Which of the following is not a block-cipher mode of operation?
2021
Which of the following is not a block-cipher mode of operation?
Answer: C. Stream Cipher Mode — ConceptA block cipher transforms fixed-size blocks. A mode of operation is a standardized rule for applying that block transformation across a longer message…
- A.
Electronic Code Book Mode
- B.
Cipher Feedback Mode
- C.
Stream Cipher Mode
- D.
Cipher Block Chaining Mode
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Show answer & explanation
Correct answer: C
Concept
A block cipher transforms fixed-size blocks. A mode of operation is a standardized rule for applying that block transformation across a longer message while controlling how blocks, chaining values, and initialization data interact.
NIST SP 800-38A specifies confidentiality modes such as ECB, CBC, CFB, OFB, and CTR. A stream cipher is instead an encryption primitive that generates a keystream and combines it with data units.
Application
Electronic Codebook (ECB) applies the block cipher independently to each plaintext block.
Cipher Feedback (CFB) feeds ciphertext from one step into the processing of later plaintext segments.
Cipher Block Chaining (CBC) XORs each plaintext block with the previous ciphertext block before encryption.
A stream cipher produces a keystream and combines it with plaintext bit by bit or byte by byte; it is a cipher family rather than a rule for repeatedly applying a block cipher.
Cross-check and result
ECB, CFB, and CBC describe ways to operate a block cipher and are listed as modes in NIST SP 800-38A. “Stream Cipher Mode” describes a different cipher family, so it is the requested answer.