Which term best describes a key that is known only to the sender and the recipient in a symmetric encryption system?

The term that best describes a key known only to the sender and the recipient in a symmetric encryption system is "Secret Key." In symmetric encryption, the same key is utilized for both encryption and decryption, meaning that both parties must possess and keep this key secret to ensure the confidentiality of the communication. The key's secrecy prevents unauthorized individuals from decrypting the data that has been protected.

While "Shared Key" may also seem relevant, it typically implies that the key is shared between the two parties rather than being strictly secret. Both parties know the key, but the emphasis on "secret" highlights the importance of confidentiality in the context of symmetric systems.

Furthermore, "Public Key" is a term associated with asymmetric encryption systems, where one key is widely distributed (public) and a corresponding private key is kept confidential. "Session Key" refers to a temporary key created for a single session or transaction, which can also be a secret key but is not exclusively limited to the sender and recipient over time.

In summary, "Secret Key" is the most accurate term for describing a key that is strictly known to both the sender and recipient in a symmetric encryption context, emphasizing the need for secrecy in ensuring secure communication.