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comment on Master Key Copying Revealed (Matt Blaze of ATT Labs)
From: Dave Farber <dave () farber net>
Date: Fri, 24 Jan 2003 12:58:46 -0500
Date: Thu, 23 Jan 2003 09:31:36 -0500 (EST) From: Donald Eastlake 3rd <dee3 () torque pothole com> To: Dave Farber <dave () farber net> Cc: interest () pothole com Subject: [interest] Re: [IP] Master Key Copying Revealed (Matt Blaze of ATT Labs) I've never seen such a ridiculously overhyped teaser article about a very simple 150 year old weakness. Pin tumbler locks work by the key, at each cut, raising a pin so that a split in the pin aligns with the interface between and inner concentric cylinder and the outer part of the lock, one reason they are sometimes called cylinder locks. Master keying frequently works by putting two cuts in each pin. One set is present in all locks of the set so the identical master key can raise the pins so those cuts align on the cylinder boundary. The 2nd cut in each pin is in a different pattern for different locks and the individual keys use them so you can have no master key cut the same as any corresponding cut on any individual key. Pin tumber locks typically have 5 to 7 pins and 10 levels of cut for each pin. (Obviously, you can also have sub-masters by using some master cuts and some individual cuts that are common to a subset of the full mastered set. You really don't want to go to three or more cuts in a pin as you start increasing the chance that a random key will open a lock. You can also can do cross section mastering where individual keys will only fit into certain locks but the master key will fit into all, but it is usually easy to get master blanks, which are just the intersection of the individual key blanks cross sections.) If you have an individual key, key blanks, and access to a lock, you can cut trial keys. Assume 5 pins and 10 level. You take a blank and pick a pin. You cut the other 4 places the same as your working key and, for the pin you picked, try the 9 other levels. (This only takes one key blank as you can start with the highest cut and keep going down with your key cutting machine or a file.) If you find some other level of cut that opens the lock, you have found the master cut for that pin. Do this for each of the 5 pins and you now know all the master cuts having used up 5 blanks and making 45 trials. In fact, you can stop as soon as you find the master cut so on average, it would be 22.5 trials. It may be a bit harder if there are 7 pins or a bit easier if you use well known heuristics for master key design which make it harder to pick locks but also constrain the most likely search space. (Of course, master keying at all can make the lock easier to pick.) The main building at MIT has (or had) two separate key holes with separate inner cylinders in each lock. Thus one used for individual keys can be only single cut. Or you can have two concentric cylinders, one inside the other, so that pin cuts have two different levels on which to line up and engineer it to avoid this weakness but it makes the tolerances smaller unless you go to fewer different levels or longer pins. Etc. This weakness has been well know for 150 years but, so what? If you are skilled enough and/or have the right equipment, its faster to pick the lock anyway. Thanks, Donald PS: The headline is wrong. It should be Master Key Discovery, not Master Key Copying. ====================================================================== Donald E. Eastlake 3rd dee3 () torque pothole com 155 Beaver Street +1-508-634-2066(h) +1-508-851-8280(w) Milford, MA 01757 USA Donald.Eastlake () motorola com ------ End of Forwarded Message ------------------------------------- You are subscribed as interesting-people () lists elistx com To unsubscribe or update your address, click http://v2.listbox.com/member/?listname=ip Archives at: http://www.interesting-people.org/archives/interesting-people/
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