Build Some Secret Coding Devices

People have tried to keep their messages secret from prying eyes for thousands of years. Try building two secret coding and decoding devices from the past.

The Spartan Scytale:

The ancient Greeks used this method to pass messages. They wound a belt on an angle around a rod so that its edges barely touched. Then they wrote the message along the length of the rod on the belt. After the belt was unwound from the rod, the message could not be read. Then a messenger would wear the belt and travel to the message's destination. There the message could only be read after the messenger took off the belt and it was again wound around a rod of the exact diameter as the first.

Build your own Spartan Scytale.

Step 1: You need a pencil, paper, ruler, tape, scissors, and a rod of some sort. You can use a second pencil as a rod, but the larger of a diameter the rod has, the easier it will be to handle.

Step 2: Use the ruler to mark off a section of the paper that is about 11 inches long by 3/8 of an inch wide. Cut this strip out with the scissors.

Step 3: Use the tape to attach one end of the strip to the rod on an angle as shown in the diagram. Then wrap the paper around the rod so the edges meet.

Step 4: When fully wound, write a message along the length of the rod.

Step 5: Unwind the paper and remove it from the rod. Can you read it now?

Step 6: Wind the paper back around the rod so that the letters line up. Can you decode the message?

The St. Cyr Cipher Device:

The St. Cyr Cipher Device allows you to generate a cipher with a key.

Step 1: You will need papers, scissors, ruler, and a pen or pencil.

Step 2: Cut a piece of paper about a ½ inch wide and 11 inches long.

Step 3: Cut a piece of paper about 2 inches wide and 6 inches long. About a ½ inch from each end, cut slits a little more than a ½ inch wide as shown in the diagram. We will call this part the "mask" and the other piece of paper the "strip."

Step 4: Slip the strip into the mask as shown.

Step 5: On the top of the mask above the strip write the alphabet in normal order as shown. Be careful to space the letters evenly apart over the whole area above the strip.

Step 6: Move the slip so the right half shows through the mask. Mark the alphabet BACKWARDS across the strip so that the A is under the Z, the B is under the Y, etc.

Step 7: Slide the strip to the right until the last letter on the stop is just to the right of the Z on the mask. Mark the alphabet backwards on the strip again.

Step 8: You now have your encryption-decryption device. To start, pick a key to the encryption by placing a letter on the mask over another letter on the slip. These two letters are now the key. Without moving the slip, encipher the message by matching the plain text on the mask to the cipher letters on the strip one by one and writing the cipher letters down.

Step 9: Now try to decrypt the message by reversing the process. Does it work? What happens if you change the key?