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