AP CSP Day 51: Encryption & Security Tradeoffs | Cycle 2
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The key exchange problem in symmetric encryption is that both parties must share the same secret key, but transmitting it over an insecure channel risks interception. Public key cryptography solves this by allowing anyone to encrypt a message using a public key that is freely distributed, while only the holder of the corresponding private key can decrypt it. AP CSP Cycle 2 encryption questions present scenarios involving key distribution, certificate authorities, or man-in-the-middle attacks, and ask students to identify which security property (confidentiality, integrity, authentication) is protected or violated in each case.
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Public Key Cryptography: The Key Exchange Solution
The Key Exchange Problem
Symmetric encryption requires both parties to share the same secret key. But how do you securely share a key with someone over an insecure channel? You cannot encrypt the key with itself. This is the key exchange problem.
How Public Key Solves It
Public key cryptography uses two mathematically related keys: a public key (freely distributed) and a private key (kept secret). Anyone can encrypt a message using your public key, but only you can decrypt it with your private key. No secret needs to be exchanged in advance.
Practice Question
Symmetric encryption uses the same key for both encryption and decryption. Asymmetric encryption uses a public key for encryption and a different private key for decryption. Which of the following is an advantage of asymmetric encryption over symmetric encryption?
The fundamental advantage of asymmetric encryption is solving the key distribution problem. The public key can be freely shared with anyone. Only the private key must be kept secret, and it never needs to be transmitted. Symmetric encryption requires both parties to share the same secret key through a secure channel.
A) Asymmetric encryption is actually slower than symmetric due to more complex mathematical operations. C) Asymmetric encryption typically requires longer keys than symmetric for equivalent security. D) Asymmetric encryption is used in many applications including HTTPS for web browsing.
Students assume asymmetric encryption is better in every way. It solves key distribution but is slower and uses longer keys. In practice, systems often use asymmetric encryption to exchange a symmetric key, then use symmetric encryption for speed.
The key advantage of asymmetric encryption = no need to share a secret key. The key advantage of symmetric encryption = faster processing. Many real systems combine both.
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