Martin Geisler Online

Today I’m at the ETH to study for the exam in Formal Methods for Information Security, which will be next Tuesday.

I’m currently looking at BAN logic (see also Wikipedia), a nice system for proving (a limited set of) properties about security protocols. It deals with the beliefs of the participants in the protocol in a formal way. This allows you to verify that the goals of the protocol can be fulfilled based on the initial assumptions given. An example from BAN could be a rule that says:

If (P believes (Q controls X))
and (P believes (Q believes X))
then P believes X.

Here one should interpret the keyword controls as “has jurisdiction over”. An example could be a server S which has authority over the public keys for B:

If (A believes (S controls public key for B))
and (A believes (S believes public key for B = K_B))
then A believes public key for B = K_B.

So this rule expresses the trust of A in S. But for this rule to be applicable A needs to believe that S believes public key for B = K_B. There’s a rule for introducing such beliefs about other participants beliefs:

If (P believes (X is fresh))
and (P believes (Q said X))
then P believes (Q believes X)

This rule is based on an assumption that you will not say something which you do not believe. There are then more rules stating when you can believe that someone said something (for example, when that something was signed, and you believe in the key used for the signature).

From this big set of rules, one can make more and more deductions and hopefully derive the goals of the protocol. A simple, but pretty cool concept.

So… December finally arrived. Now is the time to get together, light lots of candles, drink warm tea, gløg, or something else, eat lots of candy, and generally have a nice and cozy time.

And if you don’t celebrate Christmas, then I still wish you a good time with lots of candles and candy and hot beverages :-)

Stéphanie and I will have our Christmasses apart from each other, just like last year. But this time it will be me who travels back and forth, and I’ll be back in Switzerland for the New Year. I’m looking forward to really see Switzerland covered with snow — so far we have only gotten a [little bit of snow][1] a couple of days ago. The temperature has lingered just around the freezing point since, but we haven’t had any precipitation and thus no snow.

Looking out the window, Stéphanie suddenly said: “Er det sne?!” (”Is that snow?”) And it was… here in the evening it has begun to snow. Just a little, but it counts :-) Look for your self — you’ll have to excuse the strange colors, the white balance is a bit weird here at night with the warm glow from the lamp post:

This must be a sign that winter is coming… and so is Christmas :-) I’ll go home to Denmark for the Christmas, just like last year, but I’ll be back in Switzerland again to celebrate the New Year. The plans are not finalized yet — I’ll keep you updated when I know more.

I had no connectivity to http://google.com/ today — strange experience…

First I noticed that this site would load very slowly — the big blue header would load at normal speed, but then things would stop for a while, after which the rest of the page would load. The pauses were caused by the Google Adsense advertisements I use — with no connections to the Adsense server, the browser would “hang” for a while until a timeout was reached.

It wouldn’t be so bad if it were only my site that’s broken because of this, but since Adsense is to popular many, many sites around the Internet would suddenly load at crawling speed. (A moment later…) The pages now load at quite normal speed, but without the Adsense advertisements.

While testing I tried to access google.com through a SSH tunnel into DAIMI. That worked fine, so I guess the problem might lie at my end? Trying directly I have no connection to either http://google.com/ or http://google.dk/. But http://google.ch/ works! Quite strange I think… Have any of you experienced this?

I just saw today that everybodys favorite OpenSource repository has been updated with a new layout. I think I like it, especially the new design, but the advertisements at the bottom of the page (”Find a Tech Job” etc.) are too near the content for my taste. The old layout had a better separation of content and advertisements.

But I’m glad to see that SourceForge are improving. I’ve entered PEL into the beta program for the upcoming Subversion feature, but I haven’t heard anything from them yet. I’m really looking forward to quitting CVS — SVN is so much better.