Christmas cryptography: In “A Christmas Story”, Ralphie is shown with what appears to be a 1940 edition of the official Orphan Annie decoder badge, which would be appropriate for the time in which the film is set. The prop used appears to be an original badge. 1/

I know this because I have one in my collection (Enigma machines being too rich for me). Unfortunately, this is where things go a bit off the rails, cryptographically speaking. 2/

These decoder badges had two rings, one with the numbers 1-26, and the other with a permutation of the 26 letters of the alphabet. The badge could be set to one of 26 offsets (a keyspace of under 5 bits) by rotating one ring with respect to the other. 3/

The key could be denoted by specifying a number and its corresponding letter, revealing a static monoalphabetic substitution. Ciphertext would be numeric , plantext alphabetic. On the Orphan Annie radio show, keys would be given prior to reading the plaintext numbers. 4/

This is, as any cryptographer will recognize, a terrible practice. The system only has a 5 bit key to start with, but that security is immediately negated by revealing the key! Fortunately, Orphan Annie’s adversaries were apparently dumb as rocks and never caught on. Anyway, 5/

The permutation would be changed with each year’s badge. This was presumably intended to force you to send in new boxtops each year if you wanted to keep decoding messages. (Although you could still cryptanalyze message by frequency analysis). But I digress. 6/

In the film, Ralphie is shown excitedly transcribing his first message. He is told the key is “B-2”, instructing him to set the badge so B on the letter ring lines up with the digit 2. Then the plaintext is read out: 12, 11, 2, 8, 25, 14, 11, 18, 16, 23, 12, 23, 21, 3, 25. 7/

That’s a 15 character message (traffic analysis!). Now, it’s possible that we are intended, through the magic of film grammar, to assume some time passes between the beginning and the end of the message, but it definitely starts 12, 11, 2, 8 and ends with 21, 3, 25. 8/

The punchline of the scene is that the message turns out to be “BESURETODRINKYOUROVALTINE”, and Ralphie learns hard lessons in both cryptography and capitalism. But something else is wrong here. 9/

Even assuming missing characters in the middle, we know that this couldn’t possible have been the plaintext. Remember, the key is B-2. So “BESURE…” would have to start with 2, not 12. So maybe this was a small script typo, and the key was supposed to be B-12”. But… 10/

I dutifully set my decoder badge to B-12. That yielded a decoded message of BESXRHEIDNBNKUR. At least it starts with BES, but then it just gets horribly lost. And we know that since E=11 here, the ciphertext would have to have ended with 11. It’s all just wrong. 11/

Using the codes from badges from other years works no better (I said I had a collection, remember?) It’s as if Shep just gave up after the first three characters.

I’m sorry if I’ve ruined a beloved holiday film for you. But truth matters.

Happy Holidays. 12/12

I said “plaintext” when I plainly meant “ciphertext” when referring to the numeric message above. But you knew that. In any case, the Radio Orphan Annie cipher isn’t very secure. Don’t use it to protect important secrets.