How many keys are required for two people to communicate using a symmetric cipher?

• A. Two keys
• B. One key
• C. Three keys
• D. None

Answer: (B)

In symmetric encryption, the same key is used for both encryption and decryption processes. This means that for two parties to securely communicate, they both need access to the same secret key. Since only one key is needed for this mutual encryption and decryption, the correct answer is one key. This approach is efficient because it simplifies key management between the communicating parties, as they only need to securely share one key to establish their communication channel.

The other choices do not accurately represent the requirements of symmetric encryption. For instance, two keys would imply a complexity that aligns more closely with asymmetric encryption, where one key is used for encryption and a different key for decryption. Three keys are unnecessary and complicate the communication process beyond what's required for effective symmetric encryption. Finally, stating that no keys are needed conflicts with the fundamental principles of encryption, where keys are essential for securing the data.