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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.
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package org.bouncycastle.crypto.modes;

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.DefaultBufferedBlockCipher;
import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.crypto.OutputLengthException;

/**
 * A Cipher Text Stealing (CTS) mode cipher. CTS allows block ciphers to
 * be used to produce cipher text which is the same length as the plain text.
 * 
 * This version applies the CTS algorithm from one block up, rather than following the errata update issued in 2004, where CTS mode is applied
 * from greater than 1 block up and the first block is processed using CBC mode.
 * 
 */
public class OldCTSBlockCipher
    extends DefaultBufferedBlockCipher
{
    private int     blockSize;

    /**
     * Create a buffered block cipher that uses Cipher Text Stealing
     *
     * @param cipher the underlying block cipher this buffering object wraps.
     */
    public OldCTSBlockCipher(
        BlockCipher cipher)
    {
        if ((cipher instanceof OFBBlockCipher) || (cipher instanceof CFBBlockCipher))
        {
            throw new IllegalArgumentException("CTSBlockCipher can only accept ECB, or CBC ciphers");
        }

        this.cipher = cipher;

        blockSize = cipher.getBlockSize();

        buf = new byte[blockSize * 2];
        bufOff = 0;
    }

    /**
     * return the size of the output buffer required for an update
     * an input of len bytes.
     *
     * @param len the length of the input.
     * @return the space required to accommodate a call to update
     * with len bytes of input.
     */
    public int getUpdateOutputSize(
        int len)
    {
        int total       = len + bufOff;
        int leftOver    = total % buf.length;

        if (leftOver == 0)
        {
            return total - buf.length;
        }

        return total - leftOver;
    }

    /**
     * return the size of the output buffer required for an update plus a
     * doFinal with an input of len bytes.
     *
     * @param len the length of the input.
     * @return the space required to accommodate a call to update and doFinal
     * with len bytes of input.
     */
    public int getOutputSize(
        int len)
    {
        return len + bufOff;
    }

    /**
     * process a single byte, producing an output block if necessary.
     *
     * @param in the input byte.
     * @param out the space for any output that might be produced.
     * @param outOff the offset from which the output will be copied.
     * @return the number of output bytes copied to out.
     * @exception org.bouncycastle.crypto.DataLengthException if there isn't enough space in out.
     * @exception IllegalStateException if the cipher isn't initialised.
     */
    public int processByte(
        byte        in,
        byte[]      out,
        int         outOff)
        throws DataLengthException, IllegalStateException
    {
        int         resultLen = 0;

        if (bufOff == buf.length)
        {
            resultLen = cipher.processBlock(buf, 0, out, outOff);
            System.arraycopy(buf, blockSize, buf, 0, blockSize);

            bufOff = blockSize;
        }

        buf[bufOff++] = in;

        return resultLen;
    }

    /**
     * process an array of bytes, producing output if necessary.
     *
     * @param in the input byte array.
     * @param inOff the offset at which the input data starts.
     * @param len the number of bytes to be copied out of the input array.
     * @param out the space for any output that might be produced.
     * @param outOff the offset from which the output will be copied.
     * @return the number of output bytes copied to out.
     * @exception org.bouncycastle.crypto.DataLengthException if there isn't enough space in out.
     * @exception IllegalStateException if the cipher isn't initialised.
     */
    public int processBytes(
        byte[]      in,
        int         inOff,
        int         len,
        byte[]      out,
        int         outOff)
        throws DataLengthException, IllegalStateException
    {
        if (len < 0)
        {
            throw new IllegalArgumentException("Can't have a negative input length!");
        }

        int blockSize   = getBlockSize();
        int length      = getUpdateOutputSize(len);

        if (length > 0)
        {
            if ((outOff + length) > out.length)
            {
                throw new OutputLengthException("output buffer too short");
            }
        }

        int resultLen = 0;
        int gapLen = buf.length - bufOff;

        if (len > gapLen)
        {
            System.arraycopy(in, inOff, buf, bufOff, gapLen);

            resultLen += cipher.processBlock(buf, 0, out, outOff);
            System.arraycopy(buf, blockSize, buf, 0, blockSize);

            bufOff = blockSize;

            len -= gapLen;
            inOff += gapLen;

            while (len > blockSize)
            {
                System.arraycopy(in, inOff, buf, bufOff, blockSize);
                resultLen += cipher.processBlock(buf, 0, out, outOff + resultLen);
                System.arraycopy(buf, blockSize, buf, 0, blockSize);

                len -= blockSize;
                inOff += blockSize;
            }
        }

        System.arraycopy(in, inOff, buf, bufOff, len);

        bufOff += len;

        return resultLen;
    }

    /**
     * Process the last block in the buffer.
     *
     * @param out the array the block currently being held is copied into.
     * @param outOff the offset at which the copying starts.
     * @return the number of output bytes copied to out.
     * @exception org.bouncycastle.crypto.DataLengthException if there is insufficient space in out for
     * the output.
     * @exception IllegalStateException if the underlying cipher is not
     * initialised.
     * @exception org.bouncycastle.crypto.InvalidCipherTextException if cipher text decrypts wrongly (in
     * case the exception will never get thrown).
     */
    public int doFinal(
        byte[]  out,
        int     outOff)
        throws DataLengthException, IllegalStateException, InvalidCipherTextException
    {
        if (bufOff + outOff > out.length)
        {
            throw new OutputLengthException("output buffer to small in doFinal");
        }

        int     blockSize = cipher.getBlockSize();
        int     len = bufOff - blockSize;
        byte[]  block = new byte[blockSize];

        if (forEncryption)
        {
            cipher.processBlock(buf, 0, block, 0);
            
            if (bufOff < blockSize)
            {
                throw new DataLengthException("need at least one block of input for CTS");
            }

            for (int i = bufOff; i != buf.length; i++)
            {
                buf[i] = block[i - blockSize];
            }

            for (int i = blockSize; i != bufOff; i++)
            {
                buf[i] ^= block[i - blockSize];
            }

            if (cipher instanceof CBCBlockCipher)
            {
                BlockCipher c = ((CBCBlockCipher)cipher).getUnderlyingCipher();

                c.processBlock(buf, blockSize, out, outOff);
            }
            else
            {
                cipher.processBlock(buf, blockSize, out, outOff);
            }

            System.arraycopy(block, 0, out, outOff + blockSize, len);
        }
        else
        {
            byte[]  lastBlock = new byte[blockSize];

            if (cipher instanceof CBCBlockCipher)
            {
                BlockCipher c = ((CBCBlockCipher)cipher).getUnderlyingCipher();

                c.processBlock(buf, 0, block, 0);
            }
            else
            {
                cipher.processBlock(buf, 0, block, 0);
            }

            for (int i = blockSize; i != bufOff; i++)
            {
                lastBlock[i - blockSize] = (byte)(block[i - blockSize] ^ buf[i]);
            }

            System.arraycopy(buf, blockSize, block, 0, len);

            cipher.processBlock(block, 0, out, outOff);
            System.arraycopy(lastBlock, 0, out, outOff + blockSize, len);
        }

        int offset = bufOff;

        reset();

        return offset;
    }
}