Classic Cryptography Key-Based Encryption   Symmetrical Key   Assymetrical Key   RSA   IDEA, RC2 and RC4   DES   Pretty Good Privacy    (PGP)   How public key works    mathematically   Glossary Basic Concepts in Data Encryption: Key-Based Encryption 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: For use in anything other than private encryption of files, one could not determine the authenticity of the originator of the data. This lack of protection does not allow this method to verify electronic orders and financial transactions. The private key used now has to be transmitted in a very secure channel. Spontaneous secure transmission may not be feasible. When used across a network of users, there may have to be a large number of keys to facilitate one-to-one communication between each user. This adds up to (n2) = n ( n - 1 ) / 2. Copyright ©1999 ThinkQuest Team 27158 — Developed for ThinkQuest 1999