[Cryptography] How to crypto secure speed limit signs

Joseph Ashwood ashwood at msn.com
Thu Mar 26 21:20:05 EDT 2015

Replying to both Jonathan and Bill in one message.

> Beware the closed-world fallacy:  There are a lot of existing cars
> which don't/won't grok the new high-tech signs

It would be cheaper to require those vehicles have a compatible receiver 
installed. This role can be quickly covered by the smart phone. Considering 
that the entire US roadway system is the world's largest, and has been 
storable in flash memory for well over a decade, the costs are trivial. 
Today's smart phone could store the entire Eurasia super continent and not 
miss the storage space.

> And most jurisdictions like to encourage tourism, including tourists
> driving in from other countries in their own cars

That's what a border crossing is for. It isn't a massive dataset, only a few 
MB. A quick data update at the border, and everything works just fine.

> [A distinct point... how do we power these high-tech signs?

By not moving to the horribly expensive high tech signs. Instead, we retire 
the signs.

>* The system will have to support old technology for a long time.

Not very long. Use the available technology. Sure that 1939 truck won't have 
it, but that 2015 cellphone in the cab with you certainly has the power.

> "GPS isn't good enough to get you back to your car. It's good enough to 
> get you back to the parking lot."

Differential calculations are important here. At driving speeds the 
differential calculations works to identify the road itself precisely and it 
is entirely possible to determine not only which lane the vehicle is in 
(important due to lane restrictions for some vehicles) but even to determine 
where in the vehicle the GPS receiver is located.

Just as a side note on that, Android actually has a feature that tracks 
where you parked using GPS has always been accurate to within a couple of 
parking spaces for me. Today even stationary GPS is bordering on being able 
to get you back to your car, moving GPS with differential calculation is 
good enough to get you back to the random peanut that fell under your seat 6 
months ago.

>* Any solution needs to deal with the jurisdictions which use
speeding as a revenue source.

My proposal: They're screwed. Any change to the speed limit would have to be 
registered, or the jurisdiction will have to answer for it in court, safe 
bet that a lawyer will take this case. It is my firm opinion that such 
jurisdictions should be required to find other sources of revenue, revenue 
that is actually legitimate.

I am deliberately leaving alone policy decision like Bill brings up:

> * It may well be that a majority of the drivers will not support the 
> system with the current speed limits.
> Being speed limited while trying to pass on a 2 lane highway sounds 
> suicidal.

Answers to those are policy not technology.

More information about the cryptography mailing list