Most computer data and communications are less secure than you might imagine. E-mail can be quite easily intercepted, unless it's encrypted. Other users on the system can read your files, unless they are protected - you should learn about Unix file access permissions.
The system's administrator can access all files, and the only guarantee of privacy is his pledge that he will not do so, unless the well-being of the system requires it.
You should take all this into account in deciding whether to entrust sensitive, confidential or compromising information to the computer.
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