[Pkg-bitcoin-commits] [libsecp256k1] 26/45: ecmult_const: unify endomorphism and non-endomorphism skew cases
Jonas Smedegaard
dr at jones.dk
Sat Aug 27 12:00:04 UTC 2016
This is an automated email from the git hooks/post-receive script.
js pushed a commit to branch master
in repository libsecp256k1.
commit c6191fded8b4d5b1ce5ee581a73db3f94bd1f52e
Author: Andrew Poelstra <apoelstra at wpsoftware.net>
Date: Thu Jun 30 09:59:10 2016 +0000
ecmult_const: unify endomorphism and non-endomorphism skew cases
We now do a skew correction even without the endomorphism optimization,
which costs one additional group addition but unifies a lot of code.
---
src/ecmult_const_impl.h | 67 +++++++++++++++++--------------------------------
src/tests.c | 10 ++------
2 files changed, 25 insertions(+), 52 deletions(-)
diff --git a/src/ecmult_const_impl.h b/src/ecmult_const_impl.h
index 90ac947..7a6a253 100644
--- a/src/ecmult_const_impl.h
+++ b/src/ecmult_const_impl.h
@@ -58,22 +58,24 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w) {
int global_sign;
int skew = 0;
int word = 0;
+
/* 1 2 3 */
int u_last;
int u;
-#ifdef USE_ENDOMORPHISM
int flip;
int bit;
secp256k1_scalar neg_s;
int not_neg_one;
- /* If we are using the endomorphism, we cannot handle even numbers by negating
- * them, since we are working with 128-bit numbers whose negations would be 256
- * bits, eliminating the performance advantage. Instead we use a technique from
+ /* Note that we cannot handle even numbers by negating them to be odd, as is
+ * done in other implementations, since if our scalars were specified to have
+ * width < 256 for performance reasons, their negations would have width 256
+ * and we'd lose any performance benefit. Instead, we use a technique from
* Section 4.2 of the Okeya/Tagaki paper, which is to add either 1 (for even)
- * or 2 (for odd) to the number we are encoding, then compensating after the
- * multiplication. */
- /* Negative 128-bit numbers will be negated, since otherwise they are 256-bit */
+ * or 2 (for odd) to the number we are encoding, returning a skew value indicating
+ * this, and having the caller compensate after doing the multiplication. */
+
+ /* Negative numbers will be negated to keep their bit representation below the maximum width */
flip = secp256k1_scalar_is_high(&s);
/* We add 1 to even numbers, 2 to odd ones, noting that negation flips parity */
bit = flip ^ (s.d[0] & 1);
@@ -89,11 +91,6 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w) {
global_sign = secp256k1_scalar_cond_negate(&s, flip);
global_sign *= not_neg_one * 2 - 1;
skew = 1 << bit;
-#else
- /* Otherwise, we just negate to force oddness */
- int is_even = secp256k1_scalar_is_even(&s);
- global_sign = secp256k1_scalar_cond_negate(&s, is_even);
-#endif
/* 4 */
u_last = secp256k1_scalar_shr_int(&s, w);
@@ -127,15 +124,13 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
secp256k1_ge tmpa;
secp256k1_fe Z;
+ int skew_1;
+ int wnaf_1[1 + WNAF_SIZE(WINDOW_A - 1)];
#ifdef USE_ENDOMORPHISM
secp256k1_ge pre_a_lam[ECMULT_TABLE_SIZE(WINDOW_A)];
- int wnaf_1[1 + WNAF_SIZE(WINDOW_A - 1)];
int wnaf_lam[1 + WNAF_SIZE(WINDOW_A - 1)];
- int skew_1;
int skew_lam;
secp256k1_scalar q_1, q_lam;
-#else
- int wnaf[1 + WNAF_SIZE(WINDOW_A - 1)];
#endif
int i;
@@ -145,18 +140,10 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
#ifdef USE_ENDOMORPHISM
/* split q into q_1 and q_lam (where q = q_1 + q_lam*lambda, and q_1 and q_lam are ~128 bit) */
secp256k1_scalar_split_lambda(&q_1, &q_lam, &sc);
- /* no need for zero correction when using endomorphism since even
- * numbers have one added to them anyway */
skew_1 = secp256k1_wnaf_const(wnaf_1, q_1, WINDOW_A - 1);
skew_lam = secp256k1_wnaf_const(wnaf_lam, q_lam, WINDOW_A - 1);
#else
- int is_zero = secp256k1_scalar_is_zero(scalar);
- /* the wNAF ladder cannot handle zero, so bump this to one .. we will
- * correct the result after the fact */
- sc.d[0] += is_zero;
- VERIFY_CHECK(!secp256k1_scalar_is_zero(&sc));
-
- secp256k1_wnaf_const(wnaf, sc, WINDOW_A - 1);
+ skew_1 = secp256k1_wnaf_const(wnaf_1, sc, WINDOW_A - 1);
#endif
/* Calculate odd multiples of a.
@@ -179,21 +166,15 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
/* first loop iteration (separated out so we can directly set r, rather
* than having it start at infinity, get doubled several times, then have
* its new value added to it) */
-#ifdef USE_ENDOMORPHISM
i = wnaf_1[WNAF_SIZE(WINDOW_A - 1)];
VERIFY_CHECK(i != 0);
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, i, WINDOW_A);
secp256k1_gej_set_ge(r, &tmpa);
-
+#ifdef USE_ENDOMORPHISM
i = wnaf_lam[WNAF_SIZE(WINDOW_A - 1)];
VERIFY_CHECK(i != 0);
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a_lam, i, WINDOW_A);
secp256k1_gej_add_ge(r, r, &tmpa);
-#else
- i = wnaf[WNAF_SIZE(WINDOW_A - 1)];
- VERIFY_CHECK(i != 0);
- ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, i, WINDOW_A);
- secp256k1_gej_set_ge(r, &tmpa);
#endif
/* remaining loop iterations */
for (i = WNAF_SIZE(WINDOW_A - 1) - 1; i >= 0; i--) {
@@ -202,59 +183,57 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
for (j = 0; j < WINDOW_A - 1; ++j) {
secp256k1_gej_double_nonzero(r, r, NULL);
}
-#ifdef USE_ENDOMORPHISM
+
n = wnaf_1[i];
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, n, WINDOW_A);
VERIFY_CHECK(n != 0);
secp256k1_gej_add_ge(r, r, &tmpa);
-
+#ifdef USE_ENDOMORPHISM
n = wnaf_lam[i];
ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a_lam, n, WINDOW_A);
VERIFY_CHECK(n != 0);
secp256k1_gej_add_ge(r, r, &tmpa);
-#else
- n = wnaf[i];
- VERIFY_CHECK(n != 0);
- ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, n, WINDOW_A);
- secp256k1_gej_add_ge(r, r, &tmpa);
#endif
}
secp256k1_fe_mul(&r->z, &r->z, &Z);
-#ifdef USE_ENDOMORPHISM
{
/* Correct for wNAF skew */
secp256k1_ge correction = *a;
secp256k1_ge_storage correction_1_stor;
+#ifdef USE_ENDOMORPHISM
secp256k1_ge_storage correction_lam_stor;
+#endif
secp256k1_ge_storage a2_stor;
secp256k1_gej tmpj;
secp256k1_gej_set_ge(&tmpj, &correction);
secp256k1_gej_double_var(&tmpj, &tmpj, NULL);
secp256k1_ge_set_gej(&correction, &tmpj);
secp256k1_ge_to_storage(&correction_1_stor, a);
+#ifdef USE_ENDOMORPHISM
secp256k1_ge_to_storage(&correction_lam_stor, a);
+#endif
secp256k1_ge_to_storage(&a2_stor, &correction);
/* For odd numbers this is 2a (so replace it), for even ones a (so no-op) */
secp256k1_ge_storage_cmov(&correction_1_stor, &a2_stor, skew_1 == 2);
+#ifdef USE_ENDOMORPHISM
secp256k1_ge_storage_cmov(&correction_lam_stor, &a2_stor, skew_lam == 2);
+#endif
/* Apply the correction */
secp256k1_ge_from_storage(&correction, &correction_1_stor);
secp256k1_ge_neg(&correction, &correction);
secp256k1_gej_add_ge(r, r, &correction);
+#ifdef USE_ENDOMORPHISM
secp256k1_ge_from_storage(&correction, &correction_lam_stor);
secp256k1_ge_neg(&correction, &correction);
secp256k1_ge_mul_lambda(&correction, &correction);
secp256k1_gej_add_ge(r, r, &correction);
- }
-#else
- /* correct for zero */
- r->infinity |= is_zero;
#endif
+ }
}
#endif
diff --git a/src/tests.c b/src/tests.c
index 1690c68..5300560 100644
--- a/src/tests.c
+++ b/src/tests.c
@@ -2399,9 +2399,7 @@ void test_constant_wnaf(const secp256k1_scalar *number, int w) {
secp256k1_scalar x, shift;
int wnaf[256] = {0};
int i;
-#ifdef USE_ENDOMORPHISM
int skew;
-#endif
secp256k1_scalar num = *number;
secp256k1_scalar_set_int(&x, 0);
@@ -2411,10 +2409,8 @@ void test_constant_wnaf(const secp256k1_scalar *number, int w) {
for (i = 0; i < 16; ++i) {
secp256k1_scalar_shr_int(&num, 8);
}
- skew = secp256k1_wnaf_const(wnaf, num, w);
-#else
- secp256k1_wnaf_const(wnaf, num, w);
#endif
+ skew = secp256k1_wnaf_const(wnaf, num, w);
for (i = WNAF_SIZE(w); i >= 0; --i) {
secp256k1_scalar t;
@@ -2433,10 +2429,8 @@ void test_constant_wnaf(const secp256k1_scalar *number, int w) {
}
secp256k1_scalar_add(&x, &x, &t);
}
-#ifdef USE_ENDOMORPHISM
- /* Skew num because when encoding 128-bit numbers as odd we use an offset */
+ /* Skew num because when encoding numbers as odd we use an offset */
secp256k1_scalar_cadd_bit(&num, skew == 2, 1);
-#endif
CHECK(secp256k1_scalar_eq(&x, &num));
}
--
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