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345 lines
10 KiB
C
345 lines
10 KiB
C
#include "fft.h"
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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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#include <string.h>
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/**
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* @file fft.c
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* Implementation of the additive FFT and its transpose.
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* This implementation is based on the paper from Gao and Mateer: <br>
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* Shuhong Gao and Todd Mateer, Additive Fast Fourier Transforms over Finite Fields,
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* IEEE Transactions on Information Theory 56 (2010), 6265--6272.
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* http://www.math.clemson.edu/~sgao/papers/GM10.pdf <br>
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* and includes improvements proposed by Bernstein, Chou and Schwabe here:
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* https://binary.cr.yp.to/mcbits-20130616.pdf
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*/
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static void compute_fft_betas(uint16_t *betas);
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static void compute_subset_sums(uint16_t *subset_sums, const uint16_t *set, size_t set_size);
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static void radix(uint16_t *f0, uint16_t *f1, const uint16_t *f, uint32_t m_f);
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static void radix_big(uint16_t *f0, uint16_t *f1, const uint16_t *f, uint32_t m_f);
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static void fft_rec(uint16_t *w, uint16_t *f, size_t f_coeffs, uint8_t m, uint32_t m_f, const uint16_t *betas);
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/**
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* @brief Computes the basis of betas (omitting 1) used in the additive FFT and its transpose
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*
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* @param[out] betas Array of size PARAM_M-1
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*/
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static void compute_fft_betas(uint16_t *betas) {
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size_t i;
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for (i = 0; i < PARAM_M - 1; ++i) {
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betas[i] = 1 << (PARAM_M - 1 - i);
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}
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}
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/**
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* @brief Computes the subset sums of the given set
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*
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* The array subset_sums is such that its ith element is
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* the subset sum of the set elements given by the binary form of i.
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*
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* @param[out] subset_sums Array of size 2^set_size receiving the subset sums
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* @param[in] set Array of set_size elements
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* @param[in] set_size Size of the array set
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*/
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static void compute_subset_sums(uint16_t *subset_sums, const uint16_t *set, size_t set_size) {
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size_t i, j;
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subset_sums[0] = 0;
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for (i = 0; i < set_size; ++i) {
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for (j = 0; j < (1U << i); ++j) {
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subset_sums[(1 << i) + j] = set[i] ^ subset_sums[j];
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}
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}
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}
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/**
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* @brief Computes the radix conversion of a polynomial f in GF(2^m)[x]
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*
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* Computes f0 and f1 such that f(x) = f0(x^2-x) + x.f1(x^2-x)
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* as proposed by Bernstein, Chou and Schwabe:
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* https://binary.cr.yp.to/mcbits-20130616.pdf
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*
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* @param[out] f0 Array half the size of f
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* @param[out] f1 Array half the size of f
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* @param[in] f Array of size a power of 2
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* @param[in] m_f 2^{m_f} is the smallest power of 2 greater or equal to the number of coefficients of f
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*/
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static void radix(uint16_t *f0, uint16_t *f1, const uint16_t *f, uint32_t m_f) {
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switch (m_f) {
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case 4:
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f0[4] = f[8] ^ f[12];
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f0[6] = f[12] ^ f[14];
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f0[7] = f[14] ^ f[15];
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f1[5] = f[11] ^ f[13];
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f1[6] = f[13] ^ f[14];
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f1[7] = f[15];
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f0[5] = f[10] ^ f[12] ^ f1[5];
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f1[4] = f[9] ^ f[13] ^ f0[5];
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f0[0] = f[0];
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f1[3] = f[7] ^ f[11] ^ f[15];
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f0[3] = f[6] ^ f[10] ^ f[14] ^ f1[3];
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f0[2] = f[4] ^ f0[4] ^ f0[3] ^ f1[3];
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f1[1] = f[3] ^ f[5] ^ f[9] ^ f[13] ^ f1[3];
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f1[2] = f[3] ^ f1[1] ^ f0[3];
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f0[1] = f[2] ^ f0[2] ^ f1[1];
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f1[0] = f[1] ^ f0[1];
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break;
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case 3:
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f0[0] = f[0];
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f0[2] = f[4] ^ f[6];
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f0[3] = f[6] ^ f[7];
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f1[1] = f[3] ^ f[5] ^ f[7];
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f1[2] = f[5] ^ f[6];
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f1[3] = f[7];
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f0[1] = f[2] ^ f0[2] ^ f1[1];
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f1[0] = f[1] ^ f0[1];
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break;
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case 2:
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f0[0] = f[0];
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f0[1] = f[2] ^ f[3];
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f1[0] = f[1] ^ f0[1];
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f1[1] = f[3];
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break;
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case 1:
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f0[0] = f[0];
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f1[0] = f[1];
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break;
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default:
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radix_big(f0, f1, f, m_f);
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break;
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}
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}
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static void radix_big(uint16_t *f0, uint16_t *f1, const uint16_t *f, uint32_t m_f) {
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uint16_t Q[2 * (1 << (PARAM_FFT - 2))] = {0};
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uint16_t R[2 * (1 << (PARAM_FFT - 2))] = {0};
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uint16_t Q0[1 << (PARAM_FFT - 2)] = {0};
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uint16_t Q1[1 << (PARAM_FFT - 2)] = {0};
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uint16_t R0[1 << (PARAM_FFT - 2)] = {0};
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uint16_t R1[1 << (PARAM_FFT - 2)] = {0};
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size_t i, n;
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n = 1 << (m_f - 2);
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memcpy(Q, f + 3 * n, 2 * n);
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memcpy(Q + n, f + 3 * n, 2 * n);
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memcpy(R, f, 4 * n);
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for (i = 0; i < n; ++i) {
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Q[i] ^= f[2 * n + i];
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R[n + i] ^= Q[i];
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}
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radix(Q0, Q1, Q, m_f - 1);
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radix(R0, R1, R, m_f - 1);
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memcpy(f0, R0, 2 * n);
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memcpy(f0 + n, Q0, 2 * n);
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memcpy(f1, R1, 2 * n);
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memcpy(f1 + n, Q1, 2 * n);
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}
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/**
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* @brief Evaluates f at all subset sums of a given set
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*
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* This function is a subroutine of the function fft.
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*
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* @param[out] w Array
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* @param[in] f Array
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* @param[in] f_coeffs Number of coefficients of f
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* @param[in] m Number of betas
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* @param[in] m_f Number of coefficients of f (one more than its degree)
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* @param[in] betas FFT constants
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*/
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static void fft_rec(uint16_t *w, uint16_t *f, size_t f_coeffs, uint8_t m, uint32_t m_f, const uint16_t *betas) {
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uint16_t f0[1 << (PARAM_FFT - 2)] = {0};
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uint16_t f1[1 << (PARAM_FFT - 2)] = {0};
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uint16_t gammas[PARAM_M - 2] = {0};
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uint16_t deltas[PARAM_M - 2] = {0};
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uint16_t gammas_sums[1 << (PARAM_M - 2)] = {0};
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uint16_t u[1 << (PARAM_M - 2)] = {0};
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uint16_t v[1 << (PARAM_M - 2)] = {0};
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uint16_t tmp[PARAM_M - (PARAM_FFT - 1)] = {0};
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uint16_t beta_m_pow;
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size_t i, j, k;
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// Step 1
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if (m_f == 1) {
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for (i = 0; i < m; ++i) {
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tmp[i] = PQCLEAN_HQCRMRS256_CLEAN_gf_mul(betas[i], f[1]);
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}
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w[0] = f[0];
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for (j = 0; j < m; ++j) {
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for (k = 0; k < (1U << j); ++k) {
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w[(1 << j) + k] = w[k] ^ tmp[j];
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}
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}
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return;
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}
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// Step 2: compute g
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if (betas[m - 1] != 1) {
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beta_m_pow = 1;
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for (i = 1; i < (1U << m_f); ++i) {
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beta_m_pow = PQCLEAN_HQCRMRS256_CLEAN_gf_mul(beta_m_pow, betas[m - 1]);
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f[i] = PQCLEAN_HQCRMRS256_CLEAN_gf_mul(beta_m_pow, f[i]);
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}
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}
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// Step 3
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radix(f0, f1, f, m_f);
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// Step 4: compute gammas and deltas
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for (i = 0; i + 1 < m; ++i) {
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gammas[i] = PQCLEAN_HQCRMRS256_CLEAN_gf_mul(betas[i], PQCLEAN_HQCRMRS256_CLEAN_gf_inverse(betas[m - 1]));
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deltas[i] = PQCLEAN_HQCRMRS256_CLEAN_gf_square(gammas[i]) ^ gammas[i];
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}
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// Compute gammas sums
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compute_subset_sums(gammas_sums, gammas, m - 1);
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// Step 5
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fft_rec(u, f0, (f_coeffs + 1) / 2, m - 1, m_f - 1, deltas);
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k = 1 << ((m - 1) & 0xf); // &0xf is to let the compiler know that m-1 is small.
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if (f_coeffs <= 3) { // 3-coefficient polynomial f case: f1 is constant
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w[0] = u[0];
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w[k] = u[0] ^ f1[0];
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for (i = 1; i < k; ++i) {
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w[i] = u[i] ^ PQCLEAN_HQCRMRS256_CLEAN_gf_mul(gammas_sums[i], f1[0]);
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w[k + i] = w[i] ^ f1[0];
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}
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} else {
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fft_rec(v, f1, f_coeffs / 2, m - 1, m_f - 1, deltas);
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// Step 6
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memcpy(w + k, v, 2 * k);
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w[0] = u[0];
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w[k] ^= u[0];
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for (i = 1; i < k; ++i) {
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w[i] = u[i] ^ PQCLEAN_HQCRMRS256_CLEAN_gf_mul(gammas_sums[i], v[i]);
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w[k + i] ^= w[i];
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}
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}
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}
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/**
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* @brief Evaluates f on all fields elements using an additive FFT algorithm
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*
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* f_coeffs is the number of coefficients of f (one less than its degree). <br>
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* The FFT proceeds recursively to evaluate f at all subset sums of a basis B. <br>
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* This implementation is based on the paper from Gao and Mateer: <br>
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* Shuhong Gao and Todd Mateer, Additive Fast Fourier Transforms over Finite Fields,
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* IEEE Transactions on Information Theory 56 (2010), 6265--6272.
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* http://www.math.clemson.edu/~sgao/papers/GM10.pdf <br>
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* and includes improvements proposed by Bernstein, Chou and Schwabe here:
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* https://binary.cr.yp.to/mcbits-20130616.pdf <br>
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* Note that on this first call (as opposed to the recursive calls to fft_rec), gammas are equal to betas,
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* meaning the first gammas subset sums are actually the subset sums of betas (except 1). <br>
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* Also note that f is altered during computation (twisted at each level).
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*
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* @param[out] w Array
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* @param[in] f Array of 2^PARAM_FFT elements
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* @param[in] f_coeffs Number coefficients of f (i.e. deg(f)+1)
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*/
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void PQCLEAN_HQCRMRS256_CLEAN_fft(uint16_t *w, const uint16_t *f, size_t f_coeffs) {
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uint16_t betas[PARAM_M - 1] = {0};
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uint16_t betas_sums[1 << (PARAM_M - 1)] = {0};
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uint16_t f0[1 << (PARAM_FFT - 1)] = {0};
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uint16_t f1[1 << (PARAM_FFT - 1)] = {0};
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uint16_t deltas[PARAM_M - 1] = {0};
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uint16_t u[1 << (PARAM_M - 1)] = {0};
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uint16_t v[1 << (PARAM_M - 1)] = {0};
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size_t i, k;
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// Follows Gao and Mateer algorithm
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compute_fft_betas(betas);
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// Step 1: PARAM_FFT > 1, nothing to do
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// Compute gammas sums
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compute_subset_sums(betas_sums, betas, PARAM_M - 1);
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// Step 2: beta_m = 1, nothing to do
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// Step 3
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radix(f0, f1, f, PARAM_FFT);
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// Step 4: Compute deltas
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for (i = 0; i < PARAM_M - 1; ++i) {
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deltas[i] = PQCLEAN_HQCRMRS256_CLEAN_gf_square(betas[i]) ^ betas[i];
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}
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// Step 5
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fft_rec(u, f0, (f_coeffs + 1) / 2, PARAM_M - 1, PARAM_FFT - 1, deltas);
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fft_rec(v, f1, f_coeffs / 2, PARAM_M - 1, PARAM_FFT - 1, deltas);
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k = 1 << (PARAM_M - 1);
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// Step 6, 7 and error polynomial computation
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memcpy(w + k, v, 2 * k);
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// Check if 0 is root
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w[0] = u[0];
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// Check if 1 is root
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w[k] ^= u[0];
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// Find other roots
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for (i = 1; i < k; ++i) {
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w[i] = u[i] ^ PQCLEAN_HQCRMRS256_CLEAN_gf_mul(betas_sums[i], v[i]);
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w[k + i] ^= w[i];
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}
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}
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/**
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* @brief Retrieves the error polynomial error from the evaluations w of the ELP (Error Locator Polynomial) on all field elements.
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*
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* @param[out] error Array with the error
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* @param[out] error_compact Array with the error in a compact form
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* @param[in] w Array of size 2^PARAM_M
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*/
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void PQCLEAN_HQCRMRS256_CLEAN_fft_retrieve_error_poly(uint8_t *error, const uint16_t *w) {
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uint16_t gammas[PARAM_M - 1] = {0};
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uint16_t gammas_sums[1 << (PARAM_M - 1)] = {0};
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uint16_t k;
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size_t i, index;
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compute_fft_betas(gammas);
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compute_subset_sums(gammas_sums, gammas, PARAM_M - 1);
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k = 1 << (PARAM_M - 1);
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error[0] ^= 1 ^ ((uint16_t) - w[0] >> 15);
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error[0] ^= 1 ^ ((uint16_t) - w[k] >> 15);
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for (i = 1; i < k; ++i) {
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index = PARAM_GF_MUL_ORDER - PQCLEAN_HQCRMRS256_CLEAN_gf_log(gammas_sums[i]);
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error[index] ^= 1 ^ ((uint16_t) - w[i] >> 15);
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index = PARAM_GF_MUL_ORDER - PQCLEAN_HQCRMRS256_CLEAN_gf_log(gammas_sums[i] ^ 1);
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error[index] ^= 1 ^ ((uint16_t) - w[k + i] >> 15);
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}
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}
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