78 lines
1.8 KiB
C
78 lines
1.8 KiB
C
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#include "gf.h"
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#include "parameters.h"
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#include <stdint.h>
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/**
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* @file gf.c
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* Galois field implementation with multiplication using lookup tables
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*/
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/**
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* @brief Returns the integer i such that elt = a^i where a is the primitive element of GF(2^PARAM_M).
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*
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* @returns the logarithm of the given element
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*/
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uint16_t PQCLEAN_HQC256_CLEAN_gf_log(uint16_t elt) {
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return log[elt];
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}
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/**
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* @brief Multiplies nonzero element a by element b
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*
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* @returns the product a*b
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* @param[in] a First element of GF(2^PARAM_M) to multiply (cannot be zero)
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* @param[in] b Second element of GF(2^PARAM_M) to multiply (cannot be zero)
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*/
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uint16_t PQCLEAN_HQC256_CLEAN_gf_mul(uint16_t a, uint16_t b) {
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// mask = 0xffff if neither a nor b is zero. Otherwise mask is 0.
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int16_t mask = ((log[a] | log[b]) >> PARAM_M) - 1;
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return mask & exp[PQCLEAN_HQC256_CLEAN_gf_mod(log[a] + log[b])];
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}
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/**
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* @brief Squares an element of GF(2^PARAM_M)
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*
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* @returns a^2
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* @param[in] a Element of GF(2^PARAM_M)
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*/
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uint16_t PQCLEAN_HQC256_CLEAN_gf_square(uint16_t a) {
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int16_t mask = (log[a] >> PARAM_M) - 1;
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return mask & exp[PQCLEAN_HQC256_CLEAN_gf_mod(2 * log[a])];
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}
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/**
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* @brief Computes the inverse of an element of GF(2^PARAM_M)
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*
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* @returns the inverse of a
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* @param[in] a Element of GF(2^PARAM_M)
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*/
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uint16_t PQCLEAN_HQC256_CLEAN_gf_inverse(uint16_t a) {
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return exp[PARAM_GF_MUL_ORDER - log[a]];
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}
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/**
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* @brief Returns i modulo 2^PARAM_M-1
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*
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* i must be less than 2*(2^PARAM_M-1).
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* Therefore, the return value is either i or i-2^PARAM_M+1.
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*
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* @returns i mod (2^PARAM_M-1)
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* @param[in] i The integer whose modulo is taken
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*/
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uint16_t PQCLEAN_HQC256_CLEAN_gf_mod(uint16_t i) {
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uint16_t tmp = i - PARAM_GF_MUL_ORDER;
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// mask = 0xffff if(i < PARAM_GF_MUL_ORDER)
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int16_t mask = -(tmp >> 15);
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return tmp + (mask & PARAM_GF_MUL_ORDER);
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}
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