cryptix.provider.mode
Class OFB

java.lang.Object
  xjava.security.IJCE_Traceable
      xjava.security.Cipher
          xjava.security.Mode
              cryptix.provider.mode.FeedbackMode
                  cryptix.provider.mode.OFB

All 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 $

Since:

Cryptix 2.2.2

Author:

David Hopwood, Raif S. Naffah

Field Summary

protected int	currentByte Index into the ivBlock.
protected byte[]	ivBlock Value of the current feedback register/queue/buffer.
protected byte[]	ivStart Value of the IV at initialisation phase as supplied by user.
protected int	length Size of the feedback register/queue/buffer.

Fields inherited from class xjava.security.Mode

cipher

Fields inherited from class xjava.security.Cipher

DECRYPT, ENCRYPT, UNINITIALIZED

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(java.security.Key newkey) SPI: Initializes this cipher for decryption, using the specified key.
protected void	engineInitEncrypt(java.security.Key newkey) SPI: Initializes this cipher for encryption, using the specified key.
protected void	engineSetCipher(Cipher cipher) SPI: Sets the underlying cipher.
protected int	engineUpdate(byte[] in, int inOffset, int inLen, byte[] out, int outOffset) SPI: This is the main engine method for updating data.
byte[]	getInitializationVector() Gets a copy of the starting initialization vector.
int	getInitializationVectorLength() Returns the size of the initialization vector expected by setInitializationVector.
void	setInitializationVector(byte[] iv) Sets the initialization vector.

Methods inherited from class xjava.security.Mode

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

Methods inherited from class xjava.security.Cipher

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

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

ivBlock

protected byte[] ivBlock

Value of the current feedback register/queue/buffer.

ivStart

protected byte[] ivStart

Value of the IV at initialisation phase as supplied by user.

currentByte

protected int currentByte

Index into the ivBlock.

length

protected int length

Size of the feedback register/queue/buffer.

Constructor Detail

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.

Throws:

java.lang.NullPointerException - if cipher == null

IllegalBlockSizeException - if cipher.getPlaintextBlockSize() != cipher.getCiphertextBlockSize()

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)

Throws:

java.lang.NullPointerException - if cipher == null

Method Detail

engineBlockSize

protected int engineBlockSize()

Description copied from class: Cipher

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 class Cipher

Returns:

the length in bytes of a block for this cipher.

engineInitEncrypt

protected void engineInitEncrypt(java.security.Key newkey)
                          throws java.security.KeyException

Description copied from class: Cipher

SPI: Initializes this cipher for encryption, using the specified key.

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

Specified by:

engineInitEncrypt in class Cipher

Parameters:

newkey - the key to use for encryption.

Throws:

java.security.KeyException - if the key is invalid.

engineInitDecrypt

protected void engineInitDecrypt(java.security.Key newkey)
                          throws java.security.KeyException

Description copied from class: Cipher

SPI: Initializes this cipher for decryption, using the specified key.

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

Specified by:

engineInitDecrypt in class Cipher

Parameters:

newkey - the key to use for decryption.

Throws:

java.security.KeyException - if the key is invalid.

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.

Specified by:

engineUpdate in class 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.

engineSetCipher

protected void engineSetCipher(Cipher cipher)

SPI: Sets the underlying cipher.

For example, to create an IDEA cipher in CBC mode, the cipher for "IDEA" would be passed to the mode for "CBC" using this method. It is called once, immediately after the mode object is constructed.

Subclasses that override this method (to do initialization that depends on the cipher being set) should call super.engineSetCipher(cipher) first.

Overrides:

engineSetCipher in class Mode

Parameters:

cipher - the underlying cipher object

Throws:

java.lang.NullPointerException - if cipher == null

setInitializationVector

public void setInitializationVector(byte[] iv)
                             throws java.security.InvalidParameterException

Sets the initialization vector.

Note: in JavaSoft's version of JCE, this method may only be called when the cipher is in the UNINITIALIZED state. In IJCE that is relaxed to also allow it to be called after initEncrypt/initDecrypt, but before the first call to update or crypt, provided that the IV is not set twice.

Specified by:

setInitializationVector in interface FeedbackCipher

Parameters:

iv - the initialization vector.

Throws:

java.security.InvalidParameterException - if the initialization vector is of the wrong length or has already been set.

getInitializationVector

public byte[] getInitializationVector()

Gets a copy of the starting initialization vector. It will return null if the initialization vector has not been set.

Specified by:

getInitializationVector in interface FeedbackCipher

Returns:

a copy of the initialization vector for this cipher object.

getInitializationVectorLength

public int getInitializationVectorLength()

Returns the size of the initialization vector expected by setInitializationVector. For this class, that is the block size of the underlying cipher.

Specified by:

getInitializationVectorLength in interface FeedbackCipher

Returns:

the required size of the argument to setInitializationVector.