Which statement best distinguishes a code from a cipher?

### Why this is right A code substitutes one meaning-bearing unit for another and relies on a codebook of mappings. A cipher transforms symbols by following an algorithm, and the key supplies information needed by that transformation. ### The concept For example, replacing the word “red” with “brown” is a semantic mapping, so it illustrates a code. Shifting each letter by a fixed number is a cipher because the operation acts on symbols, not on what the words mean. The terms are sometimes used loosely as synonyms, but the distinction matters when describing cryptographic methods. ### The other options - **B. A code always encrypts individual bits, while a cipher replaces entire sentences with shorter phrases.**: This reverses the distinction. Codes can map words or phrases, while ciphers use transformations on symbols. - **C. A code requires a public and private key, while a cipher requires no shared information.**: Public and private keys are a cryptographic key-pair arrangement, not a defining feature of codes. Ciphers require key information in the lesson’s description. - **D. A code is reversible only with a hash, while a cipher is reversible without a key.**: A hash is a one-way fingerprint, not a codebook. Cipher decryption generally requires the appropriate key. ### Terms - **Codebook**: A list of the substitutions used by a code. - **Cipher**: A symbol transformation performed using an algorithm and key. - **Key**: Shared or otherwise appropriate information that controls a cryptographic operation. - **Hash**: A one-way calculation that produces a data fingerprint. ### Remember Codes substitute meanings; ciphers transform symbols.