Trusting sources and securing your privacy (1): Why do David's posts look weird?

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This is the first thread in what I hope will be a series, including topics like "How can I verify you really wrote that?", "How do I sign my own writing?", "The importance to innocents of plausible deniability.", and "How to keep your private parts private." The general theme of the series is to provide information on some basic security issues and equip readers with some useful cryptographic tools. One particular goal that I have is to develop a useful written guide to getting started using GPG keypairs and why it's worth doing, for distribution to people not on this forum.

You may have noticed that there's some extra text up at the beginning of my post, and a bunch of extra stuff down at the end that looks rather like http://getfiregpg.org/images/newscreenshots/editor.png and include what ought to look like a random string of characters. What you're seeing is a "PGP signature", and what it means is there's a way of checking if the post you're reading is actually something I wrote or not. To really do that check, you'd also need my public key (the big mess-o-random-characters at the top of my profile at http://www.masterzdm.com/NCD/showprofile.php?User=3152 ) and some software that knew how to deal with such things.

About that software... there are two main parts, one that does all the actual work, and one that provides nice window dressing. An example of the window-dressing software is the FireGPG plugin (http://getfiregpg.org/) for Firefox. Even if you don't have the actual-work software, FireGPG will still clean up pages containing signatures and keys and so forth, to make them look more like http://getfiregpg.org/images/newscreenshots/inlineonapage.128.png instead of what you probably see now. I suggest you try installing it yourself, and then reload this page and my profile and see how nicely things are cleaned up.

Now, assuming you've done that, the question still stands: Why do David's posts look weird? Granted, they don't look quite as messily weird as they did before, but having a box around the message with "PGP Signed Message, Unverified" at the top, "Display original | Verify" at the bottom, and a stylized foxhead in the corner... while less ugly, that still counts as weird.

Well, to start with, you need to know that there are two "keys", my 'public key' (seen in my profile) which is out there for the whole world to see, and my 'private key' which I and I alone have access to (and I darn well better keep it that way!) Unlike normal physical keys, these have some weird properties. Since there are a lot of mnimo out here, or people comfortable/familiar with them, I'm going to make an analogy that uses magic.

Imagine you have two magical keys, one called W and the other called C. You can encase any object in a magical force-field by holding one of the keys in front of it and turning the key once around. Oddly, the C key only can be turned clockwise, and the W key can only be turned counterclockwise. Turning a key twice around sets up two force-fields, turning it thrice gives three, and so forth. Turning one key, then the other, does something quite different, however.

If you turn the C key clockwise to set up a force-field, and then turn the W key counterclockwise, the force-field you set up goes away. Moreover, that's the only way to remove such a C-made force-field (aside from using a dispel-magic ritual that takes thousands of years... if you speed time up a whole bunch). Similarly, if you turn a W key to set up a field, and then a C key, the W-made force field goes away, and that's the only way to make a W-made field go away. If you turned C twice, you'd have to turn W twice. Turning C three times and then turning W four times ends you up with just one W-made force field... the rest of the W and C turns canceled out.

Now, what could you do with a pair of such keys? Well, for one thing, if I kept the W key, and gave you the C key, then if I put a W-made forcefield around a package for you (which I can do, since I have the W-key) the only person who can get through the forcefield to open the package is you, the holder of the C key. Moreover, if you get a fielded package which opens when you turn the C key at it, you know it was sent from the person (me) with the W key. I know you're the only one to see it, and you know I'm the only one who could have sent it.

"But David", you might say, "You didn't give me a magic key, and I can still read your message!" And you'd be right... in part. My post I send where everyone can read it, but that garbage at the bottom comes from compressing down my message and then W-locking that compressed version. If you had the C-key, then you could unlock the garbage and compare the result to what you get if you compress down my message. If they're the same, then you can still conclude that I wrote the message, because I'm the only one who could have sent both versions (readable, and compressed plus W-locked) and had them match up like that. Anyone at all can now read the message, but only people with the C-key can verify I really wrote it.

"That's nice," you might object, "but to use this, I'd have to make a pair of W/C keys for me and you to use, and a pair of W/C keys for me and Karol to use, and a pair of W/C for... argh... EVERYONE I want to talk to. That's too much work." And again, you'd be right! That would be nuts! Fortunately, there's a way around that. You make one W/C pair, you keep the W key, and you give EVERYONE a copy of the same C key. Similarly, I make just one pair (I'll call mine R/U... the U looks like a C on its side) and I'll keep R and give EVERYONE a copy of the U key.

Now, if I want to send you something that only you can open, I use the C key. You may not know who it came from (everyone has a C key) but I know you're the only one who can open it. Now, if I want to send you something that you'll know came from me, I can use my R key. You may not be the only person who can open it (everyone has a U key) but you'll know I'm the only person who could have sent it. Now, if I want to send you something that you'll know came from me and I'll know only you can read, all I have to do is use both the C key (which you gave me and everyone else) and my R key (which I keep hidden), and we get both those guarantees.

Moreover, now that it's not "I have one, you have the other" but "I have one, everyone else has the other", I can use my R key on anything I want, knowing that everyone else has the U key, so everyone else can remove the force-field and read my message... PLUS they'll automatically know the message came from me! Or, I can do what I'm doing with this post: I send my message in plain text, then I send the same message crunched down and locked with my R key. Even if other people don't have a way of un-crunching the crunched version, they can at least unlock it and compare the known-to-be-from-me crunched version with what they get if they crunch the allegedly-from-me plain version.

And that, roughly speaking, is why my posts look weird. I am providing evidence that the text you read is, indeed, the text I wrote.

Please ask questions on anything that's not clear, or on any "yeah, but what about" issues, or if you have a "well if I did X, then how would you know..." idea. Some responses I'll put off to a future post in this series, others I'll reply to in this thread.
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