Advanced Placement (AP) Computer Science

What is the primary function of public key cryptography in message transmission?

1. It encrypts messages for anyone to decode

2. The user has a public key to encrypt messages and a private key to decode

3. It allows multiple users to share a single key for encryption

4. It encrypts messages but does not require keys for decryption

The correct answer is: The user has a public key to encrypt messages and a private key to decode

Public key cryptography serves the primary function of enabling secure communication by using a pair of keys: a public key and a private key. The public key is made available to anyone who wants to send a secure message, while the private key is kept secret by the recipient. When someone wants to send a confidential message, they use the recipient's public key to encrypt the message. This ensures that only the individual with the corresponding private key can decrypt and read the message. This two-key system not only facilitates secure communications but also allows for functionalities such as digital signatures, where a message can be signed with a private key allowing the recipient to verify its authenticity with the sender's public key. The other choices do not accurately reflect the nature of public key cryptography. The statement regarding encryption for anyone to decode misrepresents the purpose, as public key encryption is specifically designed so that only the holder of the private key can decrypt the message. Similarly, the claim about multiple users sharing a single key pertains more to symmetric key cryptography, which relies on one key for both encryption and decryption. Lastly, the idea that messages can be encrypted without requiring keys for decryption contradicts the fundamental principles of cryptography, where keys are essential for both processes.