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.

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.

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.