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Improve performance of _ecmult_wnaf

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- Track carry explicitly instead of adding to scalar
- Branch-free code for carry calculations
This commit is contained in:
Peter Dettman 2015-07-11 13:49:04 +10:00
parent 0cbc8600f3
commit 145cc6ea8f
1 changed files with 15 additions and 12 deletions
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25
src/ecmult_impl.h
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@ -220,6 +220,7 @@ static int secp256k1_ecmult_wnaf(int *wnaf, const secp256k1_scalar_t *a, int w)
int set_bits = 0; int set_bits = 0;
int bit = 0; int bit = 0;
int sign = 1; int sign = 1;
int carry = 0;
if (secp256k1_scalar_get_bits(&s, 255, 1)) { if (secp256k1_scalar_get_bits(&s, 255, 1)) {
secp256k1_scalar_negate(&s, &s); secp256k1_scalar_negate(&s, &s);
@ -229,26 +230,28 @@ static int secp256k1_ecmult_wnaf(int *wnaf, const secp256k1_scalar_t *a, int w)
while (bit < 256) { while (bit < 256) {
int now; int now;
int word; int word;
if (secp256k1_scalar_get_bits(&s, bit, 1) == 0) { if (secp256k1_scalar_get_bits(&s, bit, 1) == (unsigned int)carry) {
bit++; bit++;
continue; continue;
} }
now = w;
if (now > 256 - bit) {
now = 256 - bit;
}
word = secp256k1_scalar_get_bits_var(&s, bit, now) + carry;
carry = (word >> (w-1)) & 1;
word -= carry << w;
while (set_bits < bit) { while (set_bits < bit) {
wnaf[set_bits++] = 0; wnaf[set_bits++] = 0;
} }
now = w;
if (bit + now > 256) {
now = 256 - bit;
}
word = secp256k1_scalar_get_bits_var(&s, bit, now);
if (word & (1 << (w-1))) {
secp256k1_scalar_add_bit(&s, bit + w);
wnaf[set_bits++] = sign * (word - (1 << w));
} else {
wnaf[set_bits++] = sign * word; wnaf[set_bits++] = sign * word;
}
bit += now; bit += now;
} }
VERIFY_CHECK(carry == 0);
return set_bits; return set_bits;
} }