While I'm not going to promise that it'll be 100% protective, we actually tested this theory ourselves a while ago by putting our key fob in a tin and then tried to get into our car. We couldn't, which suggests the metal blocked the signal from being picked up by the car - and thus it doesn't open even with the key in range. I would test it yourself doing the same thing. Lock your car, put the key into the tin, close the lid, then try to open the car door while holding the tin. If the door won't unlock and open, it's working as it should. Sounds ridiculous but that's the way to test if it's going to work. If it doesn't, you can buy larger faraday bags for larger keys. What it tends to come down to is how well the tin or bag is sealed. If the lid isn't tightly shut then that can be the different between the signal being transmitted or not.