cryptix.provider.mode

Class OFB

Implemented Interfaces:
FeedbackCipher, Parameterized, SymmetricCipher

public class OFB
extends FeedbackMode

Implements a byte-oriented stream cipher using n-bit OFB with an n-bit-sized block cipher.

The full block size of the supplied cipher is used for the Output Feedback Mode. The bytes supplied are processed and returned immediately.

References:

  1. Bruce Schneier, "Section 9.8 Output Feedback Mode," and "Section 9.11 Choosing a Cipher Mode," Applied Cryptography, 2nd edition, John Wiley & Sons, 1996

  2. sci.crypt FAQ, "Part 5: Product Ciphers," ftp://ftp.rtfm.mit.edu/pub/usenet/news.answers/cryptography-faq/part05

  3. National Bureau of Standards (now NIST), "DES Modes of Operation," NBS FIPS PUB 81, U.S. Department of Commerce, December 1980

Copyright © 1997 Systemics Ltd on behalf of the Cryptix Development Team.
All rights reserved.

$Revision: 1.6 $

Authors:
David Hopwood
Raif S. Naffah
Since:
Cryptix 2.2.2

Field Summary

Fields inherited from class cryptix.provider.mode.FeedbackMode

currentByte, ivBlock, ivStart, length

Fields inherited from class xjava.security.Mode

cipher

Fields inherited from class xjava.security.Cipher

DECRYPT, ENCRYPT, UNINITIALIZED

Fields inherited from class xjava.security.IJCE_Traceable

tracing

Constructor Summary

OFB()
Constructs an OFB cipher, assuming that the IV will be provided via setInitializationVector.
OFB(Cipher cipher)
Constructs an OFB cipher, assuming that the IV will be provided via setInitializationVector.
OFB(Cipher cipher, byte[] iv)
Constructs a OFB cipher, using an initialization vector provided in the constructor.

Method Summary

protected int
engineBlockSize()
SPI: Returns the length of a block, in bytes.
protected void
engineInitDecrypt(Key newkey)
SPI: Initializes this cipher for decryption, using the specified key.
protected void
engineInitEncrypt(Key newkey)
SPI: Initializes this cipher for encryption, using the specified key.
protected int
engineUpdate(byte[] in, int inOffset, int inLen, byte[] out, int outOffset)
SPI: This is the main engine method for updating data.

Methods inherited from class cryptix.provider.mode.FeedbackMode

engineSetCipher, getInitializationVector, getInitializationVectorLength, setInitializationVector

Methods inherited from class xjava.security.Mode

engineGetParameter, engineSetCipher, engineSetParameter, getAlgorithms, getAlgorithms, getInstance, getInstance, toString

Methods inherited from class xjava.security.Cipher

blockSize, clone, crypt, crypt, crypt, doFinal, doFinal, doFinal, doFinal, engineBlockSize, engineCiphertextBlockSize, engineCrypt, engineGetParameter, engineInBufferSize, engineInitDecrypt, engineInitEncrypt, engineOutBufferSize, enginePlaintextBlockSize, engineSetPaddingScheme, engineSetParameter, engineUpdate, getAlgorithm, getAlgorithms, getAlgorithms, getCiphertextBlockSize, getInputBlockSize, getInstance, getInstance, getInstance, getMode, getOutputBlockSize, getPadding, getPaddingScheme, getParameter, getPlaintextBlockSize, getProvider, getState, inBufferSize, inBufferSizeFinal, initDecrypt, initEncrypt, isPaddingBlockCipher, outBufferSize, outBufferSizeFinal, setParameter, toString, update, update, update, update

Methods inherited from class xjava.security.IJCE_Traceable

disableTracing, enableTracing, traceMethod, traceResult, traceResult, traceVoidMethod

Constructor Details

OFB

public OFB()
Constructs an OFB cipher, assuming that the IV will be provided via setInitializationVector.

The IV value must be unique during the lifetime of the key. If it is not unique, and an attacker has access to a different message encrypted under the same IV and key, all of the plaintext can normally be recovered.


OFB

public OFB(Cipher cipher)
Constructs an OFB cipher, assuming that the IV will be provided via setInitializationVector. See the previous constructor for more details.
Parameters:
cipher - the cipher object to use in OFB mode.

OFB

public OFB(Cipher cipher,
           byte[] iv)
Constructs a OFB cipher, using an initialization vector provided in the constructor.

The IV value must be unique during the lifetime of the key. If it is not unique, and an attacker has access to a different message encrypted under the same IV and key, all of the plaintext can normally be recovered.

Parameters:
cipher - the block cipher to use
iv - the initial value for the shift register (IV)

Method Details

engineBlockSize

protected int engineBlockSize()
SPI: Returns the length of a block, in bytes. Ciphers for which plaintext and ciphertext blocks are the same size may override this method. Otherwise, both enginePlaintextBlockSize and engineCiphertextBlockSize should be overridden.

The value may change when initEncrypt or initDecrypt is called, but it should not change at other times.

Overrides:
engineBlockSize in interface Cipher
Returns:
the length in bytes of a block for this cipher.

engineInitDecrypt

protected void engineInitDecrypt(Key newkey)
            throws KeyException
SPI: Initializes this cipher for decryption, using the specified key.

After a call to this method, the cipher's state is set to DECRYPT.

Overrides:
engineInitDecrypt in interface Cipher
Parameters:

engineInitEncrypt

protected void engineInitEncrypt(Key newkey)
            throws KeyException
SPI: Initializes this cipher for encryption, using the specified key.

After a call to this method, the cipher's state is set to ENCRYPT.

Overrides:
engineInitEncrypt in interface Cipher
Parameters:

engineUpdate

protected int engineUpdate(byte[] in,
                           int inOffset,
                           int inLen,
                           byte[] out,
                           int outOffset)
SPI: This is the main engine method for updating data. It may be called with any size of input.

in and out may be the same array, and the input and output regions may overlap.

Overrides:
engineUpdate in interface Cipher
Parameters:
in - the input data.
inOffset - the offset into in specifying where the data starts.
inLen - the length of the subarray.
out - the output array.
outOffset - the offset indicating where to start writing into the out array.
Returns:
the number of bytes written.