Three Monkeys, Three Typewriters, Two Days

February 3, 2004

Locks and the appearance of security

The gym here has these "heavy-duty theft-resistant" locks they let you use to lock lockers (amazing concept, huh?). I was playing with one of them today. It's a basic combination lock—single dial that goes from 0 to 39, 3-number combination (X, Y, Z), and the following steps to open:

  1. Start at 0.
  2. Turn to the right one full circle and then to X.
  3. Turn to the left one full circle and then to Y.
  4. Turn to the right to Z.
  5. Turn to the left to 0.

When step 5 is done the lock opens.

The first problem is that if you replace step 4 with

  1. Turn to the right to any place past Z and before Y.

the lock still opens when step 5 is done. In my case Z and Y were "34" and "0" respectively, so it's not like they were close together....

The second problem is that the lock makes a nice loud click when Z is reached in step 4 (which admittedly makes the first problem rather academic).

I have to admit that these look more difficult to saw through than typical high-school locker locks, but from a pure codebreaking standpoint they are much easier to open (only about 400 possible combinations, since there is no need to guess Z; on a lock on which all three numbers need to be guessed there are more like 8000 combinations).

Posted by bzbarsky at February 3, 2004 10:49 PM