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62 lines
1.7 KiB
C
62 lines
1.7 KiB
C
#include "reduce.h"
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#include "params.h"
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#include <stdint.h>
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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=2^16
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*
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* Arguments: - int32_t a: input integer to be reduced; 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 * 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: 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,...,q}
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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 {0,...,q} 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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int32_t t;
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const int32_t v = (1U << 26) / KYBER_Q + 1;
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t = v * a;
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t >>= 26;
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t *= KYBER_Q;
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return a - (int16_t)t;
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}
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/*************************************************
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* Name: csubq
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*
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* Description: Conditionallly subtract q
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*
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* Arguments: - int16_t a: input integer
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*
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* Returns: a - q if a >= q, else a
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**************************************************/
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int16_t PQCLEAN_KYBER76890S_CLEAN_csubq(int16_t a) {
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a -= KYBER_Q;
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a += (a >> 15) & KYBER_Q;
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return a;
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}
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