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.