The skytale is a very simple mechanical cipher system developed by the Spartans during the 5th century. Unlike any of the cipher systems discussed thus far, the Skytale's ciphertext was unique in that it could not be transmitted without the plaintext being compromised. The skytale created ciphertext on which had to be transported by messenger. This type of cryptography is unsuitable for frequent use in the 20th century where everything is transmitted as text but, back in the 5th century when messengers delivered all of the messages, the Skytale system worked well.

So, what is the skytale system? It's a cylinder and a strip of paper.

The strip of paper is wound around the cylinder so that it is tight. The message is then written on the paper, but not following the length of the strip, but written following the length of the cylinder. When the edge of the cylinder is reached, the message can be continued on the next line of the strip of paper.

Once the message is complete, the strip of paper should be removed. Because the message was written on the paper while it was wound on the cylinder, it appears unintelligible when taken off the cylinder and looks like just a jumble of letters.

The message strip can then be sent via messenger to the reciever. The text of the message can't be sent via telegraph or radio, though, because the skytale's encoding method relies soley on how the letters were placed upon the paper. Reading the letters off of the paper will make it impossible to decipher (at least in the method intended.)

When the message reaches the recipient, they decode it using a cylinder which is identical to the one used to encode it. By wrapping the message around it, the contents of the message become clear. The theory behind the security of the skytale is that only another with a staff of the same diameter will be able to read the message.

It's fairly easy to determine what size cylinder to use, though.