elgamal.cpp

/*
 * elgamal.cpp
 *
 *  Created on: Jan 27, 2010
 *      Author: bhess
 */

#include "elgamal.h"

ECC_ElGamal::ECC_ElGamal(Ellipticcurve* E) { ell = E;}

ECC_ElGamal_Ciphertext ECC_ElGamal::encrypt_element(ECC_ElGamal_Plaintext m) {
	//ECC_ElGamal_Ciphertext eco;
	Coordinate C1, C2;
	Coordinate M = m.P;
	Coordinate P = ell->point;
	mpz_class n = ell->getOrder();
	mpz_class k = rand.rand(n - 1);
	C1 = ell->pointMultiplication(P, k);
	Coordinate kQ = ell->pointMultiplication(Q, k);
	C2 = ell->addition(M, kQ);
	return ECC_ElGamal_Ciphertext(C1, C2);
}
/*
ECC_ElGamal_Ciphertext ECC_ElGamal::encrypt_element(string m) {
	return encrypt_element(ECC_ElGamal_Plaintext(to_point(m)));
}
*/
ECC_ElGamal_Plaintext ECC_ElGamal::decrypt_element (ECC_ElGamal_Ciphertext ciphertext) {
	Coordinate dC1 = ell->pointMultiplication(ciphertext.C1, d);
	return ECC_ElGamal_Plaintext(ell->subtraction(ciphertext.C2, dC1));
}

string ECC_ElGamal::encrypt(string m) {
	return encrypt_element(ECC_ElGamal_Plaintext(to_point(m))).to_string(ell);
}

string ECC_ElGamal::decrypt(string c) {
	ECC_ElGamal_Ciphertext ct = ECC_ElGamal_Ciphertext::from_string(c, ell);
	ECC_ElGamal_Plaintext ep = decrypt_element(ct);
	return remove_padding(ep);
}

string ECC_ElGamal::remove_padding(ECC_ElGamal_Plaintext& ep) {
	string st = to_string(ep.P.X >> 8);
	string res = "";
	for (int i = 0; i < (int)st.size(); ++i) {
		res += st[st.size() - 1 - i];
	}
	return res;
}

/*
vector<ECC_ElGamal_Plaintext> ECC_ElGamal::split_msg(string msg) {
	int str_length_bits = msg.length();
	int max_point_length_effective = get_max_point_length();
	//cout << max_point_length_effective << endl;
	int vec_el = (str_length_bits % max_point_length_effective == 0 ?
			str_length_bits / max_point_length_effective : (str_length_bits / max_point_length_effective) + 1);
	vector<ECC_ElGamal_Plaintext> res(vec_el);

	for (int i = 0; i < vec_el; ++i) {
		string s = msg.substr(i * max_point_length_effective, max_point_length_effective);
		//ECC_ElGamal_Plaintext ep;
		//cout << "Part " << i << ":" << s << endl;
		//ep.P = to_point(s);
		res[i] = ECC_ElGamal_Plaintext(to_point(s));
	}
	return res;
}
*/

int ECC_ElGamal::get_max_point_length() {
	return (ell->get_bits() / 8) - 1;
}

Coordinate ECC_ElGamal::to_point(string str) {
	mpz_class res = 0;
	// 8 bit encoding of a character, as in ASCII...
	//res.set_str(str, 256);
	for (int i = 0; i < (int)str.size(); ++i) {
		res |= (int)str[i];
		if (i < (int)str.size() - 1) {
			res <<= 8;
		}
	}
	return get_point_with_padding(res, 1);
}

Coordinate ECC_ElGamal::get_point_with_padding(mpz_class str, int padding_length) {
	mpz_class str_copy;
	Coordinate c;
	do {
		// setting random padding until there exists a point
		str_copy = str;
		mpz_class pad = rand.rand(255);
		str_copy <<= 8;
		str_copy |= pad;
		c = ell->getPoint_interface(str_copy, false);
	} while(c.isInfinite());
	return c;
}

void ECC_ElGamal::generate_random_keypair() {
	d = rand.rand(ell->getOrder() - 1);
	Q = ell->pointMultiplication(ell->point, d);
}

string ECC_ElGamal::to_string(mpz_class mpz) {
	string res;
	while (mpz != 0) {
		res += (char)(mpz.get_ui() & 0xFF);
		mpz >>= 8;
	}
	return res;
}

bool ECC_ElGamal::validate_curve() {
	Coordinate m = ell->pointMultiplication(ell->point, ell->getOrder());
	return m.isInfinite();
}