83 lines
2.5 KiB
C
83 lines
2.5 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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* Legend
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*
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* N : maximal degree of the polynom
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* q : "is given" (... mod q)
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* p : "is given" (... mod p)
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* f : private key
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* Fp: inverse of "modulo p"
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* e : encrypted message
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* a : result of first multiplication (StarMultiply(f, e, a, N, q))
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* d : result of second multiplication (StarMultiply(a, Fp , d, N, p)), decrypted message
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* */
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// Require: N , q, p, secret key f , inverse polynomial Fp , and encrypted message e.
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pb_poly* ntru_decrypt(pb_poly *encr_msg, pb_poly *private_key, pb_poly *Fp, 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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// StarMultiply(f, e, a, N, q)
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pb_poly *a = build_polynom(NULL, N, context);
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pb_starmultiply(private_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) { // Make all coefficients positive
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//a->terms[i] = a->terms[i] + q;
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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) { // Shift coefficients of a into range (−q/2, q/2)
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//a->terms[i] = a->terms[i] - mp_q;
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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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printf("%s\np:%d", "Nach dem StarMultiply: ", p);
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draw_polynom(a);
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pb_poly *d = build_polynom(NULL, N, context);
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// StarMultiply(a, Fp , d, N, p)
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pb_starmultiply(a, Fp, d, context, p);
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// {Decode returns the decrypted message, d, through the argument list.}
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return d;
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}
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