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Combination Locks

© Copyright 2000, Jim Loy

I was examining the insides of a combination padlock, made by a well-known lock company. The package claimed that the lock had 64000 combinations (only one of which would open the lock). There are 40 numbers on the dial, and the combination is a sequence of three numbers. That is 40x40x40=64000. You go twice around to the right to the first number, once around to the left to the second number, and then directly to the right to the third number. A burgler could probably try 5 or 6 combinations per minute. So he/she could try 64000 combinations is about 200 hours. That is pretty secure.

But, it turns out that you don't have to be accurate with your combination. On the first two numbers, you can be one off. If the number is 10, you can use 11 (or maybe 9) and it will still work. A couple of numbers you can miss on either side, but most you can only miss on one side (high or low). So, a smart burgler can just use even numbers (or just odd numbers), and still open the lock. That is 20x20x20=8000, or about 25 hours. Still secure. It also turns out that the third number can be even less accurate. There are only about 13 or 14 spots on the dial for that third number. But that doesn't matter, because you don't have to have any clue at all with that third number. You can keep experimenting all around the dial until you find the third number. So that is 20x20=400 combinations. It takes quite a bit longer to try one of these two-number combinations (as you have to experiment with the third number), maybe half a minute. So instead of 200 hours, we end up with 200 minutes, or over 3 hours.

I opened one in under an hour. So, I suspected that maybe there were more than one combination that worked. Further experimentation showed that this was not so. Only one combination works; I had been lucky to find it quickly. So our burgler has 3 hours to try all of the combinations. It may be worth the effort, depending on the prize.

Addendum #1:

Inside the lock are three wheels. There are several notches around each wheel. There are especially deep notches which line up when the correct combination is chosen. When these deep notches are lined up, a latch fits into the notches, and permits the lock to open. As you turn the knob to the right, the three notches are not lined up, and all three wheels are eventually turning together. Then when you turn the knob to the left, one of the wheels stops (so its deepest notch is in the right place, if you got the first number of the combination right), and the other two wheels turn to the left. Then when you turn the knob to the right again, another of the wheels stops (hopefully with its deepest notch in the right place), and the third wheel turns to the right. You then stop at the third number of the combination, and all three notches are lined up in the right place, and the lock can be opened.

Normally, I have not been able to pick such a combination lock, except to try all 400 combinations (200 on average, as normally you guess the right combination long before you have tried them all). Some of these locks (maybe worn out ones) seem to give a hint about one of the numbers of the combination, by stopping at that number as you turn the dial while trying to pull open the lock.

The lock that I tore apart was a Master Lock. I am told that other brands of combination padlocks are even easier to "pick." By the way, there are sounds inside the lock, clicks. These happen when one turning wheel starts to turn another wheel. When I was in the Army, I watched a safe cracker crack a combination lock on a walkin safe. He listened to clicks as he fiddled with the dial. That should be possible with combination padlocks, I suppose. But I have not figured out how the clicks relate to the combination.

Addendum #2:

Here is another kind of combination lock (it is unlocked, in the diagram), which is often used for bicycle locks. Here I have spread the wheels out, so you can get a better view. Normally, they are very close together. When locked, the wheels are in different positions, and the rod (seen to the right) cannot be pulled out of the lock. Here we probably have 10x10x10x10=10000 combinations. Except that when you have three of the numbers right, you can experiment with the fourth number until you get it. So, you essentially only have 1000 combinations, still quit a few. But some of these locks are not machined very well. So you can turn one of the wheels until the lock gives slightly, then you can turn another wheel until it gives slightly, etc. You just have to figure out which of the wheels to try first. You can do that with some experimentation. Anyway, the really good locks of this type are well machined, and they all give a little at every number, even the wrong numbers. So they can be hard to pick.

Addendum #3:

On the left is a drawing of a key and its lock. The key fits inside a cylinder which can turn when the right key is used. Here we see five little precision tooled tumblers (in shades of gray) which are pushed down by springs. Each tumbler is in two pieces (shown in light gray and dark gray). When the correct key is in the lock, the two pieces of all of the tumblers line up so the cylinder can turn. If the wrong key is used, or if no key is used, then some of the cylinders are too low or too high, and the cylinder cannot turn. This is just a combination lock, and the correct combination is the heights of the bumps and depressions on the key. Key locks seem to generally have fewer combinations than other locks. But, the various combinations are hard to try, because they are inside a hole.

In order to pick such a lock, you need lock pick equipment (illegal to have without a permit in some places) or two paper clips bent to the right shape. Some locks are easier to pick than others, probably depending on how well machined they are. I could pick my desk drawer lock in about 15 seconds, using paper clips. One paper clip turns the lock. The other pushes the tumblers up, one at a time. When the lock is turned with a constant force, perfectly machined tumblers would all become difficult to move simultaneously. But in reality, one or two will move up with difficulty. You can move that one up to the right position, when the lock will turn slightly, and the next tumbler will now move up with difficulty. This lock picking procedure takes patience, and a delicate touch.

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