3 ms·
Detecting the amplified signal won't work. There are these things known as directional antennas... The timer idea is a good one, although a ns-accurate timer i
by TheLoneWolfling 12y ago
Detecting the amplified signal won't work. There are these things known as directional antennas...
The timer idea is a good one, although a ns-accurate timer is starting to get a bit much for something to put into a key fob. Especially give it's run off of a watch battery (power requirements) and often exposed to heat / cold (thermal drift).
- Dylan16807 12y agoWell for the example timing I gave you only need 1 part per thousand accuracy, and it's easy to get 10ppm or much better. It only needs to run during communication, anyway.
- anologwintermut 12y agoThe timer could be in the car. Protocol looks like fob pings car, car challenges fob, fob responds.
- TheLoneWolfling 12y agoNope. Dylan's comment requires a timer in both the key fob and in the car. The key fob to delay transmission of the challenge response, the car to check if there isn't too much delay in the challenge / response pair. You really need a timer in the key fob, as the processor in the key fob is often so slow (for battery / cost reasons) that an extra couple clock cycles somewhere would throw off the timing enough to make it fail.
- anologwintermut 12y agoThe usual distance bounding protocols only need a nano-second accurate timer on one device called the verifier. For example https://www.usenix.org/legacy/event/sec10/tech/full_papers/Rasmussen.pdf https://www.usenix.org/legacy/event/sec10/tech/full_papers/R... Cool trick in that one, the Prover(i.e. the key fob) does the distance measuring part of the challenge response protocol using analog only components. This means its response time is <1 nano second. So you can do it with only the car having a good timer.