Messages written with the intent that they only be viewed by certain parties. Sometimes, this is ensured simply by hiding the messages in a designated area or using a secure courier. More and more often, however, the messages are encoded so that should someone other than the party intercept the message, they would be unable to read it.

With privacy becoming more of a concern on the Internet, people are turning towards encryption to keep their messages secret. Public key encryption is quite popular, with PGPbeing the most common way to implement it.

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