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.params.ParametersWithIV; import org.bouncycastle.util.Arrays; /** * implements Cipher-Block-Chaining (CBC) mode on top of a simple cipher. */ public class CBCBlockCipher implements BlockCipher { private byte[] IV; private byte[] cbcV; private byte[] cbcNextV; private int blockSize; private BlockCipher cipher = null; private boolean encrypting; /** * Basic constructor. * * @param cipher the block cipher to be used as the basis of chaining. */ public CBCBlockCipher(BlockCipher cipher) { this.cipher = cipher; this.blockSize = cipher.getBlockSize(); this.IV = new byte[blockSize]; this.cbcV = new byte[blockSize]; this.cbcNextV = new byte[blockSize]; } /** * 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. * * @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 { boolean oldEncrypting = this.encrypting; this.encrypting = encrypting; if (params instanceof ParametersWithIV) { ParametersWithIV ivParam = (ParametersWithIV) params; byte[] iv = ivParam.getIV(); if (iv.length != blockSize) { throw new IllegalArgumentException("initialisation vector must be the same length as block size"); } System.arraycopy(iv, 0, IV, 0, iv.length); reset(); // if null it's an IV changed only. if (ivParam.getParameters() != null) { cipher.init(encrypting, ivParam.getParameters()); } else if (oldEncrypting != encrypting) { throw new IllegalArgumentException("cannot change encrypting state without providing key."); } } else { reset(); // if it's null, key is to be reused. if (params != null) { cipher.init(encrypting, params); } else if (oldEncrypting != encrypting) { throw new IllegalArgumentException("cannot change encrypting state without providing key."); } } } /** * return the algorithm name and mode. * * @return the name of the underlying algorithm followed by "/CBC". */ public String getAlgorithmName() { return cipher.getAlgorithmName() + "/CBC"; } /** * return the block size of the underlying cipher. * * @return the block size of the underlying cipher. */ public int getBlockSize() { return cipher.getBlockSize(); } /** * 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 { return (encrypting) ? encryptBlock(in, inOff, out, outOff) : decryptBlock(in, inOff, out, outOff); } /** * reset the chaining vector back to the IV and reset the underlying * cipher. */ public void reset() { System.arraycopy(IV, 0, cbcV, 0, IV.length); Arrays.fill(cbcNextV, (byte) 0); cipher.reset(); } /** * Do the appropriate chaining step for CBC 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. */ private int encryptBlock(byte[] in, int inOff, byte[] out, int outOff) throws DataLengthException, IllegalStateException { if ((inOff + blockSize) > in.length) { throw new DataLengthException("input buffer too short"); } /* * XOR the cbcV and the input, * then encrypt the cbcV */ for (int i = 0; i < blockSize; i++) { cbcV[i] ^= in[inOff + i]; } int length = cipher.processBlock(cbcV, 0, out, outOff); /* * copy ciphertext to cbcV */ System.arraycopy(out, outOff, cbcV, 0, cbcV.length); return length; } /** * Do the appropriate chaining step for CBC 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 decrypted 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. */ private int decryptBlock(byte[] in, int inOff, byte[] out, int outOff) throws DataLengthException, IllegalStateException { if ((inOff + blockSize) > in.length) { throw new DataLengthException("input buffer too short"); } System.arraycopy(in, inOff, cbcNextV, 0, blockSize); int length = cipher.processBlock(in, inOff, out, outOff); /* * XOR the cbcV and the output */ for (int i = 0; i < blockSize; i++) { out[outOff + i] ^= cbcV[i]; } /* * swap the back up buffer into next position */ byte[] tmp; tmp = cbcV; cbcV = cbcNextV; cbcNextV = tmp; return length; } }