Which mode is a block cipher implementation as a self-synchronizing stream…
2022
Which mode is a block cipher implementation as a self-synchronizing stream cipher ?
- A.
Cipher Block Chaining Mode,
- B.
Cipher Feedback Mode
- C.
Electronic Codebook Mode
- D.
Output Feedback Mode
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Correct answer: B
Answer: Cipher Feedback Mode (CFB)
Why this is correct:
Cipher Feedback Mode converts a block cipher into a self-synchronizing stream cipher. The keystream used to encrypt plaintext is derived from the block cipher output combined with previous ciphertext, so the receiver can resynchronize automatically after a limited number of lost or inserted bits/blocks.
How CFB works: the block cipher output is XORed with plaintext to produce ciphertext; previous ciphertext segments are fed back into the cipher input to generate subsequent keystream segments.
Self-synchronization property: because the keystream depends on previous ciphertext, the receiver regains correct alignment automatically after a limited number of received bits/blocks.
Why the other modes are not correct: Output Feedback (OFB) is a synchronous stream mode (keystream independent of ciphertext) and does not self-synchronize; Cipher Block Chaining (CBC) and Electronic Codebook (ECB) are block modes and are not stream ciphers.
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