Caesar Cipher

by Bosman Simanjuntak, MYP/DP Math Teacher

Do you want to send a secret message to your friend? In our MYP 4/5 mathematics class, we learned how to encrypt messages using the Caesar cipher method. Encrypting a message means turning it into a code so others can’t read it without permission. The text appears scrambled and unreadable without the right key or method to decode it. The Caesar cipher, named after Julius Caesar who used it about 2050 years ago during the Roman invasion of Britain, involves encoding messages by shifting each letter of the alphabet by a fixed number of positions (Vollmar, 2008). For instance, with a shift of 4, each letter in the original message is replaced by the letter that is four positions later in the alphabet. Thus, ‘A’ becomes ‘E’, ‘B’ becomes ‘F’, and so forth. This shift of 4 is what changes the original text into the encrypted message.

Here’s how a shift of 4 affects each letter:

For example, if we encrypt the message “MATHEMATICS IS FUN” with a shift of 4, it becomes “QEXLQIEXMWG MW JYR”. The key here is the shift of 4. Knowing this key allows you to decode the encrypted message back to its original text.

In our class, one group sent the message “PXWPGROJLQJR” to their friends. Other groups need to intercept and decode this message. Can you help? To make it more challenging, we have removed the spaces between the words.

Reference

Vollmar, P. (2008). Mathematics for the international student: Pre-diploma SL and HL (MYP 5 plus): Presumed knowledge for SL and HL courses: For use with IB Middle Years programme: Interactive student CD. Haese & Harris.