Analyzing the "A Christmas Story" Decoder Pin
2017-12-25 -  10:10
I'm pretty sure just about everybody remembers the decoder pin scene from A Christmas Story where Ralphie uses his newly received the Little Orphan Annie Secret Society decoder pin to decode a secret message stated over the radio show. That decoder pin is mentioned over and over again in many introductory cryptography classes, but I haven't been able to find any resources that show how the decoder pin worked and how the numbers or letters related with themselves.
Figuring out How the Decoder Pin Worked
In the encoded secret message over the radio consisted of only numbers, as shown in the image below. That secret message decodes to BESURETODRINKYOUROVALTINE
This allows us to assume that each letter maps to a number in a simple substitution cypher. The numbers are a bit difficult to read, but we can tell that the first 2 numbers are 12 and 11, which map to B and E. This tells us that the letters are not going to map up in order with the letters, so we need to figure out what each letter maps to, which is going to require figuring out at least 25 of the letters (the 26th letter can be assumed due to process of elimination if we have 25 letters)
The first time we see the letters and numbers in this scene, we can see that the numbers increment up in order, so 8 9 10 11 12 13 14 15. The letters are not in order though. Because of this, it's safe to assume that only the letters will be out of order and that the numbers will go from 1 to 26, then wrap back to 1.
The letters that can be seen are A C E B G H and what looks to be an F, but we'll try to find another resource to confirm that.
At the moment, we know the letter order is ????????ACEBGH????????????.
In this screenshot, we see the letters W, either an N or an M, O, R, P, and Q. Because we can't tell if that is an N or an M, we'll try finding a different resource to confirm which letter it is.
The letter order so far is now Q???????ACEBGH???????W?ORP.
Note that the pin clearly states 1940 on the back. This will be important later in this post.
The letters in this screenshot are the Q from the last screehot, S, U, T, V, Y, Z, what appears to be a Z (we'll find another resource to confirm that), and the A from the first screenshot.
This means that the letters we know know are QSUTVYZ?ACEBGH???????W?ORP.
This screenshot confirms that the letter from the last screenshot was an X, so now we know that the letters are QSUTVYZXACEBGH???????W?ORP.
In this screenshot, we see that the letter from the second screenshot was an N, while also seeing the new letters M and K.
After this screenshot, we know have the letters QSUTVYZXACEBGH?????MKWNORP.
At this point, the scene does not show any more letters, but we need to figure out at least 5 of the 6 remaining letters. We saw in the second screenshot that the year of this decoder pin was 1940, which would set the movie in the Christmas Season of 1940. That allows me to try and find pictures to figure out the remaining 6 letters.
After a lot of searching, I finally found the image below.
This image actually had all 6 missing letters on it, F, D, J, I, and then L.
With this last image, the letters in order are QSUTVYZXACEBGHFDJILMKWNORP.
Ralphie Is Really Bad at Listening
There are a lot of things the radio announcer says that Ralphie fails to do or write down correctly. To begin with, the announcer tells listeners to set their pins to B-2, but Ralphie ends up setting his pin to B-12.
Another problem is that Ralphie didn't end up writing a lot of the correct numbers, as the announcer says the numbers 14 and 8, which don't appear in the final code Ralphie writes down. There was also only one 12, which decodes to B in the code Ralphie wrote down, but 12 was said twice by the announcer. The numbers Ralphie writes down are also not in the order the announcer says them, for instance, 18 is announced before 16, but Ralphie didn't write down 16 at any point after writing down 18.
Ralphie is completely at fault for getting a "crummy commercial", as he messed up in all of the right ways to get the numbers 12 11 2 3 25 11 4 24 16 25 18 23 21 6 24 3 25 24 5 9 19 4 18 23 11, which would only decode to BESURETODRINKYOUROVALTINE if Ralphie sets the pin to B-12 instead of B-2.
Final Letter order
We can now encode and decode our own messages with the letter order we figured out, QSUTVYZXACEBGHFDJILMKWNORP.