Which of the following is a type of encryption that uses a single key to encrypt and decrypt data?

Database encryption describes where or how encryption is applied to stored data, not a classification based on the number of keys used in the encryption process.

Symmetric encryption is defined by the use of one shared key that both the sender and recipient use - the same key encrypts the plaintext and decrypts the ciphertext. Common symmetric algorithms include AES, DES, and TripleDES, all of which follow this single-key model. This exactly matches the question's description of one key used for both operations.

Asymmetric encryption uses two mathematically linked keys - a public key for encryption and a private key for decryption - not a single key for both directions.

XML encryption is a W3C specification that defines how to encrypt XML document content and is classified by its target data format, not by key structure.