Output Feedback is a stream mode of operation# of #Data Encryption Standard (DES) and #Advanced Encryption Standard (AES) where the preceding encryption output of the permuted encrypted initialisation value (feedback) is used as the input to the encryption #algorithm. The feedback is indenpendent of message thus can be computed in advance. Full blocks are used. It is pretty similar to Cipher Feedback (CFB) otherwise.
Note: Notice in stream mode, plaintexts are XORed instead of encrypted as they are not the input to the cipher.
The encryption formula is shown below:
$$ \begin{align} C_i &= p_i \oplus o_i\\ O_i &= \text{ des}_{k1} (o_{i-1})\\ O_{0} &= iv \end{align} $$
Where:
- \(C\) is the ciphertexst
- \(p\) is the plaintext
- \(\text{des}\) is the Data Encryption Standard (DES) algorithm
- \(k\) is the subkey
- \(O\) is the output of the ciphertext
- \(iv\) is the initialisation vector#
Although the bit errors will not propagate throughout the encryption scheme, it is more vulnerable to message stream Modification compare to Cipher Feedback (CFB). It is considered to be a variation of One-Time Pad#, therefore the same sequence (subkey and initialisation vector) should be thrown away once used. Furthermore, the sender and receiver need to be remained in sync.
Research has concluded that only OFB-64 should ever be used.
It is used for stream-oriented transmission over noisy channel such as satellite communication.