path: root/core/src/main/java/org/spongycastle/crypto/engines/TwofishEngine.java

package org.spongycastle.crypto.engines;

import org.spongycastle.crypto.BlockCipher;
import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.DataLengthException;
import org.spongycastle.crypto.OutputLengthException;
import org.spongycastle.crypto.params.KeyParameter;

/**
 * A class that provides Twofish encryption operations.
 *
 * This Java implementation is based on the Java reference
 * implementation provided by Bruce Schneier and developed
 * by Raif S. Naffah.
 */
public final class TwofishEngine
    implements BlockCipher
{
    private static final byte[][] P =  {
    {  // p0
        (byte) 0xA9, (byte) 0x67, (byte) 0xB3, (byte) 0xE8,
        (byte) 0x04, (byte) 0xFD, (byte) 0xA3, (byte) 0x76,
        (byte) 0x9A, (byte) 0x92, (byte) 0x80, (byte) 0x78,
        (byte) 0xE4, (byte) 0xDD, (byte) 0xD1, (byte) 0x38,
        (byte) 0x0D, (byte) 0xC6, (byte) 0x35, (byte) 0x98,
        (byte) 0x18, (byte) 0xF7, (byte) 0xEC, (byte) 0x6C,
        (byte) 0x43, (byte) 0x75, (byte) 0x37, (byte) 0x26,
        (byte) 0xFA, (byte) 0x13, (byte) 0x94, (byte) 0x48,
        (byte) 0xF2, (byte) 0xD0, (byte) 0x8B, (byte) 0x30,
        (byte) 0x84, (byte) 0x54, (byte) 0xDF, (byte) 0x23,
        (byte) 0x19, (byte) 0x5B, (byte) 0x3D, (byte) 0x59,
        (byte) 0xF3, (byte) 0xAE, (byte) 0xA2, (byte) 0x82,
        (byte) 0x63, (byte) 0x01, (byte) 0x83, (byte) 0x2E,
        (byte) 0xD9, (byte) 0x51, (byte) 0x9B, (byte) 0x7C,
        (byte) 0xA6, (byte) 0xEB, (byte) 0xA5, (byte) 0xBE,
        (byte) 0x16, (byte) 0x0C, (byte) 0xE3, (byte) 0x61,
        (byte) 0xC0, (byte) 0x8C, (byte) 0x3A, (byte) 0xF5,
        (byte) 0x73, (byte) 0x2C, (byte) 0x25, (byte) 0x0B,
        (byte) 0xBB, (byte) 0x4E, (byte) 0x89, (byte) 0x6B,
        (byte) 0x53, (byte) 0x6A, (byte) 0xB4, (byte) 0xF1,
        (byte) 0xE1, (byte) 0xE6, (byte) 0xBD, (byte) 0x45,
        (byte) 0xE2, (byte) 0xF4, (byte) 0xB6, (byte) 0x66,
        (byte) 0xCC, (byte) 0x95, (byte) 0x03, (byte) 0x56,
        (byte) 0xD4, (byte) 0x1C, (byte) 0x1E, (byte) 0xD7,
        (byte) 0xFB, (byte) 0xC3, (byte) 0x8E, (byte) 0xB5,
        (byte) 0xE9, (byte) 0xCF, (byte) 0xBF, (byte) 0xBA,
        (byte) 0xEA, (byte) 0x77, (byte) 0x39, (byte) 0xAF,
        (byte) 0x33, (byte) 0xC9, (byte) 0x62, (byte) 0x71,
        (byte) 0x81, (byte) 0x79, (byte) 0x09, (byte) 0xAD,
        (byte) 0x24, (byte) 0xCD, (byte) 0xF9, (byte) 0xD8,
        (byte) 0xE5, (byte) 0xC5, (byte) 0xB9, (byte) 0x4D,
        (byte) 0x44, (byte) 0x08, (byte) 0x86, (byte) 0xE7,
        (byte) 0xA1, (byte) 0x1D, (byte) 0xAA, (byte) 0xED,
        (byte) 0x06, (byte) 0x70, (byte) 0xB2, (byte) 0xD2,
        (byte) 0x41, (byte) 0x7B, (byte) 0xA0, (byte) 0x11,
        (byte) 0x31, (byte) 0xC2, (byte) 0x27, (byte) 0x90,
        (byte) 0x20, (byte) 0xF6, (byte) 0x60, (byte) 0xFF,
        (byte) 0x96, (byte) 0x5C, (byte) 0xB1, (byte) 0xAB,
        (byte) 0x9E, (byte) 0x9C, (byte) 0x52, (byte) 0x1B,
        (byte) 0x5F, (byte) 0x93, (byte) 0x0A, (byte) 0xEF,
        (byte) 0x91, (byte) 0x85, (byte) 0x49, (byte) 0xEE,
        (byte) 0x2D, (byte) 0x4F, (byte) 0x8F, (byte) 0x3B,
        (byte) 0x47, (byte) 0x87, (byte) 0x6D, (byte) 0x46,
        (byte) 0xD6, (byte) 0x3E, (byte) 0x69, (byte) 0x64,
        (byte) 0x2A, (byte) 0xCE, (byte) 0xCB, (byte) 0x2F,
        (byte) 0xFC, (byte) 0x97, (byte) 0x05, (byte) 0x7A,
        (byte) 0xAC, (byte) 0x7F, (byte) 0xD5, (byte) 0x1A,
        (byte) 0x4B, (byte) 0x0E, (byte) 0xA7, (byte) 0x5A,
        (byte) 0x28, (byte) 0x14, (byte) 0x3F, (byte) 0x29,
        (byte) 0x88, (byte) 0x3C, (byte) 0x4C, (byte) 0x02,
        (byte) 0xB8, (byte) 0xDA, (byte) 0xB0, (byte) 0x17,
        (byte) 0x55, (byte) 0x1F, (byte) 0x8A, (byte) 0x7D,
        (byte) 0x57, (byte) 0xC7, (byte) 0x8D, (byte) 0x74,
        (byte) 0xB7, (byte) 0xC4, (byte) 0x9F, (byte) 0x72,
        (byte) 0x7E, (byte) 0x15, (byte) 0x22, (byte) 0x12,
        (byte) 0x58, (byte) 0x07, (byte) 0x99, (byte) 0x34,
        (byte) 0x6E, (byte) 0x50, (byte) 0xDE, (byte) 0x68,
        (byte) 0x65, (byte) 0xBC, (byte) 0xDB, (byte) 0xF8,
        (byte) 0xC8, (byte) 0xA8, (byte) 0x2B, (byte) 0x40,
        (byte) 0xDC, (byte) 0xFE, (byte) 0x32, (byte) 0xA4,
        (byte) 0xCA, (byte) 0x10, (byte) 0x21, (byte) 0xF0,
        (byte) 0xD3, (byte) 0x5D, (byte) 0x0F, (byte) 0x00,
        (byte) 0x6F, (byte) 0x9D, (byte) 0x36, (byte) 0x42,
        (byte) 0x4A, (byte) 0x5E, (byte) 0xC1, (byte) 0xE0 },
    {  // p1
        (byte) 0x75, (byte) 0xF3, (byte) 0xC6, (byte) 0xF4,
        (byte) 0xDB, (byte) 0x7B, (byte) 0xFB, (byte) 0xC8,
        (byte) 0x4A, (byte) 0xD3, (byte) 0xE6, (byte) 0x6B,
        (byte) 0x45, (byte) 0x7D, (byte) 0xE8, (byte) 0x4B,
        (byte) 0xD6, (byte) 0x32, (byte) 0xD8, (byte) 0xFD,
        (byte) 0x37, (byte) 0x71, (byte) 0xF1, (byte) 0xE1,
        (byte) 0x30, (byte) 0x0F, (byte) 0xF8, (byte) 0x1B,
        (byte) 0x87, (byte) 0xFA, (byte) 0x06, (byte) 0x3F,
        (byte) 0x5E, (byte) 0xBA, (byte) 0xAE, (byte) 0x5B,
        (byte) 0x8A, (byte) 0x00, (byte) 0xBC, (byte) 0x9D,
        (byte) 0x6D, (byte) 0xC1, (byte) 0xB1, (byte) 0x0E,
        (byte) 0x80, (byte) 0x5D, (byte) 0xD2, (byte) 0xD5,
        (byte) 0xA0, (byte) 0x84, (byte) 0x07, (byte) 0x14,
        (byte) 0xB5, (byte) 0x90, (byte) 0x2C, (byte) 0xA3,
        (byte) 0xB2, (byte) 0x73, (byte) 0x4C, (byte) 0x54,
        (byte) 0x92, (byte) 0x74, (byte) 0x36, (byte) 0x51,
        (byte) 0x38, (byte) 0xB0, (byte) 0xBD, (byte) 0x5A,
        (byte) 0xFC, (byte) 0x60, (byte) 0x62, (byte) 0x96,
        (byte) 0x6C, (byte) 0x42, (byte) 0xF7, (byte) 0x10,
        (byte) 0x7C, (byte) 0x28, (byte) 0x27, (byte) 0x8C,
        (byte) 0x13, (byte) 0x95, (byte) 0x9C, (byte) 0xC7,
        (byte) 0x24, (byte) 0x46, (byte) 0x3B, (byte) 0x70,
        (byte) 0xCA, (byte) 0xE3, (byte) 0x85, (byte) 0xCB,
        (byte) 0x11, (byte) 0xD0, (byte) 0x93, (byte) 0xB8,
        (byte) 0xA6, (byte) 0x83, (byte) 0x20, (byte) 0xFF,
        (byte) 0x9F, (byte) 0x77, (byte) 0xC3, (byte) 0xCC,
        (byte) 0x03, (byte) 0x6F, (byte) 0x08, (byte) 0xBF,
        (byte) 0x40, (byte) 0xE7, (byte) 0x2B, (byte) 0xE2,
        (byte) 0x79, (byte) 0x0C, (byte) 0xAA, (byte) 0x82,
        (byte) 0x41, (byte) 0x3A, (byte) 0xEA, (byte) 0xB9,
        (byte) 0xE4, (byte) 0x9A, (byte) 0xA4, (byte) 0x97,
        (byte) 0x7E, (byte) 0xDA, (byte) 0x7A, (byte) 0x17,
        (byte) 0x66, (byte) 0x94, (byte) 0xA1, (byte) 0x1D,
        (byte) 0x3D, (byte) 0xF0, (byte) 0xDE, (byte) 0xB3,
        (byte) 0x0B, (byte) 0x72, (byte) 0xA7, (byte) 0x1C,
        (byte) 0xEF, (byte) 0xD1, (byte) 0x53, (byte) 0x3E,
        (byte) 0x8F, (byte) 0x33, (byte) 0x26, (byte) 0x5F,
        (byte) 0xEC, (byte) 0x76, (byte) 0x2A, (byte) 0x49,
        (byte) 0x81, (byte) 0x88, (byte) 0xEE, (byte) 0x21,
        (byte) 0xC4, (byte) 0x1A, (byte) 0xEB, (byte) 0xD9,
        (byte) 0xC5, (byte) 0x39, (byte) 0x99, (byte) 0xCD,
        (byte) 0xAD, (byte) 0x31, (byte) 0x8B, (byte) 0x01,
        (byte) 0x18, (byte) 0x23, (byte) 0xDD, (byte) 0x1F,
        (byte) 0x4E, (byte) 0x2D, (byte) 0xF9, (byte) 0x48,
        (byte) 0x4F, (byte) 0xF2, (byte) 0x65, (byte) 0x8E,
        (byte) 0x78, (byte) 0x5C, (byte) 0x58, (byte) 0x19,
        (byte) 0x8D, (byte) 0xE5, (byte) 0x98, (byte) 0x57,
        (byte) 0x67, (byte) 0x7F, (byte) 0x05, (byte) 0x64,
        (byte) 0xAF, (byte) 0x63, (byte) 0xB6, (byte) 0xFE,
        (byte) 0xF5, (byte) 0xB7, (byte) 0x3C, (byte) 0xA5,
        (byte) 0xCE, (byte) 0xE9, (byte) 0x68, (byte) 0x44,
        (byte) 0xE0, (byte) 0x4D, (byte) 0x43, (byte) 0x69,
        (byte) 0x29, (byte) 0x2E, (byte) 0xAC, (byte) 0x15,
        (byte) 0x59, (byte) 0xA8, (byte) 0x0A, (byte) 0x9E,
        (byte) 0x6E, (byte) 0x47, (byte) 0xDF, (byte) 0x34,
        (byte) 0x35, (byte) 0x6A, (byte) 0xCF, (byte) 0xDC,
        (byte) 0x22, (byte) 0xC9, (byte) 0xC0, (byte) 0x9B,
        (byte) 0x89, (byte) 0xD4, (byte) 0xED, (byte) 0xAB,
        (byte) 0x12, (byte) 0xA2, (byte) 0x0D, (byte) 0x52,
        (byte) 0xBB, (byte) 0x02, (byte) 0x2F, (byte) 0xA9,
        (byte) 0xD7, (byte) 0x61, (byte) 0x1E, (byte) 0xB4,
        (byte) 0x50, (byte) 0x04, (byte) 0xF6, (byte) 0xC2,
        (byte) 0x16, (byte) 0x25, (byte) 0x86, (byte) 0x56,
        (byte) 0x55, (byte) 0x09, (byte) 0xBE, (byte) 0x91  }
    };

    /**
    * Define the fixed p0/p1 permutations used in keyed S-box lookup.
    * By changing the following constant definitions, the S-boxes will
    * automatically get changed in the Twofish engine.
    */
    private static final int P_00 = 1;
    private static final int P_01 = 0;
    private static final int P_02 = 0;
    private static final int P_03 = P_01 ^ 1;
    private static final int P_04 = 1;

    private static final int P_10 = 0;
    private static final int P_11 = 0;
    private static final int P_12 = 1;
    private static final int P_13 = P_11 ^ 1;
    private static final int P_14 = 0;

    private static final int P_20 = 1;
    private static final int P_21 = 1;
    private static final int P_22 = 0;
    private static final int P_23 = P_21 ^ 1;
    private static final int P_24 = 0;

    private static final int P_30 = 0;
    private static final int P_31 = 1;
    private static final int P_32 = 1;
    private static final int P_33 = P_31 ^ 1;
    private static final int P_34 = 1;

    /* Primitive polynomial for GF(256) */
    private static final int GF256_FDBK =   0x169;
    private static final int GF256_FDBK_2 = GF256_FDBK / 2;
    private static final int GF256_FDBK_4 = GF256_FDBK / 4;

    private static final int RS_GF_FDBK = 0x14D; // field generator

    //====================================
    // Useful constants
    //====================================

    private static final int    ROUNDS = 16;
    private static final int    MAX_ROUNDS = 16;  // bytes = 128 bits
    private static final int    BLOCK_SIZE = 16;  // bytes = 128 bits
    private static final int    MAX_KEY_BITS = 256;

    private static final int    INPUT_WHITEN=0;
    private static final int    OUTPUT_WHITEN=INPUT_WHITEN+BLOCK_SIZE/4; // 4
    private static final int    ROUND_SUBKEYS=OUTPUT_WHITEN+BLOCK_SIZE/4;// 8

    private static final int    TOTAL_SUBKEYS=ROUND_SUBKEYS+2*MAX_ROUNDS;// 40

    private static final int    SK_STEP = 0x02020202;
    private static final int    SK_BUMP = 0x01010101;
    private static final int    SK_ROTL = 9;

    private boolean encrypting = false;

    private int[] gMDS0 = new int[MAX_KEY_BITS];
    private int[] gMDS1 = new int[MAX_KEY_BITS];
    private int[] gMDS2 = new int[MAX_KEY_BITS];
    private int[] gMDS3 = new int[MAX_KEY_BITS];

    /**
     * gSubKeys[] and gSBox[] are eventually used in the 
     * encryption and decryption methods.
     */
    private int[] gSubKeys;
    private int[] gSBox;

    private int k64Cnt = 0;

    private byte[] workingKey = null;

    public TwofishEngine()
    {
        // calculate the MDS matrix
        int[] m1 = new int[2];
        int[] mX = new int[2];
        int[] mY = new int[2];
        int j;

        for (int i=0; i< MAX_KEY_BITS ; i++)
        {
            j = P[0][i] & 0xff;
            m1[0] = j;
            mX[0] = Mx_X(j) & 0xff;
            mY[0] = Mx_Y(j) & 0xff;

            j = P[1][i] & 0xff;
            m1[1] = j;
            mX[1] = Mx_X(j) & 0xff;
            mY[1] = Mx_Y(j) & 0xff;

            gMDS0[i] = m1[P_00]       | mX[P_00] <<  8 |
                         mY[P_00] << 16 | mY[P_00] << 24;

            gMDS1[i] = mY[P_10]       | mY[P_10] <<  8 |
                         mX[P_10] << 16 | m1[P_10] << 24;

            gMDS2[i] = mX[P_20]       | mY[P_20] <<  8 |
                         m1[P_20] << 16 | mY[P_20] << 24;

            gMDS3[i] = mX[P_30]       | m1[P_30] <<  8 |
                         mY[P_30] << 16 | mX[P_30] << 24;
        }
    }

    /**
     * initialise a Twofish cipher.
     *
     * @param encrypting whether or not we are for encryption.
     * @param params the parameters required to set up the cipher.
     * @exception IllegalArgumentException if the params argument is
     * inappropriate.
     */
    public void init(
        boolean             encrypting,
        CipherParameters    params)
    {
        if (params instanceof KeyParameter)
        {
            this.encrypting = encrypting;
            this.workingKey = ((KeyParameter)params).getKey();
            this.k64Cnt = (this.workingKey.length / 8); // pre-padded ?
            setKey(this.workingKey);

            return;
        }

        throw new IllegalArgumentException("invalid parameter passed to Twofish init - " + params.getClass().getName());
    }

    public String getAlgorithmName()
    {
        return "Twofish";
    }

    public int processBlock(
        byte[] in,
        int inOff,
        byte[] out,
        int outOff)
    {
        if (workingKey == null)
        {
            throw new IllegalStateException("Twofish not initialised");
        }

        if ((inOff + BLOCK_SIZE) > in.length)
        {
            throw new DataLengthException("input buffer too short");
        }

        if ((outOff + BLOCK_SIZE) > out.length)
        {
            throw new OutputLengthException("output buffer too short");
        }

        if (encrypting)
        {
            encryptBlock(in, inOff, out, outOff);
        }
        else
        {    
            decryptBlock(in, inOff, out, outOff);
        }

        return BLOCK_SIZE;
    }

    public void reset()
    {
        if (this.workingKey != null)
        {
            setKey(this.workingKey);
        }
    }

    public int getBlockSize()
    {
        return BLOCK_SIZE;
    }

    //==================================
    // Private Implementation
    //==================================

    private void setKey(byte[] key)
    {
        int[] k32e = new int[MAX_KEY_BITS/64]; // 4
        int[] k32o = new int[MAX_KEY_BITS/64]; // 4 

        int[] sBoxKeys = new int[MAX_KEY_BITS/64]; // 4 
        gSubKeys = new int[TOTAL_SUBKEYS];

        if (k64Cnt < 1) 
        {
            throw new IllegalArgumentException("Key size less than 64 bits");
        }
        
        if (k64Cnt > 4)
        {
            throw new IllegalArgumentException("Key size larger than 256 bits");
        }

        /*
         * k64Cnt is the number of 8 byte blocks (64 chunks)
         * that are in the input key.  The input key is a
         * maximum of 32 bytes (256 bits), so the range
         * for k64Cnt is 1..4
         */
        for (int i=0; i<k64Cnt ; i++)
        {
            int p = i* 8;

            k32e[i] = BytesTo32Bits(key, p);
            k32o[i] = BytesTo32Bits(key, p+4);

            sBoxKeys[k64Cnt-1-i] = RS_MDS_Encode(k32e[i], k32o[i]);
        }

        int q,A,B;
        for (int i=0; i < TOTAL_SUBKEYS / 2 ; i++) 
        {
            q = i*SK_STEP;
            A = F32(q,         k32e);
            B = F32(q+SK_BUMP, k32o);
            B = B << 8 | B >>> 24;
            A += B;
            gSubKeys[i*2] = A;
            A += B;
            gSubKeys[i*2 + 1] = A << SK_ROTL | A >>> (32-SK_ROTL);
        }

        /*
         * fully expand the table for speed
         */
        int k0 = sBoxKeys[0];
        int k1 = sBoxKeys[1];
        int k2 = sBoxKeys[2];
        int k3 = sBoxKeys[3];
        int b0, b1, b2, b3;
        gSBox = new int[4*MAX_KEY_BITS];
        for (int i=0; i<MAX_KEY_BITS; i++)
        {
            b0 = b1 = b2 = b3 = i;
            switch (k64Cnt & 3)
            {
                case 1:
                    gSBox[i*2]       = gMDS0[(P[P_01][b0] & 0xff) ^ b0(k0)];
                    gSBox[i*2+1]     = gMDS1[(P[P_11][b1] & 0xff) ^ b1(k0)];
                    gSBox[i*2+0x200] = gMDS2[(P[P_21][b2] & 0xff) ^ b2(k0)];
                    gSBox[i*2+0x201] = gMDS3[(P[P_31][b3] & 0xff) ^ b3(k0)];
                break;
                case 0: // 256 bits of key
                    b0 = (P[P_04][b0] & 0xff) ^ b0(k3);
                    b1 = (P[P_14][b1] & 0xff) ^ b1(k3);
                    b2 = (P[P_24][b2] & 0xff) ^ b2(k3);
                    b3 = (P[P_34][b3] & 0xff) ^ b3(k3);
                    // fall through, having pre-processed b[0]..b[3] with k32[3]
                case 3: // 192 bits of key
                    b0 = (P[P_03][b0] & 0xff) ^ b0(k2);
                    b1 = (P[P_13][b1] & 0xff) ^ b1(k2);
                    b2 = (P[P_23][b2] & 0xff) ^ b2(k2);
                    b3 = (P[P_33][b3] & 0xff) ^ b3(k2);
                    // fall through, having pre-processed b[0]..b[3] with k32[2]
                case 2: // 128 bits of key
                    gSBox[i*2]   = gMDS0[(P[P_01]
                        [(P[P_02][b0] & 0xff) ^ b0(k1)] & 0xff) ^ b0(k0)];
                    gSBox[i*2+1] = gMDS1[(P[P_11]
                        [(P[P_12][b1] & 0xff) ^ b1(k1)] & 0xff) ^ b1(k0)];
                    gSBox[i*2+0x200] = gMDS2[(P[P_21]
                        [(P[P_22][b2] & 0xff) ^ b2(k1)] & 0xff) ^ b2(k0)];
                    gSBox[i*2+0x201] = gMDS3[(P[P_31]
                        [(P[P_32][b3] & 0xff) ^ b3(k1)] & 0xff) ^ b3(k0)];
                break;
            }
        }

        /* 
         * the function exits having setup the gSBox with the 
         * input key material.
         */
    }

    /**
     * Encrypt the given input starting at the given offset and place
     * the result in the provided buffer starting at the given offset.
     * The input will be an exact multiple of our blocksize.
     *
     * encryptBlock uses the pre-calculated gSBox[] and subKey[]
     * arrays.
     */
    private void encryptBlock(
        byte[] src, 
        int srcIndex,
        byte[] dst,
        int dstIndex)
    {
        int x0 = BytesTo32Bits(src, srcIndex) ^ gSubKeys[INPUT_WHITEN];
        int x1 = BytesTo32Bits(src, srcIndex + 4) ^ gSubKeys[INPUT_WHITEN + 1];
        int x2 = BytesTo32Bits(src, srcIndex + 8) ^ gSubKeys[INPUT_WHITEN + 2];
        int x3 = BytesTo32Bits(src, srcIndex + 12) ^ gSubKeys[INPUT_WHITEN + 3];

        int k = ROUND_SUBKEYS;
        int t0, t1;
        for (int r = 0; r < ROUNDS; r +=2)
        {
            t0 = Fe32_0(x0);
            t1 = Fe32_3(x1);
            x2 ^= t0 + t1 + gSubKeys[k++];
            x2 = x2 >>>1 | x2 << 31;
            x3 = (x3 << 1 | x3 >>> 31) ^ (t0 + 2*t1 + gSubKeys[k++]);

            t0 = Fe32_0(x2);
            t1 = Fe32_3(x3);
            x0 ^= t0 + t1 + gSubKeys[k++];
            x0 = x0 >>>1 | x0 << 31;
            x1 = (x1 << 1 | x1 >>> 31) ^ (t0 + 2*t1 + gSubKeys[k++]);
        }

        Bits32ToBytes(x2 ^ gSubKeys[OUTPUT_WHITEN], dst, dstIndex);
        Bits32ToBytes(x3 ^ gSubKeys[OUTPUT_WHITEN + 1], dst, dstIndex + 4);
        Bits32ToBytes(x0 ^ gSubKeys[OUTPUT_WHITEN + 2], dst, dstIndex + 8);
        Bits32ToBytes(x1 ^ gSubKeys[OUTPUT_WHITEN + 3], dst, dstIndex + 12);
    }

    /**
     * Decrypt the given input starting at the given offset and place
     * the result in the provided buffer starting at the given offset.
     * The input will be an exact multiple of our blocksize.
     */
    private void decryptBlock(
        byte[] src, 
        int srcIndex,
        byte[] dst,
        int dstIndex)
    {
        int x2 = BytesTo32Bits(src, srcIndex) ^ gSubKeys[OUTPUT_WHITEN];
        int x3 = BytesTo32Bits(src, srcIndex+4) ^ gSubKeys[OUTPUT_WHITEN + 1];
        int x0 = BytesTo32Bits(src, srcIndex+8) ^ gSubKeys[OUTPUT_WHITEN + 2];
        int x1 = BytesTo32Bits(src, srcIndex+12) ^ gSubKeys[OUTPUT_WHITEN + 3];

        int k = ROUND_SUBKEYS + 2 * ROUNDS -1 ;
        int t0, t1;
        for (int r = 0; r< ROUNDS ; r +=2)
        {
            t0 = Fe32_0(x2);
            t1 = Fe32_3(x3);
            x1 ^= t0 + 2*t1 + gSubKeys[k--];
            x0 = (x0 << 1 | x0 >>> 31) ^ (t0 + t1 + gSubKeys[k--]);
            x1 = x1 >>>1 | x1 << 31;

            t0 = Fe32_0(x0);
            t1 = Fe32_3(x1);
            x3 ^= t0 + 2*t1 + gSubKeys[k--];
            x2 = (x2 << 1 | x2 >>> 31) ^ (t0 + t1 + gSubKeys[k--]);
            x3 = x3 >>>1 | x3 << 31;
        }

        Bits32ToBytes(x0 ^ gSubKeys[INPUT_WHITEN], dst, dstIndex);
        Bits32ToBytes(x1 ^ gSubKeys[INPUT_WHITEN + 1], dst, dstIndex + 4);
        Bits32ToBytes(x2 ^ gSubKeys[INPUT_WHITEN + 2], dst, dstIndex + 8);
        Bits32ToBytes(x3 ^ gSubKeys[INPUT_WHITEN + 3], dst, dstIndex + 12);
    }

    /* 
     * TODO:  This can be optimised and made cleaner by combining
     * the functionality in this function and applying it appropriately
     * to the creation of the subkeys during key setup.
     */
    private int F32(int x, int[] k32)
    {
        int b0 = b0(x);
        int b1 = b1(x);
        int b2 = b2(x);
        int b3 = b3(x);
        int k0 = k32[0];
        int k1 = k32[1];
        int k2 = k32[2];
        int k3 = k32[3];

        int result = 0;
        switch (k64Cnt & 3)
        {
            case 1:
                result = gMDS0[(P[P_01][b0] & 0xff) ^ b0(k0)] ^
                         gMDS1[(P[P_11][b1] & 0xff) ^ b1(k0)] ^
                         gMDS2[(P[P_21][b2] & 0xff) ^ b2(k0)] ^
                         gMDS3[(P[P_31][b3] & 0xff) ^ b3(k0)];
                break;
            case 0: /* 256 bits of key */
                b0 = (P[P_04][b0] & 0xff) ^ b0(k3);
                b1 = (P[P_14][b1] & 0xff) ^ b1(k3);
                b2 = (P[P_24][b2] & 0xff) ^ b2(k3);
                b3 = (P[P_34][b3] & 0xff) ^ b3(k3);
            case 3: 
                b0 = (P[P_03][b0] & 0xff) ^ b0(k2);
                b1 = (P[P_13][b1] & 0xff) ^ b1(k2);
                b2 = (P[P_23][b2] & 0xff) ^ b2(k2);
                b3 = (P[P_33][b3] & 0xff) ^ b3(k2);
            case 2:
                result = 
                gMDS0[(P[P_01][(P[P_02][b0]&0xff)^b0(k1)]&0xff)^b0(k0)] ^ 
                gMDS1[(P[P_11][(P[P_12][b1]&0xff)^b1(k1)]&0xff)^b1(k0)] ^
                gMDS2[(P[P_21][(P[P_22][b2]&0xff)^b2(k1)]&0xff)^b2(k0)] ^
                gMDS3[(P[P_31][(P[P_32][b3]&0xff)^b3(k1)]&0xff)^b3(k0)];
            break;
        }
        return result;
    }

    /**
     * Use (12, 8) Reed-Solomon code over GF(256) to produce
     * a key S-box 32-bit entity from 2 key material 32-bit
     * entities.
     *
     * @param    k0 first 32-bit entity
     * @param    k1 second 32-bit entity
     * @return     Remainder polynomial generated using RS code
     */
    private int RS_MDS_Encode(int k0, int k1)
    {
        int r = k1;
        for (int i = 0 ; i < 4 ; i++) // shift 1 byte at a time
        {
            r = RS_rem(r);
        }
        r ^= k0;
        for (int i=0 ; i < 4 ; i++)
        {
            r = RS_rem(r);
        }

        return r;
    }

    /**
     * Reed-Solomon code parameters: (12,8) reversible code:<p>
     * <pre>
     * g(x) = x^4 + (a+1/a)x^3 + ax^2 + (a+1/a)x + 1
     * </pre>
     * where a = primitive root of field generator 0x14D
     */
    private int RS_rem(int x)
    {
        int b = (x >>> 24) & 0xff;
        int g2 = ((b << 1) ^ 
                 ((b & 0x80) != 0 ? RS_GF_FDBK : 0)) & 0xff;
        int g3 = ((b >>> 1) ^ 
                 ((b & 0x01) != 0 ? (RS_GF_FDBK >>> 1) : 0)) ^ g2 ;
        return ((x << 8) ^ (g3 << 24) ^ (g2 << 16) ^ (g3 << 8) ^ b);
    }
        
    private int LFSR1(int x)
    {
        return (x >> 1) ^ 
                (((x & 0x01) != 0) ? GF256_FDBK_2 : 0);
    }

    private int LFSR2(int x)
    {
        return (x >> 2) ^
                (((x & 0x02) != 0) ? GF256_FDBK_2 : 0) ^
                (((x & 0x01) != 0) ? GF256_FDBK_4 : 0);
    }

    private int Mx_X(int x)
    {
        return x ^ LFSR2(x);
    } // 5B

    private int Mx_Y(int x)
    {
        return x ^ LFSR1(x) ^ LFSR2(x);
    } // EF

    private int b0(int x)
    {
        return x & 0xff;
    }

    private int b1(int x)
    {
        return (x >>> 8) & 0xff;
    }

    private int b2(int x)
    {
        return (x >>> 16) & 0xff;
    }

    private int b3(int x)
    {
        return (x >>> 24) & 0xff;
    }

    private int Fe32_0(int x)
    {
        return gSBox[ 0x000 + 2*(x & 0xff) ] ^
               gSBox[ 0x001 + 2*((x >>> 8) & 0xff) ] ^
               gSBox[ 0x200 + 2*((x >>> 16) & 0xff) ] ^
               gSBox[ 0x201 + 2*((x >>> 24) & 0xff) ];
    }
    
    private int Fe32_3(int x)
    {
        return gSBox[ 0x000 + 2*((x >>> 24) & 0xff) ] ^
               gSBox[ 0x001 + 2*(x & 0xff) ] ^
               gSBox[ 0x200 + 2*((x >>> 8) & 0xff) ] ^
               gSBox[ 0x201 + 2*((x >>> 16) & 0xff) ];
    }
    
    private int BytesTo32Bits(byte[] b, int p)
    {
        return ((b[p] & 0xff)) | 
             ((b[p+1] & 0xff) << 8) |
             ((b[p+2] & 0xff) << 16) |
             ((b[p+3] & 0xff) << 24);
    }

    private void Bits32ToBytes(int in,  byte[] b, int offset)
    {
        b[offset] = (byte)in;
        b[offset + 1] = (byte)(in >> 8);
        b[offset + 2] = (byte)(in >> 16);
        b[offset + 3] = (byte)(in >> 24);
    }
}