jPaillier

A Java implementation of the Paillier cryptosystem.

This library was developed as part of the research project CoPPDA (Corporate Privacy Preserving Data Analysis, http://www.enterprise-application-development.org/projects/coppda.html). To see it in action have a look at ppid3 where it is used to implement a privacy preserving ID3 decision tree generation algorithm.

WARNING: This library was developed for research only and is by no means implemented in a cryptographically secure way.

Usage

create a key pair:

    KeyPairBuilder keygen = new KeyPairBuilder();
    keyPair = keygen.generateKeyPair();

encryption:

	PublicKey publicKey = keyPair.getPublicKey();
    BigInteger ciphertext = publicKey.encrypt(plainData);

decrypt a ciphertext:

    BigInteger decryptedData = keyPair.decrypt(ciphertext);

Homomorphic Addition

An interesting feature of the Paillier cryptosystem is its homomophic properties. The following example demonstrates how this library can be used to compute the homomorphic addition of two plaintext input values.

	BigInteger plainA = BigInteger.valueOf(102);
	BigInteger plainB = BigInteger.valueOf(203);

	BigInteger encryptedA = publicKey.encrypt(plainA);
	BigInteger encryptedB = publicKey.encrypt(plainB);
	
	BigInteger encryptedProduct = encryptedA.multiply(encryptedB).mod(publicKey.getnSquared());

	BigInteger additionResult = keypair.decrypt(encryptedProduct);
		
	// additionResult = 102 + 203 = 305

See the references for details. More examples can be found in the file: HomomorphicPropertiesTest.

References

• Public-Key Cryptosystems Based on Composite Degree Residuosity Classes
• Wikipedia article