The

**video below**shows how the combination locking system works and the

**Steps**following the video give details on the construction of the cabinet and the locking system.

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The cabinet was made as a wedding gift for my son and daughter in law. The cabinet itself follows conventional design and construction techniques - what makes the anniversary cabinet different is the combination locking mechanism. The four drawers require you to know a code (combination) in order to open them. Each drawer has a different combination. The idea then, is to give the combination for one drawer at a time (on the wedding anniversary). To get things going though the combination for the first drawer (the bottom one) is given "free" at the time the gift is presented so that the general operation can be explained and tested. In this case the guests at the wedding reception were given a sheet of paper and an envelope so that they could write a note to the bride and groom. The envelopes were sealed and placed at random in the top three drawers. So on the anniversary when the combination is revealed the contents (envelopes or other gifts) can be accessed. Of course this may not work as intended, as the level of bribery re getting the combination early could come into play.

The**video below** shows how the combination locking system works and the **Steps** following the video give details on the construction of the cabinet and the locking system.

The

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Mortise and tenon joints, pocket holes, and dovetails are the primary joinery methods used in the construction of the cabinet and drawers. But any kind of joinery can be used to make a cabinet/drawer for the purposes of including a combination lock like the one described in this instructable.

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*Since the locked positions of the dowel can be thought of as continuous representations between 0.0 and 1.0, we can ignore them for this calculation, but being able to divide them into discrete chunks will be useful later on.

1111111 is the largest number that can be represented in 7 bits, and that number in decimal is 127. If we include 0000000 (0), which represents all of the dowels aligned as intended, that brings the total up to 128 possible arrangements per drawer. It's not terrible security, actually, considering that an apartment door that has been set up to work on a master key system might accept as many as 35 other keys.

If we assume a fair level of precision and no angling of the cuts that might help to align dowels that are close but not on the money, we get 16 possible positions for each of the seven dials, of which eight are duplicates of their opposite position. That means 8^7 possible positions or 2,097,152 total positions if an attacker knows how the chest is made and is smart enough to only move the rod on an arbitrary 180 degree slice to prevent duplicating their efforts.

In a six drawer system, that means that as long as no two drawers share the exact same dowel position to open, there are 128 x 6 combinations that will open at least one drawer, half of which can be thrown out by an attacker, leaving 64 x 6, or (Remember that 1111111 will open a drawer just as easily as 0000000, so why try both?), or 384 of the 2,097,152 total possible combinations. Assuming this, that means that there are still 2,096,768 combinations that will not open any drawer. If I've carried all my ones and put my decimal points in the right places, that makes the odds of putting in a code at random that opens one of the six drawers 3:16,384 or roughly 1 in 5,461.

If the machining is extremely tight, you could increase the number of possible positions to 32 or even 64, though you might need to use steel or brass rods to achieve this level of precision, and at 64 positions per dial you would increase the number of possible positions to 4,398,046,511,104, or roughly 4.4 trillion total combinations while still only allowing 768 total combinations that would open a drawer at random, or only a single position per drawer if you only notch one side of the rod so that it doesn't operate in opposing positions. Friction locking the rods when the cover is in place would effectively prevent an attacker approximating the correct position and coaxing it in to place with brute force.

Anyone who does mathematics or cryptography for a living is invited to double-check my work, but I think I got it right. A motivated attacker could feel their way from right to left, or left to right, or outside to middle by turning the knobs a half step at a time in pairs until they got some movement, significantly reducing the number of guesses required to get in. This attack could be thwarted by putting each drawer on a guide track with gliders or bearing slides to reduce lateral movement, but might be overkill depending on the actual level of security desired.

One could simply smash the chest or drill out the dowels, but that's not really the point, is it? I hope I answered your question, and I think I got my numbers right. Again, if you're reading this and you're a cryptographer or professional mathematician, feel free to correct anything I've gotten wrong!

Possible improvement a hutch or a fire box, (Lord forbid that someone would ever need it.)

If you had two, you'd have 8 * 8 combinations (8 positions for the second bar for each of the 8 positions for the first bar), or 64 combinations.

if you had three locking bars, you'd have 8 * 8 * 8 combinations (all of the combinations you had for the first two bars would be repeated for each possible spot on the third bar), or 512 combinations.

Continuing on, each bar adds a factor of 8 to the number of combinations, for a total of 8 * 8 * 8 * 8 * 8 * 8 * 8 = 2,097,152 possible combinations.

But, if you take into account that your "flats" don't have to be exactly on the eights, and really wherever you felt like putting them, then you have an infinite number of combinations.

if there are 8 positions on each spindle (not using any positions in between pips)

2 of them will open the lock and 6 will not.

so if we combine the 2 opens into one permutation (since if it is in either position it will open) and take the 6 locked positions that will give us 7 per spindle

7 * 7 * 7 * 7 * 7 * 7 * 7 = 823,543

Let's do a simplified example of two knobs. Assume the first knob opens on 1. That means it would also open on 5, but wouldn't on 2, 3, 4, 6, 7 or 8.

Now the second knob opens on 2, and therefore also 6, but not on 1, 3, 4, 5, 7, or 8.

You have 8 * 8=64 possible combinations of numbers. But only the combinations 1,2; 1,6; 5,2; 5;6 will open it.

We still have 64 combinations. The fact that you have 4 solutions instead of just one, doesn't change the number of combinations.

your gunna need more drawers Nlinventor :)

amazing concept

Lorddrake. Really appreciate it.Not obvious unless you remove a drawer and look at the bottom. Each drawer has a collage of family pictures secured in place with a plexiglass cover.