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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. Note: this package includes the NTRU encryption algorithms.
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package org.bouncycastle.crypto.modes;

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.BufferedBlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.OutputLengthException;
import org.bouncycastle.crypto.params.ParametersWithIV;
import org.bouncycastle.util.Pack;

/**
 * Implementation of DSTU7624 XTS mode
 */
public class KXTSBlockCipher
    extends BufferedBlockCipher
{
    /*
     * Constants for GF(2^m) operations
     *
     * GF(2 ^ 128) -> x^128 + x^7 + x^2 + x + 1
     * GF(2 ^ 256) -> x^256 + x^10 + x^5 + x^2 + 1
     * GF(2 ^ 512) -> x^512 + x^8 + x^5 + x^2 + 1
     */
    private static final long RED_POLY_128 = 0x0087L;
    private static final long RED_POLY_256 = 0x0425L;
    private static final long RED_POLY_512 = 0x0125L;

    protected static long getReductionPolynomial(int blockSize)
    {
        switch (blockSize)
        {
        case 16:
            return RED_POLY_128;
        case 32:
            return RED_POLY_256;
        case 64:
            return RED_POLY_512;
        default:
            throw new IllegalArgumentException("Only 128, 256, and 512 -bit block sizes supported"); 
        }
    }

    private final int blockSize;
    private final long reductionPolynomial;
    private final long[] tw_init, tw_current;
    private int counter;

    public KXTSBlockCipher(BlockCipher cipher)
    {
//        super(cipher);
        this.cipher = cipher;

        this.blockSize = cipher.getBlockSize();
        this.reductionPolynomial = getReductionPolynomial(blockSize);
        this.tw_init = new long[blockSize >>> 3];
        this.tw_current = new long[blockSize >>> 3];
        this.counter = -1;
    }

    public int getOutputSize(int length)
    {
        return length;
    }

    public int getUpdateOutputSize(int len)
    {
        return len;
    }

    public void init(boolean forEncryption, CipherParameters parameters)
    {
        if (!(parameters instanceof ParametersWithIV))
        {
            throw new IllegalArgumentException("Invalid parameters passed");
        }

        ParametersWithIV ivParam = (ParametersWithIV)parameters;
        parameters = ivParam.getParameters();

        byte[] iv = ivParam.getIV();

        /*
         * TODO We need to check what the rule is supposed to be for IVs that aren't exactly one block.
         * 
         * Given general little-endianness, presumably a short IV should be right-padded with zeroes.
         */
        if (iv.length != blockSize)
        {
            throw new IllegalArgumentException("Currently only support IVs of exactly one block");
        }

        byte[] tweak = new byte[blockSize];
        System.arraycopy(iv, 0, tweak, 0, blockSize);

        cipher.init(true, parameters);
        cipher.processBlock(tweak, 0, tweak, 0);

        cipher.init(forEncryption, parameters);
        Pack.littleEndianToLong(tweak, 0, tw_init);
        System.arraycopy(tw_init, 0, tw_current, 0, tw_init.length);
        counter = 0;
    }

    public int processByte(byte in, byte[] out, int outOff)
    {
        /*
         * TODO This class isn't really behaving like a BufferedBlockCipher yet
         */
        throw new IllegalStateException("unsupported operation");
    }

    public int processBytes(byte[] input, int inOff, int len, byte[] output, int outOff)
    {
        if (input.length - inOff < len)
        {
            throw new DataLengthException("Input buffer too short");
        }
        if (output.length - inOff < len)
        {
            throw new OutputLengthException("Output buffer too short");
        }
        if (len % blockSize != 0)
        {
            throw new IllegalArgumentException("Partial blocks not supported");
        }

        for (int pos = 0; pos < len; pos += blockSize)
        {
            processBlock(input, inOff + pos, output, outOff + pos);
        }

        return len;
    }

    private void processBlock(byte[] input, int inOff, byte[] output, int outOff)
    {
        /*
         * A somewhat arbitrary limit of 2^32 - 1 blocks
         */
        if (counter == -1)
        {
            throw new IllegalStateException("Attempt to process too many blocks");
        }

        ++counter;

        /*
         * Multiply tweak by 'alpha', which is just 2
         */
        GF_double(reductionPolynomial, tw_current);

        byte[] tweak = new byte[blockSize];
        Pack.longToLittleEndian(tw_current, tweak, 0);

        byte[] buffer = new byte[blockSize];
        System.arraycopy(tweak, 0, buffer, 0, blockSize);

        for (int i = 0; i < blockSize; ++i)
        {
            buffer[i] ^= input[inOff + i];
        }

        cipher.processBlock(buffer, 0, buffer, 0);

        for (int i = 0; i < blockSize; ++i)
        {
            output[outOff + i] = (byte)(buffer[i] ^ tweak[i]);
        }
    }

    public int doFinal(byte[] output, int outOff)
    {
        reset();

        return 0;
    }

    public void reset()
    {
//        super.reset();
        cipher.reset();

        System.arraycopy(tw_init, 0, tw_current, 0, tw_init.length);
        counter = 0;
    }

    private static void GF_double(long redPoly, long[] z)
    {
        long c = 0;
        for (int i = 0; i < z.length; ++i)
        {
            long zVal = z[i];
            long bit = zVal >>> 63;
            z[i] = (zVal << 1) ^ c;
            c = bit;
        }

        z[0] ^= redPoly & -c;
    }
}