Secret Key (or Symmetric) Technique

The symmetric algorithm is the classical technique that has been know about since at least Roman Times when Juleps Caesar used a very simple technique to protect his messages as they traveled by courier. The basis of the symmetric algorithm is that a single key and the same algorithm are used for both encipherment and decipherment. Thus


This complete mapping places some constraints on the algorithms that can be used since the mapping of the E function from the plaintext onto the ciphertext must be symmetrical with the mapping of the same function from the ciphertext to the plaintext. When a symmetric key algorithm is used for sending information the recipient will know that the data came from the authentic sender if the correct secret key can be used to decipher the message and it makes sense. This provides authentication of origin, that is, the technique allows the recipient to be confident that the identity of the sender is known. As long as the key really is a secret known only to the authorized sender and recipient. To ensure that the deciphered data is not gibberish the plaintext usually contains some easily identifiable pattern so that it can be checked by a computer. The protection provided by the symmetric algorithm relies on keeping the key a secret, confined to those who need to carry out the encipherment or decipherment of the data. For instance, if this technique was to be used to provide confidentiality on a piece of data (to protect it from a disclosure threat), then every object that needed to access the data would have a copy of the key. If these objects were distributed around the system then that key would have to be sent to each of the objects. When symmetric keys need to be distributed this must be done with confidentiality. The most widely used symmetric algorithm is known as DES (Data Encryption Standard) which was originally approved by the US government for commercial use; though this approval has now lapsed. Since no other algorithms have been approved (nor are they likely to be) and since a number of hardware implementations of this algorithm have been produced it is widely used in Banking and some allied commercial applications. Symmetric algorithms can be made to work quite fast and are considered to be quite satisfactory for use on processing quite large amounts of data for confidentiality and integrity. They are used to protect information on storage systems as well as during transfer between computer systems.