http://www.bouncycastle.org/java.html: The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8.
  • The Legion of the Bouncy Castle Inc. 
package org.bouncycastle.jce.interfaces;

import java.math.BigInteger;

import javax.crypto.interfaces.DHPrivateKey;

public interface ElGamalPrivateKey
    extends ElGamalKey, DHPrivateKey
{
    public BigInteger getX();
}