pqc/crypto_sign/dilithium4/clean/reduce.c

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#include <stdint.h>
#include "params.h"
#include "reduce.h"
/*************************************************
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* Name: PQCLEAN_DILITHIUM4_CLEAN_montgomery_reduce
*
* Description: For finite field element a with 0 <= a <= Q*2^32,
* compute r \equiv a*2^{-32} (mod Q) such that 0 <= r < 2*Q.
*
* Arguments: - uint64_t: finite field element a
*
* Returns r.
**************************************************/
uint32_t PQCLEAN_DILITHIUM4_CLEAN_montgomery_reduce(uint64_t a) {
uint64_t t;
t = a * QINV;
t &= (1ULL << 32) - 1;
t *= Q;
t = a + t;
t >>= 32;
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return (uint32_t)t;
}
/*************************************************
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* Name: PQCLEAN_DILITHIUM4_CLEAN_reduce32
*
* Description: For finite field element a, compute r \equiv a (mod Q)
* such that 0 <= r < 2*Q.
*
* Arguments: - uint32_t: finite field element a
*
* Returns r.
**************************************************/
uint32_t PQCLEAN_DILITHIUM4_CLEAN_reduce32(uint32_t a) {
uint32_t t;
t = a & 0x7FFFFF;
a >>= 23;
t += (a << 13) - a;
return t;
}
/*************************************************
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* Name: PQCLEAN_DILITHIUM4_CLEAN_csubq
*
* Description: Subtract Q if input coefficient is bigger than Q.
*
* Arguments: - uint32_t: finite field element a
*
* Returns r.
**************************************************/
uint32_t PQCLEAN_DILITHIUM4_CLEAN_csubq(uint32_t a) {
a -= Q;
a += ((int32_t)a >> 31) & Q;
return a;
}
/*************************************************
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* Name: PQCLEAN_DILITHIUM4_CLEAN_freeze
*
* Description: For finite field element a, compute standard
* representative r = a mod Q.
*
* Arguments: - uint32_t: finite field element a
*
* Returns r.
**************************************************/
uint32_t PQCLEAN_DILITHIUM4_CLEAN_freeze(uint32_t a) {
a = PQCLEAN_DILITHIUM4_CLEAN_reduce32(a);
a = PQCLEAN_DILITHIUM4_CLEAN_csubq(a);
return a;
}