Digital Moose Tracks

I saw this lock at Walmart for $8 and couldn’t resist, since its so different than regular combination locks. 

Operation

Operating the lock is amazingly simple and elegant.  Rather than rotating the dial like a traditional lock you push it in the cardinal directions.  This has two big benefits.  Firstly it requires much less dexterity to use then a traditional combination since you only have the be able to push the lock in 4 directions rather than be able to rotate the lock to 39 different angles.  Secondly it allows you to enter even fairly long combinations in under 10 seconds.  The lock is also user settable to any combination you want, in less than 30 seconds.  (Assuming you don’t choose a 37 move combo or something)

How it works

The internals of the lock are truly clever.  I won’t bother going over them since there is this nice pdf that already does that.  And there is this great flash visualizer that shows you how the internals of the lock work in operation.

Security

Because of the way the lock works there are only 7,501 states the internal mechanism can be in.   So although there are lots, 4^8, combinations of length 8 all of them map to one of the 7,501 states.  More troubling however is that a combination of length 8 may produced a state that is the same as a combination of length 4.  The table below summarizes the probable ‘badness’ of combos of different lengths.  It shows that combos of length 7 seem to be the sweet spot.

*Bad combos mean a combination that can be open by another combination shorter than 5 moves.

Since all the combinations of length 4 or less can be tried in about 20 minutes, 249*5sec, having a combination that can be open with a combination of 4 or less opens you up to a brute force attack.  Unfortunately these bad combos are not simple to recognize as such.  For instance the combo up,up,right,up,down,down,left,right is the same as right,right,right,right. 

Conclusion

Although there is a small chance, that you can accidently pick a bad combination for the advertised purpose of protecting a high school locker it should be more than adequate security and the easy of use is much better than a traditional lock.