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. 
package org.bouncycastle.crypto.modes.gcm;

public interface GCMExponentiator
{
    void init(byte[] x);
    void exponentiateX(long pow, byte[] output);
}