St Thomas of Canterbury, 5 July 2019

This was our showpiece week at Bletchley Juniors Codebreaking Club, as we tackled some frequency analysis. I promised the children I would show them how to decode a ciphertext without knowing the key, and this was it.

The plaintext was three paragraphs summarising what the children had learnt in the six months since we started this club. I used a Caesar cipher with a shift of 7, but kept this information from the children. (Why 7? It was a lucky number to the Ancient Greeks!). I explained that some letters in English texts occur much more frequently than others (E, T, A and O) and others letters occur very infrequently (X, Q and Z). I showed them a frequency distribution chart of the whole alphabet.

I got the children to count the frequency of the letters in the ciphertext. I wrote the answers on post-it notes. The children then arranged these in order: the most frequent were L, A, H and V.  I suggested it was very likely that L represented E, A was T, H was A and V was O. The children found every L in the ciphertext and wrote E above it. They repeated this for A, H and V.  I said it was very unlikely there would be an exact match between the frequency of our ciphertext and the frequency of English texts. I steered them towards a list of the most common three letter words that I had supplied them with. The children could predict where the word ‘the’ was in the ciphertext: they then found every O and wrote H above it. Returning to the list of three letter words, the children could predict where the word ‘and’ was in the ciphertext; they then found every U and wrote N above it, and found every K and wrote D above it.

From this point on it was a case of pointing the children towards some partially complete words in the ciphertext and encouraging them to work out what that word was, then transferring this knowledge to all of the ciphertext. There was a point when we felt we had reached the summit of the mountain, and it was all downhill from there. Eventually I was able to read out the whole plaintext message.

One thing that did not go well was the timing: we over ran by 20 minutes, as it proved impossible for me to predict how long the activity would take. I did gently push the children to get the activity finished before leaving. The lesson I learnt was that I could have split the activity across two weeks, or I could have written a shorter plaintext. Perhaps I was too ambitious: you live and learn.

One other thing to bear in mind was that it is very easy to miss letters when counting them, and when looking for them to write the real letter above them. I did this in practice, and the children did it during the session. I think the solution is to know that this will happen and to allow time to check the children’s work as they go along.

As we over ran there was no time to summarise what we had achieved in the session. One parent did tell me later that their child was intrigued by frequency analysis. It was good to know we had provided another enjoyable session.

St Thomas of Canterbury, 28 June 2019

At Bletchley Juniors Codebreaking Club this week we tried three new ciphers!

Our first was the Atbash cipher. I encoded a group of words and asked the children to work out the connection between the words. I used these groups: DC Comics superheroes, the sun, the moon, and the Seven Wonders of the Solar System. This last one had a ‘match the picture’ round. I explained that I chose the moon as it is nearly 50 years since a man first landed on the moon. The words in my moon group were: full, half, Apollo, lunar, crescent, gibbous, waxing and waning.

The second was the Jefferson cipher wheel. I was in possession of a replica wheel. I showed the children that each disk has a random sequence of each letter of the alphabet, which can be removed and replaced in an agreed order. The plaintext is then spelled out on one row of the wheel. Every other row will look like gibberish. One of these rows is chosen as the ciphertext. The recipient of the coded message sets their wheel to the ciphertext and turns the wheel around until they find the one row which make sense, which is the plaintext. The activity was to decode the Seven Wonders of the Ancient World, then match each to its picture. The children found it quite hard to manipulate the wheel: some of the disks were stiff and hard to turn, and others were a little too loose. This made it hard to line up each row in a straight line. With some help the children were able to decode the Great Pyramid of Giza and the Colossus of Rhodes. As we were running out of time I read out the names of the other five wonders, and we went straight to the pictures to match everything up.

Our third cipher was the Ancient Runes. There were some mythological beasts and monsters that we did not have time to look at last week, so I reused them and encoded them in the Runes. I do love a ‘match the picture’ round so included one here. After decoding such beasts as the Basilisk, Manticore, Chimera, Werewolf and Griffin the children were successful in matching it all up.


St Thomas of Canterbury, 21 June 2019

At Bletchley Juniors Codebreaking Club this week we used the Date Shift Cipher. I chose some famous dates from history e.g. Shakespeare’s Date of Death, the Battle of Hastings and The Fall of the Berlin, and turned the date into a repeating keyword. I explained that this was another example of a polyalphabetic substitution cipher.

As an example, the Raising of the Mary Rose was on 11 October 1982. This turns into 111082 when the month is turned into a number and only the last two digits of the year are used. The activity was to decode information about mythological beasts and monsters. To encode, each letter in the plaintext was shifted forwards according to the number in the keyword e.g. for 111082 and Centaur, the C was shifted by 1 to make D; the e by 1 to make f; the n by 1 to make o; the t by 0 so stays as t; the a by 8 to make i; the u by 2 to make k; the r by 1 to make s. So Centaur becomes Dfotikr. Decoding means shifting backwards along the alphabet by the number in the keyword.

I gave the children a short paragraph on different mythological beasts and monsters, with the name and some of the key descriptive words encoded. Once a set of beasts was decoded I gave out a set of pictures, asking the children to match the beast with its picture.  The children enjoyed this chance to build on their knowledge of mythology from around the world.


St Thomas of Canterbury, 14 June 2019

At Bletchley Juniors Codebreaking Club this week we used the Vigenere cipher. I reminded the children about the Caesar cipher we used many weeks ago. I explained the difference between a monoalphabetic and a polyalphabetic substitution cipher, and how polyalphabetic ones are more secure, especially when they use a keyword.  I used an example to demonstrate how the Vigenere cipher works. The plaintext word I had chosen – headlice – has two e’s and they were encoded into different ciphertext letters. The ciphertext had two z’s and they encoded different plaintext letters. I pointed out the difference with the Caesar cipher, when every plaintext letter would only correspond to the same ciphertext letter.

Our activity this week was disgusting facts about the human body. The children’s eyes lit up when I told them we would be covering blood, brains and guts. The children had to decode a ciphertext word in a sentence, read the sentence and decide if it was true or false. We then told them if they were correct or not. My favourite example, which is true: when you blush, the inside of your stomach does too.

St Thomas of Canterbury, 7 June 2019

At Bletchley Juniors Codebreaking Club this week we used the Dancing Men Cipher, from the Sherlock Holmes short story of the same name.


Our activity was to make a set of Ancient Greece Top Trumps playing cards. I had made the cards, featuring an image, a description and the statistics, but without the name of the god, goddess, mortal or creature. The children had to decode the name using the cipher, then write it on the card. There were 34 cards in total, which the children completed between them. We then had a game of Top Trumps. The magic category was the most popular.  We finished with a matching pairs memory game: the pairs were all characters from Ancient Greece. These activities really helped to consolidate the children’s growing knowledge of Ancient Greece mythology.

St Thomas of Canterbury, 24 May 2019

At Bletchley Juniors Codebreaking Club this week we practised some vexillology i.e. the study of flags. I bought some Harry Potter World theme park maps. I then wrote a sequence of instructions to navigate the way around the theme park. I added North and West coordinates to the maps. I then encoded the locations of various places on the map using flag designs, with colours representing numbers. The children were able to successfully decode the flag colours as map coordinates, then read the coordinates on the map to find the location. I threw in some questions about each location, to extend those children who were Harry Potter fans. The other children enjoyed the combination of flags and maps, and having their first go at reading map coordinates.

St Thomas of Canterbury, 17 May 2019

At Bletchley Juniors Codebreaking Club this week I introduced the children to a Cardan Grille, making sure they knew that this use of the grille refers to a type of window not to a way of cooking sausages.

I explained that the grille can be used eight different ways i.e. it has two faces, and can be rotated four times.  The activity this week was collective nouns e.g. a pride of lions. I prepared squares containing 36 letters in a 6 x 6 square. I made grilles with 12 windows. I arranged the letters so that in only one of the eight possible options was there a word that made sense. The words were all the nouns i.e. animals, birds or fish. Once the children had decoded the set of nouns, I gave them pictures to match to the nouns.

cardan grille2
My grille looked a bit like this, except with 12 windows. 

The next task was to decode the collective words. I had encoded these using Shadow’s Code, which we used several weeks ago but had not been seen by some of the children who have joined our club since then. Once the collective words were decoded, the children had to match this to the noun.  This was the hardest part of the activity and required a bit of a steer from us. 

Some of our favourite collective nouns were: a smack of jellyfish; a crash of hippos; a shiver of sharks, and a fever of stingrays.