IMPORTANT: The information in the [Anarchist's Cookbook: Picking Master Locks|Anarchist's Cookbook] on this subject is now [obsolete]. It claims that the new locks are [Making things foolproof results in better fools|foolproof]; the following should disprove that.

GETTING THE LAST NUMBER

The [Master Lock] should have two parts: the body (the part that has the dial on it) and the U-shaped [shackle]. Pull up on the shackle (or down on the body, if it's locked to something), but not all the way. Knowing how much to pull is a matter of [practice]: it has to be enough that this next part will work, but not so much that the dial won't turn.

With the shackle being pulled, turn the dial. It should "stick" in certain places. There should be twelve sticking places; finding all twelve isn't necessary, but it's [practice makes perfect|good practice]. Exactly ONE of the sticking places is the correct last number; the other eleven were put there by the Master Lock corporation to keep you from doing [breaking and entering|exactly what you're trying to do]. How to tell them apart:

Seven sticking points will be between two marks—between 12 and 13, between 34 and 35, whatever. These are all fake.
Four of them will all have the same [digit] in the ones place: 7, 17, 27, and 37 could be four that you find. These are also fake.
One of them won't fit into the above [category|categories], like, say, 30. This is the last number of the combination.

GETTING THE FIRST AND SECOND NUMBERS

Sorry, the skill part's over; it's all [calculation] and [brute force] from here on in. Let's say that (as above) the last number is 30. Everyone remember [modulo] division? No? Go read up on it, it's critical to this next step. Now, the last number modulo 4 is going to be either 0, 1, 2, or three; in this case,

30 mod 4 = 2.

The first number mod 4 equals the last number mod 4. That is, the remainder of the first number when divided by 4 is also going to be 2. Which limits our first number [possibility|possibilities] to:

2,6,10,14,18,22,26,30,34,38.

And the second number mod 4 equals the last number mod 4 [plus or minus] 2. So in our case, the second number mod 4 is 0 (since 2-2=0, and 2+2=0, since we're in mod-4-world). So our second number possibilities are:

0,4,8,12,16,20,24,28,32,36,40.

Which leaves us with (10*10*1)=100 possible combinations, which is a lot, but it's a lot less than the 1500 combinations you'd have otherwise.

A [cautionary] note: Be really, really, really sure that you've got the right last number before you continue. Few things suck more than running through all 100 combinations to discover that you'd been [barking up the wrong tree] the whole time.

Source: http://www.people.fas.harvard.edu/~hillson/master_lock.html

A highly worrying [addendum]: Yesterday (5/23/02), I was working on a lock with only eleven sticking points—all of which were fake. If anyone else finds something like this, please let me know.

Cracking a Master Lock