Monday, September 13, 2010

Testing a Watchdog Timer

I recently got a query about how to test a watchdog timer. This is a special case of the more general question of how you test for a fault that is never supposed to happen. This is a tricky topic in general, but the simple answer (if you can figure out how to do it easily) is to insert the fault intentionally into your system and see what happens.

Here are some ideas that I've seen or thought up that you may find helpful:

(1) Set the watchdog timer period to shorter and shorter periods until it trips in normal operation. This will give you an idea how close to the edge you are. But, it is easy to make a mistake changing the watchdog period back to the normal value. So it isn't a good test for nearly-final code.

(2) Add a timeout loop (for example, a do-nothing loop that you have made sure won't be removed by your optimizer). Increase the timeout value until the watchdog timer trips. This similarly gives you an idea how close to the edge you are in terms of timing, but with a nicer level of detail. It also has the advantage of testing operation without modifying the watchdog code itself.

(3) Use a jumper that, when inserted, activates a time-wasting task (similar to idea #2). The idea is when a jumper is installed it enables the running of a task that wastes so much time it is guaranteed to trip the watchdog. When the jumper is removed, that task doesn't run and the system operates properly.  You can insert the jumper during system test to make sure the watchdog function works properly.  (So just jumping to the watchdog handling code isn't the idea -- you have to simulate a situation of CPU overload for this to be a realistic test.)  When you ship the system, you make sure the jumper has been removed. Just to avoid problems, put the watchdog test first on the outgoing test plan instead of last. That way the jumper is sure to be removed before shipment. If you want to be really clever, that same jumper hardware could be used to disable the watchdog in early testing, and code could be changed to have it trip the watchdog as the system nears completion.  But whether you want to be that tricky is a matter of taste and the type of system you are building.

Wednesday, September 1, 2010

Using Car Remote Entry Key Fobs For Payments?

There is a news story that suggests people will be using the remote keyless entry systems for their cars as payment systems.  (So now your car key transmitter might have "lock", "unlock", and "pay" buttons.)  Do you think this is a good idea?

I did some design work on a previous generation of this technology. Cryptographic algorithms I developed jointly with Alan Finn were in a lot of model year 1994-2004 cars. What struck me about this market was the extreme cost sensitivity of things. No way could we afford industrial strength crypto.  The competition all had laughable crypto or made what I consider rookie mistakes (for example, using the same manufacturer key in all devices).  I never heard that my algorithm was broken during its designed 10-year life (but I'll bet the NSA had a nice chuckle when they saw it).  But other algorithms have proved to be insecure. For example, the Keeloq system has become the target of numerous attacks.  While the first published attacks are relatively recent, people likely knew about the weaknesses and could have attacked them a lot earlier if they had wanted to do so. While technology has changed over the years, these important lessons probably are the same:
  • You have to have real security experts work on these products. You can't just put something together without really knowing your stuff or you will make rookie mistakes.
  • You have to have some appreciation for security by the customers buying the systems (often it is a car manufacturer deciding how good is good enough). Too often decisions are made on "cheap and not obviously bad" rather than "is actually secure." One of the best things that happened to me in my experience was that the customer had someone (his name is Tom) who understood crypto and was willing to back us when I said we couldn't go below a certain cost threshold without compromising security beyond the required level.
  • You have to use real crypto now, not cheesy crypto. Attackers have gained in sophistication and if someone has time to attack an old Keeloq system they'll attack your system.
  • Once you let a key fob control money, it's a lot more attractive to attack. So it is sure to be attacked.
  • Cars have a 10-15 year life, multi-year production runs, and easily a 3-5 year lead time. So you have to plan for your approach to be secure 20-25 years from now. That's hard to do in any system, much less one that is supposed to be inexpensive.
I predict next generation devices will have vulnerabilities due to failure to appreciate the above bullets. It has happened in the past, and it will happen in the future. I don't plan to use my key fob transmitter to make payments any time soon, and I used to design these things for a living. But then again, as long as you read about problems in the news before you're the victim of an attack, maybe you'll be OK.