http://www.bouncycastle.org/java.html: 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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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.

/**
 * 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.
 */
package org.bouncycastle.crypto.modes;

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.BufferedBlockCipher;
import org.bouncycastle.crypto.DataLengthException;
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 class implements the NIST version as documented in "Addendum to NIST SP 800-38A, Recommendation for Block Cipher Modes of Operation: Three Variants of Ciphertext Stealing for CBC Mode"



/**
 * 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.
 * <p>
 *     This class implements the NIST version as documented in "Addendum to NIST SP 800-38A, Recommendation for Block Cipher Modes of Operation: Three Variants of Ciphertext Stealing for CBC Mode"
 * </p>
 */
public class NISTCTSBlockCipher
    extends BufferedBlockCipher
{
    public static final int CS1 = 1;
    public static final int CS2 = 2;
    public static final int CS3 = 3;

    private final int type;
    private final int blockSize;

    
Create a buffered block cipher that uses NIST Cipher Text Stealing

Params:
  • type – type of CTS mode (CS1, CS2, or CS3)
  • cipher – the underlying block cipher used to create the CBC block cipher this cipher uses..
/**
     * Create a buffered block cipher that uses NIST Cipher Text Stealing
     *
     * @param type type of CTS mode (CS1, CS2, or CS3)
     * @param cipher the underlying block cipher used to create the CBC block cipher this cipher uses..
     */
    public NISTCTSBlockCipher(
        int type,
        BlockCipher cipher)
    {
        this.type = type;
        this.cipher = new CBCBlockCipher(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.

Params:
  • len – the length of the input.
Returns:the space required to accommodate a call to update
with len bytes of input.
/**
     * 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.

Params:
  • len – the length of the input.
Returns:the space required to accommodate a call to update and doFinal
with len bytes of input.
/**
     * 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.

Params:
  • in – the input byte.
  • out – the space for any output that might be produced.
  • outOff – the offset from which the output will be copied.
Throws:
Returns:the number of output bytes copied to out.
/**
     * 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.

Params:
  • in – the input byte array.
  • inOff – the offset at which the input data starts.
  • len – the number of bytes to be copied out of the input array.
  • out – the space for any output that might be produced.
  • outOff – the offset from which the output will be copied.
Throws:
Returns:the number of output bytes copied to out.
/**
     * 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.

Params:
  • out – the array the block currently being held is copied into.
  • outOff – the offset at which the copying starts.
Throws:
Returns:the number of output bytes copied to out.
/**
     * 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)
        {
            if (bufOff < blockSize)
            {
                throw new DataLengthException("need at least one block of input for NISTCTS");
            }

            if (bufOff > blockSize)
            {
                byte[]  lastBlock = new byte[blockSize];

                if (this.type == CS2 || this.type == CS3)
                {
                    cipher.processBlock(buf, 0, block, 0);

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

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

                    if (this.type == CS2 && len == blockSize)
                    {
                        System.arraycopy(block, 0, out, outOff, blockSize);

                        System.arraycopy(lastBlock, 0, out, outOff + blockSize, len);
                    }
                    else
                    {
                        System.arraycopy(lastBlock, 0, out, outOff, blockSize);

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

                    System.arraycopy(buf, bufOff - len, lastBlock, 0, len);
                    cipher.processBlock(lastBlock, 0, lastBlock, 0);
                    System.arraycopy(lastBlock, 0, out, outOff + len, blockSize);
                }
            }
            else
            {
                cipher.processBlock(buf, 0, block, 0);

                System.arraycopy(block, 0, out, outOff, blockSize);
            }
        }
        else
        {
            if (bufOff < blockSize)
            {
                throw new DataLengthException("need at least one block of input for CTS");
            }

            byte[]  lastBlock = new byte[blockSize];

            if (bufOff > blockSize)
            {
                if (this.type == CS3 || (this.type == CS2 && ((buf.length - bufOff) % blockSize) != 0))
                {
                    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);
                }
                else
                {
                    BlockCipher c = ((CBCBlockCipher)cipher).getUnderlyingCipher();

                    c.processBlock(buf, bufOff - blockSize, lastBlock, 0);

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

                    if (len != blockSize)
                    {
                        System.arraycopy(lastBlock, len, block, len, blockSize - len);
                    }

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

                    System.arraycopy(block, 0, out, outOff, blockSize);

                    for (int i = 0; i != len; i++)
                    {
                        lastBlock[i] ^= buf[i];
                    }

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

                System.arraycopy(block, 0, out, outOff, blockSize);
            }
        }

        int offset = bufOff;

        reset();

        return offset;
    }
}