Java tutorial
package org.bouncycastle.crypto.modes; import org.bouncycastle.crypto.BlockCipher; import org.bouncycastle.crypto.CipherParameters; import org.bouncycastle.crypto.DataLengthException; import org.bouncycastle.crypto.StreamBlockCipher; import org.bouncycastle.crypto.params.ParametersWithIV; import org.bouncycastle.util.Arrays; /** * implements a Cipher-FeedBack (CFB) mode on top of a simple cipher. */ public class CFBBlockCipher extends StreamBlockCipher { private byte[] IV; private byte[] cfbV; private byte[] cfbOutV; private byte[] inBuf; private int blockSize; private BlockCipher cipher = null; private boolean encrypting; private int byteCount; /** * Basic constructor. * * @param cipher the block cipher to be used as the basis of the * feedback mode. * @param bitBlockSize the block size in bits (note: a multiple of 8) */ public CFBBlockCipher(BlockCipher cipher, int bitBlockSize) { super(cipher); this.cipher = cipher; this.blockSize = bitBlockSize / 8; this.IV = new byte[cipher.getBlockSize()]; this.cfbV = new byte[cipher.getBlockSize()]; this.cfbOutV = new byte[cipher.getBlockSize()]; this.inBuf = new byte[blockSize]; } /** * 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, CipherParameters params) throws IllegalArgumentException { this.encrypting = encrypting; 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(); // if null it's an IV changed only. if (ivParam.getParameters() != null) { cipher.init(true, ivParam.getParameters()); } } else { reset(); // if it's null, key is to be reused. if (params != null) { cipher.init(true, params); } } } /** * return the algorithm name and mode. * * @return the name of the underlying algorithm followed by "/CFB" * and the block size in bits. */ public String getAlgorithmName() { return cipher.getAlgorithmName() + "/CFB" + (blockSize * 8); } protected byte calculateByte(byte in) throws DataLengthException, IllegalStateException { return (encrypting) ? encryptByte(in) : decryptByte(in); } private byte encryptByte(byte in) { if (byteCount == 0) { cipher.processBlock(cfbV, 0, cfbOutV, 0); } byte rv = (byte) (cfbOutV[byteCount] ^ in); inBuf[byteCount++] = rv; if (byteCount == blockSize) { byteCount = 0; System.arraycopy(cfbV, blockSize, cfbV, 0, cfbV.length - blockSize); System.arraycopy(inBuf, 0, cfbV, cfbV.length - blockSize, blockSize); } return rv; } private byte decryptByte(byte in) { if (byteCount == 0) { cipher.processBlock(cfbV, 0, cfbOutV, 0); } inBuf[byteCount] = in; byte rv = (byte) (cfbOutV[byteCount++] ^ in); if (byteCount == blockSize) { byteCount = 0; System.arraycopy(cfbV, blockSize, cfbV, 0, cfbV.length - blockSize); System.arraycopy(inBuf, 0, cfbV, cfbV.length - blockSize, blockSize); } return rv; } /** * return the block size we are operating at. * * @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. * * @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 { processBytes(in, inOff, blockSize, out, outOff); return blockSize; } /** * Do the appropriate processing for CFB mode encryption. * * @param in the array containing the data to be encrypted. * @param inOff offset into the in array the data starts at. * @param out the array the encrypted 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 encryptBlock(byte[] in, int inOff, byte[] out, int outOff) throws DataLengthException, IllegalStateException { processBytes(in, inOff, blockSize, out, outOff); return blockSize; } /** * Do the appropriate processing for CFB mode decryption. * * @param in the array containing the data to be decrypted. * @param inOff offset into the in array the data starts at. * @param out the array the encrypted 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 decryptBlock(byte[] in, int inOff, byte[] out, int outOff) throws DataLengthException, IllegalStateException { processBytes(in, inOff, blockSize, out, outOff); return blockSize; } /** * Return the current state of the initialisation vector. * * @return current IV */ public byte[] getCurrentIV() { return Arrays.clone(cfbV); } /** * reset the chaining vector back to the IV and reset the underlying * cipher. */ public void reset() { System.arraycopy(IV, 0, cfbV, 0, IV.length); Arrays.fill(inBuf, (byte) 0); byteCount = 0; cipher.reset(); } }