The Ancient Greeks are famous for the gods, their cultural and sporting events like the olympics and their philosophers. They were also amazing inventors. One very simple Ancient Greek invention is the Scytale. It is a simple encryption device for quickly writing secret messages.
The Scytale is a clever way to do computation, using physical properties of the world instead of having to do calculation or symbol manipulation in your head. It is an early device All you need is a ribbon (or long strip of paper) a cylinder and a pen that writes on the ribbon.
How to encrypt a message using a Scytale
Fasten the end of the ribbon to one end of the cylinder with tape. Wrap the ribbon tightly in a spiral round the cylinder, so that it covers the whole cylinder, with no overlap. Stick down the end of the ribbon at the end of the tube.
Now write the message along the tube starting at the beginning of the tape, with each letter on the next section of the tape. When you get to the end of the cylinder, turn the cylinder (eg a quarter turn) and carry on writing the letters of the message from the start of the cylinder again, and so on. Pad out the end of the message with random letters to fill up the ribbon if there is space at the end.
Now unwrap the ribbon, and you are holding an encrypted (ie scrambled) version of your message. Send the ribbon (and so the secret message) to whoever the message is for. Anyone who intercepts it will just find gobbledygook.
An Example
For example, if I wanted to send the message “HELLO WORLD” using a Scytale long enough for 3 letters along its length and where the diameter was enough so that I could write 4 letters round the cylinder (so turning it 90 degrees each time I got to the end), then I would write:
H E L
then turn the cylinder 90 degrees and write
L O W
then turn the cylinder 90 degrees and write
O R L
then turn the cylinder 90 degrees and write
D E E
(note here I have added extra Es to pad out the message – though random letters would be better (or perhaps add some extra joke message on the end).
The unwrapped ribbon would then contain the encrypted (so scrambled message) which is essentially reading down the above columns:
H L O D H O R E L W L E
How to decrypt a message using a Scytale
How does the other person read the secret message? All they need is a cylinder (so a Scytale) with the same diameter as yours. They wrap the ribbon round their cylinder and read the message off along the length of the cylinder.
Transposition Ciphers
A cipher is just an algorithm for making (encrypting) secret messages or for unscrambling (decrypting) them. The Scytale is a very early example of a device that implements a particular cipher, so algorithm, in analogue way – using physical properties of the world. You could implement the algorithm by hand (or on a computer) but you do not need to.
The kind of cipher that a Scytale implements is called a Transposition cipher. It is just a way of making a secret message by moving the letters of the message around – it is essentially an anagram. It contrasts with a substitution cipher, where the letters of the message are swapped for new symbols.
Other Shapes – Prisms
The Ancient Greeks used cylinders as Scytales, but other 3-D shapes could be used too….they just have to be prisms. For example, a triangular prism could be used giving three sides per rotation to write letters on. Try different kinds of prisms. Of course whoever you are sending the message too needs to have an identical prism to read the message.
Some kinds of chocolate come in prism shaped packets so make a fairly easy to find and tasty source of prism Scytales…and another way to do computation with chocolate.
Teaching London Computing: A RESOURCE HUB from CAS LONDON & CS4FN 