Advanced Placement (AP) Computer Science

In cryptography, what does a cipher represent?

1. A method for decoding text.

2. A set of rules for encoding information.

3. A type of encryption algorithm.

4. A specific key for securing data.

The correct answer is: A set of rules for encoding information.

A cipher represents a set of rules for encoding information. In cryptography, a cipher is the algorithm or procedure that transforms plain text into ciphertext, which is a form of text that is not intelligible without the corresponding decryption key or method. This transformation can involve various techniques, such as substituting letters or rearranging their positions based on predefined rules. Understanding this concept is essential because the effectiveness of communication security relies heavily on these encoding rules. Ciphers dictate how information is transformed, ensuring that only authorized parties can understand the content. This foundational aspect of cryptography underlies many methods used in secure communications today. Other options, while related to various aspects of cryptography, do not capture the core definition of a cipher. For instance, decoding text refers to the process of converting ciphertext back into readable text, which is separate from the rules used to encode it. Similarly, while a type of encryption algorithm may utilize ciphers, the choice of which algorithm to use is not what constitutes a cipher itself. Lastly, a specific key for securing data is part of the cryptographic process but does not define what a cipher is, as ciphers focus on how the encoding is performed rather than the keys involved.