Advanced Placement (AP) Computer Science

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Enhance your readiness for the AP Computer Science exam with insightful quizzes. Deepen your understanding and master key concepts as you prepare for a successful test experience!

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What is a one-time pad encryption?

A method using a pad that can be reused
A cipher that is limited to specific characters
A technique requiring a key as long as the message
A system that involves multiple keys for encryption

The correct answer is: A technique requiring a key as long as the message

One-time pad encryption is a cryptographic technique that involves using a key that is as long as the message being encrypted. This key is used only once and is completely random, making the encryption theoretically unbreakable when implemented correctly. The process involves combining the plaintext message with the key material using an operation such as modular addition. The uniqueness and randomness of the one-time pad’s key ensure that each character of the plaintext is transformed into a different character of the ciphertext, and because the key is not reused, it provides perfect secrecy. This means that even if a hacker gains access to an encrypted message, without the key, there is no way to decipher the message. In contrast, the other options describe characteristics of other encryption methods or misunderstandings about the one-time pad concept. For example, reusing keys compromises security; limiting the character set does not align with the principles of a one-time pad; and multiple keys for encryption indicate a different system entirely that does not adhere to the one-time pad’s fundamental principles.