Decoder Ring

In technical lingo, any device/documentation/etc. which provides a mapping from one thing to another, in order to make something go from cryptic to sensible - for example, a "cheat sheet" which explained the structure of an IP packet might be called an "IP decoder ring". Typically, a "decoder ring" is compact and easy to use; with the mapping conspicuous, obvious, and in one place, and little if any other content. (The various IETF standard describing IpProtocol? would not constitute a decoder ring; the meaning of IP packets being buried in much other verbiage).

Also known as a "magic decoder ring" or "secret decoder ring".

Derives from the following usage in pop-culture:

In America, probably elsewhere, cereal manufacturers used prizes to sell their cereal to children. One of the "canonical prizes" is a secret decoder ring. A "decoder ring" usually implemented a simple CaesarCipher (mapping letters to numbers) with a spinning wheel containing the letter and an outer ring containing the numbers (or the other way around); I've seen both a ring on the finger where the whole thing spins, and a ring with a large top area that contains the wheel. (Technically, this allowed for up to 26 distinct Caeser ciphers.) As you might imagine, these are typically quite clumsy. The manufacturer would then include messages on the box in the given Caesar cipher, and the children would presumably have fun decoding them... except that they were typically only more marketing messages.

Some images of these things can be found at http://home.ca.inter.net/~hagelin/children.html - for instance, see the "Chex" ring.

This was lampooned in ChristmasStoryMovie, wherein the protagonist in the movie, an eight-year-old boy named Ralphie growing up Indiana in the 1940's, sends away for a "secret decoder ring" from a popular radio serial (Little Orphan Annie), and after anxiously waiting the typical six-to-eight weeks, decodes a secret message that says "BE SURE TO DRINK YOUR OVALTINE". ("Ovaltine" is a foul-tasting chocolate additive for milk that was sponsoring the radio serial; in modern times, it is advertised as being good for you due to vitamin addition, and that gee golly, it doesn't taste that bad considering how healthy it is! In other words, its marketed at parents, not kids, a sure sign that kids don't want it in our society.) If you've never seen this movie, it's hilarious. Based on the book "In God We Trust, All Others Pay Cash" by the late Jean Shepherd. It gently skewers numerous AmericanCulturalAssumptions.

This explanation prompted by just such a usage on RevisedTerminology.