We'll begin with GNU Privacy Guard (GPG). This is a free open source implementation of Phil Zimmermann's Pretty Good Privacy, which he created back in 1991. You can use either one of them to either encrypt or cryptographically sign files or messages. In this section, we'll focus strictly on GPG.

There are some advantages of using GPG:

• It uses strong, hard-to-crack encryption algorithms.
• It uses the private/public key scheme, which eliminates the need to transfer a password to a message or file recipient in a secure manner. Instead, just send along your public key, which is useless to anyone other than the intended recipient.
• You can use GPG to just encrypt your own files for your own use, the same as you'd use any other encryption utility.
• It can be used to encrypt email messages, allowing you to have true end-to-end encryption for sensitive emails.
• There are a few GUI-type frontends available to make it somewhat easier to use.

But, as you might know, there are also some disadvantages:

• Using public keys instead of passwords is great when you work directly only with people who you implicitly trust. But, for anything beyond that, such as distributing a public key to the general population so that everyone can verify your signed messages, you're dependent upon a web-of-trust model that can be very hard to set up.
• For the end-to-end encryption of email, the recipients of your email must also have GPG set up on their systems, and know how to use it. That might work in a corporate environment, but lots of luck getting your friends to set that up. (I've never once succeeded in getting someone else to set up email encryption.)
• If you use a standalone email client, such as Mozilla Thunderbird, you can install a plugin that will encrypt and decrypt messages automatically. But, every time a new Thunderbird update is released, the plugin breaks, and it always takes a while before a new working version gets released.

Even with its numerous weaknesses, GPG is still one of the best ways to share encrypted files and emails. GPG comes preinstalled on both Ubuntu Server and CentOS. So, you can use either of your virtual machines for these demos.