Relatively soon I'll start digitally signing all my messages. I thought that some of those out there that use Outlook Express might ask some questions, so here the questions are followed by the answers (the following stolen shamelessly from Peter Harkins):
What is with the attachments you keep sending? They have no file formats and I can't open any of them. What am I doing wrong?
I've started getting this question fairly regularly, so I've written an explanation. If you want the short version, just read the first paragraph. If you want to know the how and why, read the rest. It's easy reading and you'll learn how e-mail works, but there won't be a quiz on this material. Here's your copy & paste of the explanation, (insert name here):
The attachments are digital signatures. E-mails can be spoofed or altered so I sign messages to evidence that I wrote the message and that it wasn't altered (deliberately or not) during delivery. It's also possible (if the other person also uses signatures) to encrypt e-mail so that a private conversation is actually, you know, private -- an e-mail will pass through several systems before it reaches its destination, and all of them could read it if they wanted.
Long answer continues:
This gets right at major problem with e-mail: when people think about e-mail, they think of sending letters. All the icons in e-mail programs reinforce this, so it's a reasonable assumption.
It's also totally wrong: e-mail is much more like a postcard, and not at all private. You give it to somebody who reads it and passes it onto somebody else. They read it and pass it on. After enough passing, it shows up in the recipient's inbox.
Hopefully these people only read enough to pass it along, but they could read the whole thing, send out lots of copies or save copies. All these behaviors actually happen all the time for legitimate reasons (and aren't impeded by the tools I use): spam or virus scanners read all of your e-mails, mailing lists exist to replicate e-mail, and you probably want your ISP to save your e-mail until you come along to check your mail.
Anyone along the way (and you have no idea who they will be when the message leaves your hands) might be unable to deliver it and delete it. Hopefully they'll send you back an e-mail (which also may or may not arrive) and tell you what happened. You've almost certainly seen these - they're from "postmaster" or "mailer-daemon" and have lots of geeky techspeak to make your eyes glaze over.
That this is all possible sounds really crazy until you learn that SMTP (Simple Mail Transfer Protocol) the protocol that dicates how sending e-mail works, dates back to 1981 (well, with that name, an important part is from 1973). It was a different Internet then -- there were far fewer high-speed links and computers weren't online all the time. Quite often it was a case of "you can't get there from here", so it made sense to pass your e-mail on to someone who would hang onto it long enough for the next guy to come online and pass it along. Some computers would only come online late at night when it was cheaper, so it wasn't uncommon for an e-mail to take a week or just disappear entirely.
Let's get back to the signature attachments. I'm not going to get into the math of it (and boy is there a lot of it) but each signature is generated, based on the message contents, from a private key only I have. I give out a public key that people can use to check signatures or encrypt e-mails to me. There's plugins for just about every e-mail client.
The reason these plugins aren't turned on by default is that most people don't want to deal with the complications it can introduce. There's one or two complicated parts to the system and it means more to learn for users and more work for tech support. People also, wrongly, think something along the lines of "I don't need that, I have nothing to hide." They've got plenty to hide: the e-mail they get from their bank, the discussion of their personal life, their business dealings. If people use it only for the important stuff, just using it becomes suspicious and distrusftul and brings further scrutiny. It's important to use it regularly and expect others to do the same.
So that's why I sign my e-mails. It just makes all kinds of sense.
If you don't ever want to deal with them, that's tough. I'm not going to stop using privacy and security tools because it's an incredibly minor inconvenience to those who don't. I'm just as likely to carry around fistfulls of cash because a wallet or debit card is inconvenient or give up pants because zipping them up is so much added work. I'd have to be reckless to carry on my correspondence in pencil on postcards. I'll be happy to field any questions. As you may have guessed, this is something I think is really neat and important, so pardon my verbosity.
(Yes, for anyone who reads the whole thing, that "(insert name here)" is my idea of humor.)