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What is cryptography? It protects a message or file from being read by an eavesdropper who has no other means of access to either the original text of what is protected, or the key with which it is encrypted. How does it do it? You use a program to encrypt it; the program will change the letters into symbols and other weird characters, so when someone opens the file they cannot read it. If you encrypt something and send it to someone the other person will need to have the program to decrypt it.

What can I encrypt? You can use it to encrypt your e-mail that prevents people from reading it in transit. You can use it to protect the entire contents of your hard disk by encrypting everything. But if you use the Internet, there is at present, no encryption technique available to prevent your ISP (Internet Service Provider) from monitoring your connection, and seeing what web sites you visit. Theoretically, however, you could make a secure connection with your browser to a web site that then serves as a browser, accessing web pages for you that you request, and then sending them to your browser in encrypted form. There are anonymizing services on the Internet for other functions, but these services have been subjected to legal action at times, and required to give up their logs.
Since the Internet is terribly new and complicated, the issue involved can be made clearer by illustrating it in terms of older technology. You can use a code, or a voice scrambler, to keep someone eavesdropping on a telephone conversation of yours from understanding what you are saying.

Although cryptography is fascinating and glamorous, because of its association with such things as espionage, diplomacy, and the higher levels of the military, it has a limited but important role in the area of computer security.