Cryptography Tutorials - Herong's Tutorial Examples - Version 5.40, by Dr. Herong Yang
What is OFB (Output FeedBack) Operation Mode?
This section describes what is OFB (Output FeedBack) Operation Mode - each plaintext block is XORed with the current output block to be the ciphertext block. The current output block is the encrypted version of the previous output block.
OFB (Output FeedBack) operation mode can be described with notations defined earlier as the following formula and diagram:
C[i] = P[i] XOR O[i] O[i] = Ek(O[i-1]) O = E(IV) IV | -----| -----| Ek() / Ek() / Ek() |--O |--O |--O | | | P--XOR P--XOR P--XOR | | | C C C
As you can see from the formula and the diagram, in OFB mode, each block of plaintext is XORed with the current output block to generate the current ciphertext block. The current output block is obtained by applying the encryption process on the previous output block. Note that for the first block, the Initial Vector (IV) is used as the previous output block.
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