brute forcing combination locks (Re: Bruce Schneier, Sandia, FBI and the REAL WORLD)

[Adam Back <aba@dcs.ex.ac.uk>]

Bill Payne <billp@nmol.com> writes:
> Bruce Schneier wrote 
>
>    and a burglar willing to try all 10,000 is guaranteed to break 
>    into your house.
> 
> Sandia employees Jack Hudson and Jack Menako, both in my division
> when Sandia transferred me to break electronic locks for the FBI/ERF
> [Engineering Research Facility, Quantico, VA], were TRYING to 
> defeat combination locks on file cabinets.
> 
> Menako built a frame to connect a stepper-motor to the combination
> dial.
> 
> The stepper motor was wried to a PC.
> 
> Hudson wrote the software to try all possible combinations.
> 
> What happened, IN FACT, was that the combination lock wore-out
> before the combination which opened the lock was reached.
> 
> Combinations locks are NOT ENGINEERED for such heavy use.

Interesting.

When I was at [x] they had (and still do I think, hence obscuring
name!) 0-9 digit key pad combination locks.  I noticed by just
casually using various permutations each time I used the locks that
there was something anomalous about the way the key pads worked.  If
the code was 6789 you could get in by typing 56789, or by typing
456789 etc.

Clearly this gives you almost a 4 x reduction in the search space.

So I hacked up some code to compute the minimal universal door entry
sequence number.  Joy of joys the "universal code" as it was dubbed
fitted easily on 1/2 a sheet of A4 paper at 66 lines by 80 characters
per line, and we figured (myself and entertained colleagues) with
semi-covert experimentation that with a bit of practice you could
break a lock in around 10 mins manually or something like that.  It
looked quite esthetically pleasing also.

The sequence looked something like this:

01234567890124568902346780...

which would try combinations in this sequence:

0123
1234
2345
3456
4567
5678
6789
7890
8901
9012
0124
1245
2456
4568
5689
6890
8902
9023
0234
2346
3467
4678
6780
...

I was not able to do it quite the theoretical minimal number of
permutations of 2503, but it was only 3 or 4 extra digits I think.  I
am not sure if you could do better than this, but it was a
computational trade off, my algorithm was recursive, and back tracked
as it was; I just chopped it off and demanded best solution after 1
hours CPU or whatever.

I might have the universal code and source code around somewhere,
can't lay my hands on it right now.

We dubbed this sheet of paper the "universal door code", and
considered pinning it beside the lock :-)

These locks looked pretty cheap, and didn't suffer the mechanical
failure problem you describe, we probably gave them enough stress
testing in our semi-covert experiments.

Also it occurs to me that a duty cycle of 10k operations isn't that
high.  I am left wondering if perhaps you were pushing the units too
hard -- would you have been able to break the lock before mechanical
failure if you had slowed the rotation rate of your mechanical dial
turner?  Also, perhaps your mechanical setup was too rough, putting
abnormal strain through clunky motion or something?


Also, in fact these 0-9 digit 4 digit locks at [x] I think had other
problems also, under certain circumstances you were able to type a
spurious digit between the digits of the 4 digit code (eg code is
1234, typing 15234 would let you in!)  If we had been able to find a
rule governing this behaviour, an even shorter universal code would
have been obtainable.  However it seemed erratic, and interest waned
around this point somewhere.

They also had a few hex versions with 0-9A-F and 4 digits, which I
also calculated a universal code for.  They seemed not vulnerable to
the spurious digit flaw described in the above paragraph of the 0-9
units.

Adam
-- 
Now officially an EAR violation...
Have *you* exported RSA today? --> http://www.dcs.ex.ac.uk/~aba/rsa/

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