For much of history, encryption algorithms were symmetrical, which means that the same key wasused to both encrypt and decrypt a message. This means that the sender and receiver had to agree inadvance on the key. Symmetrical key encryption is also called private key encryption. There are a widevariety of symmetrical encryption algorithms. The most widely used symmetrical encryption algorithmwas the DES, which was sanctioned by the National Institute of Standards and Technology （NIST） foruse with unclassified government documents. The DES employed 56-bit keys. While DES is still used,other algorithms have been invented because of its susceptibility to brute force attack. For example, RC2,RC4, and RC5 are a series of encryption algorithms invented by RSA Data Security. Their keys range inlength up to 2,048 bits. One difficulty with symmetrical or private key encryption is that many Internet messages are sentbetween people who have never met. Another difficulty is that web servers are accessible to manypeople. If a server's private key is distributed to thousands of users, there is no way that the key willremain secret for long. For these reasons, a new type of algorithm, called public key encryption, wasinvented in 1976 by Whitfield Diffie and Martin Hellman.

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