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PaillierScheme.java
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Tue, Jan 7, 00:36

PaillierScheme.java
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/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package crypto.paillier;
import java.math.BigInteger;
import java.util.Random;
import crypto.CryptoException;
/**
*
* @author raisaro
*/
public class PaillierScheme {
private boolean canEncrypt; // Whether encryption can take place.
private boolean canDecrypt; // Whether decryption can take place.
private BigInteger x; // Private Key a random integer € [1,n^2/2]
private BigInteger g; // Encryption base.
private BigInteger h; // Encryption base.
private BigInteger n; // Public key.
private BigInteger nsqr; // The value n^2 (n squared).
private Random prng; // Random number generator.
public PaillierScheme(BigInteger N, BigInteger G, BigInteger H, BigInteger X){
canEncrypt = true;
canDecrypt = true;
n = N;
g = G;
h = H;
x = X;
nsqr = n.multiply(n);
prng = new Random( System.currentTimeMillis() );
}
public PaillierScheme(BigInteger N, BigInteger G, BigInteger H){
canEncrypt = true;
canDecrypt = false;
n = N;
g = G;
h = H;
nsqr = n.multiply(n);
prng = new Random( System.currentTimeMillis() );
}
/**
* Encrypts the given plaintext to produce ciphertext.
*
* @param plain
*
* @param gPOWr
*
* @param hPOWr
*
* @return The encrypted value of the plaintext, modulo n^2.
*
* @throws CryptoException CryptoException If the encryption cannot be performed or
* there is an error while encrypting
* the plaintext.
*/
public BigInteger[] encrypt( BigInteger plain, BigInteger gPOWr, BigInteger hPOWr ) throws CryptoException{
BigInteger[] cipher = new BigInteger[2];
BigInteger cipherA; // The ciphertext A.
BigInteger cipherB; // The ciphertext B.
cipherA = gPOWr;
cipherB = hPOWr.multiply(((plain.multiply(n).mod(nsqr)).add(BigInteger.ONE)).mod(nsqr)).mod(nsqr);
cipher[0] = cipherA;
cipher[1] = cipherB;
return cipher;
}
/**
* Encrypts the given plaintext to produce ciphertext.
*
* @param plain The plaintext.
*
* @return The encrypted value of the plaintext, modulo n^2.
*
* @exception CryptoException If the encryption cannot be performed or
* there is an error while encrypting
* the plaintext.
*/
public BigInteger[] encrypt( BigInteger plain ) throws CryptoException{
BigInteger[] cipher = new BigInteger[2];
BigInteger cipherA; // The ciphertext A.
BigInteger cipherB; // The ciphertext B.
long rand; // Value generated by PRNG.
BigInteger r; // Random value used to hide plaintext.
if( !canEncrypt ) {
throw new CryptoException( "Cannot encrypt value " +
plain + " because no public key was defined." );
}
do{
r = new BigInteger(1024, new Random());
r = r.mod(n.divide(BigInteger.valueOf(4)));
r = r.add(BigInteger.ONE);
cipherA = g.modPow( r, nsqr );
}while((crypto.Utils.gcd(cipherA.longValue(),nsqr.longValue()) != 1)||(r.equals(BigInteger.ZERO)));
cipherB = (h.modPow(r, nsqr)).multiply(((plain.multiply(n).mod(nsqr)).add(BigInteger.ONE)).mod(nsqr)).mod(nsqr);
// System.out.println("A = "+cipherA);
// System.out.println("B = "+cipherB);
cipher[0] = cipherA;
cipher[1] = cipherB;
return cipher;
}
/**
* Decrypts the given ciphertext, which was previously encrypted using
* the public key given to this instance of the class. This method will
* throw an exception if no decryption keys were defined.
*
* @param cipher The ciphertext.
*
* @return The decrypted value of the plaintext, modulo n.
*
* @exception CryptoException If the decryption fails.
*/
public BigInteger decrypt( BigInteger[] cipher ) throws CryptoException {
BigInteger plain; // The plaintext.
BigInteger cipherA = cipher[0];
BigInteger cipherB = cipher[1];
BigInteger Ainv; // Intermediate result during decryption.
if( !canDecrypt ) {
throw new CryptoException( "Cannot decrypt value " +
cipherB.longValue() + " because no private key was defined." );
}
Ainv = (cipherA.modPow(x,nsqr)).modInverse(nsqr);
plain = (((cipherB.multiply(Ainv).mod(nsqr)).subtract(BigInteger.ONE)).mod(nsqr)).divide(n);
return plain;
}
/**
* Partially Decrypts the given ciphertext, which was previously encrypted using
* the public key given to this instance of the class. This method will
* throw an exception if no decryption keys were defined. The private key used
* must be part of the original private key
* @param cipher
* @param x1
* @return The modified cipher text which can be finally decrypted by the other
* part of the original private key
* @throws CryptoException
*/
public BigInteger[] proxyDecription( BigInteger[] cipher) throws CryptoException{
BigInteger cipherA = cipher[0];
BigInteger cipherB = cipher[1];
BigInteger[] new_cipher = new BigInteger[2];
BigInteger new_cipherB;
BigInteger Ainv;
if( !canDecrypt ) {
throw new CryptoException( "Cannot decrypt value " +
cipherB.longValue() + " because no private key was defined." );
}
Ainv = (cipherA.modPow(x,nsqr)).modInverse(nsqr);
new_cipherB = (cipherB.multiply(Ainv)).mod(nsqr);
new_cipher[0] = cipherA;
new_cipher[1] = new_cipherB;
return new_cipher;
}
}

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