A file is encrypted in fixed-size groups of bits, and each output group has the same size as its input group. Which method does this describe?

### Why this is right A block cipher processes fixed-length groups called blocks. Its transformation produces a ciphertext block the same size as the corresponding plaintext block, with the secret key controlling the transformation. ### The concept A block cipher handles data in blocks rather than one individual digit at a time. Messages longer than one block need a mode of operation to specify how the blocks are processed. Depending on the mode, a block cipher can behave in a way that resembles a stream cipher. A stream cipher, by contrast, transforms individual digits or units in sequence. ### The other options - **B. A hash function**: A hash produces a data fingerprint and is not described as reversible block encryption. - **C. A codebook**: A codebook lists semantic substitutions, not fixed-size cryptographic transformations. - **D. A digital certificate**: A certificate binds an identity to a public key; it does not encrypt blocks of file data. ### Terms - **Block cipher**: A symmetric cipher that transforms fixed-length groups of bits. - **Mode of operation**: A method for applying a block cipher to data longer than one block. - **Stream cipher**: A cipher that transforms individual digits or units one at a time. ### Remember Fixed-size groups indicate a block cipher; one-unit-at-a-time processing indicates a stream cipher.