A Symmetric key is a single key that is used with the operations of a symmetric encryption scheme. For example, such a key may be used in a symmetric encryption algorithm for both encryption and decryption.
The best-known symmetric algorithms are faster than the best-known public key algorithms. Thus, they are preferred when processing large amounts of data. Some of the more common symmetric algorithms are RC4, Triple DES (Data Encryption Standard), and Advanced Encryption Standard (AES).
Because much of the activity involving symmetric keys relates to keeping them secret, it is important to keep the number of people who possess a particular symmetric key as small as possible. We recommend that only one or two people possess any one symmetric key.