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During the early years of the United States of America, Thomas Jefferson, then ambassador to France, needed a way of communicating securely back to the U.S.A. He is credited with inventing the "Jefferson Wheel Cipher", a variant of the Vigenere cipher.
For Jefferson's purposes, a set of wooden disks were created. Each disk was put on a common spindle using a hole in the middle of the disks. Around the outer edge of the disk were placed the twenty six characters of the alphabet in a random order. Each disk had on it's face a number so that the disks could be organized in a predetermined order.
To encode a message, you would stack the disks on the spindle in the "correct" order. Then, you would spin the disks around until your plain text appeared all in one row across the face of the device. You would then choose any OTHER row as your encrypted text.
To decode, you would once again stack the disks on the spindle in the predetermined order and spin them around until the encrypted text lines up all in one row across the face of the device. You would then scan all rows by spinning the entire device until you found something that looked like English along one of the other rows. This was your plain text message.
In either case, you would repeat the process for as many times as necessary until the entirety of your message was translated.
This method is still pretty secure, given that no decoder had access to either the disks nor the stacking order. It is quick and easy to implement, and your average child can grasp how to do it.
In this section, I'll describe how you can generate your own set of Jefferson wheels suitable for playing around with your kids. First, you will need to print out this web page and make enough copies so that all of the kids involved will have his or her own copy (a few extras don't hurt).
Then, cut each of the strips out of the printout below and keep the number at the top attached for now.
Then you will need to get a large stack of those little paper/plastic cups that are commonly used by children as they brush their teeth. Dixie, Solo, and many others make the same basic cup. Stack two or three of these together and glue or staple them together.
The slips below should then be hot glued around the rim of the stacked cups and the number at the top of the slip should go inside the cup.
Then, by stacking a given sequence of cups together, you can perform the same basic encoding/decoding scheme as the original Jefferson Wheel Cipher.
Here is a random set of slips, just for you. Enjoy!
Others on the Net
ThinkQuest has an operational example of a M-64 machine, a some what modernized version of Jefferson's original, used in the period between WW1 and WW2.