Nate Duehr wrote:
> It's all about finding a balance, but I don't think the iButton lock
> buys you much over a decent quality mechanical lock.  Heck, the
> iButton requires power, metal gears don't... it actually has some
> very distinct disadvantages.

People should also consider that a mechanical key is merely the embodiment
of a small password.  A standard pin and tumbler lock has a fixed number of
pins, usually 6 for a normal house lock.  For manufacturing and accuracy
reasons, each of these pins will be unlocked at one of a small number of
fixed levels for that model of lock.  Five levels is common.  Therefore the
key is merely a mechanical embodiment of a 6 digit base 5 password.  The
chance of any random key opening the lock is 1 / (5 ** 6) = .0064%, or 1 in
16K keys.

There are other wrinkles however.  In a large installation with many rooms
or buildings like a school, there are usually master keys that open all
locks or a set of locks.  Sometimes there are hierachical master keys, like
one for each building, with high level master keys that open any lock in a
group of buildings or the whole installation.  This is done by adding more
cuts to the pins of each lock.  This means that each pin in each lock
actually responds to more than one cut level on the key.  There are now a
large number of possible keys that could open a particular lock.  If each
pin has 3 cuts, then the chance of a random cut level matching a pin are
3/5.  For all 6 pins, the chance of a random key opening the lock is (3/5)
** 6 = 4.7%, or about 1 in 21 keys.

And it gets worse.  Again in large organizations, various locks need to be
changed on a regular basis as people come and go, rooms take on new uses,
things break, etc.  Removing and replacing the whole lock as you would do
with your house front door is way too tedious and expensive.  Instead, the
locks essentially have two independent tumblers, one to open the lock and
the other to remove the core.  The locksmith (a full time job at a large
installation) has a "core puller" key that is used to remove and replace
lock cores in the doors.  This can be done in matter of seconds.

Someone that is very familiar with a particular series of key at such an
installation can look at a key and immediately read off the cut levels.  I
know that because I've done it.  A two second close up look of the key is
all that's required.

When I was at RPI from 1974 thru 1980, the key system was divided into 5
different trees.  There was a "grand master" key for each tree, plus
building masters and sometimes departmental master keys within buildings.
By the time I left, I had figured out how to make 3 of the 5 grand master
keys, and a core puller that worked for two of the trees.  The biggest
breakthru came when the key guy came to change the locks of one of the labs
I was working in.  I already knew about core pullers and master keys, but
played dumb.  I said something like "Gee, this key looks just like any other
key but can take the lock out!?".  That allowed me to hold it in my hand for
a couple of seconds, during which time I read off and memorized the cut
levels.  As soon as he left I wrote them down and made my own core puller.
After that treeing out to more locks is trivial since you can take out the
core and determine the various cut levels that would unlock each pin.  After
examining only a few locks, the grand master cut levels are usually pretty
obvious.


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