The Internet has become a global forum for the exchange of all kinds of information, both public and private. Personal discourse, political and legal debate, commerce, and more all take place on the Internet, and the range of information that occupies its bandwidth is rapidly growing. Before the Internet's population explosion, usernames and passwords were considered sufficient to protect private information. However, as the Internet assumes an increasingly important role in society, the value of its content rises--and so does the incentive to steal information.

Simple passwords may help prevent unauthorized parties from requesting information, but they do not prevent them from eavesdropping as the data--or even the password itself--makes its way from router to router toward its destination. In fact, even though the data take only milliseconds to jump from one router to the next, the routers themselves may keep copies in cache for some time afterward. As long as they are readable in transit, your business correspondence, credit card number, personal documents, or travel plans can easily be picked up by a third party. If you run an online service, the privacy and security of hundreds or even thousands of users is in your hands.

To address this problem, Netscape developed the Secure Sockets Layer (SSL) protocol for authentication and encryption over TCP/IP networks, including the Internet. Under this protocol, clients and servers can reliably authenticate each other and then exchange encrypted data that only they can decode. The key that unlocks the encrypted data is itself encrypted, leaving no means for an unauthorized party to read the information.

SSL encryption uses ciphers, which differ from codes in that they are systematic and mathematical rather than arbitrary and symbolic. The strongest ciphers are based on the random generation of two large prime numbers. When the two prime numbers are multiplied, the product is a number so large that ordinary computers cannot factor it back to the two original primes. When data is encrypted using a mathematical formula and the extremely large product, it cannot be deciphered without the two prime numbers. As long as the original prime numbers are kept secret, no one can decipher an SSL-encrypted message without an extravagant expediture of resources. The large numbers used to encrypt and decrypt data are called keys. Their complexity also makes them useful for authentication purposes.

See "Keys and Certificates" for more detailed information about how SSL encryption and authentication work.