package com;
 
/**
 * 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 3 of the License, 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, see <http://www.gnu.org/licenses/>.
 */
 
import java.math.*;
import java.util.*;
 
/**
 * Paillier Cryptosystem <br>
 * <br>
 * References: <br>
 * [1] Pascal Paillier,
 * "Public-Key Cryptosystems Based on Composite Degree Residuosity Classes,"
 * EUROCRYPT'99. URL:
 * <a  rel="external nofollow" >http:
 * //www.gemplus.com/smart/rd/publications/pdf/Pai99pai.pdf</a><br>
 *
 * [2] Paillier cryptosystem from Wikipedia. URL:
 * <a  rel="external nofollow" >http://en.
 * wikipedia.org/wiki/Paillier_cryptosystem</a>
 *
 * @author Kun Liu (kunliu1@cs.umbc.edu)
 * @version 1.0
 */
public class Paillier {
 
 /**
  * p and q are two large primes. lambda = lcm(p-1, q-1) =
  * (p-1)*(q-1)/gcd(p-1, q-1).
  */
 private BigInteger p, q, lambda;
 /**
  * n = p*q, where p and q are two large primes.
  */
 public BigInteger n;
 /**
  * nsquare = n*n
  */
 public BigInteger nsquare;
 /**
  * a random integer in Z*_{n^2} where gcd (L(g^lambda mod n^2), n) = 1.
  */
 private BigInteger g;
 /**
  * number of bits of modulus
  */
 private int bitLength;
 
 /**
  * Constructs an instance of the Paillier cryptosystem.
  *
  * @param bitLengthVal
  *   number of bits of modulus
  * @param certainty
  *   The probability that the new BigInteger represents a prime
  *   number will exceed (1 - 2^(-certainty)). The execution time of
  *   this constructor is proportional to the value of this
  *   parameter.
  */
 public Paillier(int bitLengthVal, int certainty) {
  KeyGeneration(bitLengthVal, certainty);
 }
 
 /**
  * Constructs an instance of the Paillier cryptosystem with 512 bits of
  * modulus and at least 1-2^(-64) certainty of primes generation.
  */
 public Paillier() {
  KeyGeneration(512, 64);
 }
 
 /**
  * Sets up the public key and private key.
  *
  * @param bitLengthVal
  *   number of bits of modulus.
  * @param certainty
  *   The probability that the new BigInteger represents a prime
  *   number will exceed (1 - 2^(-certainty)). The execution time of
  *   this constructor is proportional to the value of this
  *   parameter.
  */
 public void KeyGeneration(int bitLengthVal, int certainty) {
  bitLength = bitLengthVal;
  /*
   * Constructs two randomly generated positive BigIntegers that are
   * probably prime, with the specified bitLength and certainty.
   */
  p = new BigInteger(bitLength / 2, certainty, new Random());
  q = new BigInteger(bitLength / 2, certainty, new Random());
 
  n = p.multiply(q);
  nsquare = n.multiply(n);
 
  g = new BigInteger("2");
  lambda = p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE))
    .divide(p.subtract(BigInteger.ONE).gcd(q.subtract(BigInteger.ONE)));
  /* check whether g is good. */
  if (g.modPow(lambda, nsquare).subtract(BigInteger.ONE).divide(n).gcd(n).intValue() != 1) {
   System.out.println("g is not good. Choose g again.");
   System.exit(1);
  }
 }
 
 /**
  * Encrypts plaintext m. ciphertext c = g^m * r^n mod n^2. This function
  * explicitly requires random input r to help with encryption.
  *
  * @param m
  *   plaintext as a BigInteger
  * @param r
  *   random plaintext to help with encryption
  * @return ciphertext as a BigInteger
  */
 public BigInteger Encryption(BigInteger m, BigInteger r) {
  return g.modPow(m, nsquare).multiply(r.modPow(n, nsquare)).mod(nsquare);
 }
 
 /**
  * Encrypts plaintext m. ciphertext c = g^m * r^n mod n^2. This function
  * automatically generates random input r (to help with encryption).
  *
  * @param m
  *   plaintext as a BigInteger
  * @return ciphertext as a BigInteger
  */
 public BigInteger Encryption(BigInteger m) {
  BigInteger r = new BigInteger(bitLength, new Random());
  return g.modPow(m, nsquare).multiply(r.modPow(n, nsquare)).mod(nsquare);
 
 }
 
 /**
  * Decrypts ciphertext c. plaintext m = L(c^lambda mod n^2) * u mod n, where
  * u = (L(g^lambda mod n^2))^(-1) mod n.
  *
  * @param c
  *   ciphertext as a BigInteger
  * @return plaintext as a BigInteger
  */
 public BigInteger Decryption(BigInteger c) {
  BigInteger u = g.modPow(lambda, nsquare).subtract(BigInteger.ONE).divide(n).modInverse(n);
  return c.modPow(lambda, nsquare).subtract(BigInteger.ONE).divide(n).multiply(u).mod(n);
 }
 
 /**
  * sum of (cipher) em1 and em2
  *
  * @param em1
  * @param em2
  * @return
  */
 public BigInteger cipher_add(BigInteger em1, BigInteger em2) {
  return em1.multiply(em2).mod(nsquare);
 }
 
 /**
  * main function
  *
  * @param str
  *   intput string
  */
 public static void main(String[] str) {
  /* instantiating an object of Paillier cryptosystem */
  Paillier paillier = new Paillier();
  /* instantiating two plaintext msgs */
  BigInteger m1 = new BigInteger("20");
  BigInteger m2 = new BigInteger("60");
  /* encryption */
  BigInteger em1 = paillier.Encryption(m1);
  BigInteger em2 = paillier.Encryption(m2);
  /* printout encrypted text */
  System.out.println(em1);
  System.out.println(em2);
  /* printout decrypted text */
  System.out.println(paillier.Decryption(em1).toString());
  System.out.println(paillier.Decryption(em2).toString());
 
  /*
   * test homomorphic properties -> D(E(m1)*E(m2) mod n^2) = (m1 + m2) mod
   * n
   */
  // m1+m2,求明文數(shù)值的和
  BigInteger sum_m1m2 = m1.add(m2).mod(paillier.n);
  System.out.println("original sum: " + sum_m1m2.toString());
  // em1+em2，求密文數(shù)值的乘
  BigInteger product_em1em2 = em1.multiply(em2).mod(paillier.nsquare);
  System.out.println("encrypted sum: " + product_em1em2.toString());
  System.out.println("decrypted sum: " + paillier.Decryption(product_em1em2).toString());
 
  /* test homomorphic properties -> D(E(m1)^m2 mod n^2) = (m1*m2) mod n */
  // m1*m2,求明文數(shù)值的乘
  BigInteger prod_m1m2 = m1.multiply(m2).mod(paillier.n);
  System.out.println("original product: " + prod_m1m2.toString());
  // em1的m2次方，再mod paillier.nsquare
  BigInteger expo_em1m2 = em1.modPow(m2, paillier.nsquare);
  System.out.println("encrypted product: " + expo_em1m2.toString());
  System.out.println("decrypted product: " + paillier.Decryption(expo_em1m2).toString());
 
  //sum test
  System.out.println("--------------------------------");
  Paillier p = new Paillier();
  BigInteger t1 = new BigInteger("21");System.out.println(t1.toString());
  BigInteger t2 = new BigInteger("50");System.out.println(t2.toString());
  BigInteger t3 = new BigInteger("50");System.out.println(t3.toString());
  BigInteger et1 = p.Encryption(t1);System.out.println(et1.toString());
  BigInteger et2 = p.Encryption(t2);System.out.println(et2.toString());
  BigInteger et3 = p.Encryption(t3);System.out.println(et3.toString());
  BigInteger sum = new BigInteger("1");
  sum = p.cipher_add(sum, et1);
  sum = p.cipher_add(sum, et2);
  sum = p.cipher_add(sum, et3);
  System.out.println("sum: "+sum.toString());
  System.out.println("decrypted sum: "+p.Decryption(sum).toString());
  System.out.println("--------------------------------");
 }
}