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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.
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package org.spongycastle.crypto.engines;

import org.spongycastle.crypto.CipherParameters;
import org.spongycastle.crypto.DataLengthException;
import org.spongycastle.crypto.OutputLengthException;
import org.spongycastle.crypto.StreamCipher;
import org.spongycastle.crypto.params.KeyParameter;
import org.spongycastle.crypto.params.ParametersWithIV;

/**
 * HC-256 is a software-efficient stream cipher created by Hongjun Wu. It 
 * generates keystream from a 256-bit secret key and a 256-bit initialization 
 * vector.
 * 
 * http://www.ecrypt.eu.org/stream/p3ciphers/hc/hc256_p3.pdf
 * 

 * Its brother, HC-128, is a third phase candidate in the eStream contest.
 * The algorithm is patent-free. No attacks are known as of today (April 2007). 
 * See
 * 
 * http://www.ecrypt.eu.org/stream/hcp3.html
 * 
 */
public class HC256Engine
    implements StreamCipher
{
    private int[] p = new int[1024];
    private int[] q = new int[1024];
    private int cnt = 0;

    private int step()
    {
        int j = cnt & 0x3FF;
        int ret;
        if (cnt < 1024)
        {
            int x = p[(j - 3 & 0x3FF)];
            int y = p[(j - 1023 & 0x3FF)];
            p[j] += p[(j - 10 & 0x3FF)]
                + (rotateRight(x, 10) ^ rotateRight(y, 23))
                + q[((x ^ y) & 0x3FF)];

            x = p[(j - 12 & 0x3FF)];
            ret = (q[x & 0xFF] + q[((x >> 8) & 0xFF) + 256]
                + q[((x >> 16) & 0xFF) + 512] + q[((x >> 24) & 0xFF) + 768])
                ^ p[j];
        }
        else
        {
            int x = q[(j - 3 & 0x3FF)];
            int y = q[(j - 1023 & 0x3FF)];
            q[j] += q[(j - 10 & 0x3FF)]
                + (rotateRight(x, 10) ^ rotateRight(y, 23))
                + p[((x ^ y) & 0x3FF)];

            x = q[(j - 12 & 0x3FF)];
            ret = (p[x & 0xFF] + p[((x >> 8) & 0xFF) + 256]
                + p[((x >> 16) & 0xFF) + 512] + p[((x >> 24) & 0xFF) + 768])
                ^ q[j];
        }
        cnt = cnt + 1 & 0x7FF;
        return ret;
    }

    private byte[] key, iv;
    private boolean initialised;

    private void init()
    {
        if (key.length != 32 && key.length != 16)
        {
            throw new IllegalArgumentException(
                "The key must be 128/256 bits long");
        }

        if (iv.length < 16)
        {
            throw new IllegalArgumentException(
                "The IV must be at least 128 bits long");
        }

        if (key.length != 32)
        {
            byte[] k = new byte[32];

            System.arraycopy(key, 0, k, 0, key.length);
            System.arraycopy(key, 0, k, 16, key.length);

            key = k;
        }

        if (iv.length < 32)
        {
            byte[] newIV = new byte[32];

            System.arraycopy(iv, 0, newIV, 0, iv.length);
            System.arraycopy(iv, 0, newIV, iv.length, newIV.length - iv.length);

            iv = newIV;
        }

        idx = 0;
        cnt = 0;

        int[] w = new int[2560];

        for (int i = 0; i < 32; i++)
        {
            w[i >> 2] |= (key[i] & 0xff) << (8 * (i & 0x3));
        }

        for (int i = 0; i < 32; i++)
        {
            w[(i >> 2) + 8] |= (iv[i] & 0xff) << (8 * (i & 0x3));
        }

        for (int i = 16; i < 2560; i++)
        {
            int x = w[i - 2];
            int y = w[i - 15];
            w[i] = (rotateRight(x, 17) ^ rotateRight(x, 19) ^ (x >>> 10))
                + w[i - 7]
                + (rotateRight(y, 7) ^ rotateRight(y, 18) ^ (y >>> 3))
                + w[i - 16] + i;
        }

        System.arraycopy(w, 512, p, 0, 1024);
        System.arraycopy(w, 1536, q, 0, 1024);

        for (int i = 0; i < 4096; i++)
        {
            step();
        }

        cnt = 0;
    }

    public String getAlgorithmName()
    {
        return "HC-256";
    }

    /**
     * Initialise a HC-256 cipher.
     *
     * @param forEncryption whether or not we are for encryption. Irrelevant, as
     *                      encryption and decryption are the same.
     * @param params        the parameters required to set up the cipher.
     * @throws IllegalArgumentException if the params argument is
     *                                  inappropriate (ie. the key is not 256 bit long).
     */
    public void init(boolean forEncryption, CipherParameters params)
        throws IllegalArgumentException
    {
        CipherParameters keyParam = params;

        if (params instanceof ParametersWithIV)
        {
            iv = ((ParametersWithIV)params).getIV();
            keyParam = ((ParametersWithIV)params).getParameters();
        }
        else
        {
            iv = new byte[0];
        }

        if (keyParam instanceof KeyParameter)
        {
            key = ((KeyParameter)keyParam).getKey();
            init();
        }
        else
        {
            throw new IllegalArgumentException(
                "Invalid parameter passed to HC256 init - "
                    + params.getClass().getName());
        }

        initialised = true;
    }

    private byte[] buf = new byte[4];
    private int idx = 0;

    private byte getByte()
    {
        if (idx == 0)
        {
            int step = step();
            buf[0] = (byte)(step & 0xFF);
            step >>= 8;
            buf[1] = (byte)(step & 0xFF);
            step >>= 8;
            buf[2] = (byte)(step & 0xFF);
            step >>= 8;
            buf[3] = (byte)(step & 0xFF);
        }
        byte ret = buf[idx];
        idx = idx + 1 & 0x3;
        return ret;
    }

    public int processBytes(byte[] in, int inOff, int len, byte[] out,
                             int outOff) throws DataLengthException
    {
        if (!initialised)
        {
            throw new IllegalStateException(getAlgorithmName()
                + " not initialised");
        }

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

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

        for (int i = 0; i < len; i++)
        {
            out[outOff + i] = (byte)(in[inOff + i] ^ getByte());
        }

        return len;
    }

    public void reset()
    {
        init();
    }

    public byte returnByte(byte in)
    {
        return (byte)(in ^ getByte());
    }

    private static int rotateRight(
        int     x,
        int     bits)
    {
        return (x >>> bits) | (x << -bits);
    }
}