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71 lines
2.0 KiB
C
71 lines
2.0 KiB
C
#include "reduce.h"
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#include "params.h"
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static const uint32_t qinv = 7679; // -inverse_mod(q,2^18)
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static const uint32_t rlog = 18;
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/*************************************************
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* Name: montgomery_reduce
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*
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* Description: Montgomery reduction; given a 32-bit integer a, computes
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* 16-bit integer congruent to a * R^-1 mod q,
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* where R=2^18 (see value of rlog)
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*
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* Arguments: - uint32_t a: input unsigned integer to be reduced; has to be in
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*{0,...,2281446912}
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*
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* Returns: unsigned integer in {0,...,2^13-1} congruent to a * R^-1 modulo
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*q.
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**************************************************/
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uint16_t PQCLEAN_KYBER768_montgomery_reduce(uint32_t a) {
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uint32_t u;
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u = (a * qinv);
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u &= ((1 << rlog) - 1);
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u *= KYBER_Q;
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a = a + u;
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return a >> rlog;
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}
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/*************************************************
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* Name: barrett_reduce
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*
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* Description: Barrett reduction; given a 16-bit integer a, computes
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* 16-bit integer congruent to a mod q in {0,...,11768}
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*
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* Arguments: - uint16_t a: input unsigned integer to be reduced
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*
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* Returns: unsigned integer in {0,...,11768} congruent to a modulo q.
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**************************************************/
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uint16_t PQCLEAN_KYBER768_barrett_reduce(uint16_t a) {
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uint32_t u;
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u = a >> 13; //((uint32_t) a * sinv) >> 16;
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u *= KYBER_Q;
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a -= u;
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return a;
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}
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/*************************************************
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* Name: freeze
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*
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* Description: Full reduction; given a 16-bit integer a, computes
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* unsigned integer a mod q.
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*
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* Arguments: - uint16_t x: input unsigned integer to be reduced
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*
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* Returns: unsigned integer in {0,...,q-1} congruent to a modulo q.
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**************************************************/
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uint16_t PQCLEAN_KYBER768_freeze(uint16_t x) {
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uint16_t m, r;
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int16_t c;
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r = PQCLEAN_KYBER768_barrett_reduce(x);
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m = r - KYBER_Q;
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c = m;
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c >>= 15;
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r = m ^ ((r ^ m) & c);
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return r;
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}
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