[wolrah]

That would only work if the car has also not seen any further valid messages since the one that was blocked, so I'm not seeing how this technique could be useful for anything other than preventing someone walking away from their car from successfully locking it and being able to go through the car before replaying the signal and locking it yourself to cover your tracks.

Even then if they're the sort of person who does the double click for horn/flash thing they'll just assume their fob batteries are dying and return to the vehicle until they're close enough to defeat the jammer.

If the rolling code is predictable and the attacker can generate their own valid "next message" that's an entirely different matter, but a pure replay is only useful in very specific situations.

[londons_explore]

The typical process, which I see active in London pretty frequently, is to jam and record everything on 433.92 Mhz.

Then, when you have received at least one lock and unlock request for a car, start retransmitting lock and unlock requests, but delayed by one request.

That way, the owner of the car thinks their key is working, and it unlocks and relocks reliably, but in fact the attacker always has one code 'spare'

Then, at 3am they come use their spare code to unlock the car and steal stuff or drive it off (the car only needs to see to fob present briefly to start the engine, but I believe that bit is done with a relay attack. I see the guys who do this waving their suitcase antenna around peoples doors regularly).

One giveaway to know on my car that such an attack is underway is that if you press the lock button 5 times in a row, it would normally flash the indicators with every press. However, when an attack is underway, it will only flash the indicators the first time, and won't do it again till after the next unlock, which makes sense because the command (lock or unlock) is not independent of the rolling code.

[wolrah]

> Then, when you have received at least one lock and unlock request for a car, start retransmitting lock and unlock requests, but delayed by one request.

For this story to play out with a non-spoofable implementation the attackers would have to be present for and able to jam/intercept every use of the fob between the first one in their sequence and the present.

I'm not saying it's not technically possible, but any use of the fob that slips through the cracks resets the sequence so if the target takes the vehicle somewhere else or even just uses the fob while close enough to not be jammed the idea falls apart. Unless the target is in the habit of going back out to their parked car to retrieve things and then locking it back up it seems like that'd be a rare catch.

> (the car only needs to see to fob present briefly to start the engine, but I believe that bit is done with a relay attack. I see the guys who do this waving their suitcase antenna around peoples doors regularly).

Relay attacks on pushbutton start vehicles are an entirely different matter as they do involve two-way communication and if you have the ability to start a car with one you also have the ability to unlock it with the same method. I'm just talking about traditional long range one-way fobs here.