Instructables

Anniversary cabinet with a wooden combination lock

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Picture of Anniversary cabinet with a wooden combination lock
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.
 
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Step 1: Video showing how the combination locking system works on the anniversary cabinet

Step 2: Some stages of cabinet construction

Picture of Some stages of cabinet construction
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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. 

Step 3: Making the lock rod plates

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Each drawer has a "lock rod plate" that mounts with two screws to the drawer. The drill press and the table saw (with a sled) make this operation easy.  I used scrap wood spacers to keep the spacing between openings consistent.
churusaa8 months ago
There are 128 possible combinations for each drawer based on a little simple binary math. Because each rod has a position where it is correct (1), and its opposite (0), each rod's unlocked position can be said to represent a binary bit. The total number of working combinations can be worked out by determining what the largest number is that could be represented by the number of binary bits corresponding to the number of dowels. The range here is 0000000 to 1111111, with the possibilities in between being variants such as 1011010 or 1000001, and so-on, with each rod being aligned either as intended or in the reversed position.

*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!
nlinventor (author)  churusaa8 months ago
Hi churusaa - Wow, thanks for the work on figuring this out! As you can see from others there is disagreement in the final calculated outcome and approach to solving the problem. Not being into this kind of determination (not a clue) I appreciate all input that results in some answer as to how many possible combinations are involved to open one of the drawers. If you have access to others with like abilities perhaps you might pass the page link on to them to see if your particular calculations can be verified. If you do get a chance to do this please let me know. Thanks again.
tseay9 months ago
Thanks for sharing certainly inspirational. Enter in a contest here.
Possible improvement a hutch or a fire box, (Lord forbid that someone would ever need it.)
nlinventor (author)  tseay9 months ago
Thanks tseay - I entered an Instructibles contest some time ago and won first place. Got real nice drill and impact driver as a prize.
This is ingenious. I love the design and the thought that went into it. The lock mechanism is a work of art. Did you design the lock yourself or is it based off of a model? Either way, the details you put into it are incredible.
nlinventor (author)  Haunted Spider2 years ago
Haunted Spider - Yes I designed the locking system myself and this is the second generation of the idea. The first cabinet (a totally different cabinet design as it was a jewellry cabinet) worked fine but it was easier to "break the code" as it didn't have the secondary locking system (closing the lock rod cover to enable opening the drawers). Thanks for the comments!
tseay nlinventor9 months ago
I was thinking this would be a good jewelry cabinet. It seems like this might be a historical idea and I love it. But if this is an original idea then filing a patent is fairly simple to do. Since you likely already have the drawings that define your design. Just stamp a patent pending on your documents and make copies and file. This is awesome.
gtramp1 year ago
Love it, so many little details, very skilled and thoughtful
nlinventor (author)  gtramp1 year ago
gtramp - Thanks!
kaway272 years ago
Great ideas and nice design. If you have numbers instead of little dots on each node, it would be easier for you to remember the combination.
nlinventor (author)  kaway272 years ago
Right now I refer to points of the compass when I am thinking or talking about the different settings. But numbers would definitely be easier.
mrmath2 years ago
Assuming 8 positions for each "flat", there would be 8 possible combinations if you had only one locking bar.

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.
1048576, because the exact opposite position also opens it. Divide by 2 :)
actually it would be 823,543

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
You're treating the two separate opens into one. You could do the same with the ones that don't open, saying that they are just.

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.
so that would give you 1,048,576 combinations and 128 possible solutions?

your gunna need more drawers Nlinventor :)
nlinventor (author)  Lorddrake2 years ago
Interesting math discussion - will wait in case there is a new number before I setup to make a zillion drawers :)
Good catch. If the cutouts were offset, you would have the 2+ million, but with the offsets centered, you do have to divide by two, and you end up with your number.
nlinventor (author)  mrmath2 years ago
Like the 2 million plus but like infinity even better! Thanks!
You're welcome!
Lorddrake2 years ago
5 stars

amazing concept
nlinventor (author)  Lorddrake2 years ago
Thank you very much Lorddrake. Really appreciate it.
meralgia2 years ago
how will you prevent the recipient of the cabinet from watching the video? if you pause it just so, it would be pretty easy to steal the combinations ahead of time.
nlinventor (author)  meralgia2 years ago
They already knew all combinations before I did the video and instructables. It's hard to keep a secret!
poofrabbit2 years ago
This is very cool, since you have a secret compartment you might think about trying to enter it into the spy contest too! Wish I had wood tools, this is very very cool and I would love one! Nice idea, execution and instructable! 5 Stars!
nlinventor (author)  poofrabbit2 years ago
I'm posting pictures of the three secret compartments on the actual wedding anniversary - today! All three compartments have false bottoms. The center one is accessed using a magnet or magnetic material. The left and right hand false bottoms are raised by pressing down on the back of the false bottoms. I just posted a video on youtube showing all of this. You can view it here: http://youtu.be/OAZMn_OmYuw
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nlinventor (author)  nlinventor2 years ago
BUT WAIT, THERE'S MORE!

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.
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nlinventor (author)  poofrabbit2 years ago
Thanks poofrabbit! Also thanks for the suggestion re entering into the spy contest. Do you know the process to do this for my project... what I'm wondering is should I modify the current entry to add the secret compartments (I thought I read somewhere that you are not allowed to add photos/videos after the project is published?) or would I have to do a completely new instructable showing the secret compartments?
Servelan2 years ago
Lovely cabinet - worthy of being passed down. Might be handy to document the combination somehow on the piece, but ordinarily hidden so that it doesn't become a variant of The Musgrave Ritual: 'Whose was it?' 'Grandpa's' ... 'How was it opened?' 'First the second, then the fourth, then six and three'. : )
nlinventor (author)  Servelan2 years ago
Good point. Not shown in the instructable are three separate "secret" compartments - maybe put the combinations there - oh yeah, probably not a good idea :)
mrmath2 years ago
Gorgeous would work, and an evil, evil idea! :)
nlinventor (author)  mrmath2 years ago
Thanks mrmath - I didn't mean to be evil though :). With your user name maybe you can answer the question re the number of combinations (no pressure though).
I just posted the answer as a new comment, and can't delete it for some reason. But it's there for you.