What is a defining feature of a block cipher?

What is a defining feature of a block cipher?

• A. Encrypts continuous streams of data
• B. Uses public-key encryption
• C. Encrypts block of data of fixed size
• D. Operates on bytes individually

Answer: (C)

Block ciphers are designed to transform fixed-size chunks of data. They take a block of plaintext of a predetermined length (for example, 128 bits in AES) and produce a ciphertext block of the same size using a secret key. When encrypting longer messages, these blocks are processed in sequence and often linked through a mode of operation (like CBC or CTR) to securely handle multiple blocks. This fixed-block processing is what defines a block cipher, distinguishing it from stream ciphers that encrypt data as a continuous stream or from public-key (asymmetric) schemes.

Block ciphers are designed to transform fixed-size chunks of data. They take a block of plaintext of a predetermined length (for example, 128 bits in AES) and produce a ciphertext block of the same size using a secret key. When encrypting longer messages, these blocks are processed in sequence and often linked through a mode of operation (like CBC or CTR) to securely handle multiple blocks. This fixed-block processing is what defines a block cipher, distinguishing it from stream ciphers that encrypt data as a continuous stream or from public-key (asymmetric) schemes.