The trick Julius Caesar used to play

Julius Caesar adopted a tricky method of sending messages to his generals through messengers. What if his enemies caught hold of the messenger and read the message? They will then know Caesar’s plans. Caesar’s trick was to change the letters in the message. For instance, he could write “D” instead of “A”, “E” instead of “B” and so on. Every letter would be replaced by another three places further down in the alphabetical order.

What do you do with “Y”? What is three places beyond “Y”? The trick is to think of the letters being arranged in a ring so that “A” follows “Z” and “B” follows “A”. In this ring you can go three places from “Y” and find “B”. The letter three places ahead of “Z” will be “C”.

To encipher a message, that is to make it secret, we first choose a key. I recommend that you choose a whole number between 1 and 6 as the key. Suppose you choose 3 as the key. Now replace every letter in “TEXT” by one which is 3 steps away along the ring. Do you get “WHAW”? You can send a secret message to a general if you have told him the key the last time you met him. Without the key he would go nuts!

Deciphering a secret message requires that you replace every letter in the secret message by one that is 3 steps back, if 3 is the key. Try and decipher “RSHQ”. You should get “OPEN”. You are now a cryptographer! That is a person who wears a hoodie and dark glasses and tries to read off other people’s secret messages!

I will tell you another secret that only cryptographers know. When they want to make a message a secret, they remove all the spaces between words and then only apply the key. Like this:

Message: I LIKE ICE CREAM

Modified message ready to be enciphered:  ILIKEICECREAM

Secret message: LOLNHLFHFUHDP (using 3 as the key)

It is a common practice to write the secret message with a space after every five letters. This space means nothing and is ignored before deciphering. Grouping into five-letter blocks is only to make it convenient for you to read. So, we can write the final secret form of the message as

LOLNH LFHFU HDP

The last group has only three letters. That is okay.

Now try to decipher the message below. Its key is not 3, but some other number. Your task is to try different numbers between 1 and 6 and see which is the key. Then you can read off the secret!

STSJJ IKTWM TTINJ FSIIF WPLQF XXJX

Srinivasan Ramani