Advanced Placement (AP) Computer Science

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What type of key is used by the receiver in public key cryptography to decode the message?

Public key
Private key
Shared key
Session key

The correct answer is: Private key

In public key cryptography, the receiver uses their private key to decode a message that has been encrypted with their public key. The public key is freely distributed and can be shared with anyone who wants to send a secure message to the receiver. When the sender encrypts the message with the receiver's public key, only the corresponding private key can decrypt it. This method ensures that even if someone intercepts the encrypted message, they cannot decrypt it without access to the private key, thereby maintaining the confidentiality of the communication. The private key is kept secret by the receiver, adding an additional layer of security. In contrast, the other keys mentioned—public key, shared key, and session key—serve different purposes in various encryption scenarios, but it is specifically the private key that is essential for the decryption of messages in public key cryptography.