gnu.crypto.mode
Class OFB

java.lang.Object
  gnu.crypto.mode.BaseMode
      gnu.crypto.mode.OFB

All Implemented Interfaces:

java.lang.Cloneable, IBlockCipher, IMode

public class OFB

extends BaseMode

implements java.lang.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:

• OFB Encryption:
  • I₁ = IV;
  • I_j = O_{j -1} for j = 2...n;
  • O_j = CIPH_K(I_j) for j = 1, 2...n;
  • C_j = P_j XOR O_j for j = 1, 2...n.
• OFB Decryption:
  • I₁ = IV;
  • I_j = O_{j -1} for j = 2...n;
  • O_j = CIPH_K(I_j) for j = 1, 2...n;
  • P_j = C_j XOR O_j for j = 1, 2...n.

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 j^th plaintext block, then the j^th output of the forward cipher function can be determined easily from the j^th ciphertext block of the message. This information allows the j^th 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: 1.2 $

Field Summary

Fields inherited from class gnu.crypto.mode.BaseMode

cipher, cipherBlockSize, iv, lock, modeBlockSize, name, state

Fields inherited from interface gnu.crypto.mode.IMode

DECRYPTION, ENCRYPTION, IV, MODE_BLOCK_SIZE, STATE

Fields inherited from interface gnu.crypto.cipher.IBlockCipher

CIPHER_BLOCK_SIZE, KEY_MATERIAL

Constructor Summary

(package private)

OFB(IBlockCipher underlyingCipher, int cipherBlockSize) Trivial package-private constructor for use by the Factory class.

Method Summary

java.lang.Object	clone() Returns a clone of this instance.
void	decryptBlock(byte[] in, int i, byte[] out, int o) Decrypts exactly one block of ciphertext.
void	encryptBlock(byte[] in, int i, byte[] out, int o) 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.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

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 java.lang.Object clone()

Description copied from interface: IBlockCipher

Returns a clone of this instance.

Specified by:

clone in interface 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 i,
                         byte[] out,
                         int o)

Description copied from interface: IBlockCipher

Encrypts exactly one block of plaintext.

Specified by:

encryptBlock in interface IBlockCipher

Specified by:

encryptBlock in class BaseMode

decryptBlock

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

Description copied from interface: IBlockCipher

Decrypts exactly one block of ciphertext.

Specified by:

decryptBlock in interface IBlockCipher

Specified by:

decryptBlock in class BaseMode