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bignum: add assumption about i to bn_multiply_reduce_step

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Pavol Rusnak 2019-01-23 15:45:25 +01:00
parent c34e8ab3bd
commit 5cb2c71f08
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1 changed files with 2 additions and 0 deletions
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bignum.c
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@ -425,6 +425,7 @@ void bn_multiply_long(const bignum256 *k, const bignum256 *x, uint32_t res[18])
// auxiliary function for multiplication.
// reduces res modulo prime.
// assumes i >= 8 and i <= 16
// assumes res normalized, res < 2^(30(i-7)) * 2 * prime
// guarantees res normalized, res < 2^(30(i-8)) * 2 * prime
void bn_multiply_reduce_step(uint32_t res[18], const bignum256 *prime, uint32_t i) {
@ -436,6 +437,7 @@ void bn_multiply_reduce_step(uint32_t res[18], const bignum256 *prime, uint32_t
// 0 <= coef < 2^31
// subtract (coef * 2^(30k) * prime) from res
// note that we unrolled the first iteration
assert(i >= 8 && i <= 16);
uint32_t j;
uint32_t coef = (res[i] >> 16) + (res[i + 1] << 14);
uint64_t temp = 0x2000000000000000ull + res[i - 8] - prime->val[0] * (uint64_t)coef;