Which of the following is not a Cipher mode in Cryptography?

Concept

A block cipher (e.g. AES, DES) encrypts only one fixed-size block at a time. A "mode of operation" is the standardised scheme that specifies HOW that block cipher is repeatedly applied to encrypt a message longer than one block. The standard confidentiality modes are defined in NIST SP 800-38A: ECB, CBC, CFB, OFB and CTR. A stream cipher, by contrast, is a different FAMILY of cipher that encrypts data one bit/byte at a time and is not a "mode" applied on top of a block cipher.

Apply to each option

• Electronic Code Book (ECB): a standard block cipher mode — each block encrypted independently.

• Cipher Feedback (CFB): a standard block cipher mode — makes a block cipher behave like a self-synchronising stream of segments.

• Cipher Block Chaining (CBC): a standard block cipher mode — each block is XORed with the previous ciphertext block before encryption.

• Stream Cipher Mode: there is no such block cipher mode of operation. "Stream cipher" names a separate category of algorithm, not a way of running a block cipher.

Result

ECB, CFB and CBC are all genuine block cipher modes of operation, so the option that is NOT a cipher mode of operation is Stream Cipher Mode.