St Thomas of Canterbury, 8 March 2019

I keep on taking the children back to the Ancient World in Bletchley Juniors Codebreaking Club, and this week was no exception as we went to Ancient Greece. Polybius (c200 BC to c118 BC) was a historian who developed a substitution cipher using a grid and coordinates. I sneaked in a little bit of maths by asking the children what a square number is. We recapped all the squares numbers up to 6 x 6. I asked how many letters there are in the alphabet. When I got the answer of 26 I asked which square number is closest. The children were very quick to answer 5 x 5 =25. I explained that Polybius used a 5 x 5 grid which he filled with each letter of the alphabet. On the whiteboard I stuck a grid I had prepared earlier. I pointed out that two letters need to share a cell as 26 is one more than 25: these two letters are I and J. (With Polybius the Greek alphabet had 24 letters so he had one cell left empty). I pointed out that the grid has the numbers 1 to 5 along the bottom and also up the side.

Polybius2

I then used an example to practise with:

32 / 53 / 22 / 44 / 33 / 24       44 / 32      34 / 55 / 22 / 55

I explained that the first number (3 in this example) means go to 3 on the numbers along the bottom (or the x axis for those of us who can’t resist a bit of maths), and that the second number (2 in this example) means go to 2 on the numbers up the side (that will be the y axis). Where the two numbers meet is the letter, in this case ‘s’. The whole example decodes as ‘spring is here’. We then practised turning a word into code, using ‘code’ as the example. It encodes as 35 / 43 / 45 / 55.

I gave each child their own ciphertext to decode. The challenge was to work out what the words had in common. One was on Lego, another was Lego Brands. Others were mythical creatures, steam locomotives, Disney princesses, maps, Minecraft and Power Rangers. Yes, each child had a ciphertext which matched their interests! For those that finished early, I had other ciphertexts based on recipes for slime and jelly worms, but this time using a 6 x 6 Polybius Square, which happily has space for all the letters of the alphabet and every digit from 0 to 9 . I have since learnt from a parent that one child spent time at the weekend finishing decoding their ciphertext as they enjoyed it so much: truly a thumbs up for us!

polybius
He loved squares

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