80 lines
2.2 KiB
C
80 lines
2.2 KiB
C
/*
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* Copyright (C) 2014 FH Bielefeld
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*
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* This file is part of a FH Bielefeld project.
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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* MA 02110-1301 USA
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*/
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#include "ntru_decrypt.h"
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/**
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* Decryption of the given Polynom with the private key, its inverse
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* and the fitting ntru_context
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*
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* @param encr_msg encrypted polynom with maximum length of N from
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* the given context
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* @param priv_key the polynom containing the private key to decrypt
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* the message
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* @param priv_key_inv the inverse polynome to the private key
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* @param context the ntru_context
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* @param decr_msg may contain the decrypted polynome at some point
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* @returns the decrypted polynome at the moment
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*
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*
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*/
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pb_poly* ntru_decrypt(pb_poly *encr_msg, pb_poly *priv_key,
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pb_poly *priv_key_inv, ntru_context *context, char ** decr_msg){
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unsigned int q = context->q;
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unsigned int p = context->p;
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unsigned int N = context->N;
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unsigned int i;
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pb_poly *a = build_polynom(NULL, N);
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pb_starmultiply(priv_key, encr_msg, a, context, q);
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mp_int mp_q;
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mp_int mp_qdiv2;
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mp_int zero;
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init_integer(&mp_q);
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init_integer(&mp_qdiv2);
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init_integer(&zero);
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MP_SET_INT(&mp_q, q);
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mp_div_2(&mp_q, &mp_qdiv2);
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mp_zero(&zero);
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for(i = 0; i < N; i++){
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if(mp_cmp(&(a->terms[i]),&zero) == MP_LT) {
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mp_add((&a->terms[i]),&mp_q,(&a->terms[i]));
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}
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if(mp_cmp(&(a->terms[i]), &mp_qdiv2) == MP_GT) {
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mp_sub((&a->terms[i]),&mp_q,(&a->terms[i]));
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}
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}
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pb_poly *d = build_polynom(NULL, N);
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pb_starmultiply(a, priv_key_inv, d, context, p);
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pb_normalize(d,-1,1,context);
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return d;
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}
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