gnu.crypto.mode

Class OFB

public class OFB extends BaseMode implements Cloneable

The Output Feedback (OFB) mode is a confidentiality mode that requires a unique IV for every message that is ever encrypted under the given key. The OFB mode is defined as follows:

In OFB encryption, the IV is transformed by the forward cipher function to produce the first output block. The first output block is exclusive-ORed with the first plaintext block to produce the first ciphertext block. The first output block is then transformed by the forward cipher function to produce the second output block. The second output block is exclusive-ORed with the second plaintext block to produce the second ciphertext block, and the second output block is transformed by the forward cipher function to produce the third output block. Thus, the successive output blocks are produced from enciphering the previous output blocks, and the output blocks are exclusive-ORed with the corresponding plaintext blocks to produce the ciphertext blocks.

In OFB decryption, the IV is transformed by the forward cipher function to produce the first output block. The first output block is exclusive-ORed with the first ciphertext block to recover the first plaintext block. The first output block is then transformed by the forward cipher function to produce the second output block. The second output block is exclusive-ORed with the second ciphertext block to produce the second plaintext block, and the second output block is also transformed by the forward cipher function to produce the third output block. Thus, the successive output blocks are produced from enciphering the previous output blocks, and the output blocks are exclusive-ORed with the corresponding ciphertext blocks to recover the plaintext blocks.

In both OFB encryption and OFB decryption, each forward cipher function (except the first) depends on the results of the previous forward cipher function; therefore, multiple forward cipher functions cannot be performed in parallel. However, if the IV is known, the output blocks can be generated prior to the availability of the plaintext or ciphertext data.

The OFB mode requires a unique IV for every message that is ever encrypted under the given key. If, contrary to this requirement, the same IV is used for the encryption of more than one message, then the confidentiality of those messages may be compromised. In particular, if a plaintext block of any of these messages is known, say, the jth plaintext block, then the jth output of the forward cipher function can be determined easily from the jth ciphertext block of the message. This information allows the jth plaintext block of any other message that is encrypted using the same IV to be easily recovered from the jth ciphertext block of that message.

Confidentiality may similarly be compromised if any of the input blocks to the forward cipher function for the encryption of a message is used as the IV for the encryption of another message under the given key.

References:

  1. Recommendation for Block Cipher Modes of Operation Methods and Techniques, Morris Dworkin.

Version: $Revision: 1.3 $

Constructor Summary
OFB(IBlockCipher underlyingCipher, int cipherBlockSize)

Trivial package-private constructor for use by the Factory class.

Method Summary
Objectclone()
voiddecryptBlock(byte[] in, int i, byte[] out, int o)
voidencryptBlock(byte[] in, int i, byte[] out, int o)
voidsetup()
voidteardown()

Constructor Detail

OFB

OFB(IBlockCipher underlyingCipher, int cipherBlockSize)

Trivial package-private constructor for use by the Factory class.

Parameters: underlyingCipher the underlying cipher implementation. cipherBlockSize the underlying cipher block size to use.

Method Detail

clone

public Object clone()

decryptBlock

public void decryptBlock(byte[] in, int i, byte[] out, int o)

encryptBlock

public void encryptBlock(byte[] in, int i, byte[] out, int o)

setup

public void setup()

teardown

public void teardown()
Copyright © 2001, 2002, 2003 Free Software Foundation, Inc. All Rights Reserved.