Which statement describes a block cipher?

• A. A block cipher encrypts large chunks or blocks of data at a time.
• B. A block cipher encrypts one bit at a time.
• C. A block cipher does not use keys.
• D. A block cipher is used only for hashing.

Answer: (A)

Block ciphers operate on fixed-size blocks of data (such as 64 or 128 bits) and transform each block using a secret key. This means they encrypt data in chunks, and for longer messages you apply a mode of operation to process multiple blocks in sequence, which is how secure encryption with a block cipher is achieved. This contrasts with stream ciphers, which encrypt data one bit or byte at a time using a keystream. Block ciphers do use keys, so saying they don’t would be incorrect, and they are not used for hashing. So the statement that they encrypt large chunks or blocks of data at a time is the correct description.