ntru_decrypt.c

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/*
 * Copyright (C) 2014 FH Bielefeld
 *
 * This file is part of a FH Bielefeld project.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA  02110-1301  USA
 */

#include "ntru_ascii_poly.h"
#include "ntru_decrypt.h"
#include "ntru_params.h"
#include "ntru_poly.h"
#include "ntru_poly_ascii.h"
#include "ntru_string.h"

#include <stdbool.h>
#include <string.h>

#include <fmpz_poly.h>
#include <fmpz.h>


/*------------------------------------------------------------------------*/

void
ntru_decrypt_poly(
        const fmpz_poly_t encr_msg,
        const fmpz_poly_t priv_key,
        const fmpz_poly_t priv_key_inv,
        fmpz_poly_t out_bin,
        const ntru_params *params)
{
    fmpz_poly_t a,
                priv_key_tmp,
                priv_key_inv_tmp,
                encr_msg_tmp;

    if (!encr_msg || !priv_key || !priv_key_inv || !out_bin || !params)
        NTRU_ABORT_DEBUG("Unexpected NULL parameters");

    fmpz_poly_init(a);
    fmpz_poly_zero(a);

    /*
     * make sure all are shifted to
     * [-q/2, q/2]
     */
    fmpz_poly_init(priv_key_tmp);
    fmpz_poly_init(priv_key_inv_tmp);
    fmpz_poly_init(encr_msg_tmp);
    fmpz_poly_set(priv_key_tmp, priv_key);
    fmpz_poly_set(priv_key_inv_tmp, priv_key_inv);
    fmpz_poly_set(encr_msg_tmp, encr_msg);
    fmpz_poly_mod(priv_key_tmp, params->q);
    fmpz_poly_mod(priv_key_inv_tmp, params->q);
    fmpz_poly_mod(encr_msg_tmp, params->q);

    poly_starmultiply(priv_key_tmp, encr_msg_tmp, a, params, params->q);
    fmpz_poly_mod(a, params->q);
    poly_starmultiply(a, priv_key_inv_tmp, out_bin, params, params->p);
    fmpz_poly_mod(out_bin, params->p);

    fmpz_poly_clear(a);
    fmpz_poly_clear(priv_key_tmp);
    fmpz_poly_clear(priv_key_inv_tmp);
    fmpz_poly_clear(encr_msg_tmp);
}

/*------------------------------------------------------------------------*/

string *
ntru_decrypt_string(
        const string *encr_msg,
        const fmpz_poly_t priv_key,
        const fmpz_poly_t priv_key_inv,
        const ntru_params *params)
{
    uint32_t i = 0;
    string *decr_msg;
    fmpz_poly_t **poly_array;

    if (!encr_msg || !encr_msg->len)
        NTRU_ABORT_DEBUG("Unexpected NULL parameters");

    poly_array = base64_to_poly_arr(encr_msg, params);

    while (*poly_array[i]) {
        ntru_decrypt_poly(*poly_array[i],
                    priv_key,
                    priv_key_inv,
                    *poly_array[i],
                    params);
        i++;
    }

    decr_msg = bin_poly_arr_to_ascii((const fmpz_poly_t **)poly_array,
            i, params);

    poly_delete_array(poly_array);

    return decr_msg;
}

/*------------------------------------------------------------------------*/