Internet email is about as secure as sending a postcard--any postal clerks along the delivery path can read your message if they wanted to since there's no envelope to protect it. So if you're concerned about your private email or files falling into the wrong hands, encryption is the solution. Encryption will scramble your message so that only the holder of the secret decryption key will be able to read it. The de facto standard for encryption is the PGP (Pretty Good Privacy) program, written by Phil Zimmerman. Actually pretty good privacy is a pretty serious understatement. If you use a sufficiently long password key, the computing power required to crack the code that PGP uses becomes astronomically large.
Are You a Crypto Criminal?
PGP is such a good program that the United States government has classified it as a "munition" and made exporting it illegal, for reasons of national security. Apparently the feds are worried that the bad guys will be able to correspond in a way they can't decode. Although it is not illegal to send a message that has been encoded by PGP, you can't export (via email, FTP, or any other means) the PGP sofware from the United States, except to Canada (or from Canada, except to the United States), without a license from the federal government. The one strange exception to this rule is that printed books containing the PGP source code can be exported.
It is also illegal to use PGP in some countries (it's legal in the United States), so if you are an evil terrorist or are plotting the overthrow of your government, check with your local authorities first before using PGP. The Crypto Law Survey at http://rechten.uvt.nl/koops/cryptolaw summarizes the legalities of PGP around the world.
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