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Increase sparsity of pippenger fixed window naf representation

This commit is contained in:
Jonas Nick 2018-02-27 21:34:08 +00:00
parent cd329dbc3e
commit 6dbb007869
2 changed files with 14 additions and 2 deletions
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13
src/ecmult_impl.h
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@ -607,6 +607,19 @@ static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) {
} else {
wnaf[pos] = (val + sign) ^ sign;
}
/* Set a coefficient to zero if it is 1 or -1 and the proceeding digit
* is strictly negative or strictly positive respectively. Only change
* coefficients at previous positions because above code assumes that
* wnaf[pos - 1] is odd.
*/
if (pos >= 2 && ((wnaf[pos - 1] == 1 && wnaf[pos - 2] < 0) || (wnaf[pos - 1] == -1 && wnaf[pos - 2] > 0))) {
if (wnaf[pos - 1] == 1) {
wnaf[pos - 2] += 1 << w;
} else {
wnaf[pos - 2] -= 1 << w;
}
wnaf[pos - 1] = 0;
}
++pos;
}
VERIFY_CHECK(pos == WNAF_SIZE(w));

3
src/tests.c
View File

@ -3022,8 +3022,7 @@ void test_fixed_wnaf(const secp256k1_scalar *number, int w) {
for (i = WNAF_SIZE(w)-1; i >= 0; --i) {
secp256k1_scalar t;
int v = wnaf[i];
CHECK(v != 0); /* check nonzero */
CHECK(v & 1); /* check parity */
CHECK(v == 0 || v & 1); /* check parity */
CHECK(v > -(1 << w)); /* check range above */
CHECK(v < (1 << w)); /* check range below */