Symmetrical Key

Traditional encryption techniques rely on both the sender and the receiver of a message knowing a particular secret key. The sender encrypts the message with a particular cryptosystem using the secret key and the receiver decrypts the message with the same key. This method of encryption is called private key, or symmetric, encryption. It relies on no one else being able to find out what the private key is.

This is fine if encrypted files are only stored on the hard disk (in which case only the encryptor needs to know the key) or if one is sending data to someone close by, say in the same office, in which case one can let the key be known personally. If the receiver is not close by, however, then the key must somehow be sent. It could be sent by courier or told over the telephone or even sent separately to the actual message. All these methods involve trusting a third party not to divulge the secret key. Furthermore, the receiver of the message must also be trusted, as they could decrypt any message sent using the same key.

There are two types of symmetric encryption algorithms: stream ciphers and block ciphers. Stream ciphers can encrypt a single bit of plaintext at a time whereas block ciphers encrypt multiple bits of data (normally 64 bits)--called a block. Symmetric ciphers are generally faster than public key ciphers. Some symmetric ciphers, such as DES, can be implemented in hardware. This makes them very fast.

Disadvantages include: