St Thomas of Canterbury, 8 February 2019

This week at Bletchley Juniors we learnt about The Shadow’s Code.


The Shadow was a pulp fiction hero from 1930s America. He dressed all in black, lurked in the shadows, and leapt out to perform heroic deeds in his war on crime. Stories about The Shadow often included codes, and the following one from ‘The Chain of Death‘ is rather cunning.


Each letter of the alphabet has its own symbol. There are four direction symbols (straight up, turn right, upside down and turn left) which show which way up the key needs to be held. This means each letter can be encoded up to four different ways, depending on how many lines of symmetry the symbol has (E and F both have two lines of symmetry, so only encode in one way). This is an example of a polyalphabetic substitution cipher. Is the fact that the most frequent letter in the alphabet – E – always encodes the same way a bit of a flaw?

I had great fun preparing the material for this session. I chose song lyrics from Disney films as my ciphertext, and enjoyed listening to the songs on You Tube to help me decide which ones to include. I have to include a link to my favourite – ‘I Just Can’t Wait To Be King‘ from The Lion King.


Some of the children found this code harder than I expected them to. We usually do a decoding example together on the whiteboard before I hand out the ciphertexts, but that wasn’t possible this week: I drew each symbol on the whiteboard but did not put a letter next to each symbol due to the fact that it depends which way up the key is held. The children did get the hang of it, once they remembered to always look for the direction symbol. I had made this easier by using red for the direction symbol and black for the letter symbols.

Our second code this week was the Knot Code. This ancient code involves tying knots in a piece of thread or string, and sewing it inside an item of clothing to be worn by the messenger, where it would be found, removed and decoded by the recipient. For the key, I made a long strip of paper and wrote on it every letter of the alphabet at 4cm intervals. For the ciphertext I used the name of Disney films. I took a long piece of string, lined up one end to the left edge of the paper strip, held down the string until it reached the first letter of the ciphertext and tied a knot at this point. I then moved this knot to the left until it touched the left edge of the paper strip, and again held down the string until it reached the second letter I wanted and tied a knot. I repeated this process until I had encoded each ciphertext. If your ciphertext includes lots of letters from the end of the alphabet you will end up with a very long piece of string! The children enjoyed exploring this unusual method of communicating a message. One boy asked to take home a long piece of string and the key so he could encode his own message at home.

We are taking a three week break from Bletchley Juniors due to an inset day and half-term.

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