Editor's Note: The following is an excerpt from chapter 11, "Keeping Your Data Private", of Peter van der Linden's Guide to Linux, published August 2005 by Prentice Hall, ISBN 0-13-187284-2.

This is the third and final part of my series of articles on public key encryption in Linux. In this article, I describe how to integrate GnuPG with the Thunderbird e-mail client so you can send and receive secure private e-mail. Part 1 of this series is available here, and Part 2 is here.

Thunderbird is a popular e-mail client based on just the e-mail part of the all-in-one Mozilla program, itself a descendant of the Netscape Navigator suite. You add a small library plug-in called Enigmail to the Mozilla or Thunderbird mail client to provide seamless integration with GnuPG (GPG). Other e-mail clients can work with GnuPG too; consult their documentation for details.

The name Enigmail relates to the German Enigma encryption device used in WWII, which was cracked by a brilliant Polish mathematician and exploited by the British to track and sink Nazi U-Boats. Enigmail provides secure communication by scrambling e-mail text with GPG so only the intended recipient can read it.

You need to find out the version number of Mozilla or Thunderbird that you are using. There are different Enigmail versions for the different Mozilla versions, and the wrong one will fail with a mysterious and unhelpful error message.

Now that you know your mailer version number, browse the Enigmail download page at enigmail.mozdev.org/download.html.

If you scroll down the page, you'll find several versions of the downloads for different operating systems and different mail clients. Make sure that you are on the part of the page that shows Downloads for Linux x86-32.

You are going to download and install an Enigmail module. If you're still using Mozilla mail, the installation will be slightly different, and you also might need to download an Enigmime module.

Find the Enigmail module that matches your operating system and mailer application version, as shown in figure 3-3. The Web page has improved a lot recently but still can be confusing.

Figure 1. The Enigmail Download Page

It is very easy to download the wrong version of the file. If you do, the mailer will not be able to handle encryption, and you will not get any sensible error messages. If things aren't working, this is the first thing to double-check.

Right-click on the link for that Enigmail module and select Save Link As. Save the linked file to your desktop so it is easy to find again.

The downloaded file will have a name like enigmail-0.92.1-tb-linux.xpi. An ".xpi" file is a Mozilla format for mailer and browser add-ons. It's just a ZIP archive that contains several Javascript files with a bunch of instructions that tell the Mozilla application how to add new menu items and what programs to run when the new menu items are clicked. If an Enigmime module is listed, download that too.

You're ready to install the downloaded Enigmail file (and Enigmime if it is listed in that part of the Web page). Start Thunderbird, then choose Tools > Extensions > Install. Browse for the Enigmail ".xpi" file you just downloaded and install it, as shown in Figure 2. Finally completely exit and restart Thunderbird to make Enigmail available.

Figure 2. Click Open to Install an .xpi File

When you restart Thunderbird mail, note that there is a new menu item, Enigmail, shown in Figure 3.

Figure 3. Thunderbird's Enigmail Menu

You now need to complete a very small amount of configuration to be able to process encrypted mail. Choose Enigmail > Preferences to bring up the window shown in Figure 4.

Figure 4. Thunderbird's Enigmail Preferences Menu

Enter the path to the program in the GnuPG executable path field and click OK. If you can't recall where it was installed, use your file manager to search for GnuPG by name.

Bring up the Thunderbird Enigmail Preferences panel in Figure 5 and click the Debugging tab shown there. Fill in your own e-mail address in the Test e-mail field, and click the Test button. You will be prompted for your GPG passphrase.

Figure 5. Test the E-mail Integration

The test button doesn't actually send mail (so it is a bug to label the button "test em-ail"), but it goes through the motions of making sure that Thunderbird can talk to GPG. If everything is working together, you will see a dialog box indicating that is so.

This message means the mailer and GPG are talking to each other. If the mailer can't make contact with the Enigmail library, it will pop up a different dialog. That's usually because you installed a version of Enigmail or Enigmime that doesn't match your e-mail client. Uninstall Enigmail by choosing Tools > Extensions > Uninstall button, and try again.

The last part of configuring Thunderbird for encrypted mail is to tell Thunderbird what your GPG identity is. The easiest way to do that is to compose an e-mail, ask for encryption and let it prompt you for the information it needs.

In Thunderbird, hold down the Shift key and click Write to compose a new mail message. The shift key turns off HTML formatting for the e-mail. When you're encrypting a message, you don't want all kinds of HTML markup littered throughout it. You can turn off the use of HTML in mail for good, by choosing Edit > Account Settings > Composition & Addressing > Compose messages in HTML format.

In the new compose e-mail window, fill in the fields To, Subject and Body, then click OpenPGP. The message in Figure 6 will appear. Click Yes.

Figure 6. Thunderbird Prompts You to Configure Security

The window shown in Figure 7 will appear. If the e-mail identity you are using in Thunderbird matches the e-mail identity that you used when you set up your GPG key, select Enable OpenPGP support (Enigmail) for this identity.

Figure 7. Telling Thunderbird Your GPG Key ID

If the mail addresses are not the same, select Use specific OpenPGP key ID. Then click Select Key and select the appropriate key ID from the list that appears. When you have made those settings, click OK.

The configuration screens will disappear, leaving your compose window and a small dialog window offering you several choices, including Encrypt Message. Make your selection, then click OK. Finally, click Send to send the mail. Only the body and attachments of the email will be encrypted, so don't give your secret plans away in the subject line.

As an e-mail is sent across the Internet to its addressee, anyone who administers any host that it passes through can make a copy of the mail. If the email is encrypted, this copy will look something like the mail shown in the following example. It is effectively immune to casual eavesdropping efforts.

An encrypted e-mail as seen by an eavesdropper:

From pvdl@gmail.com Mon Apr 4 23:41:12 2005 +0000
Message-ID: <4251D097.70701@gmail.com>
Date: Mon, 04 Apr 2005 16:41:11 -0700
From: Peter van der Linden
User-Agent: Mozilla Thunderbird 1.0.2 (X11/20050317)
X-Accept-Language: en-us, en
To: Peter van der Linden
Subject: test message re cheese
X-Enigmail-Version: 0.91.0.0

-----BEGIN PGP MESSAGE-----

Charset: ISO-8859-1
Version: GnuPG v1.4.1 (GNU/Linux)
Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org
hQEOA7XFUy1wsQ8yEAQAsh+0abSkGnGZV2DiLESQ9Wl5pwGx1ApDUh2bMCHjwWw9
FGzT3keACkmFUJfo+eSMpcdX6zDMzeG0N/eGjuHACYbQZjBDBDUd/+o2pP6GuGmA
D6ZwnNXi8NDWDXlyqHefPDKDCFuX3D8ZDkzmowepzJTCtsFvpGB6aMkAIx75OQgD
/RX8FvYd6v+clYGOzS+FwyncwHRjRw5uyKXwb/PGZd4yZ2wyVkNTIxqlS2gtTdx9

-----END PGP MESSAGE-----

After the one time setup, when mail comes in for you, Thunderbird and Enigmail work together to decrypt it automatically. Thunderbird tells Enigmail "Hey, I have an encrypted mail here." Enigmail looks to see if it was encoded with the public key of the user who is reading the e-mail. If so, it decodes it with the private key and hands the result back to Mozilla. The resulting clear text is automatically displayed. If Enigmail doesn't have the private key to decode the mail, it displays a small key icon with the key in two broken pieces.

Encryption can be used to guarantee e-mail authenticity, as well as to conceal messages. Just as you sign a bank check or a letter written on paper to guarantee that it was issued by you, you can use PKE to "sign" an e-mail message to assure readers that it came from you. You can sign arbitrary files (programs, operating system patches, word processor files) to assure readers that they came from you.

The system is designed so that public key and private key are symmetric. You can use either one to encode and use the other to decode. If someone encrypts a file with his private key, anyone can decode it using the sender's public key. The only key that will decode the file correctly is the sender's public key, so you know that the file really did come from the sender (or someone with access to the sender's private key).

Encryption falls short of an absolute guarantee of authenticity because codes can be broken, people bribed, keyboards tapped, etc. But you can put a lot more trust in the authenticity of signed e-mail compared with unsigned.

For convenience of readers, you might send an e-mail in plain text, with an attachment containing the encrypted version. The recipient can read the plain text immediately and check that it is accurate by decrypting the attachment and comparing it to the plain text.

You don't even have to encrypt the plain text. It's enough to get a checksum summary of the plain text (known as a message digest) and encrypt that. For e-mail, the message digest is typically in two pieces added before and after the body of the e-mail. With non-e-mail files, the message digest typically is kept in a separate file, but with a name that relates it to the file for which it is a summary.

When a file has been signed, the recipient has an easy way to check if the file contents have been altered at all after the signing. Calculate the digest of the message that was received. There are several popular digest programs: SHA1, SHA2 and MD5. Use the sender's public key to decrypt the digest that was received. Calculate the checksum using the same checksum program that the sender used. If the message has been altered, the decrypted message digest will not exactly match the calculated message digest.

Signing is the reason we don't refer to a public key as a locking key, nor to a private key as an unlocking key. Depending on whether you are encrypting or signing, you will process the message with the recipient's public key or your private key. When I want to send Alice a confidential message, I use her public key to encrypt the message.

But if Alice wants to sign a reply to me, she will use her private key to encrypt it. If she wants to encrypt a reply to me, she will use my public key to encrypt the message. Both public and private keys can be used for locking and unlocking. Whichever key you used for locking, you use the other key for unlocking.

Enigmail gives an option to sign e-mail as well as encrypting it. You can also sign files with GPG commands. If you have a file called contract.txt, you can sign it with this command:

gpg --armor --clearsign contract.txt

That will leave the original file unchanged and create a new file, contract.txt.asc, that contains the old file plus the signature information. When the other party gets that contract.txt.asc file from me, she can verify that she is reading the same thing that I signed by running this command:

gpg --verify contract.txt.asc

The output from the command will say whether the signature matches the text or not, and hence whether it has been tampered with or not.

People often sign files or e-mail that they encrypt. That way, only the intended recipient can read it, and the recipient knows that you are definitely the person who sent it, too. Computerized signatures based on encryption are far more reliable than written signatures that are forged on a daily basis by people with criminal intent. But computerized signatures are only as good as the encryption scheme and key length you use. For GPG, that's a pretty good assurance, until you start to look at all the interfaces outside GPG that can be subverted.

GPG is the strongest part of a system that has weak parts where humans are involved. Rather than encrypting individual files, the approach should be to transparently encrypt entire filesystems at block level. Some products do this, but they are not yet in widespread use. One is sourceforge.net/projects/loop-aes.

You might have found GPG to be somewhat inconvenient to use. That's a common reaction. In practice the barrier of GPG is high enough that the program is not in use by the majority of Linux users. It's quite rare to get encrypted mail. A program called seahorse (seahorse.sourceforge.net) is a GUI for GPG key management. It's good as far as it goes, which isn't far.

The lack of a good, simple user interface for GPG definitely is holding back widespread adoption of public key encryption. In contrast, the HTTPS protocol is trivial to use and virtually universal in e-commerce. Perhaps GPG is not the right starting point when looking for easy-to-use encryption. Or perhaps the right way to look at it is that you endure the one-time pain of GPG setup and then Thunderbird makes mail encryption easy.

There are more features to GnuPG than I have covered in this series. You can find out about these features by reading the references. You should be careful looking at the Mozilla mail documentation for encryption, as it can be misleading. Because Mozilla is an everything-at-once combined e-mail client, news reader, HTML editor and browser, the documentation that you reference from the e-mail help menu may actually take you to a file that was intended for the Web browser help.

This happens in Mozilla 1.5 if you search for encryption. You get some totally inapplicable material talking about certificates, which really relates to how the browser deals with HTTPS. It has nothing to do with e-mail encryption. This is yet another reason why three or four small fast applications with shared libraries are usually better than one big multifunction application.

Here are some links to further sources of information.