Advanced Placement (AP) Computer Science

Which of the following describes a polyalphabetic cipher?

1. A method that uses a single alphabet throughout

2. A technique that alters the alphabet during encryption

3. A simple substitution cipher.

4. A system that encrypts using number codes only

The correct answer is: A technique that alters the alphabet during encryption

A polyalphabetic cipher is characterized by the use of multiple substitution alphabets during the encryption process. This means that different letters may be substituted for the same letter at different points in the message, depending on the position or a key. This technique enhances security compared to simpler ciphers, as it makes frequency analysis significantly more difficult for an attacker. In contrast, using a single alphabet throughout describes a monoalphabetic cipher, where each letter in the plaintext is always replaced by the same letter in the ciphertext. A simple substitution cipher typically belongs to this monoalphabetic category. The option referring to number codes only does not pertain to ciphers in the context of alphabets but rather suggests a purely numerical encryption method. Thus, the correct description of a polyalphabetic cipher is one that emphasizes the alteration of the alphabet used during the encryption process, making the chosen answer accurate.