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bignum: constant time implementation for bn_mod()

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Roman Zeyde 2015-08-31 20:55:02 +03:00
parent e67018178b
commit 437f8b3856
1 changed files with 4 additions and 21 deletions
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25
bignum.c
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@ -229,27 +229,10 @@ void bn_mult_k(bignum256 *x, uint8_t k, const bignum256 *prime)
// assumes x partly reduced, guarantees x fully reduced.
void bn_mod(bignum256 *x, const bignum256 *prime)
{
int i = 8;
uint32_t temp;
// compare numbers
while (i >= 0 && prime->val[i] == x->val[i]) i--;
// if equal
if (i == -1) {
// set x to zero
bn_zero(x);
} else {
// if x is greater
if (x->val[i] > prime->val[i]) {
// substract p from x
temp = 0x40000000u;
for (i = 0; i < 9; i++) {
temp += x->val[i] - prime->val[i];
x->val[i] = temp & 0x3FFFFFFF;
temp >>= 30;
temp += 0x3FFFFFFFu;
}
}
}
const int flag = bn_is_less(x, prime); // x < prime
bignum256 temp;
bn_subtract(x, prime, &temp); // temp = x - prime
bn_cmov(x, flag, x, &temp);
}
// auxiliary function for multiplication.