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-rw-r--r--src/ecmult_gen_impl.h134
1 files changed, 96 insertions, 38 deletions
diff --git a/src/ecmult_gen_impl.h b/src/ecmult_gen_impl.h
index 849452c7a1..4697753ac8 100644
--- a/src/ecmult_gen_impl.h
+++ b/src/ecmult_gen_impl.h
@@ -1,5 +1,5 @@
/**********************************************************************
- * Copyright (c) 2013, 2014 Pieter Wuille *
+ * Copyright (c) 2013, 2014, 2015 Pieter Wuille, Gregory Maxwell *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or http://www.opensource.org/licenses/mit-license.php.*
**********************************************************************/
@@ -10,36 +10,23 @@
#include "scalar.h"
#include "group.h"
#include "ecmult_gen.h"
+#include "hash_impl.h"
-typedef struct {
- /* For accelerating the computation of a*G:
- * To harden against timing attacks, use the following mechanism:
- * * Break up the multiplicand into groups of 4 bits, called n_0, n_1, n_2, ..., n_63.
- * * Compute sum(n_i * 16^i * G + U_i, i=0..63), where:
- * * U_i = U * 2^i (for i=0..62)
- * * U_i = U * (1-2^63) (for i=63)
- * where U is a point with no known corresponding scalar. Note that sum(U_i, i=0..63) = 0.
- * For each i, and each of the 16 possible values of n_i, (n_i * 16^i * G + U_i) is
- * precomputed (call it prec(i, n_i)). The formula now becomes sum(prec(i, n_i), i=0..63).
- * None of the resulting prec group elements have a known scalar, and neither do any of
- * the intermediate sums while computing a*G.
- */
- secp256k1_ge_storage_t prec[64][16]; /* prec[j][i] = 16^j * i * G + U_i */
-} secp256k1_ecmult_gen_consts_t;
-
-static const secp256k1_ecmult_gen_consts_t *secp256k1_ecmult_gen_consts = NULL;
+static void secp256k1_ecmult_gen_context_init(secp256k1_ecmult_gen_context_t *ctx) {
+ ctx->prec = NULL;
+}
-static void secp256k1_ecmult_gen_start(void) {
+static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context_t *ctx) {
secp256k1_ge_t prec[1024];
secp256k1_gej_t gj;
secp256k1_gej_t nums_gej;
- secp256k1_ecmult_gen_consts_t *ret;
int i, j;
- if (secp256k1_ecmult_gen_consts != NULL)
+
+ if (ctx->prec != NULL) {
return;
+ }
- /* Allocate the precomputation table. */
- ret = (secp256k1_ecmult_gen_consts_t*)checked_malloc(sizeof(secp256k1_ecmult_gen_consts_t));
+ ctx->prec = (secp256k1_ge_storage_t (*)[64][16])checked_malloc(sizeof(*ctx->prec));
/* get the generator */
secp256k1_gej_set_ge(&gj, &secp256k1_ge_const_g);
@@ -85,42 +72,113 @@ static void secp256k1_ecmult_gen_start(void) {
}
for (j = 0; j < 64; j++) {
for (i = 0; i < 16; i++) {
- secp256k1_ge_to_storage(&ret->prec[j][i], &prec[j*16 + i]);
+ secp256k1_ge_to_storage(&(*ctx->prec)[j][i], &prec[j*16 + i]);
}
}
+ secp256k1_ecmult_gen_blind(ctx, NULL);
+}
- /* Set the global pointer to the precomputation table. */
- secp256k1_ecmult_gen_consts = ret;
+static int secp256k1_ecmult_gen_context_is_built(const secp256k1_ecmult_gen_context_t* ctx) {
+ return ctx->prec != NULL;
}
-static void secp256k1_ecmult_gen_stop(void) {
- secp256k1_ecmult_gen_consts_t *c;
- if (secp256k1_ecmult_gen_consts == NULL)
- return;
+static void secp256k1_ecmult_gen_context_clone(secp256k1_ecmult_gen_context_t *dst,
+ const secp256k1_ecmult_gen_context_t *src) {
+ if (src->prec == NULL) {
+ dst->prec = NULL;
+ } else {
+ dst->prec = (secp256k1_ge_storage_t (*)[64][16])checked_malloc(sizeof(*dst->prec));
+ memcpy(dst->prec, src->prec, sizeof(*dst->prec));
+ dst->initial = src->initial;
+ dst->blind = src->blind;
+ }
+}
- c = (secp256k1_ecmult_gen_consts_t*)secp256k1_ecmult_gen_consts;
- secp256k1_ecmult_gen_consts = NULL;
- free(c);
+static void secp256k1_ecmult_gen_context_clear(secp256k1_ecmult_gen_context_t *ctx) {
+ free(ctx->prec);
+ secp256k1_scalar_clear(&ctx->blind);
+ secp256k1_gej_clear(&ctx->initial);
+ ctx->prec = NULL;
}
-static void secp256k1_ecmult_gen(secp256k1_gej_t *r, const secp256k1_scalar_t *gn) {
- const secp256k1_ecmult_gen_consts_t *c = secp256k1_ecmult_gen_consts;
+static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context_t *ctx, secp256k1_gej_t *r, const secp256k1_scalar_t *gn) {
secp256k1_ge_t add;
secp256k1_ge_storage_t adds;
+ secp256k1_scalar_t gnb;
int bits;
int i, j;
- secp256k1_gej_set_infinity(r);
+ memset(&adds, 0, sizeof(adds));
+ *r = ctx->initial;
+ /* Blind scalar/point multiplication by computing (n-b)G + bG instead of nG. */
+ secp256k1_scalar_add(&gnb, gn, &ctx->blind);
add.infinity = 0;
for (j = 0; j < 64; j++) {
- bits = secp256k1_scalar_get_bits(gn, j * 4, 4);
+ bits = secp256k1_scalar_get_bits(&gnb, j * 4, 4);
for (i = 0; i < 16; i++) {
- secp256k1_ge_storage_cmov(&adds, &c->prec[j][i], i == bits);
+ /** This uses a conditional move to avoid any secret data in array indexes.
+ * _Any_ use of secret indexes has been demonstrated to result in timing
+ * sidechannels, even when the cache-line access patterns are uniform.
+ * See also:
+ * "A word of warning", CHES 2013 Rump Session, by Daniel J. Bernstein and Peter Schwabe
+ * (https://cryptojedi.org/peter/data/chesrump-20130822.pdf) and
+ * "Cache Attacks and Countermeasures: the Case of AES", RSA 2006,
+ * by Dag Arne Osvik, Adi Shamir, and Eran Tromer
+ * (http://www.tau.ac.il/~tromer/papers/cache.pdf)
+ */
+ secp256k1_ge_storage_cmov(&adds, &(*ctx->prec)[j][i], i == bits);
}
secp256k1_ge_from_storage(&add, &adds);
secp256k1_gej_add_ge(r, r, &add);
}
bits = 0;
secp256k1_ge_clear(&add);
+ secp256k1_scalar_clear(&gnb);
+}
+
+/* Setup blinding values for secp256k1_ecmult_gen. */
+static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context_t *ctx, const unsigned char *seed32) {
+ secp256k1_scalar_t b;
+ secp256k1_gej_t gb;
+ secp256k1_fe_t s;
+ unsigned char nonce32[32];
+ secp256k1_rfc6979_hmac_sha256_t rng;
+ int retry;
+ if (!seed32) {
+ /* When seed is NULL, reset the initial point and blinding value. */
+ secp256k1_gej_set_ge(&ctx->initial, &secp256k1_ge_const_g);
+ secp256k1_gej_neg(&ctx->initial, &ctx->initial);
+ secp256k1_scalar_set_int(&ctx->blind, 1);
+ }
+ /* The prior blinding value (if not reset) is chained forward by including it in the hash. */
+ secp256k1_scalar_get_b32(nonce32, &ctx->blind);
+ /** Using a CSPRNG allows a failure free interface, avoids needing large amounts of random data,
+ * and guards against weak or adversarial seeds. This is a simpler and safer interface than
+ * asking the caller for blinding values directly and expecting them to retry on failure.
+ */
+ secp256k1_rfc6979_hmac_sha256_initialize(&rng, seed32 ? seed32 : nonce32, 32, nonce32, 32, NULL, 0);
+ /* Retry for out of range results to achieve uniformity. */
+ do {
+ secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
+ retry = !secp256k1_fe_set_b32(&s, nonce32);
+ retry |= secp256k1_fe_is_zero(&s);
+ } while (retry);
+ /* Randomize the projection to defend against multiplier sidechannels. */
+ secp256k1_gej_rescale(&ctx->initial, &s);
+ secp256k1_fe_clear(&s);
+ do {
+ secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
+ secp256k1_scalar_set_b32(&b, nonce32, &retry);
+ /* A blinding value of 0 works, but would undermine the projection hardening. */
+ retry |= secp256k1_scalar_is_zero(&b);
+ } while (retry);
+ secp256k1_rfc6979_hmac_sha256_finalize(&rng);
+ memset(nonce32, 0, 32);
+ secp256k1_ecmult_gen(ctx, &gb, &b);
+ secp256k1_scalar_negate(&b, &b);
+ ctx->blind = b;
+ ctx->initial = gb;
+ secp256k1_scalar_clear(&b);
+ secp256k1_gej_clear(&gb);
}
#endif
generated by cgit v1.2.3 (git 2.26.2) at 2024-11-12 19:49:07 +0000