My 3D printer produces a decoder ring: R TFFC CZKKCE UVMZTV!
In point of fact I let the ring cool too long, and then I had to separate the parts out with a pair of vise grips, which damaged the finish. But the ring does work
A Caesar Cipher is a simple substitution code. Each authorized user carries a ring such as this, which consists of two concentric, freely-spinning rings nested one inside the other. Both the message sender and recipient know that a message sent or enciphered on a given day has all of its letters set a certain number of place values off from the other… Thus, in this example, A of plaintext equals R of enciphered text. There are only 26 letters, no punctuation, and no variations in spelling. It’s a pretty simple code to break, and yet it gives 25 basic codes that can be used in order, or in a randomized order, to prevent casual but unwarranted snooping in information.