Secure E-Mail

Can Someone Read My E-Mail?

Can someone else read my e-mail?

Umm -- could you be more precise?  Who can read my e-mail?
Lots of people, starting with the system administrator of every machine your mail touches, plus anyone who has hacked those machines, plus anyone who is eavesdropping on any of the networks passed. (There are many more ways for a hacker to get at it, too....)

Ulp.  Let's take this one at a time.  Why are so many system administrators involved?
E-mail rarely goes directly.  Most PC mailers are so-called "thin clients".  They do the formatting (and often minimal formatting at that), but never deliver the mail directly.  Instead, they pass it off to a mail server for actual delivery.  The mail server may pass the mail on to a higher-level server, especially for Internet email which passes through a firewall machine.

Inbound mail is vulnerable as well. There are often multiple levels of relaying (including, of course, the firewall) before it reaches the end-user's mail server. From there, the user generally uses something like POP3 to download it to the PC.

UNIX users go through a similar dance, though in their case the mail is likely to be read by someone who has logged in remotely to the mail server, either via rlogin or via an X window.

The real point, though, is that the mail will sit on the disk of each of these machines for some amount of time. That makes it easy pickings for anyone with privileged access to those machines.

Even the originating and receiving users machines are vulnerable, if not directly then via their network usage. For example, if the user relies on a network file server, that machine has the actual mail. Similarly, if a computer is backed up over the network, the backup server has copies. Are all of these machines secure?

What about the network? Can someone "wiretap" it?
Absolutely, though that's a hard way to get at the mail. It's much easier to (ab)use the network to gain access to a user's account, and proceed from there.

What sorts of network abuse?
That's getting into the whole area of network security, about which entire books have been written. But let me give a few examples.

First, of course, an attacker can pick up your password. This is old technology, and works very well. And bear in mind that many POP3 clients transmit your password every time they check for mail.

For UNIX users, IP spoofing is an easy way to gain access to the server. Failing that, it's often easy to connect to a user's X server and read keystrokes and screen images that way.

Okay, I'm convinced. Can I use encrypted email?
Sure. But unless you use it properly, it's not going to do you much good.

Here we go again. Could you please be more precise?
Boy, some folks are never satisfied.

Let's start with what we've already talked about. If you're logging in to a remote machine to read your mail, you're still toast, because the mail will traverse the net after you've decrypted it. For that matter, the password you use to unlock your secret key will traverse the net, too. Or maybe the attacker will just pick up your keystrokes from your X server.

Are you saying that only UNIX users shouldn't use encrypted e-mail?
Oh, no. PC users can be just as vulnerable, though to different attacks. For starters, remember what I said before about networked file systems and backup servers. And have you shared your own disk partitions?

Moving on from there, consider the lovely things that can be done with tailored viruses. There have also been a number of bugs in Web browsers that permitted access to arbitrary files on the machine.

I give up. What should I do?
Ah, that's the right question. Here are some guidelines:

What is a good e-mail security package?
Look first at its general characteristics:

Can I get away with doing less?
As with any security system, that depends on who your enemies are. If your main concern is eavesdropping on the public Internet or at your recipient's site, you may be able to disregard some of these precautions. If your own site is at all dangerous -- and that's actually fairly likely -- yes, you do need to be very careful.

Do I need a certificate?
Certificates are a way of assessing the authenticity of a public key. In other words, they only matter if get the key from a dubious source, such as a public key server or directory. If you get the key from someone you trust -- and in particular if the owner gives it to you directly (not by email!), you don't need a certificate.

If you do use certificates, make sure you trust the issuer. You can buy your own certificate-issuing program, but the machine it runs on has to be very secure; if it gets compromised, your entire secure e-mail infrastructure may be compromised.

What secure mail package should I use? My Web browser supports secure email; can I use it?
I'm loathe to recommend specific products. In general, though, smaller is better. Complexity is the enemy of correctness -- and hence in this case, of security. That tends to suggest that browser-based solutions are not ideal. On the other hand, some of the latest be-all, do-all mailers aren't any smaller. See,1282,12249,00.html for an example of how software bugs can subvert cryptography.

I should add that with proper software structuring techniques, a large program may be able to provide adequate protection of the security-sensitive pieces. But that's hard, and requires a degree of self-discipline that's relatively rare. Stand-alone programs, though sometimes less convenient to use, are much more likely to be secure.

There is one class of secure email program to avoid at all costs. These are intended for use when you wish to send email to someone who doesn't have a decryption program. These encryptors package up the ciphertext in a self-extracting executable and send it to the recipient.

Now -- consider the security model from the recipient's perspective. At a time when they think that someone might be reading their email, they're supposed to (a) run a program sent to them in an unauthenticated fashion, and (b) type a secret into this program of unknown origin. And this is quite apart from the more general class of bad things that can happen from trusting emailed executables.

Regrettably, several otherwise-responsible vendors have introduced such products. Don't just trust the brand name; some ideas are stupid, no matter who sells them.