* The designated CALG block cipher should be used in OFB (Output Feedback * Block) mode in the ISO variant, as described in The Handbook of Applied * Cryptography, algorithm 7.20. *
* Let k be the block size of the chosen symmetric key block
* cipher algorithm; e.g. for AES this is 128 bits or
* 16 octets. The OFB mode used shall be of length/size
* k.
*
* It is recommended that block ciphers operating in OFB mode be used with an
* Initial Vector (the mode's IV). In such a mode of operation - OFB with key
* re-use - the IV need not be secret. For the mechanism in question the IVs
* shall be a random octet sequence of k bytes.
*
* The input data to the confidentiality protection algorithm shall be a
* multiple of the symmetric cipher block size k. When the input
* length is not a multiple of k octets, the data shall be padded
* according to the following scheme:
*
* Assuming the length of the input is l octets,
* (k - (l mod k)) octets, all having the value
* (k - (l mod k)), shall be appended to the original data. In
* other words, the input is padded at the trailing end with one of the
* following sequences:
*
* * 01 -- if l mod k = k-1 * 02 02 -- if l mod k = k-2 * ... * ... * ... * k k ... k k -- if l mod k = 0 **
* The padding can be removed unambiguously since all input is padded and no
* padding sequence is a suffix of another. This padding method is well-defined
* if and only if k < 256 octets, which is the case with
* symmetric key block ciphers today, and in the forseeable future.
*/
public final class CALG
{
private Assembly assembly;
private Object modeNdx; // initialisation key of the cascade's attributes
private int blockSize; // the underlying cipher's blocksize == IV length
private int keySize; // the underlying cipher's key size (in bytes).
/** Private constructor to enforce instantiation through Factory method. */
private CALG(final int blockSize, final int keySize, final Object modeNdx,
final Assembly assembly)
{
super();
this.blockSize = blockSize;
this.keySize = keySize;
this.modeNdx = modeNdx;
this.assembly = assembly;
}
/**
* Returns an instance of a SASL-SRP CALG implementation.
*
* @param algorithm the name of the symmetric cipher algorithm.
* @return an instance of this object.
*/
static synchronized CALG getInstance(final String algorithm)
{
final IBlockCipher cipher = CipherFactory.getInstance(algorithm);
final int blockSize = cipher.defaultBlockSize();
final int keySize = cipher.defaultKeySize();
final Cascade ofbCipher = new Cascade();
IMode ofbMode = ModeFactory.getInstance(Registry.OFB_MODE,
cipher,
blockSize);
Stage modeStage = Stage.getInstance(ofbMode, Direction.FORWARD);
final Object modeNdx = ofbCipher.append(modeStage);
final IPad pkcs7 = PadFactory.getInstance(Registry.PKCS7_PAD);
final Assembly asm = new Assembly();
asm.addPreTransformer(Transformer.getCascadeTransformer(ofbCipher));
asm.addPreTransformer(Transformer.getPaddingTransformer(pkcs7));
return new CALG(blockSize, keySize, modeNdx, asm);
}
/**
* Initialises a SASL-SRP CALG implementation.
*
* @param kdf the key derivation function.
* @param iv the initial vector value to use.
* @param dir whether this CALG is used for encryption or decryption.
*/
public void init(final KDF kdf, final byte[] iv, final Direction dir)
throws SaslException
{
final byte[] realIV;
if (iv.length == blockSize)
realIV = iv;
else
{
realIV = new byte[blockSize];
if (iv.length > blockSize)
System.arraycopy(iv, 0, realIV, 0, blockSize);
else // shouldnt happen
System.arraycopy(iv, 0, realIV, 0, iv.length);
}
final HashMap modeAttributes = new HashMap();
final byte[] sk = kdf.derive(keySize);
modeAttributes.put(IBlockCipher.KEY_MATERIAL, sk);
modeAttributes.put(IMode.IV, realIV);
final HashMap attributes = new HashMap();
attributes.put(Assembly.DIRECTION, dir);
attributes.put(modeNdx, modeAttributes);
try
{
assembly.init(attributes);
}
catch (TransformerException x)
{
throw new SaslException("getInstance()", x);
}
}
/**
* Encrypts or decrypts, depending on the mode already set, a designated array
* of bytes and returns the result.
*
* @param data the data to encrypt/decrypt.
* @return the decrypted/encrypted result.
* @throws ConfidentialityException if an exception occurs duirng the process.
*/
public byte[] doFinal(final byte[] data) throws ConfidentialityException
{
return doFinal(data, 0, data.length);
}
/**
* Encrypts or decrypts, depending on the mode already set, a designated array
* of bytes and returns the result.
*
* @param data the data to encrypt/decrypt.
* @param offset where to start in data.
* @param length how many bytes to consider in data.
* @return the decrypted/encrypted result.
* @throws ConfidentialityException if an exception occurs duirng the process.
*/
public byte[] doFinal(final byte[] data, final int offset, final int length)
throws ConfidentialityException
{
final byte[] result;
try
{
result = assembly.lastUpdate(data, offset, length);
}
catch (TransformerException x)
{
throw new ConfidentialityException("doFinal()", x);
}
return result;
}
}