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In , a scytale (; also transliterated skytale, skutálē "baton, cylinder", also σκύταλον]] skútalon) is a used to perform a transposition cipher, consisting of a cylinder with a strip of parchment wound around it on which is written a message. The , and the in particular, are said to have used this cipher to communicate during campaigns.

The recipient uses a rod of the same on which the parchment is wrapped to read the message.


Encrypting
Suppose the rod allows one to write four letters around in a circle and five letters down the side of it. The could be: "I am hurt very badly help".

To encrypt, one simply writes across the leather:

_____________________________________________________________
     
      |   |   |   |   |   |  |
      | I | a | m | h | u |  |
    __| r | t | v | e | r |__|
   |  | y | b | a | d | l |
   |  | y | h | e | l | p |
   |  |   |   |   |   |   |
     
_____________________________________________________________

so the becomes, "Iryyatbhmvaehedlurlp" after unwinding.


Decrypting
To decrypt, all one must do is wrap the leather strip around the rod and read across. An example ciphertext is: "Iryyatbhmvaehedlurlp". Every fourth letter will appear on the same line. After the re-insertion of spaces, the plaintext becomes: "I am hurt very badly help".


History
From indirect evidence, the scytale was first mentioned by the Greek poet , who lived in the 7th century BC. Other Greek and Roman writers during the following centuries also mentioned it; however, it was not until Apollonius of Rhodes (middle of the 3rd century BC) that a clear indication of its use as a cryptographic device appeared. A description of how it operated is not known from before (50–120 AD):

Due to difficulties in reconciling the description of Plutarch with the earlier accounts, and circumstantial evidence such as the cryptographic weakness of the device, several authors have suggested that the scytale was used for conveying messages in and that Plutarch's description is mythological.


Message authentication hypothesis
An alternative hypothesis is that the scytale was used for message authentication rather than encryption.
(1999). 9780472110643, U. Michigan Press.
Only if the sender wrote the message around a scytale of the same diameter as the receiver's would the receiver be able to read it. It would therefore be difficult for enemy spies to inject false messages into the communication between two commanders.

Nonetheless, any person intercepting a scytale message, and having heard about the method, could with little difficulty find out the rod size needed (a kind of brute-force attack); once knowing that, it would be easy to supplant the sender and forge new messages.


See also


Further reading
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