provotum / security

The ElGamal encryption and proof components

Provotum Security

Credits: The encryption and proof logic is heavily inspired by the Adder Voting System.

Requirements

• Java 8
• Maven
• Install the Java Cryptography Extension available at http://www.oracle.com/technetwork/java/javase/downloads/jce8-download-2133166.html
• Put them into the appropriate directory (overwrite if already existing) as described at https://stackoverflow.com/questions/37741142/how-to-install-unlimited-strength-jce-for-java-8-in-os-x. You may get an exception Caused by: java.security.InvalidKeyException: Illegal key size or default parameters otherwise.

Installation

• Clone this repo and cd into it: git clone [e-mail]:provotum/security.git && cd security
• Run mvn clean install to install the application and run the corresponding tests

Development

Main Interfaces

• IHomomorphicEncryption This interface ensures the signature for all implementations of a particular kind of homomorphic encryption. As a generic parameter, it requires the kind of ciphertext it operates on.
• IHomomorphicCipherText Homomorphic cipher texts allow to operate on each other, abstracting the concrete mathematical details from the caller. It requires a concrete ciphertext as generic parameter.
• IMembershipProof The interface for a membership proof requires a class implementing \texttt{IHomomorphicCiphertext} as generic parameter, restricting the classes it is able to generate proofs for.

Main Implementations

• ElGamal additive Ciphertext: An exponential ElGamal homomorphic ciphertext in the form of

    E(m) = (g^r, h^r * g^m), with
    
    g = generator
    m = message</li>
    h = g^x i.e. the public key whereas x = private key
    r = [0, q-1]
    
    which operates in an additive manner over the encrypted plaintexts:
    
    E(m1) * E(m2) = (g^(r1+r2), h^(r1+r2) * g^(m1+m2))
                  = E(m1 + m2)

• ElGamal Encryption The encryption and decryption component for the above documented ciphertext.
• Non-interactive Membership Proof The non-interactive membership proof allowing to prove that a certain ElGamal ciphertext actually contains a particular cleartext value.