diff --git a/src/ecmult_impl.h b/src/ecmult_impl.h index 608d19b..7f3d5cd 100644 --- a/src/ecmult_impl.h +++ b/src/ecmult_impl.h @@ -567,7 +567,9 @@ static size_t secp256k1_strauss_max_points(secp256k1_scratch *scratch) { */ static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) { int skew = 0; - int pos = 1; + int pos; + int max_pos; + int last_w; const secp256k1_scalar *work = s; if (secp256k1_scalar_is_zero(s)) { @@ -582,14 +584,24 @@ static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) { } wnaf[0] = secp256k1_scalar_get_bits_var(work, 0, w) + skew; + /* Compute last window size. Relevant when window size doesn't divide the + * number of bits in the scalar */ + last_w = WNAF_BITS - (WNAF_SIZE(w) - 1) * w; - while (pos * w < WNAF_BITS) { - int now = w; - int val; - if (now + pos * w > WNAF_BITS) { - now = WNAF_BITS - pos * w; + /* Store the position of the first nonzero word in max_pos to allow + * skipping leading zeros when calculating the wnaf. */ + for (pos = WNAF_SIZE(w) - 1; pos > 0; pos--) { + int val = secp256k1_scalar_get_bits_var(work, pos * w, pos == WNAF_SIZE(w)-1 ? last_w : w); + if(val != 0) { + break; } - val = secp256k1_scalar_get_bits_var(work, pos * w, now); + wnaf[pos] = 0; + } + max_pos = pos; + pos = 1; + + while (pos <= max_pos) { + int val = secp256k1_scalar_get_bits_var(work, pos * w, pos == WNAF_SIZE(w)-1 ? last_w : w); if ((val & 1) == 0) { wnaf[pos - 1] -= (1 << w); wnaf[pos] = (val + 1); @@ -611,7 +623,6 @@ static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) { } ++pos; } - VERIFY_CHECK(pos == WNAF_SIZE(w)); return skew; } diff --git a/src/tests.c b/src/tests.c index ed88811..13d1a91 100644 --- a/src/tests.c +++ b/src/tests.c @@ -3040,7 +3040,20 @@ void test_fixed_wnaf(const secp256k1_scalar *number, int w) { CHECK(secp256k1_scalar_eq(&x, &num)); } -void test_fixed_wnaf_zero(int w) { +/* Checks that the first 8 elements of wnaf are equal to wnaf_expected and the + * rest is 0.*/ +void test_fixed_wnaf_small_helper(int *wnaf, int *wnaf_expected, int w) { + int i; + for (i = WNAF_SIZE(w)-1; i >= 8; --i) { + CHECK(wnaf[i] == 0); + } + for (i = 7; i >= 0; --i) { + CHECK(wnaf[i] == wnaf_expected[i]); + } +} + +void test_fixed_wnaf_small(void) { + int w = 4; int wnaf[256] = {0}; int i; int skew; @@ -3048,12 +3061,49 @@ void test_fixed_wnaf_zero(int w) { secp256k1_scalar_set_int(&num, 0); skew = secp256k1_wnaf_fixed(wnaf, &num, w); - for (i = WNAF_SIZE(w)-1; i >= 0; --i) { int v = wnaf[i]; CHECK(v == 0); } CHECK(skew == 0); + + secp256k1_scalar_set_int(&num, 1); + skew = secp256k1_wnaf_fixed(wnaf, &num, w); + for (i = WNAF_SIZE(w)-1; i >= 1; --i) { + int v = wnaf[i]; + CHECK(v == 0); + } + CHECK(wnaf[0] == 1); + CHECK(skew == 0); + + { + int wnaf_expected[8] = { 0xf, 0xf, 0xf, 0xf, 0xf, 0xf, 0xf, 0xf }; + secp256k1_scalar_set_int(&num, 0xffffffff); + skew = secp256k1_wnaf_fixed(wnaf, &num, w); + test_fixed_wnaf_small_helper(wnaf, wnaf_expected, w); + CHECK(skew == 0); + } + { + int wnaf_expected[8] = { -1, -1, -1, -1, -1, -1, -1, 0xf }; + secp256k1_scalar_set_int(&num, 0xeeeeeeee); + skew = secp256k1_wnaf_fixed(wnaf, &num, w); + test_fixed_wnaf_small_helper(wnaf, wnaf_expected, w); + CHECK(skew == 1); + } + { + int wnaf_expected[8] = { 1, 0, 1, 0, 1, 0, 1, 0 }; + secp256k1_scalar_set_int(&num, 0x01010101); + skew = secp256k1_wnaf_fixed(wnaf, &num, w); + test_fixed_wnaf_small_helper(wnaf, wnaf_expected, w); + CHECK(skew == 0); + } + { + int wnaf_expected[8] = { -0xf, 0, 0xf, -0xf, 0, 0xf, 1, 0 }; + secp256k1_scalar_set_int(&num, 0x01ef1ef1); + skew = secp256k1_wnaf_fixed(wnaf, &num, w); + test_fixed_wnaf_small_helper(wnaf, wnaf_expected, w); + CHECK(skew == 0); + } } void run_wnaf(void) { @@ -3067,7 +3117,7 @@ void run_wnaf(void) { n.d[0] = 2; test_constant_wnaf(&n, 4); /* Test 0 */ - test_fixed_wnaf_zero(4); + test_fixed_wnaf_small(); /* Random tests */ for (i = 0; i < count; i++) { random_scalar_order(&n);