#include "bch.h" #include "fft.h" #include "gf.h" #include "parameters.h" #include "vector.h" #include #include /** * @file bch.c * Constant time implementation of BCH codes */ static void unpack_message(uint8_t *message_unpacked, const uint64_t *message); static void lfsr_encode(uint8_t *codeword, const uint8_t *message); static void pack_codeword(uint64_t *codeword, const uint8_t *codeword_unpacked); static size_t compute_elp(uint16_t *sigma, const uint16_t *syndromes); static void message_from_codeword(uint64_t *message, const uint64_t *codeword); static void compute_syndromes(uint16_t *syndromes, const uint64_t *vector); static void compute_roots(uint64_t *error, const uint16_t *sigma); /** * @brief Unpacks the message message to the array message_unpacked where each byte stores a bit of the message * * @param[out] message_unpacked Array of VEC_K_SIZE_BYTES bytes receiving the unpacked message * @param[in] message Array of PARAM_K bytes storing the packed message */ static void unpack_message(uint8_t *message_unpacked, const uint64_t *message) { for (size_t i = 0; i < (VEC_K_SIZE_64 - (PARAM_K % 64 != 0)); ++i) { for (size_t j = 0; j < 64; ++j) { message_unpacked[j + 64 * i] = (message[i] >> j) & 0x0000000000000001; } } for (int8_t j = 0; j < PARAM_K % 64; ++j) { message_unpacked[j + 64 * (VEC_K_SIZE_64 - 1)] = (message[VEC_K_SIZE_64 - 1] >> j) & 0x0000000000000001; } } /** * @brief Encodes the message message to a codeword codeword using the generator polynomial bch_poly of the code * * @param[out] codeword Array of PARAM_N1 bytes receiving the codeword * @param[in] message Array of PARAM_K bytes storing the message to encode */ static void lfsr_encode(uint8_t *codeword, const uint8_t *message) { uint8_t gate_value = 0; uint8_t bch_poly[PARAM_G] = PARAM_BCH_POLY; // Compute the Parity-check digits for (int16_t i = PARAM_K - 1; i >= 0; --i) { gate_value = message[i] ^ codeword[PARAM_N1 - PARAM_K - 1]; for (size_t j = PARAM_N1 - PARAM_K - 1; j; --j) { codeword[j] = codeword[j - 1] ^ (-gate_value & bch_poly[j]); } codeword[0] = gate_value; } // Add the message memcpy(codeword + PARAM_N1 - PARAM_K, message, PARAM_K); } /** * @brief Packs the codeword from an array codeword_unpacked where each byte stores a bit to a compact array codeword * * @param[out] codeword Array of VEC_N1_SIZE_BYTES bytes receiving the packed codeword * @param[in] codeword_unpacked Array of PARAM_N1 bytes storing the unpacked codeword */ static void pack_codeword(uint64_t *codeword, const uint8_t *codeword_unpacked) { for (size_t i = 0; i < (VEC_N1_SIZE_64 - (PARAM_N1 % 64 != 0)); ++i) { for (size_t j = 0; j < 64; ++j) { codeword[i] |= ((uint64_t) codeword_unpacked[j + 64 * i]) << j; } } for (size_t j = 0; j < PARAM_N1 % 64; ++j) { codeword[VEC_N1_SIZE_64 - 1] |= ((uint64_t) codeword_unpacked[j + 64 * (VEC_N1_SIZE_64 - 1)]) << j; } } /** * @brief Encodes a message message of PARAM_K bits to a BCH codeword codeword of PARAM_N1 bits * * Following @cite lin1983error (Chapter 4 - Cyclic Codes), * We perform a systematic encoding using a linear (PARAM_N1 - PARAM_K)-stage shift register * with feedback connections based on the generator polynomial bch_poly of the BCH code. * * @param[out] codeword Array of size VEC_N1_SIZE_BYTES receiving the encoded message * @param[in] message Array of size VEC_K_SIZE_BYTES storing the message */ void PQCLEAN_HQC256_CLEAN_bch_code_encode(uint64_t *codeword, const uint64_t *message) { uint8_t message_unpacked[PARAM_K]; uint8_t codeword_unpacked[PARAM_N1] = {0}; unpack_message(message_unpacked, message); lfsr_encode(codeword_unpacked, message_unpacked); pack_codeword(codeword, codeword_unpacked); } /** * @brief Computes the error locator polynomial (ELP) sigma * * This is a constant time implementation of Berlekamp's simplified algorithm (see @cite joiner1995decoding).
* We use the letter p for rho which is initialized at -1/2.
* The array X_sigma_p represents the polynomial X^(2(mu-rho))*sigma_p(X).
* Instead of maintaining a list of sigmas, we update in place both sigma and X_sigma_p.
* sigma_copy serves as a temporary save of sigma in case X_sigma_p needs to be updated.
* We can properly correct only if the degree of sigma does not exceed PARAM_DELTA. * This means only the first PARAM_DELTA + 1 coefficients of sigma are of value * and we only need to save its first PARAM_DELTA - 1 coefficients. * * @returns the degree of the ELP sigma * @param[out] sigma Array of size (at least) PARAM_DELTA receiving the ELP * @param[in] syndromes Array of size (at least) 2*PARAM_DELTA storing the syndromes */ static size_t compute_elp(uint16_t *sigma, const uint16_t *syndromes) { uint16_t sigma_copy[PARAM_DELTA - 1] = {0}; uint16_t X_sigma_p[PARAM_DELTA + 1] = {0}; uint16_t d_p, d, dd; uint16_t mask; int32_t pp; // 2*rho size_t deg_sigma, deg_sigma_p, deg_sigma_copy, deg_X_sigma_p; d = syndromes[0]; sigma[0] = 1; X_sigma_p[1] = 1; deg_sigma = 0; deg_sigma_p = 0; d_p = 1; pp = -1; for (size_t mu = 0; mu < PARAM_DELTA; ++mu) { // Save sigma in case we need it to update X_sigma_p memcpy(sigma_copy, sigma, 2 * (PARAM_DELTA - 1)); deg_sigma_copy = deg_sigma; dd = PQCLEAN_HQC256_CLEAN_gf_mul(d, PQCLEAN_HQC256_CLEAN_gf_inverse(d_p)); // 0 if(d == 0) for (size_t i = 1; (i <= 2 * mu + 1) && (i <= PARAM_DELTA); ++i) { sigma[i] ^= PQCLEAN_HQC256_CLEAN_gf_mul(dd, X_sigma_p[i]); } deg_X_sigma_p = 2 * mu - pp + deg_sigma_p; // mask = 0xffff if(d != 0) and 0 otherwise mask = -((uint16_t) - d >> 15); // mask2 &= 0xffff if(deg_X_sigma_p > deg_sigma) and 0 otherwise mask &= -((uint16_t) (deg_sigma - deg_X_sigma_p) >> 15); deg_sigma ^= mask & (deg_sigma ^ deg_X_sigma_p); if (mu == PARAM_DELTA - 1) { break; } // Update pp, d_p and X_sigma_p if needed pp ^= mask & (pp ^ (2 * mu)); d_p ^= mask & (d_p ^ d); for (size_t i = PARAM_DELTA - 1; i; --i) { X_sigma_p[i + 1] = X_sigma_p[i - 1]; X_sigma_p[i + 1] ^= mask & (X_sigma_p[i + 1] ^ sigma_copy[i - 1]); } X_sigma_p[1] = 0; X_sigma_p[0] = 0; deg_sigma_p ^= mask & (deg_sigma_p ^ deg_sigma_copy); // Compute the next discrepancy d = syndromes[2 * mu + 2]; for (size_t i = 1; (i <= 2 * mu + 1) && (i <= PARAM_DELTA); ++i) { d ^= PQCLEAN_HQC256_CLEAN_gf_mul(sigma[i], syndromes[2 * mu + 2 - i]); } } return deg_sigma; } /** * @brief Retrieves the message message from the codeword codeword * * Since we performed a systematic encoding, the message is the last PARAM_K bits of the codeword. * * @param[out] message Array of size VEC_K_SIZE_BYTES receiving the message * @param[in] codeword Array of size VEC_N1_SIZE_BYTES storing the codeword */ static void message_from_codeword(uint64_t *message, const uint64_t *codeword) { uint64_t mask1 = (uint64_t) (0xffffffffffffffff << ((PARAM_N1 - PARAM_K) % 64)); uint64_t mask2 = (uint64_t) (0xffffffffffffffff >> (64 - (PARAM_N1 - PARAM_K) % 64)); size_t index = (PARAM_N1 - PARAM_K) / 64; for (size_t i = 0; i < VEC_K_SIZE_64 - 1; ++i) { message[i] = (codeword[index] & mask1) >> ((PARAM_N1 - PARAM_K) % 64); message[i] |= (codeword[++index] & mask2) << (64 - (PARAM_N1 - PARAM_K) % 64); } // Last byte (8-val % 8 is the number of bits given by message1) message[VEC_K_SIZE_64 - 1] = (codeword[index] & mask1) >> ((PARAM_N1 - PARAM_K) % 64); ++index; if (index < VEC_N1_SIZE_64) { message[VEC_K_SIZE_64 - 1] |= (codeword[index] & mask2) << (64 - (PARAM_N1 - PARAM_K) % 64); } } /** * @brief Computes the 2^PARAM_DELTA syndromes from the received vector vector * * Syndromes are the sum of powers of alpha weighted by vector's coefficients. * To do so, we use the additive FFT transpose, which takes as input a family w of GF(2^PARAM_M) elements * and outputs the weighted power sums of these w.
* Therefore, this requires twisting and applying a permutation before feeding vector to the PQCLEAN_HQC256_CLEAN_fft transpose.
* For more details see Berstein, Chou and Schawbe's explanations: * https://binary.cr.yp.to/mcbits-20130616.pdf * * @param[out] syndromes Array of size 2^(PARAM_FFT_T) receiving the 2*PARAM_DELTA syndromes * @param[in] vector Array of size VEC_N1_SIZE_BYTES storing the received word */ static void compute_syndromes(uint16_t *syndromes, const uint64_t *vector) { uint16_t w[1 << PARAM_M]; PQCLEAN_HQC256_CLEAN_fft_t_preprocess_bch_codeword(w, vector); PQCLEAN_HQC256_CLEAN_fft_t(syndromes, w, 2 * PARAM_DELTA); } /** * @brief Computes the error polynomial error from the error locator polynomial sigma * * See function PQCLEAN_HQC256_CLEAN_fft for more details. * * @param[out] error Array of VEC_N1_SIZE_BYTES elements receiving the error polynomial * @param[in] sigma Array of 2^PARAM_FFT elements storing the error locator polynomial */ static void compute_roots(uint64_t *error, const uint16_t *sigma) { uint16_t w[1 << PARAM_M] = {0}; // w will receive the evaluation of sigma in all field elements PQCLEAN_HQC256_CLEAN_fft(w, sigma, PARAM_DELTA + 1); PQCLEAN_HQC256_CLEAN_fft_retrieve_bch_error_poly(error, w); } /** * @brief Decodes the received word * * This function relies on four steps: *
    *
  1. The first step, done by additive FFT transpose, is the computation of the 2*PARAM_DELTA syndromes. *
  2. The second step is the computation of the error-locator polynomial sigma. *
  3. The third step, done by additive FFT, is finding the error-locator numbers by calculating the roots of the polynomial sigma and takings their inverses. *
  4. The fourth step is the correction of the errors in the received polynomial. *
* For a more complete picture on BCH decoding, see Shu. Lin and Daniel J. Costello in Error Control Coding: Fundamentals and Applications @cite lin1983error * * @param[out] message Array of size VEC_K_SIZE_BYTES receiving the decoded message * @param[in] vector Array of size VEC_N1_SIZE_BYTES storing the received word */ void PQCLEAN_HQC256_CLEAN_bch_code_decode(uint64_t *message, uint64_t *vector) { uint16_t syndromes[1 << PARAM_FFT_T] = {0}; uint16_t sigma[1 << PARAM_FFT] = {0}; uint64_t error[(1 << PARAM_M) / 8] = {0}; // Calculate the 2*PARAM_DELTA syndromes compute_syndromes(syndromes, vector); // Compute the error locator polynomial sigma // Sigma's degree is at most PARAM_DELTA but the FFT requires the extra room compute_elp(sigma, syndromes); // Compute the error polynomial error compute_roots(error, sigma); // Add the error polynomial to the received polynomial PQCLEAN_HQC256_CLEAN_vect_add(vector, vector, error, VEC_N1_SIZE_64); // Retrieve the message from the decoded codeword message_from_codeword(message, vector); }