[Cryptography] How to crypto secure speed limit signs

[Stephan Neuhaus]

On 2015-03-25 14:39, 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?

I'm not criticising, but instead trying to understand the situation.

Are you comparing the cost of creating a single QR code and signing that 
and then printing out hundreds of copies with the cost of creating many 
QR different codes, signing each one and then printing it out just once?

I think that the second alternative will be more expensive, but not 
because printing or signing is the deciding factor. Rather, I think that 
the difference in price comes when you have to train the people who 
install the signs that they should install the right signs in the right 
places.

But I'll be interested to hear what others think.

Fun,

Stephan