How do I encrypt email?

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Summary: Encrypting email is surprisingly difficult. We'll look at a practical solution that anyone can use, as well as the way it "should" work.

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I wish this was easier, and it should be easier.

But the fact is encrypting email is cumbersome and solutions that should work easily don't work consistently.

I'll give you three alternatives, starting with the one that's easiest for just about anyone to use.

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The issue is simple: even if you communicate with your mail server through an https or secure connection, your message is stored in plain, unencrypted text on the server, and it is likely to be transmitted onto the recipient's server unencrypted as well. In addition, you don't control how your recipient connects to their email. If they don't use a secure connection, then they could easily be downloading your email in the clear.

The only solution for total security is to encrypt the email message itself.

Unfortunately, that's not as easy as it sounds.

Encrypted Attachments

This is by far the most practical solution available today for the average user. It ain't pretty, but it works.

• Write your message in a program like Notepad, Word or whatever text or document editor you prefer.

• Save that message to a file on your hard disk. (example: message.txt for notepad, or message.doc for a Word document)

• Encrypt that file using a tool like AxCrypt, 7-Zip, or even GnuPG.

• Email the encrypted file as an attachment to its destination.

• Convey the password or encryption key to the destination by some other means. This is important, because if a malicious individual was able to capture the encrypted package, then they might well be able to capture the encryption key if you sent it using the same mechanism. Pick up the phone, send a letter, or use completely different email accounts on both ends using completely different computers in completely different locations.

There are, naturally, a couple of caveats.

Make sure that you chose an encryption program that your recipient can use. AxCrypt, for example, is Windows only, so it will present a problem if your recipient is using a Mac. Your recipient will need to be able to run the corresponding decryption tool.

Make sure you choose an appropriately difficult password/passphrase - just like account login passwords, encryption passwords are subject to brute force attacks as well. The simpler your password, the easier it might be for someone to guess it and decrypt your message.

With "how it can most easily work" out of the way, let's look at two alternatives to "how it should work".

Certificate Based Encryption

Buried in the advanced security dialogs for Outlook, as well as the security setting in Thunderbird's accounts dialogs, as well as similar places in some other email clients is the concept of using digital certificates to sign and encrypt email.

These are, essentially, the same kind of certificates that are used to protect https websites, and in general they're purchased - yes, purchased - pretty much the same way. Like website certificates, these certificates can be used for two purposes:

• To "sign" your messages in such a way that you are confirmed as the author of the message, and that the message has not changed since you signed it. (This isn't encryption, it's validation.)

• To allow people to send you messages that are encrypted such that only you can decrypt them.

That last one is a little counter intuitive. To encrypt a message, you don't use your own encryption certificate, you use that of the person you're sending the encrypted message to. That way, only they can decrypt it.

This is a fine approach, and once set up can be nearly transparent.

Unfortunately, it suffers from a couple of problems that have prevented widespread adoption: not all email programs support it, and certificates either cost money, or are cumbersome to obtain.

PGP/GPG Based Encryption

PGP (Pretty Good Privacy) and GPG (Gnu Privacy Guard, the open-source equivalent) are very similar to the certificate based encryption scheme above. The rely on public key cryptography which, in a nutshell, boils down to this:

• Two really large numbers, A and B, are created that share a special relationship

• Anything encrypted with "A" can only be decrypted by "B", and anything encrypted with "B" can only be decrypted by "A".

• You make one of them, either A or B, public and keep the other one private.

Let's say I make my "A" number public (which I have), and keep the "B" number private - only I have it.

You can now encrypt a message to my using my public key "A". Only I can decrypt it with my private key "B".

Conversely, I can encrypt (or sign) a message using my private key "B", and anyone can decrypt it - but since they can, they know beyond any doubt (as long as I've kept my private key private) that only I could have created the message.

PGP and GPG are just standard techniques and tools to manage those two "numbers", more commonly called keys.

The good news about GPG is that all the parts are free. The bad news it can be fairly geeky. I wrote an article about using it some years ago: How do I send encrypted email? that walks through the steps. I don't actually recommend it for the average user, but if you're at all technical, or just enjoy this kind of geekery, you can see what it entails.

Most email programs do not include support for GPG/PGP keys. Thunderbird does have an extension, "Engimail", that adds support quite nicely.

The Weakest Link

I have to include a word about trust.

All the encryption in the world won't help a whit if you can't trust the person at the receiving end. They have to keep passwords safe, if you use them, and they have to keep your private message private, if that's the intent of your encryption. They also have to practice safe computing - your message will be decrypted on their computer, and hence visible to any malware that might be present thereon. Heck, if their computer is stolen and they happened to keep the decrypted message for some reason, that's now in the hands of someone else.

So in addition to doing the right things yourself with your sensitive information, make sure that your recipient has an appropriate level of understanding as well.

That's something that technology just can't fix.

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Article C3659 - February 25, 2009