is.logi.crypto.modes
Class EncryptOFB

java.lang.Object
  |
  +--is.logi.crypto.Crypto
        |
        +--is.logi.crypto.modes.EncryptMode
              |
              +--is.logi.crypto.modes.EncryptOFB

public class EncryptOFB
extends EncryptMode

Output Fedback Mode iterates the encryption routine on the IV and xors the resulting stream with the plaintext to produce the ciphertext.

This is as fast as ECB or CBC mode, but has the streaming properties of CFB mode. In addition, the stream xored with the plaintext is precalculated in a separate thread, to give better response (although this does not not lower the total time spent calculating).

However, since (with plaintext P, ciphertext C and xor-stream S)

P = C ^ S,

the opponent can alter the plaintext received by changing

C' = C ^ A,

so that

P' = C' ^ S = C ^ A ^ S = P ^ A

is seen. But the opponent will not be able to learn P or control what P' is.

Author:
Logi Ragnarsson (logir@hi.is)
See Also:
DecryptOFB

Fields inherited from class is.logi.crypto.Crypto
BIT, cdsPath, keySource, NIBBLE, primeCertainty, random
 
Constructor Summary
EncryptOFB(CipherKey key, int bufSize)
          Create a new OFB-mode encrypt session with the specified key.
EncryptOFB(int bufSize)
          Create a new OFB-mode encrypt session with no key.
 
Method Summary
 byte[] encrypt(byte[] source, int i, int length)
          Send bytes to the EncryptOFB object for encryption.
 byte[] flush()
          Pads the internal buffer, encrypts it and returns the ciphertext.
 CipherKey getKey()
          Return the key used for encryption.
 void setKey(CipherKey key)
          Set the key to use for encryption.
 
Methods inherited from class is.logi.crypto.modes.EncryptMode
flush
 
Methods inherited from class is.logi.crypto.Crypto
binString, binString, equal, equalRelaxed, equalSub, fromHexNibble, fromHexString, fromString, fromString, hexString, hexString, hexString, hexString, makeClass, makeInt, makeLong, makeSessionKey, pastSpace, pickBits, pickBits, readInt, writeBytes, writeBytes, writeInt
 
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

EncryptOFB

public EncryptOFB(CipherKey key,
                  int bufSize)
Create a new OFB-mode encrypt session with the specified key. A buffer of bufSize bytes is created to hold a pre-calculated xor-stream.

EncryptOFB

public EncryptOFB(int bufSize)
Create a new OFB-mode encrypt session with no key. No encryption can be performed until the setKey() method has been called.

A buffer of bufSize bytes is created to hold a pre-calculated xor-stream.

Method Detail

getKey

public CipherKey getKey()
Return the key used for encryption.
Overrides:
getKey in class EncryptMode

setKey

public void setKey(CipherKey key)
Set the key to use for encryption.
Overrides:
setKey in class EncryptMode

flush

public byte[] flush()
Pads the internal buffer, encrypts it and returns the ciphertext. Since CBF mode doesn't use an internal buffer, an empty array is returned.
Overrides:
flush in class EncryptMode

encrypt

public byte[] encrypt(byte[] source,
                      int i,
                      int length)
Send bytes to the EncryptOFB object for encryption.

Encrypt length bytes from source, starting at i and return the ciphertext.

Overrides:
encrypt in class EncryptMode


Copyright 1997-1999 Logi Ragnarsson