For our third week of Bletchley Juniors we tackled the Caesar cipher. I started with a very brief overview of who Julius Caesar was, including looking at a map of the Roman Empire (any excuse to get a map out). I explained that Caesar shifted each letter forward by 3 (A becomes D) in order to send coded messages. I wrote out the alphabet on the whiteboard, all on one line. As a group we encoded the plaintext ‘julias caesar’ as MXOLXV FDHVDU; we decoded the ciphertext ZDV ERUQ LQ URPH as ‘was born in rome’ by shifting each letter back by 3. I gave every child the opportunity to both encode and decode a letter.
I then showed the children a Caesar cipher wheel, explaining that it could do both the adding and subtracting for them, saving time. I gave each child a cardboard sheet with the blank template of the inner and outer wheels printed on it. The children wrote the alphabet on both wheels, cut them out and pinned them together. I showed them how to set the inner wheel to a shift of 3. I gave them each a ciphertext to decode, which took the form of a statement which was either true or false e.g. there are five different chess pieces (false, there are six). Each child was able to successfully decode their ciphertext. I then asked them to use their wheel to encode a message to someone in the group in ciphertext, then swap over and decode into plaintext. There was just about enough time for this before the session was over. As with the first week, the children enjoyed the chance to make something with their hands, as well as learning more about ciphers.