75 lines
1.8 KiB
C
75 lines
1.8 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: For finite field element a with 0 <= a <= Q*2^32,
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* compute r \equiv a*2^{-32} (mod Q) such that 0 <= r < 2*Q.
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*
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* Arguments: - uint64_t: finite field element a
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*
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* Returns r.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUMIII_montgomery_reduce(uint64_t a) {
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uint64_t t;
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t = a * QINV;
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t &= (1ULL << 32) - 1;
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t *= Q;
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t = a + t;
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t >>= 32;
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return t;
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}
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/*************************************************
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* Name: reduce32
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*
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* Description: For finite field element a, compute r \equiv a (mod Q)
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* such that 0 <= r < 2*Q.
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*
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* Arguments: - uint32_t: finite field element a
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*
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* Returns r.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUMIII_reduce32(uint32_t a) {
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uint32_t t;
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t = a & 0x7FFFFF;
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a >>= 23;
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t += (a << 13) - a;
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return t;
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}
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/*************************************************
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* Name: csubq
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*
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* Description: Subtract Q if input coefficient is bigger than Q.
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*
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* Arguments: - uint32_t: finite field element a
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*
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* Returns r.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUMIII_csubq(uint32_t a) {
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a -= Q;
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a += ((int32_t)a >> 31) & Q;
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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: For finite field element a, compute standard
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* representative r = a mod Q.
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*
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* Arguments: - uint32_t: finite field element a
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*
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* Returns r.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUMIII_freeze(uint32_t a) {
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a = PQCLEAN_DILITHIUMIII_reduce32(a);
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a = PQCLEAN_DILITHIUMIII_csubq(a);
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return a;
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}
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