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The ring was a plastic ring that you could slip on your finger. On the top of the ring were two circles, one inside the other, with the letters of the alphabet printed on them. The inner circle was fixed, but you could rotate the outer circle so that the letters on it would line up with the letters on the inner circle in different ways. The face of this ring looked like this:
Besides the secret message, we were also given the weekly secret setting for the ring. For instance, one week we would be told to rotate the outer circle until the letter M on the outer circle was next to the J on the inner circle. After the ring was set, we figured out the secret message by finding each letter of the secret message on the outer ring and replace it by the corresponding letter on the inner ring.
It was also possible for two junior space cadets to write secret messages to each other. All they had to do was to agree on a secret setting for their messages, such as "put the A on the outer circle next to the K on the inner circle" and keep this private information secret. To write a secret message to each other, they first set the ring with the secret setting, then taking each letter of the message they want to send (this is called the clear text) they find the letter on the inner circle and replace it with the corresponding letter on the outer circle. To read the secret message, the ring is set to the secret setting, then every letter of secret message is found on the outer circle and replaced by the corresponding letter on the inner circle, just as was done with the messages from the TV show.
These decoder rings are now hard to find, they have become collector's items. But, every once in a while you can still get one as a toy in a cereal box or a Cracker Jack box. However, we don't have to have secret decoder rings since we can use clock arithmetic to do the same thing that the decoder rings did.
Since we are going to use clock arithmetic, we need to convert the letters to numbers. We will always use the following table, both to convert letters to numbers and numbers back to letters (we will use this table often, so you might want to make your own copy of it):
In order to figure out a secret message, when we know the secret number, we first convert the letters to numbers and then subtract the secret number mod 26, and finally convert back to letters. For example, if the secret number is 5, then the secret message DTZ FWJ XRFWY is figured out this way: "D" = 3, 3-5 mod 26 = -2 mod 26 = 24 ="Y", "T" = 19, 19-5 mod 26 = 14 = "O", "Z" = 25, 25 -5 mod 26 = 20 = "U", "F" = 5, 5-5 mod 26 = 0 = "A", "W" = 22, 22-5 mod 26 = 17 = "R", "J" = 9, 9-5 mod 26 = 4 = "E". So, our secret message starts out with YOU ARE ... now, figure out the rest of this secret message.
You can now write secret messages to your friends, all you have to do is pick a different secret number to share with each friend. In the next section we'll see how to use more clock arithmetic to make an even better secret decoder ring (without the ring).
The message is, "DRINK BOSCO ITS GOOD FOR YOU".
I should explain: Bosco was the brand name of a chocolate malted powder that you mixed with milk to get a chocolate milk drink. Bosco was the sponsor of the Captain Midnight and the Space Cadets show. So, this secret message was really a commercial.
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