gnu.javax.crypto.mode
Class OFB
- Cloneable, IBlockCipher, IMode
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;
- Ij = Oj -1 for j = 2...n;
- Oj = CIPHK(Ij) for j = 1, 2...n;
- Cj = Pj XOR Oj for j = 1, 2...n.
OFB Decryption:
- I1 = IV;
- Ij = Oj -1 for j = 2...n;
- Oj = CIPHK(Ij) for j = 1, 2...n;
- Pj = Cj XOR Oj 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:
-
Recommendation for Block Cipher Modes of Operation Methods and Techniques,
Morris Dworkin.
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.
|
blockSizes, clone, currentBlockSize, decryptBlock, defaultBlockSize, defaultKeySize, encryptBlock, init, keySizes, name, reset, selfTest, setup, teardown, update |
clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait |
setup
public void setup()
The initialisation phase of the concrete mode implementation.
- setup in interface BaseMode
OFB.java --
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