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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 and up.

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package org.bouncycastle.crypto.modes;

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.OutputLengthException;
import org.bouncycastle.crypto.StreamBlockCipher;
import org.bouncycastle.crypto.params.ParametersWithIV;
import org.bouncycastle.util.Arrays;

/**
 * Implementation of DSTU7624 CTR mode
 */
public class KCTRBlockCipher
    extends StreamBlockCipher
{
    private byte[] iv;
    private byte[] ofbV;
    private byte[] ofbOutV;

    private int             byteCount;

    private boolean initialised;
    private BlockCipher engine;

    public KCTRBlockCipher(BlockCipher engine)
    {
        super(engine);

        this.engine = engine;
        this.iv = new byte[engine.getBlockSize()];
        this.ofbV = new byte[engine.getBlockSize()];
        this.ofbOutV = new byte[engine.getBlockSize()];
    }

    public void init(boolean forEncryption, CipherParameters params)
        throws IllegalArgumentException
    {
        this.initialised = true;

        if (params instanceof ParametersWithIV)
        {
            ParametersWithIV ivParam = (ParametersWithIV)params;
            byte[] iv = ivParam.getIV();
            int diff = this.iv.length - iv.length;

            Arrays.fill(this.iv, (byte)0);
            System.arraycopy(iv, 0, this.iv, diff, iv.length);
            params = ivParam.getParameters();
        }
        else
        {
            throw new IllegalArgumentException("invalid parameter passed");
        }
 
        if (params != null)
        {
            engine.init(true, params);
        }

        reset();
    }

    public String getAlgorithmName()
    {
        return engine.getAlgorithmName() + "/KCTR";
    }

    public int getBlockSize()
    {
        return engine.getBlockSize();
    }

    protected byte calculateByte(byte b)
    {
        if (byteCount == 0)
        {
            incrementCounterAt(0);

            checkCounter();

            engine.processBlock(ofbV, 0, ofbOutV, 0);

            return (byte)(ofbOutV[byteCount++] ^ b);
        }

        byte rv = (byte)(ofbOutV[byteCount++] ^ b);

        if (byteCount == ofbV.length)
        {
            byteCount = 0;
        }

        return rv;
    }

    public int processBlock(byte[] in, int inOff, byte[] out, int outOff)
        throws DataLengthException, IllegalStateException
    {
        if (in.length - inOff < getBlockSize())
        {
            throw new DataLengthException("input buffer too short");
        }
        if (out.length - outOff < getBlockSize())
        {
            throw new OutputLengthException("output buffer too short");
        }
        
        processBytes(in, inOff, getBlockSize(), out, outOff);

        return getBlockSize();
    }

    public void reset()
    {
        if (initialised)
        {
            engine.processBlock(this.iv, 0, ofbV, 0);
        }
        engine.reset();
        byteCount = 0;
    }

    private void incrementCounterAt(int pos)
    {
        int i = pos;
        while (i < ofbV.length)
        {
            if (++ofbV[i++] != 0)
            {
                break;
            }
        }
    }

    private void checkCounter()
    {
        // TODO:
        // if the IV is the same as the blocksize we assume the user knows what they are doing
//        if (IV.length < ofbV.length)
//        {
//            for (int i = 0; i != IV.length; i++)
//            {
//                if (ofbV[i] != IV[i])
//                {
//                    throw new IllegalStateException("Counter in KCTR mode out of range.");
//                }
//            }
//        }
    }
}