Which cryptographic operation works on data arranged in blocks?

• A. Stream Ciphers
• B. Encoding
• C. Hash Functions
• D. Block Mode Ciphers

Answer: (D)

Block processing is used when data is handled in fixed-size units. A block cipher takes each block of plaintext and encrypts it with a secret key, producing a block of ciphertext of the same size. To handle longer messages, these block operations are used in modes of operation that chain or sequence multiple blocks, which is exactly what block mode ciphers refer to.

In contrast, stream ciphers work on data as a continuous stream, bit by bit or byte by byte, generating a keystream and combining it with the plaintext. Encoding is about data representation rather than securing it, and hash functions produce a fixed-size digest from input data without encrypting it. So, when the data is organized into blocks, the appropriate cryptographic operation is block mode ciphers.