Which cipher uses a random key equal in length to the message and used only once?

• A. AES
• B. One-Time Pad
• C. DES
• D. CBC Mode

Answer: (B)

Perfect secrecy is achievable when the key is as long as the message and used only once. That is the One-Time Pad. With a truly random key that matches the plaintext length, each possible plaintext is equally likely to produce any given ciphertext, so knowing the ciphertext reveals no information about the plaintext without the exact key. The decryption is simply applying XOR between the ciphertext and the same random key, recovering the original message. The key must be truly random, as long as the message, and never reused; reuse or predictable randomness breaks the secrecy guarantees.

Other approaches use fixed-length keys that are much shorter than the message and rely on computational difficulty rather than absolute secrecy. For example, AES and DES use secret keys of fixed sizes (DES with 56-bit, AES with 128/192/256-bit) and do not require the key to match the message length. They depend on the hardness of breaking the algorithm rather than on perfect secrecy. CBC mode is a mode of operation that uses a secret key with an IV; it’s not defined by a key length equal to the message and is still vulnerable if the key is reused or not managed securely.