One aspect of RPM is that you can get a package from the Internet, and easily install it. But what do you know about that package file? Is the organization listed as being the ``vendor'' of the package really the organization that built it? Did someone make unauthorized changes to it? Can you trust that, if installed, it won't mail a copy of your password file to a system cracker?
Features built into RPM allow you to make sure that the package file you've just gotten won't cause you problems once it's installed, whether the package was corrupted by line noise when you downloaded it, or something more sinister happened to it.
The command rpm -K (The option --checksig is equivalent) verifies a package file. Using this command, it is easy to make sure the file has not been changed in any way. rpm -K can also be used to make sure that the package was actually built by the organization listed as being the package's vendor. That's all very impressive, but how does it do that? Well, it just needs help from some ``Pretty Good'' software.
The ``Pretty Good'' software we're referring to is known as ``Pretty Good Privacy'', or PGP. While all the information on PGP could fill a book (or several), we've provided a quick introduction to help you get started.
If PGP is new to you, a quick glance through Appendix should get you well on your way to understanding, building, and installing PGP. If, on the other hand, you've got PGP already installed and have sent an encrypted message or two, you're probably more than ready to continue with this chapter.