Secret Message Decoder with Ratchet (Caesar cipher)

This is a toy cipher machine so kids can exchange secret messages. It’s similar to the popular radio promotional toys from the 1930s. The two disks have a ratchet mechanism to keep the inner disk from slipping after you’ve set it, making the toy easier to use.

I've put a demonstration video up at https://youtu.be/DDKSZtagUIQ

To encrypt a message, first agree with your friend on a key letter, for example ‘E’, then turn the inner disk so the inner letter ‘A’ lines up with the outer, key letter (‘E’). Then read each message letter on the inner disk and write down the corresponding secret letter from the outer disk. For example, when the inner letter ‘A’ is aligned with the outer letter ‘E’, the message “HAPPY BIRTHDAY!” becomes the secret message “LETTC FMVXLHEC!”

To decrypt a message, turn the inner disk as before, so the inner letter ‘A’ aligns with the outer, key letter (for example, ‘E’). Then read each secret letter on the outer disk and write down the corresponding message