So, I was thinking about W2R's childhood story of kids spending hours trying to open a safe with a dial combination lock. Is it feasible? How much time it would take if you did a brute force attack by trying all combinations?
Before answering that question, one must understand how the lock works. Basically, a personal fire safe like my Sentry safe has a lock not unlike the common padlock made by Master Lock. Both employ a set of 3 disks with notches that must be lined up for a lever to engage for unlocking. This is best explained by a youtube video using a wooden model.
To see the actual internal mechanism of a Sentry safe, fast forward to 3:30 in the following video.
The dial has graduations from 0 to 99. However, the slots of the 3 wheels are intentionally cut wider than the intruding lever to allow some slop. A tight tolerance would make the lock too finicky to open, and that would frustrate users. I have found that I can deviate about +-2 from the given combination and the lock still opens.
So, we do not have to try every number for each of the combinations, but perhaps only one every 3 graduation marks. Then we have 33 x 33 x 33 = 35,937 combinations to try.
How long would it take to try 1 of the possibilities? Note that turning the dial for the first 2 combinations numbers takes the longest. That may take perhaps 15 to 30 seconds. Then, one just sequentially tries all 33 possibilities for the last number. That would take perhaps 60 seconds.
Then, the time to try all possibilities would be 33 x 33 x (30 secs + 60 secs) = 98,010 seconds or 27 hours maximum. On the average, it should be half that, or 14 hours.
If the lock can be opened with a slop of +-2 on the dial, then it would take only 25 x 25 x (30 sec + 60 sec) = 56,250 sec or 16 hours max. On the average, that's a mere 8 hours.
A team of youngsters can take turn to open a safe if they approach it systematically. The task is still nowhere long enough to keep them occupied over a summer. One patient kid can tackle this by him/herself even.
