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36 lines
1.4 KiB
C
36 lines
1.4 KiB
C
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/********************************************************************************************
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* FrodoKEM: Learning with Errors Key Encapsulation
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*
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* Abstract: noise sampling functions
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*********************************************************************************************/
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#include <stdint.h>
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#include "api.h"
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#include "common.h"
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#include "params.h"
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static uint16_t CDF_TABLE[CDF_TABLE_LEN] = CDF_TABLE_DATA;
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void PQCLEAN_FRODOKEM640SHAKE_OPT_sample_n(uint16_t *s, size_t n) {
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// Fills vector s with n samples from the noise distribution which requires 16 bits to sample.
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// The distribution is specified by its CDF.
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// Input: pseudo-random values (2*n bytes) passed in s. The input is overwritten by the output.
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size_t i;
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unsigned int j;
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for (i = 0; i < n; ++i) {
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uint16_t sample = 0;
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uint16_t prnd = s[i] >> 1; // Drop the least significant bit
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uint16_t sign = s[i] & 0x1; // Pick the least significant bit
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// No need to compare with the last value.
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for (j = 0; j < (unsigned int)(CDF_TABLE_LEN - 1); j++) {
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// Constant time comparison: 1 if CDF_TABLE[j] < s, 0 otherwise. Uses the fact that CDF_TABLE[j] and s fit in 15 bits.
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sample += (uint16_t)(CDF_TABLE[j] - prnd) >> 15;
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}
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// Assuming that sign is either 0 or 1, flips sample iff sign = 1
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s[i] = ((-sign) ^ sample) + sign;
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}
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}
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