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gnu.javax.crypto.mode
Class CFB  view CFB download CFB.java

java.lang.Object
  extended bygnu.javax.crypto.mode.BaseMode
      extended bygnu.javax.crypto.mode.CFB
All Implemented Interfaces:
java.lang.Cloneable, gnu.javax.crypto.cipher.IBlockCipher, IMode

public class CFB
extends BaseMode

The cipher feedback mode. CFB mode is a stream mode that operates on s bit blocks, where 1 <= s <= b, if b is the underlying cipher's block size. Encryption is:

 I[1] = IV
 I[j] = LSB(b-s, I[j-1]) | C[j-1]   for j = 2...n
 O[j] = CIPH(K, I[j])               for j = 1,2...n
 C[j] = P[j] ^ MSB(s, O[j])         for j = 1,2...n
 

And decryption is:

 I[1] = IV
 I[j] = LSB(b-s, I[j-1]) | C[j-1]   for j = 2...n
 O[j] = CIPH(K, I[j])               for j = 1,2...n
 P[j] = C[j] ^ MSB(s, O[j])         for j = 1,2...n
 

CFB mode requires an initialization vector, which need not be kept secret.

References:

  1. Bruce Schneier, Applied Cryptography: Protocols, Algorithms, and Source Code in C, Second Edition. (1996 John Wiley and Sons) ISBN 0-471-11709-9.
  2. Recommendation for Block Cipher Modes of Operation Methods and Techniques, Morris Dworkin.


Field Summary
private  byte[] scratch
          The output block from the block cipher.
private  byte[] shiftRegister
          The shift register, the input block to the block cipher.
 
Fields inherited from class gnu.javax.crypto.mode.BaseMode
cipher, cipherBlockSize, iv, lock, modeBlockSize, name, state
 
Fields inherited from interface gnu.javax.crypto.mode.IMode
DECRYPTION, ENCRYPTION, IV, MODE_BLOCK_SIZE, STATE
 
Fields inherited from interface gnu.javax.crypto.cipher.IBlockCipher
CIPHER_BLOCK_SIZE, KEY_MATERIAL
 
Constructor Summary
private CFB(CFB that)
          Cloneing constructor.
(package private) CFB(gnu.javax.crypto.cipher.IBlockCipher underlyingCipher, int cipherBlockSize)
          Package-private constructor for the factory class.
 
Method Summary
 java.lang.Object clone()
          Returns a clone of this instance.
 void decryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
          Decrypts exactly one block of ciphertext.
 void encryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
          Encrypts exactly one block of plaintext.
 void setup()
          The initialisation phase of the concrete mode implementation.
 void teardown()
          The termination phase of the concrete mode implementation.
 
Methods inherited from class gnu.javax.crypto.mode.BaseMode
blockSizes, currentBlockSize, defaultBlockSize, defaultKeySize, init, keySizes, name, reset, selfTest, update
 
Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

shiftRegister

private byte[] shiftRegister
The shift register, the input block to the block cipher.


scratch

private byte[] scratch
The output block from the block cipher.

Constructor Detail

CFB

CFB(gnu.javax.crypto.cipher.IBlockCipher underlyingCipher,
    int cipherBlockSize)
Package-private constructor for the factory class.


CFB

private CFB(CFB that)
Cloneing constructor.

Method Detail

clone

public java.lang.Object clone()
Description copied from interface: gnu.javax.crypto.cipher.IBlockCipher

Returns a clone of this instance.

Specified by:
clone in interface gnu.javax.crypto.cipher.IBlockCipher
Specified by:
clone in class BaseMode

setup

public void setup()
Description copied from class: BaseMode
The initialisation phase of the concrete mode implementation.

Specified by:
setup in class BaseMode

teardown

public void teardown()
Description copied from class: BaseMode
The termination phase of the concrete mode implementation.

Specified by:
teardown in class BaseMode

encryptBlock

public void encryptBlock(byte[] in,
                         int inOffset,
                         byte[] out,
                         int outOffset)
Description copied from interface: gnu.javax.crypto.cipher.IBlockCipher

Encrypts exactly one block of plaintext.

Specified by:
encryptBlock in interface gnu.javax.crypto.cipher.IBlockCipher
Specified by:
encryptBlock in class BaseMode

decryptBlock

public void decryptBlock(byte[] in,
                         int inOffset,
                         byte[] out,
                         int outOffset)
Description copied from interface: gnu.javax.crypto.cipher.IBlockCipher

Decrypts exactly one block of ciphertext.

Specified by:
decryptBlock in interface gnu.javax.crypto.cipher.IBlockCipher
Specified by:
decryptBlock in class BaseMode