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pqcrypto/crypto_kem/newhope512cpa/clean/reduce.c

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2019-05-08 16:33:08 +01:00
#include "reduce.h"
#include "params.h"
static const uint32_t qinv = 12287; // -inverse_mod(p,2^18)
static const uint32_t rlog = 18;
/*************************************************
* Name: verify
*
* Description: Montgomery reduction; given a 32-bit integer a, computes
* 16-bit integer congruent to a * R^-1 mod q,
* where R=2^18 (see value of rlog)
*
* Arguments: - uint32_t a: input unsigned integer to be reduced; has to be in {0,...,1073491968}
*
* Returns: unsigned integer in {0,...,2^14-1} congruent to a * R^-1 modulo q.
**************************************************/
uint16_t PQCLEAN_NEWHOPE512CPA_CLEAN_montgomery_reduce(uint32_t a) {
uint32_t u;
u = (a * qinv);
u &= ((1 << rlog) - 1);
u *= NEWHOPE_Q;
a = a + u;
return a >> 18;
}