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. 
package org.bouncycastle.crypto.modes;

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.params.ParametersWithIV;


implements a Output-FeedBack (OFB) mode on top of a simple cipher.

/**
 * implements a Output-FeedBack (OFB) mode on top of a simple cipher.
 */
public class OFBBlockCipher
    implements BlockCipher
{
    private byte[]          IV;
    private byte[]          ofbV;
    private byte[]          ofbOutV;

    private final int             blockSize;
    private final BlockCipher     cipher;

    
Basic constructor.

Params:
  • cipher – the block cipher to be used as the basis of the
feedback mode.
  • blockSize – the block size in bits (note: a multiple of 8)
/**
     * Basic constructor.
     *
     * @param cipher the block cipher to be used as the basis of the
     * feedback mode.
     * @param blockSize the block size in bits (note: a multiple of 8)
     */
    public OFBBlockCipher(
        BlockCipher cipher,
        int         blockSize)
    {
        this.cipher = cipher;
        this.blockSize = blockSize / 8;

        this.IV = new byte[cipher.getBlockSize()];
        this.ofbV = new byte[cipher.getBlockSize()];
        this.ofbOutV = new byte[cipher.getBlockSize()];
    }

    
return the underlying block cipher that we are wrapping.

Returns:the underlying block cipher that we are wrapping.
/**
     * return the underlying block cipher that we are wrapping.
     *
     * @return the underlying block cipher that we are wrapping.
     */
    public BlockCipher getUnderlyingCipher()
    {
        return cipher;
    }

    
Initialise the cipher and, possibly, the initialisation vector (IV).
If an IV isn't passed as part of the parameter, the IV will be all zeros.
An IV which is too short is handled in FIPS compliant fashion.

Params:
  • encrypting – if true the cipher is initialised for
 encryption, if false for decryption.
  • params – the key and other data required by the cipher.
Throws:
/**
     * Initialise the cipher and, possibly, the initialisation vector (IV).
     * If an IV isn't passed as part of the parameter, the IV will be all zeros.
     * An IV which is too short is handled in FIPS compliant fashion.
     *
     * @param encrypting if true the cipher is initialised for
     *  encryption, if false for decryption.
     * @param params the key and other data required by the cipher.
     * @exception IllegalArgumentException if the params argument is
     * inappropriate.
     */
    public void init(
        boolean             encrypting, //ignored by this OFB mode
        CipherParameters    params)
        throws IllegalArgumentException
    {
        if (params instanceof ParametersWithIV)
        {
                ParametersWithIV ivParam = (ParametersWithIV)params;
                byte[]      iv = ivParam.getIV();

                if (iv.length < IV.length)
                {
                    // prepend the supplied IV with zeros (per FIPS PUB 81)
                    System.arraycopy(iv, 0, IV, IV.length - iv.length, iv.length); 
                    for (int i = 0; i < IV.length - iv.length; i++)
                    {
                        IV[i] = 0;
                    }
                }
                else
                {
                    System.arraycopy(iv, 0, IV, 0, IV.length);
                }

                reset();

                cipher.init(true, ivParam.getParameters());
        }
        else
        {
                reset();

                cipher.init(true, params);
        }
    }

    
return the algorithm name and mode.

Returns:the name of the underlying algorithm followed by "/OFB"
and the block size in bits
/**
     * return the algorithm name and mode.
     *
     * @return the name of the underlying algorithm followed by "/OFB"
     * and the block size in bits
     */
    public String getAlgorithmName()
    {
        return cipher.getAlgorithmName() + "/OFB" + (blockSize * 8);
    }

    
    
return the block size we are operating at (in bytes).

Returns:the block size we are operating at (in bytes).
/**
     * return the block size we are operating at (in bytes).
     *
     * @return the block size we are operating at (in bytes).
     */
    public int getBlockSize()
    {
        return blockSize;
    }

    
Process one block of input from the array in and write it to
the out array.

Params:
  • in – the array containing the input data.
  • inOff – offset into the in array the data starts at.
  • out – the array the output data will be copied into.
  • outOff – the offset into the out array the output will start at.
Throws:
Returns:the number of bytes processed and produced.
/**
     * Process one block of input from the array in and write it to
     * the out array.
     *
     * @param in the array containing the input data.
     * @param inOff offset into the in array the data starts at.
     * @param out the array the output data will be copied into.
     * @param outOff the offset into the out array the output will start at.
     * @exception DataLengthException if there isn't enough data in in, or
     * space in out.
     * @exception IllegalStateException if the cipher isn't initialised.
     * @return the number of bytes processed and produced.
     */
    public int processBlock(
        byte[]      in,
        int         inOff,
        byte[]      out,
        int         outOff)
        throws DataLengthException, IllegalStateException
    {
        if ((inOff + blockSize) > in.length)
        {
            throw new DataLengthException("input buffer too short");
        }

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

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

        //
        // XOR the ofbV with the plaintext producing the cipher text (and
        // the next input block).
        //
        for (int i = 0; i < blockSize; i++)
        {
            out[outOff + i] = (byte)(ofbOutV[i] ^ in[inOff + i]);
        }

        //
        // change over the input block.
        //
        System.arraycopy(ofbV, blockSize, ofbV, 0, ofbV.length - blockSize);
        System.arraycopy(ofbOutV, 0, ofbV, ofbV.length - blockSize, blockSize);

        return blockSize;
    }

    
reset the feedback vector back to the IV and reset the underlying
cipher.

/**
     * reset the feedback vector back to the IV and reset the underlying
     * cipher.
     */
    public void reset()
    {
        System.arraycopy(IV, 0, ofbV, 0, IV.length);

        cipher.reset();
    }
}