge_storage adds;
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secp256k1_scalar gnb;
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int bits;
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int i, j;
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memset(&adds, 0, sizeof(adds));
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*r = ctx->initial;
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/* Blind scalar/point multiplication by computing (n-b)G + bG instead of nG. */
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secp256k1_scalar_add(&gnb, gn, &ctx->blind);
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add.infinity = 0;
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for (j = 0; j < 64; j++) {
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bits = secp256k1_scalar_get_bits(&gnb, j * 4, 4);
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for (i = 0; i < 16; i++) {
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/** This uses a conditional move to avoid any secret data in array indexes.
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* _Any_ use of secret indexes has been demonstrated to result in timing
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* sidechannels, even when the cache-line access patterns are uniform.
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* See also:
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* "A word of warning", CHES 2013 Rump Session, by Daniel J. Bernstein and Peter Schwabe
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* (https://cryptojedi.org/peter/data/chesrump-20130822.pdf) and
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* "Cache Attacks and Countermeasures: the Case of AES", RSA 2006,
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* by Dag Arne Osvik, Adi Shamir, and Eran Tromer
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* (http://www.tau.ac.il/~tromer/papers/cache.pdf)
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*/
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secp256k1_ge_storage_cmov(&adds, &(*ctx->prec)[j][i], i == bits);
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}
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secp256k1_ge_from_storage(&add, &adds);
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secp256k1_gej_add_ge(r, r, &add);
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}
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bits = 0;
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secp256k1_ge_clear(&add);
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secp256k1_scalar_clear(&gnb);
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}
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/* Setup blinding values for secp256k1_ecmult_gen. */
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static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const unsigned char *seed32) {
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secp256k1_scalar b;
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secp256k1_gej gb;
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secp256k1_fe s;
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unsigned char nonce32[32];
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secp256k1_rfc6979_hmac_sha256 rng;
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int retry;
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unsigned char keydata[64] = {0};
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if (seed32 == NULL) {
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/* When seed is NULL, reset the initial point and blinding value. */
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secp256k1_gej_set_ge(&ctx->initial, &secp256k1_ge_const_g);
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secp256k1_gej_neg(&ctx->initial, &ctx->initial);
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secp256k1_scalar_set_int(&ctx->blind, 1);
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}
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/* The prior blinding value (if not reset) is chained forward by including it in the hash. */
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secp256k1_scalar_get_b32(nonce32, &ctx->blind);
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/** Using a CSPRNG allows a failure free interface, avoids needing large amounts of random data,
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* and guards against weak or adversarial seeds. This is a simpler and safer interface than
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* asking the caller for blinding values directly and expecting them to retry on failure.
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*/
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memcpy(keydata, nonce32, 32);
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if (seed32 != NULL) {
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memcpy(keydata + 32, seed32, 32);
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}
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secp256k1_rfc6979_hmac_sha256_initialize(&rng, keydata, seed32 ? 64 : 32);
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memset(keydata, 0, sizeof(keydata));
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/* Retry for out of range results to achieve uniformity. */
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do {
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secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
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retry = !secp256k1_fe_set_b32(&s, nonce32);
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retry |= secp256k1_fe_is_zero(&s);
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} while (retry); /* This branch true is cryptographically unreachable. Requires sha256_hmac output > Fp. */
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/* Randomize the projection to defend against multiplier sidechannels. */
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secp256k1_gej_rescale(&ctx->initial, &s);
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secp256k1_fe_clear(&s);
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do {
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secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
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secp256k1_scalar_set_b32(&b, nonce32, &retry);
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/* A blinding value of 0 works, but would undermine the projection hardening. */
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retry |= secp256k1_scalar_is_zero(&b);
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} while (retry); /* This branch true is cryptographically unreachable. Requires sha256_hmac output > order. */
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secp256k1_rfc6979_hmac_sha256_finalize(&rng);
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memset(nonce32, 0, 32);
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secp256k1_ecmult_gen(ctx, &gb, &b);
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secp256k1_scalar_negate(&b, &b);
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ctx->blind = b;
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ctx->initial = gb;
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secp256k1_scalar_clear(&b);
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secp256k1_gej_clear(&gb);
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}
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#endif /* SECP256K1_ECMULT_GEN_IMPL_H */
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