The CipherTextStealing.java Java example source code
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* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
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package com.sun.crypto.provider;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.ShortBufferException;
/**
* This class represents ciphers in cipher text stealing (CTS) mode.
* <br>CTS provides a way to allow block ciphers to operate on partial
* blocks without padding, and all bits of the message go through
* the encryption algorithm, rather than simply being XOR'd.
* <br>More details can be found in RFC 2040 section 8 "Description
* of RC5-CTS".
*
* <p>This mode is implemented independently of a particular cipher.
* Ciphers to which this mode should apply (e.g., DES) must be
* <i>plugged-in using the constructor.
*
* <p>NOTE#1: CTS requires the input data to be at least one block
* long. Thus, callers of this class has to buffer the input data
* to make sure the input data passed to encryptFinal()/decryptFinal()
* is not shorter than a block.
* <p>NOTE#2: This class does not deal with buffering or padding
* just like all other cipher mode implementations.
*
* @author Valerie Peng
*/
final class CipherTextStealing extends CipherBlockChaining {
CipherTextStealing(SymmetricCipher embeddedCipher) {
super(embeddedCipher);
}
/**
* Gets the name of this feedback mode.
*
* @return the string <code>CBC
*/
String getFeedback() {
return "CTS";
}
/**
* Performs the last encryption operation.
*
* <p>The input plain text plain, starting at
* <code>plainOffset and ending at
* <code>(plainOffset + len - 1), is encrypted.
* The result is stored in <code>cipher, starting at
* <code>cipherOffset.
*
* <p>It is the application's responsibility to make sure that
* <code>plainLen is a multiple of the embedded cipher's block size,
* as any excess bytes are ignored.
*
* @param plain the buffer with the input data to be encrypted
* @param plainOffset the offset in <code>plain
* @param plainLen the length of the input data
* @param cipher the buffer for the result
* @param cipherOffset the offset in <code>cipher
* @return the number of bytes placed into <code>cipher
*/
int encryptFinal(byte[] plain, int plainOffset, int plainLen,
byte[] cipher, int cipherOffset)
throws IllegalBlockSizeException {
if (plainLen < blockSize) {
throw new IllegalBlockSizeException("input is too short!");
} else if (plainLen == blockSize) {
encrypt(plain, plainOffset, plainLen, cipher, cipherOffset);
} else {
// number of bytes in the last block
int nLeft = plainLen % blockSize;
if (nLeft == 0) {
encrypt(plain, plainOffset, plainLen, cipher, cipherOffset);
// swap the last two blocks after encryption
int lastBlkIndex = cipherOffset + plainLen - blockSize;
int nextToLastBlkIndex = lastBlkIndex - blockSize;
byte[] tmp = new byte[blockSize];
System.arraycopy(cipher, lastBlkIndex, tmp, 0, blockSize);
System.arraycopy(cipher, nextToLastBlkIndex,
cipher, lastBlkIndex, blockSize);
System.arraycopy(tmp, 0, cipher, nextToLastBlkIndex,
blockSize);
} else {
int newPlainLen = plainLen - (blockSize + nLeft);
if (newPlainLen > 0) {
encrypt(plain, plainOffset, newPlainLen, cipher,
cipherOffset);
plainOffset += newPlainLen;
cipherOffset += newPlainLen;
}
// Do final CTS step for last two blocks (the second of which
// may or may not be incomplete).
byte[] tmp = new byte[blockSize];
// now encrypt the next-to-last block
for (int i = 0; i < blockSize; i++) {
tmp[i] = (byte) (plain[plainOffset+i] ^ r[i]);
}
byte[] tmp2 = new byte[blockSize];
embeddedCipher.encryptBlock(tmp, 0, tmp2, 0);
System.arraycopy(tmp2, 0, cipher,
cipherOffset+blockSize, nLeft);
// encrypt the last block
for (int i=0; i<nLeft; i++) {
tmp2[i] = (byte)
(plain[plainOffset+blockSize+i] ^ tmp2[i]);
}
embeddedCipher.encryptBlock(tmp2, 0, cipher, cipherOffset);
}
}
return plainLen;
}
/**
* Performs decryption operation.
*
* <p>The input cipher text cipher, starting at
* <code>cipherOffset and ending at
* <code>(cipherOffset + len - 1), is decrypted.
* The result is stored in <code>plain, starting at
* <code>plainOffset.
*
* <p>It is the application's responsibility to make sure that
* <code>cipherLen is a multiple of the embedded cipher's block
* size, as any excess bytes are ignored.
*
* <p>It is also the application's responsibility to make sure that
* <code>init has been called before this method is called.
* (This check is omitted here, to avoid double checking.)
*
* @param cipher the buffer with the input data to be decrypted
* @param cipherOffset the offset in <code>cipherOffset
* @param cipherLen the length of the input data
* @param plain the buffer for the result
* @param plainOffset the offset in <code>plain
* @return the number of bytes placed into <code>plain
*/
int decryptFinal(byte[] cipher, int cipherOffset, int cipherLen,
byte[] plain, int plainOffset)
throws IllegalBlockSizeException {
if (cipherLen < blockSize) {
throw new IllegalBlockSizeException("input is too short!");
} else if (cipherLen == blockSize) {
decrypt(cipher, cipherOffset, cipherLen, plain, plainOffset);
} else {
// number of bytes in the last block
int nLeft = cipherLen % blockSize;
if (nLeft == 0) {
// swap the last two blocks before decryption
int lastBlkIndex = cipherOffset + cipherLen - blockSize;
int nextToLastBlkIndex =
cipherOffset + cipherLen - 2*blockSize;
byte[] tmp = new byte[2*blockSize];
System.arraycopy(cipher, lastBlkIndex, tmp, 0, blockSize);
System.arraycopy(cipher, nextToLastBlkIndex,
tmp, blockSize, blockSize);
int cipherLen2 = cipherLen-2*blockSize;
decrypt(cipher, cipherOffset, cipherLen2, plain, plainOffset);
decrypt(tmp, 0, 2*blockSize, plain, plainOffset+cipherLen2);
} else {
int newCipherLen = cipherLen-(blockSize+nLeft);
if (newCipherLen > 0) {
decrypt(cipher, cipherOffset, newCipherLen, plain,
plainOffset);
cipherOffset += newCipherLen;
plainOffset += newCipherLen;
}
// Do final CTS step for last two blocks (the second of which
// may or may not be incomplete).
// now decrypt the next-to-last block
byte[] tmp = new byte[blockSize];
embeddedCipher.decryptBlock(cipher, cipherOffset, tmp, 0);
for (int i = 0; i < nLeft; i++) {
plain[plainOffset+blockSize+i] =
(byte) (cipher[cipherOffset+blockSize+i] ^ tmp[i]);
}
// decrypt the last block
System.arraycopy(cipher, cipherOffset+blockSize, tmp, 0,
nLeft);
embeddedCipher.decryptBlock(tmp, 0, plain, plainOffset);
//System.arraycopy(r, 0, tmp, 0, r.length);
for (int i=0; i<blockSize; i++) {
plain[plainOffset+i] = (byte)
(plain[plainOffset+i]^r[i]);
}
}
}
return cipherLen;
}
}
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