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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.

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package org.bouncycastle.crypto.modes.kgcm;

import org.bouncycastle.math.raw.Interleave;

/**
 * Utilities for the GF(2^m) field with corresponding extension polynomial:
 *  * GF (2^256) -> x^256 + x^10 + x^5 + x^2 + 1
 * 
 * The representation is little-endian arrays of 64-bit words
*/
public class KGCMUtil_256
{
    public static final int SIZE = 4;

    public static void add(long[] x, long[] y, long[] z)
    {
        z[0] = x[0] ^ y[0];
        z[1] = x[1] ^ y[1];
        z[2] = x[2] ^ y[2];
        z[3] = x[3] ^ y[3];
    }

    public static void copy(long[] x, long[] z)
    {
        z[0] = x[0];
        z[1] = x[1];
        z[2] = x[2];
        z[3] = x[3];
    }

    public static boolean equal(long[] x, long[] y)
    {
        long d = 0L;
        d |= x[0] ^ y[0];
        d |= x[1] ^ y[1];
        d |= x[2] ^ y[2];
        d |= x[3] ^ y[3];
        return d == 0L;
    }

    public static void multiply(long[] x, long[] y, long[] z)
    {
        long x0 = x[0], x1 = x[1], x2 = x[2], x3 = x[3];
        long y0 = y[0], y1 = y[1], y2 = y[2], y3 = y[3];
        long z0 = 0, z1 = 0, z2 = 0, z3 = 0, z4 = 0;

        for (int j = 0; j < 64; ++j)
        {
            long m0 = -(x0 & 1L); x0 >>>= 1;
            z0 ^= (y0 & m0);
            z1 ^= (y1 & m0);
            z2 ^= (y2 & m0);
            z3 ^= (y3 & m0);

            long m1 = -(x1 & 1L); x1 >>>= 1;
            z1 ^= (y0 & m1);
            z2 ^= (y1 & m1);
            z3 ^= (y2 & m1);
            z4 ^= (y3 & m1);

            long c = y3 >> 63;
            y3 = (y3 << 1) | (y2 >>> 63);
            y2 = (y2 << 1) | (y1 >>> 63);
            y1 = (y1 << 1) | (y0 >>> 63);
            y0 = (y0 << 1) ^ (c & 0x425L);
        }

        long y4 = y3;
        y3 = y2;
        y2 = y1;
        y1 = y0 ^ (y4 >>> 62) ^ (y4 >>> 59) ^ (y4 >>> 54);
        y0 = y4 ^ (y4 <<   2) ^ (y4 <<   5) ^ (y4 <<  10);

        for (int j = 0; j < 64; ++j)
        {
            long m2 = -(x2 & 1L); x2 >>>= 1;
            z0 ^= (y0 & m2);
            z1 ^= (y1 & m2);
            z2 ^= (y2 & m2);
            z3 ^= (y3 & m2);

            long m3 = -(x3 & 1L); x3 >>>= 1;
            z1 ^= (y0 & m3);
            z2 ^= (y1 & m3);
            z3 ^= (y2 & m3);
            z4 ^= (y3 & m3);

            long c = y3 >> 63;
            y3 = (y3 << 1) | (y2 >>> 63);
            y2 = (y2 << 1) | (y1 >>> 63);
            y1 = (y1 << 1) | (y0 >>> 63);
            y0 = (y0 << 1) ^ (c & 0x425L);
        }

        z0 ^= z4 ^ (z4 <<   2) ^ (z4 <<   5) ^ (z4 <<  10);
        z1 ^=      (z4 >>> 62) ^ (z4 >>> 59) ^ (z4 >>> 54);      

        z[0] = z0; z[1] = z1; z[2] = z2; z[3] = z3;
    }

    public static void multiplyX(long[] x, long[] z)
    {
        long x0 = x[0], x1 = x[1], x2 = x[2], x3 = x[3];
        long m = x3 >> 63;
        z[0] = (x0 << 1) ^ (m & 0x425L);
        z[1] = (x1 << 1) | (x0 >>> 63);
        z[2] = (x2 << 1) | (x1 >>> 63);
        z[3] = (x3 << 1) | (x2 >>> 63);
    }

    public static void multiplyX8(long[] x, long[] z)
    {
        long x0 = x[0], x1 = x[1], x2 = x[2], x3 = x[3];
        long c = x3 >>> 56;
        z[0] = (x0 << 8) ^ c ^ (c << 2) ^ (c << 5) ^ (c << 10);
        z[1] = (x1 << 8) | (x0 >>> 56);
        z[2] = (x2 << 8) | (x1 >>> 56);
        z[3] = (x3 << 8) | (x2 >>> 56);
    }

    public static void one(long[] z)
    {
        z[0] = 1;
        z[1] = 0;
        z[2] = 0;
        z[3] = 0;
    }

    public static void square(long[] x, long[] z)
    {
        long[] t  = new long[SIZE << 1];
        for (int i = 0; i < SIZE; ++i)
        {
            Interleave.expand64To128(x[i], t, i << 1);
        }

        int j = SIZE << 1;
        while (--j >= SIZE)
        {
            long n = t[j];
            t[j - SIZE    ] ^= n ^ (n <<   2) ^ (n <<   5) ^ (n <<  10);
            t[j - SIZE + 1] ^=     (n >>> 62) ^ (n >>> 59) ^ (n >>> 54);      
        }

        copy(t, z);
    }

    public static void x(long[] z)
    {
        z[0] = 2;
        z[1] = 0;
        z[2] = 0;
        z[3] = 0;
    }

    public static void zero(long[] z)
    {
        z[0] = 0;
        z[1] = 0;
        z[2] = 0;
        z[3] = 0;
    }
}