Secret decoder ring

A secret decoder ring (or secret decoder) is a device that allows one to decode a simple substitution cipher—or to encrypt a message by working in the opposite direction.^[1]

As inexpensive toys, secret decoders have often been used as promotional items by retailers, as well as radio and television programs, from the 1930s through to the current day. Decoders, whether badges or rings, are an entertaining way for children to tap into a common fascination with encryption, ciphers, and secret codes, and are used to send hidden messages back and forth to one another.