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.crypto.modes.gcm;

public class BasicGCMMultiplier
    implements GCMMultiplier
{
    private long[] H;

    public void init(byte[] H)
    {
        this.H = GCMUtil.asLongs(H);
    }

    public void multiplyH(byte[] x)
    {
        long[] t = GCMUtil.asLongs(x);
        GCMUtil.multiply(t, H);
        GCMUtil.asBytes(t, x);
    }
}