A form of encryption where the same key is used to encrypt and decrypt the text that is to be sent. This means that the sender and the receiver must have exchanged this key some time before the message has been sent and that this transfer must occur over a secure channel. The key used for symmetric key encryption is often known as a secret key. The algorithms used for encryption with this type of scheme can be either block cipher algorithms or stream cipher algorithms. The former encrypt a block of data at a time, while the latter encrypt data on a character-by-character basis. Symmetric key encryption algorithms are usually much more efficient than their main competitor, the algorithms used in public key encryption. The most popular algorithms and technologies associated with symmetric key encryption in current use include the Digital Signature Standard, RSA, and the Diffie Hellman key agreement. See also Data Encryption Standard, RC2, RC4, RC5, and International Data Encryption Algorithm.