What characterizes a block cipher in symmetric encryption?

• A. It transforms a single bit of plaintext
• B. It encrypts bits in a continuous stream
• C. It transforms blocks of plaintext bits as a whole
• D. It requires multiple keys for encryption

Answer: (C)

A block cipher is characterized by its method of processing data in fixed-size blocks rather than singular bits or streams. In this approach, the plaintext is divided into fixed-size blocks, typically 64 or 128 bits, and each block is encrypted as a whole using the same key, resulting in an equally sized block of ciphertext. This method allows for more efficient and secure encryption because the entire block can undergo complex transformations, enhancing the overall security of the system.

The process provides a structured approach to encryption that contrasts with stream ciphers, which handle data bit by bit, making block ciphers suitable for scenarios where data integrity and security are paramount. Therefore, the characteristic of transforming blocks of plaintext bits as a whole is what distinctly defines a block cipher in the realm of symmetric encryption.