Which of the following best defines symmetric encryption?

Symmetric encryption is best defined by its use of a single secret key for both encrypting and decrypting data. This means that the same key is utilized by both the sender and the recipient to secure the information being transmitted. This method is efficient because the encryption and decryption processes are often faster compared to asymmetric encryption, which uses a pair of keys. The reliance on one key also necessitates that both parties keep this key private to maintain the confidentiality of the encrypted data.

The other choices do not accurately reflect the principles of symmetric encryption. The option regarding two keys pertains to asymmetric encryption, where a public key is used for encryption and a private key is used for decryption. Similarly, the requirement of unique keys for each user suggests a more complex key management scheme typical in asymmetric encryption. Lastly, stating that symmetric encryption does not provide data confidentiality is incorrect, as maintaining confidentiality is one of its primary purposes; the key must remain secret to prevent unauthorized access to the encrypted information.