Introduction: Cardboard Cryptex Safe!

About: I'm a highschooler who is interested in technology, science, and engineering. In my spare time I work on projects that allow me to learn new skills and concepts.

A cryptex is a portable vault used to hide messages and valuables. Most cryptexes are built from robust materials such as metal and wood. However, a cryptex can also be built out of cardboard. A cardboard cryptex is easier to build than a metal or wooden one. Obviously, a cardboard cryptex is just for show and is not intended to be used as a real safe, but it is a cool craft to make, and it will show off one's gluing skills!

Step 1: A Video Overview

Here's a short little video that gives you an idea of how to build the cryptex. I also have more videos on my blog @

Step 2: Tools and Materials


  • Cardboard (about 1mm thick, I used a Special K Cereal box)
  • Two cardboard tubes (toilet paper tubes, it's not disgusting when its used for crafts!)
  • Slightly bendy cardboard (I used a Cheerios box)
  • Paper
  • Hot glue sticks (2-3)


  • Glue stick
  • Hot glue gun
  • Compass (The one that draws circles)
  • Scissors
  • Exact-o knife or similar (dollar stores have cheap alternatives)
  • Box cutter

Step 3: Cutting the Cardboard

We're going to start this project off by cutting some cardboard circles:

The following dimensions are diameters. The radii are bracketed and underlined.


  • 2 80mm (40mm radius) circles

  • 4 80mm (40mm radius) circles with 45mm (22.5mm radius) holes

  • 4 79mm (39.5mm radius) circles with 45mm (22.5mm radius) holes

  • 4 80mm (40mm radius) circles with 65mm (32.5mm radius) holes

  • 4 62mm (31mm radius) circles with 45mm (22.5mm radius) holes

  • 1 80mm (40mm radius) circle with a 45mm (22.5mm radius) hole

  • 1 80mm (40mm radius) circle with a 44mm (22mm radius) hole

  • 1 43mm (21.5mm radius) circle

  • 1 42mm (21mm radius) circle

Be sure to mark a center hole on the circles (with the compass) as they are needed later.

Some circles need to have holes inside them. I found it best to go over the outside of the hole with an Exact-o knife first, then finish it off with the box cutter.

We are also going to need to cut some cardboard strips out of slightly flexible cardboard. The length for each strip is 251.3mm, but I opted to trim each strip after I glued it on.


  • 2 25mm wide strips
  • 4 20mm wide strips

We will also need several small rectangles for the teeth of the key:


  • 8 13mm x 9mm rectangles

Step 4: Constructing the Key

The key of the cryptex is the part that slides into the base of the cryptex. It is also the part that has teeth (adding the teeth will come later).

Here are the cardboard parts to make the key:

  • 80mm circle
  • 80mm circle with 44mm hole
  • 42mm circle
  • 25mm strip
  • 114mm tube

First start with the 80mm circle. Take the back side (the part with any text or pictures) and use your compass to draw a 42mm circle in the center. Hot glue the 42mm cardboard circle onto the back of the 80mm circle where you have just drawn the same size circle.

Take a cardboard tube and mark 25mm from the bottom. Then fit and glue is snugly over the 42mm circle that is glued to the 80mm one.

Then, glue the 80mm circle with the 44mm hole on to the 25mm mark you made on the cardboard tube.

Lastly, wrap the 25mm wide strip around the two 80mm circles and glue it on.

Step 5: Building the Base

The base is very similar to the key of the cryptex.

Here are the parts you will need:

  • 80mm circle
  • 80mm circle with 45mm hole
  • 43mm circle
  • 25mm strip
  • 114mm tube with 10cm slot

Just like the key, center the 43mm circle onto the backside of the 80mm circle and glue it on. Use a ruler and cut a slot parallel to the tube. Cut a bit off the tube (~5 mm) so you end up with a 10mm gap in your tube. The tube should fit smoothly over the key of the cryptex.

Fit the 80mm circle with the 45mm hole onto the tube and sand the hole until the tube fits with no problems. Glue the ring 25mm from the bottom of the tube.

Then fit the tube over the 43mm circle that is glued onto the 80mm circle and glue the tube on. As before, cover up the 25mm gap with a 25mm strip of cardboard.

Step 6: Constructing the Rings and Spacers

The rings are constructed from three circles and each come with a spacer.

Here are the parts to building four rings:

  • 4 62mm circles with 45mm holes (spacer, notched)
  • 4 79mm circles with 45mm holes (notched)
  • 4 80mm circles with 45mm holes (notched)
  • 4 80mm circles with 65mm holes
  • 4 20mm strips

The notched parts have 10x10mm notches cut out which line up once the ring is constructed.

Start by gluing the 80mm circle with 65mm hole on to the 79mm circle with the 45mm hole. This outer circle is used to hide the spacer. Cut out the 10x10mm notch that is illustrated in the photos above. Do this step four times.

Use the circles you just built to trace and cut out notches from the 80mm circles with 45mm holes. Glue the 20mm strips around the 79mm circles. Then, glue the 80mm circles with 45mm holes onto the other side of the strips, making sure the notches are lined up. This gives you a completed ring.

The spacers are the 62mm circles with 45mm holes and they also have notches cut out. They will be glued onto the base and will fit within the 80mm circles with the 65mm holes. That way, the rings can turn, but the spacers will keep them from rubbing up too much.

Three of the four spacers are optional! The only one you really need is the outermost one, however, adding all 4 spacers would mean less friction between the rings.

Step 7: Adding Teeth to the Key

The key will have teeth that will lock onto the base of the cryptex if any of the rings are rotated the wrong way.

You will need 8 13x9mm cardboard rectangles to build the 4 teeth.

Take your ring and mark out a 9mm line parallel to the tube of the key. Then put the rings and spacers onto the base in the order of ring, spacer, ring, and so on... Lining up the key with the base would give you the location to put the keys. They keys go into the inside area of the rings, the edges that are closer to the key. Take a look at the pictures to give you an idea. The key should be able to slide in with the teeth, but the teeth should not be crushed by the rings or spacers.

Glue the teeth in two layers, adding one rectangle first, then the second one on top.

Step 8: Assembly

Now it is time to put everything together! Make sure the rings and the key fit smoothly onto the base, if they don't, sand them down with some sandpaper (this creates a nice finish on the edges as well!). Be sure to mark out where the notches are on the rings, or you will have a hard time opening the cryptex later!

Take the base and put on a ring (spacer side up). Then put a spacer and glue it on. Repeat this pattern until you have four rings and four spacers on the base. The rings should spin freely; they should also have a spacer keeping the last ring from falling off. They key should slide in easily as well. Putting the key in and messing around the letters would lock the cryptex and it should not open until the letters are returned to where the notches match the teeth on the key. In essence, the cryptex is finished!

Step 9: Decorating With Paper

I added some paper strips to cover up the rings, base, and key. I added the letters before I cut out the paper, but after I applied it to the cryptex, I discovered that the pencil-drawn letters were barely visible. Highlighting them with a red sharpie did the trick.

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Epilog Challenge VI

Participated in the
Epilog Challenge VI