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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.

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package org.bouncycastle.crypto.modes.gcm;

public class BasicGCMExponentiator
    implements GCMExponentiator
{
    private long[] x;

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

    public void exponentiateX(long pow, byte[] output)
    {
        // Initial value is little-endian 1
        long[] y = GCMUtil.oneAsLongs();

        if (pow > 0)
        {
            long[] powX = new long[GCMUtil.SIZE_LONGS];
            GCMUtil.copy(x, powX);

            do
            {
                if ((pow & 1L) != 0)
                {
                    GCMUtil.multiply(y, powX);
                }
                GCMUtil.square(powX, powX);
                pow >>>= 1;
            }
            while (pow > 0);
        }

        GCMUtil.asBytes(y, output);
    }
}