45 lines
1.4 KiB
C
45 lines
1.4 KiB
C
#include "params.h"
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#include "reduce.h"
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#include <stdint.h>
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/*************************************************
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* Name: PQCLEAN_KYBER76890S_CLEAN_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, where R=2^16
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*
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* Arguments: - int32_t a: input integer to be reduced;
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* has to be in {-q2^15,...,q2^15-1}
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*
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* Returns: integer in {-q+1,...,q-1} congruent to a * R^-1 modulo q.
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**************************************************/
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int16_t PQCLEAN_KYBER76890S_CLEAN_montgomery_reduce(int32_t a) {
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int32_t t;
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int16_t u;
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u = (int16_t)(a * (int64_t)QINV);
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t = (int32_t)u * KYBER_Q;
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t = a - t;
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t >>= 16;
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return (int16_t)t;
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}
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/*************************************************
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* Name: PQCLEAN_KYBER76890S_CLEAN_barrett_reduce
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*
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* Description: Barrett reduction; given a 16-bit integer a, computes
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* centered representative congruent to a mod q in {-(q-1)/2,...,(q-1)/2}
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*
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* Arguments: - int16_t a: input integer to be reduced
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*
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* Returns: integer in {-(q-1)/2,...,(q-1)/2} congruent to a modulo q.
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**************************************************/
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int16_t PQCLEAN_KYBER76890S_CLEAN_barrett_reduce(int16_t a) {
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int16_t t;
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const int16_t v = ((1U << 26) + KYBER_Q / 2) / KYBER_Q;
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t = ((int32_t)v * a + (1 << 25)) >> 26;
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t *= KYBER_Q;
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return a - t;
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}
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