[Cryptography] How to crypto secure speed limit signs

Ray Dillinger bear at sonic.net
Wed Mar 25 12:01:21 EDT 2015

On 03/25/2015 06:39 AM, Henry Baker wrote:
> FYI -- In order to keep these Fords from being spoofed, we would need to "secure" each speed limit sign with a crypto signature.  Presumably, this could be done with a QR code. 
> But how to avoid a "replay" attack -- i.e., cloning an existing sign & installing it somewhere else?  Should the QR code crypto sign the sign's GPS coordinates?  Wouldn't that make speed limit signs pretty expensive to manufacture & install?
> Ditto with all kinds of other street signs.
> http://www.theregister.co.uk/2015/03/25/ford_auto_speed_limit_obedience_fuel_starvation/

Spoofing could be a real problem.  Although I haven't heard of
any cases where somebody made trouble by replacing a speed limit
sign yet, most people simply ignore speed limit signs that are
obviously wrong.  If it's a six-lane interstate highway with
miles between offramps and no construction, a human would know
darn well that a 20-MPH speed limit is a fake.  An automatic
system?  Not so much.  It'll take at least a couple instances
of some joker putting up a 20-MPH sign on a highway just to
see the ensuing chaos before Ford gets the "ignore speed limit
signs when...." rules built into their systems, along with a
GPS database that tells the car what the real speed limits are.

It should be easier to build speed-limit information into
the GPS database that all the new cars use as a map.  I would
think that actually relying completely on the sign would be a
temporary measure until they can reliably get timely info
about changes in speed limit into their geographic database.

I would totally like to have that system on my car.  I haven't
had a speeding ticket in years, but I'd be happier knowing I
didn't need to worry so much about getting one accidentally.

And it's just making another baby step toward the self-driving
car.  We've been taking those steps for a while now; new
commercial trucks, and some cars, already have forward and
backward looking radar with automatic braking, and lane sensors
that can sound an alarm if they're drifting out of their marked
lane.  A few more increments like this and we'll be getting
to commercially available self-drive systems.


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