> > But if they have physical access to the iButton, what is stopping them > from just using it instead trying to make a copy? > > If you lose a set of keys in real life, you change your locks. You > could do the same thing with the iButton system, make it easy for you > to grant/revoke access based on serial numbers. Much quicker than > getting a locksmitch to install a new front door lock. :) > > Alex As an example, lets say you are at work and go to the restroom, leaving your keys on your desk. While you're gone for 5 minutes no coworker could get your ibutton, drive to your residence, gain access, and return the keys such that you would not notice. But 5 minutes is more than enough to copy the serial number for use at a later time. Other examples would be in the airport, at the gym, leaving your keys in an unlocked car, or at the pool. Perhaps I'm more careless with my keys than most people, but just today alone I can think of twice that an unsavory individual might have been able to copy an ibutton's serial without my knowledge. If the entire ibutton is missing obviously it would need to lose its access privileges. But if a copy is made there would be no way of knowing, and therefore the lock wouldn't be changed, even after multiple break-ins. In theory, some sort of rolling code would sound the alarm that someone has gained unauthorized access, even if it wouldn't stop the initial access from occuring. It would act as a broken window or crowbarred door, and prevent multiple entries. Standard warning signs would be conspicuously absent from an ibutton authorized system, since no physical damage would be done. Access logs (if kept) could offer an after-the-fact explanation if you remember to review them, and remember what you were doing all day. I still think it's a good idea to have a double-check system like the one I've described. A simple increment-on-use syncronized counter would be acceptable, but far easier to bypass or work around than a more complicated unpublished algorithm. In the end it might not make the system any more secure against an initial attack, but it would at least alert you if someone else had been / is currently there, and that its time to change the lock to resecure the system. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body