Asymmetric (Dual Key) Lock

How the Lock Works

The lock plate has three positions. The center position is unlocked, while both left and right are locked. One key can only turn the lock ring clockwise, and the other key con only turn the lock ring anti-clockwise. Either key can be used to first lock the box. Once the box is closed and turned with one key, that key will no longer do anything else. Only the other key can turn the lock ring back to center to open the box. (Turning further will lock the box in the other direction.)

While an interesting mechanism on its own, I designed this lock to demonstrate asymmetric-key cryptography to students. By putting different messages in the box, it is also possible to illustrate message signing, certificate exchange, and other asymmetric processes.

Printing

Print one of everything except the parts labeled “option”. The box can be built using the Box opton for a fully 3D printed model. Or, the Tube Ring and Tube Cap can be glued onto a 3-inch (75mm) cardboard mailing tube to make a container of any size.

Designed and tested with PLA at 0.20mm layer height. Other materials may make the pawl ratcheting action too strong, requiring adjustment of the pawl finger width.

The Cover can be laser-cut from acrylic or printed in a transparent material to show the mechanism.

Other Materials Required

• four 3mm dia. x 2mm high rare earth magnets
• six #2 x ¼" (or M2.5 x 6mm) wood or sheet metal screws

Assembly

Press three rare earth magnets into the Baseplate. Make sure that the poles of the magnets are all facing the same direction. Press the remaining magnet into the Latch Ring with the poles in the opposite direction so that it is attracted to each of the magnets in the Baseplate. Place the Latch Ring into the Baseplate with its magnet over the center magnet in the Baseplate.

Place the Lower Pawl and then the Upper Pawl in the center. The raised disks on each one go downward and make sure that the fingers of the two pawls are facing in opposite directions.

Check for proper operation using the keys, then fasten the Cover onto the Baseplate using the six screws.

If using the cardboard tube option, glue the Tube Ring and Tube Cover onto opposite ends of the cardboard tube.

Further Work

Additional pawl sets using shapes other than a hexagon can be made for additional locks. The only requirement is that the lower key must be able to turn freely in the upper pawl.

I would like to design a version that does not require the magnets but relies on a printed plastic detent and catch. However, the action is delicate, since too little resistance will allow the key to work in the backwards direction and too much resistance can make it hard to stop in the center position.