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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.engines;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.CryptoServicesRegistrar;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.OutputLengthException;
import org.bouncycastle.crypto.StreamCipher;
import org.bouncycastle.crypto.constraints.DefaultServiceProperties;
import org.bouncycastle.crypto.params.KeyParameter;

public class RC4Engine
    implements StreamCipher
{
    private final static int STATE_LENGTH = 256;

    /*
     * variables to hold the state of the RC4 engine
     * during encryption and decryption
     */

    private byte[]      engineState = null;
    private int         x = 0;
    private int         y = 0;
    private byte[]      workingKey = null;
    private boolean     forEncryption;

    public RC4Engine()
    {
        CryptoServicesRegistrar.checkConstraints(new DefaultServiceProperties(getAlgorithmName(), 20));
    }

    /**
     * initialise a RC4 cipher.
     *
     * @param forEncryption 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             forEncryption, 
        CipherParameters     params)
    {
        if (params instanceof KeyParameter)
        {
            /* 
             * RC4 encryption and decryption is completely
             * symmetrical, so the 'forEncryption' is 
             * irrelevant.
             */
            this.workingKey = ((KeyParameter)params).getKey();
            this.forEncryption = forEncryption;
            setKey(workingKey);

            CryptoServicesRegistrar.checkConstraints(new DefaultServiceProperties(getAlgorithmName(), 20, params, Utils.getPurpose(forEncryption)));

            return;
        }

        throw new IllegalArgumentException("invalid parameter passed to RC4 init - " + params.getClass().getName());
    }

    public String getAlgorithmName()
    {
        return "RC4";
    }

    public byte returnByte(byte in)
    {
        x = (x + 1) & 0xff;
        y = (engineState[x] + y) & 0xff;

        // swap
        byte tmp = engineState[x];
        engineState[x] = engineState[y];
        engineState[y] = tmp;

        // xor
        return (byte)(in ^ engineState[(engineState[x] + engineState[y]) & 0xff]);
    }

    public int processBytes(
        byte[]     in, 
        int     inOff, 
        int     len, 
        byte[]     out, 
        int     outOff)
    {
        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++)
        {
            x = (x + 1) & 0xff;
            y = (engineState[x] + y) & 0xff;

            // swap
            byte tmp = engineState[x];
            engineState[x] = engineState[y];
            engineState[y] = tmp;

            // xor
            out[i+outOff] = (byte)(in[i + inOff]
                    ^ engineState[(engineState[x] + engineState[y]) & 0xff]);
        }

        return len;
    }

    public void reset()
    {
        setKey(workingKey);
    }

    // Private implementation

    private void setKey(byte[] keyBytes)
    {
        workingKey = keyBytes;

        // System.out.println("the key length is ; "+ workingKey.length);

        x = 0;
        y = 0;

        if (engineState == null)
        {
            engineState = new byte[STATE_LENGTH];
        }

        // reset the state of the engine
        for (int i=0; i < STATE_LENGTH; i++)
        {
            engineState[i] = (byte)i;
        }
        
        int i1 = 0;
        int i2 = 0;

        for (int i=0; i < STATE_LENGTH; i++)
        {
            i2 = ((keyBytes[i1] & 0xff) + engineState[i] + i2) & 0xff;
            // do the byte-swap inline
            byte tmp = engineState[i];
            engineState[i] = engineState[i2];
            engineState[i2] = tmp;
            i1 = (i1+1) % keyBytes.length; 
        }
    }
}