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https://github.com/henrydcase/pqc.git
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116 lines
3.3 KiB
C
116 lines
3.3 KiB
C
#include "rounding.h"
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/*************************************************
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* Name: power2round
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*
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* Description: For finite field element a, compute a0, a1 such that
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* a mod Q = a1*2^D + a0 with -2^{D-1} < a0 <= 2^{D-1}.
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* Assumes a to be standard representative.
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*
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* Arguments: - uint32_t a: input element
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* - uint32_t *a0: pointer to output element Q + a0
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*
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* Returns a1.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUM2_AVX2_power2round(uint32_t a, uint32_t *a0) {
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int32_t t;
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/* Centralized remainder mod 2^D */
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t = a & ((1U << D) - 1);
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t -= (1U << (D - 1)) + 1;
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t += (t >> 31) & (1U << D);
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t -= (1U << (D - 1)) - 1;
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*a0 = Q + t;
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a = (a - t) >> D;
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return a;
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}
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/*************************************************
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* Name: PQCLEAN_DILITHIUM2_AVX2_decompose
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*
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* Description: For finite field element a, compute high and low bits a0, a1 such
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* that a mod Q = a1*ALPHA + a0 with -ALPHA/2 < a0 <= ALPHA/2 except
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* if a1 = (Q-1)/ALPHA where we set a1 = 0 and
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* -ALPHA/2 <= a0 = a mod Q - Q < 0. Assumes a to be standard
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* representative.
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*
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* Arguments: - uint32_t a: input element
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* - uint32_t *a0: pointer to output element Q + a0
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*
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* Returns a1.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUM2_AVX2_decompose(uint32_t a, uint32_t *a0) {
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int32_t t, u;
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/* Centralized remainder mod ALPHA */
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t = a & 0x7FFFF;
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t += (a >> 19) << 9;
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t -= ALPHA / 2 + 1;
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t += (t >> 31) & ALPHA;
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t -= ALPHA / 2 - 1;
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a -= t;
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/* Divide by ALPHA (possible to avoid) */
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u = a - 1;
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u >>= 31;
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a = (a >> 19) + 1;
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a -= u & 1;
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/* Border case */
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*a0 = Q + t - (a >> 4);
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a &= 0xF;
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return a;
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}
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/*************************************************
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* Name: PQCLEAN_DILITHIUM2_AVX2_make_hint
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*
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* Description: Compute hint bit indicating whether the low bits of the
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* input element overflow into the high bits. Inputs assumed to be
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* standard representatives.
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*
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* Arguments: - uint32_t a0: low bits of input element
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* - uint32_t a1: high bits of input element
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*
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* Returns 1 if high bits of a and b differ and 0 otherwise.
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**************************************************/
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unsigned int PQCLEAN_DILITHIUM2_AVX2_make_hint(const uint32_t a0, const uint32_t a1) {
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if (a0 <= GAMMA2 || a0 > Q - GAMMA2 || (a0 == Q - GAMMA2 && a1 == 0)) {
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return 0;
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}
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return 1;
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}
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/*************************************************
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* Name: PQCLEAN_DILITHIUM2_AVX2_use_hint
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*
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* Description: Correct high bits according to hint.
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*
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* Arguments: - uint32_t a: input element
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* - unsigned int hint: hint bit
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*
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* Returns corrected high bits.
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**************************************************/
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uint32_t PQCLEAN_DILITHIUM2_AVX2_use_hint(const uint32_t a, const unsigned int hint) {
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uint32_t a0, a1;
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a1 = PQCLEAN_DILITHIUM2_AVX2_decompose(a, &a0);
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if (hint == 0) {
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return a1;
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}
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if (a0 > Q) {
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return (a1 + 1) & 0xF;
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}
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return (a1 - 1) & 0xF;
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/* If decompose does not divide out ALPHA:
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if(hint == 0)
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return a1;
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else if(a0 > Q)
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return (a1 + ALPHA) % (Q - 1);
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else
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return (a1 - ALPHA) % (Q - 1);
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*/
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}
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