pqc/crypto_kem/hqc-128-1-cca2/leaktime/bch.c

/**
 * @file bch.c
 * Constant time implementation of BCH codes
 */

#include "bch.h"
#include "fft.h"
#include "gf.h"
#include "parameters.h"
#include "vector.h"
#include <stdint.h>
#include <string.h>

static void unpack_message(uint8_t *message_unpacked, const uint8_t *message);
static void lfsr_encode(uint8_t *codeword, const uint8_t *message);
static void pack_codeword(uint8_t *codeword, const uint8_t *codeword_unpacked);
static size_t compute_elp(uint16_t *sigma, const uint16_t *syndromes);
static void message_from_codeword(uint8_t *message, const uint8_t *codeword);
static void compute_syndromes(uint16_t *syndromes, const uint8_t *vector);
static void compute_roots(uint8_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 uint8_t *message) {
    for (size_t i = 0; i < (VEC_K_SIZE_BYTES - (PARAM_K % 8 != 0)); ++i) {
        for (size_t j = 0; j < 8; ++j) {
            message_unpacked[j + 8 * i] = (message[i] >> j) & 0x01;
        }
    }

    for (int8_t j = 0; j < PARAM_K % 8; ++j) {
        message_unpacked[j + 8 * (VEC_K_SIZE_BYTES - 1)] = (message[VEC_K_SIZE_BYTES - 1] >> j) & 0x01;
    }
}



/**
 * @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(uint8_t *codeword, const uint8_t *codeword_unpacked) {
    for (size_t i = 0; i < (VEC_N1_SIZE_BYTES - (PARAM_N1 % 8 != 0)); ++i) {
        for (size_t j = 0; j < 8; ++j) {
            codeword[i] |= codeword_unpacked[j + 8 * i] << j;
        }
    }

    for (size_t j = 0; j < PARAM_N1 % 8; ++j) {
        codeword[VEC_N1_SIZE_BYTES - 1] |= codeword_unpacked[j + 8 * (VEC_N1_SIZE_BYTES - 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_HQC1281CCA2_LEAKTIME_bch_code_encode(uint8_t *codeword, const uint8_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). <br>
 * We use the letter p for rho which is initialized at -1/2. <br>
 * The array X_sigma_p represents the polynomial X^(2(mu-rho))*sigma_p(X). <br>
 * Instead of maintaining a list of sigmas, we update in place both sigma and X_sigma_p. <br>
 * sigma_copy serves as a temporary save of sigma in case X_sigma_p needs to be updated. <br>
 * 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) {
    sigma[0] = 1;
    size_t deg_sigma = 0;
    size_t deg_sigma_p = 0;
    uint16_t sigma_copy[PARAM_DELTA - 1] = {0};
    size_t deg_sigma_copy = 0;
    uint16_t X_sigma_p[PARAM_DELTA + 1] = {0, 1};
    int32_t pp = -1; // 2*rho
    uint16_t d_p = 1;
    uint16_t d = syndromes[0];

    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;

        uint16_t dd = PQCLEAN_HQC1281CCA2_LEAKTIME_gf_mul(d, PQCLEAN_HQC1281CCA2_LEAKTIME_gf_inverse(d_p)); // 0 if(d == 0)
        for (size_t i = 1; (i <= 2 * mu + 1) && (i <= PARAM_DELTA); ++i) {
            sigma[i] ^= PQCLEAN_HQC1281CCA2_LEAKTIME_gf_mul(dd, X_sigma_p[i]);
        }

        size_t deg_X = 2 * mu - pp; // 2*(mu-rho)
        size_t deg_X_sigma_p = deg_X + deg_sigma_p;

        // mask1 = 0xffff if(d != 0) and 0 otherwise
        int16_t mask1 = -((uint16_t) - d >> 15);

        // mask2 = 0xffff if(deg_X_sigma_p > deg_sigma) and 0 otherwise
        int16_t mask2 = -((uint16_t) (deg_sigma - deg_X_sigma_p) >> 15);

        // mask12 = 0xffff if the deg_sigma increased and 0 otherwise
        int16_t mask12 = mask1 & mask2;
        deg_sigma = (mask12 & deg_X_sigma_p) ^ (~mask12 & deg_sigma);

        if (mu == PARAM_DELTA - 1) {
            break;
        }

        // Update pp, d_p and X_sigma_p if needed
        pp = (mask12 & (2 * mu)) ^ (~mask12 & pp);
        d_p = (mask12 & d) ^ (~mask12 & d_p);
        for (size_t i = PARAM_DELTA - 1; i; --i) {
            X_sigma_p[i + 1] = (mask12 & sigma_copy[i - 1]) ^ (~mask12 & X_sigma_p[i - 1]);
        }
        X_sigma_p[1] = 0;
        X_sigma_p[0] = 0;
        deg_sigma_p = (mask12 & deg_sigma_copy) ^ (~mask12 & deg_sigma_p);

        // Compute the next discrepancy
        d = syndromes[2 * mu + 2];
        for (size_t i = 1; (i <= 2 * mu + 1) && (i <= PARAM_DELTA); ++i) {
            d ^= PQCLEAN_HQC1281CCA2_LEAKTIME_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(uint8_t *message, const uint8_t *codeword) {
    int32_t val = PARAM_N1 - PARAM_K;

    uint8_t mask1 = 0xff << val % 8;
    uint8_t mask2 = 0xff >> (8 - val % 8);
    size_t index = val / 8;

    for (size_t i = 0; i < VEC_K_SIZE_BYTES - 1; ++i) {
        uint8_t message1 = (codeword[index] & mask1) >> val % 8;
        uint8_t message2 = (codeword[++index] & mask2) << (8 - val % 8);
        message[i] = message1 | message2;
    }

    // Last byte (8-val % 8 is the number of bits given by message1)
    if ((PARAM_K % 8 == 0) || (8 - val % 8 < PARAM_K % 8)) {
        uint8_t message1 = (codeword[index] & mask1) >> val % 8;
        uint8_t message2 = (codeword[++index] & mask2) << (8 - val % 8);
        message[VEC_K_SIZE_BYTES - 1] = message1 | message2;
    } else {
        uint8_t message1 = (codeword[index] & mask1) >> val % 8;
        message[VEC_K_SIZE_BYTES - 1] = message1;
    }
}



/**
 * @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. <br>
 * Therefore, this requires twisting and applying a permutation before feeding vector to the fft transpose. <br>
 * 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 uint8_t *vector) {
    uint16_t w[1 << PARAM_M];

    PQCLEAN_HQC1281CCA2_LEAKTIME_fft_t_preprocess_bch_codeword(w, vector);
    PQCLEAN_HQC1281CCA2_LEAKTIME_fft_t(syndromes, w, 2 * PARAM_DELTA);
}



/**
 * @brief Computes the error polynomial error from the error locator polynomial sigma
 *
 * See function fft for more details.
 *
 * @param[out] err 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(uint8_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_HQC1281CCA2_LEAKTIME_fft(w, sigma, PARAM_DELTA + 1);
    PQCLEAN_HQC1281CCA2_LEAKTIME_fft_retrieve_bch_error_poly(error, w);
}



/**
 * @brief Decodes the received word
 *
 * This function relies on four steps:
 *    <ol>
 *    <li> The first step, done by additive FFT transpose, is the computation of the 2*PARAM_DELTA syndromes.
 *    <li> The second step is the computation of the error-locator polynomial sigma.
 *    <li> 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.
 *    <li> The fourth step is the correction of the errors in the received polynomial.
 *    </ol>
 * 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_HQC1281CCA2_LEAKTIME_bch_code_decode(uint8_t *message, uint8_t *vector) {
    uint16_t syndromes[1 << PARAM_FFT_T];
    uint16_t sigma[1 << PARAM_FFT] = {0};
    uint8_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_HQC1281CCA2_LEAKTIME_vect_add(vector, vector, error, VEC_N1_SIZE_BYTES);

    // Retrieve the message from the decoded codeword
    message_from_codeword(message, vector);
}