Email is sent across the Internet as plain text, which means that almost anyone can read your private emails and sensitive information. We’ve already covered before how to send encrypted emails with Mozilla Thunderbird, and while Thunderbird is a cross-platform email client that will work on Mac OS X, it just might not be your favorite email application.

If you’re concerned about your email’s security, this hack shows four easy steps to configure Apple’s Mail.app email client to send and receive encrypted emails.

Step 1: Installing MacGPG

Grab the latest installation package of MacGPG, a Mac OS X port of GnuPG. This utility uses a strong public-key encryption algorithm to protect your data. As of writing, the stable version of MacGPG is 1.4.7. Once downloaded, simply double-click on the disk image and open it. Launch the installer (the .mpkg file) and follow the instructions.

Step 2: Creating a GPG Key

Before installing GPGMail and sending encrypted emails, you’ll need to generate a private and public key pair. The public key is used to send you encrypted emails, and the private key is used to decrypted the encrypted emails. To generate a new key pair, download GPG Keychain Access, which is available from the MacGPG web site. Download the application and launch it. You’ll be presented with a dialog that will ask you for your private key. Click on the Generate button to create a new key pair and follow the instructions.

Step 3: Installing GPGMail

Now that you have prepared the key and installed MacGPG, you’re ready to install GPGMail. GPGMail is a plugin for Apple’s Mail.app, providing a front-end to GPG for some operations. It allows you to read and write encrypted messages, as well as electronically signed messages. As of writing, the latest stable release is 1.1.2.

Installing GPGMail is easy. Just download the latest disk image and double-click it. Once the installation is completed, launch Mail.app to enable the plug-in. If everything goes well, you should see a new section in the Preferences panel. Make sure to select your account’s key.

Step 4: Sending and Receiving Encrypted Emails

Now, you’re ready to send emails. When composing a new email message, you’ll see two additional checkboxes: one for singing an email message and another for encrypting it. When receiving an encrypted email message, Mail.app will automatically prompt you for your private key’s passphrase, and will display the decrypted message.