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// -*-c++-*-
/* $Id: elgamal.h,v 1.1 2006/02/25 02:02:40 mfreed Exp $ */
/*
*
* Copyright (C) 2006 Michael J. Freedman (mfreedman at alum.mit.edu)
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2, or (at
* your option) any later version.
*
* This program 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
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
* USA
*
*/
#ifndef _ELGAMAL_H_
#define _ELGAMAL_H_ 1
#include "homoenc.h"
class elgamal_pub : public virtual homoenc_pub {
public:
const bigint p; /* Prime modulus: 2q + 1 */
const bigint q; /* Prime modulus */
const bigint g; /* Random generator of Z^*_p */
const bigint gr; /* Generator^r */
const size_t nbits, abits; /* Size of p, r */
const bigint p1;
const bigint q1;
elgamal_pub (const bigint &pp, const bigint &qq,
const bigint &gg, const bigint &ggr, size_t aabits);
virtual const size_t &mod_size () const { return nbits; }
virtual const bigint &ptext_modulus () const { return q; }
virtual const bigint &ctext_modulus () const { return p; }
virtual bool encrypt (crypt_ctext *c, const bigint &msg, bool recover) const;
virtual bool encrypt (crypt_ctext *c, const str &msg, bool recover) const
{ return homoenc_pub::encrypt (c, msg, recover); }
virtual crypt_keytype ctext_type () const { return CRYPT_ELGAMAL; }
// Resulting ciphertext is sum of msgs' corresponding plaintexts
// ctext = ctext1*ctext2 ==> ptext = ptext1 + ptext2
virtual void add (crypt_ctext *c, const crypt_ctext &msg1,
const crypt_ctext &msg2) const;
// Resulting ciphertext is msg's corresponding plaintext * constant
// ctext = ctext1^const ==> ptext = const * ptext1
virtual void mult (crypt_ctext *c, const crypt_ctext &msg,
const bigint &cons) const;
};
class elgamal_priv : public elgamal_pub, public virtual homoenc_priv {
public:
const bigint r; /* Random interger \in Z^*_p */
const bigint i2; /* 2^-1 mod q */
elgamal_priv (const bigint &pp, const bigint &qq,
const bigint &gg, const bigint &rr);
static ptr<elgamal_priv> make (const bigint &p,
const bigint &g,
const bigint &r);
str decrypt (const crypt_ctext &msg, size_t msglen,
bool recover = true) const;
};
// abits gives length of random moduli [e.g., 160 bits for 1024-bit keys]
elgamal_priv elgamal_keygen (size_t nbits, size_t abits, u_int iter = 32);
#endif /* !_ELGAMAL_H_ */
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