Caesar Cipher Part 1

Suppose you want to send a secret message to a friend. Maybe you want to give them directions to a secret clubhouse you built in the woods. Something like “walk for five minutes, turn left, then walk for three minutes, the club house is up in the brown tree.”

However, if you just write down this information and put it in your friend’s mailbox, it is possible that one of your enemies could intercept the message and gain access to your secret fortress! How can we avoid this situation?

The best method is some sort of code or encryption. Encryption is a method of taking a regular message and making it difficult to read. Decryption is a method of taking an encrypted message and making it easy to read.

Time for an example. The simplest example of encryption is the Caesar Cipher. Caesar used it thousands of years ago to communicate with his generals in such a way that if his enemies intercepted a message, they would not be able to read it.
Here is how the process works for a short message. For example, suppose the message is simply the word “dog.”

Just to note, the alphabet is provided here as a reference for those of you who forget your ABC’s.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Step 1: Split the word up into each letter. D O G

Step 2: Count up three letters from each letter.

D goes to G:    D -> E -> F -> G

O goes to R

G goes to J

Step 3: Send the encrypted message. D O G is encrypted as G R J

That’s it. The word grj is nonsense to anyone who doesn’t know about how we encrypted the message. In addition, decryption of the message is really easy. All we do is reverse the process.

Step 1: Split the word up into each letter. G R J

Step 2: Count down three letters from each letter.

G goes to D:    G -> F -> E -> D

R goes to O

J goes to G

Step 3: G R J is decrypted as D O G

Now back to our clubhouse situation. If you want to communicate the secret instructions, just talk to your friend ahead of time and tell him you will be using a Caesar Cipher. Then your message is encrypted as follows:

“walk for five minutes, turn left, then walk for three minutes, the club house is up in the brown tree.”

“zdon iru ilyh plqxwhv, wxuq ohiw, wkhq zdon iru wkuhh plqxwhv, wkh foxe krxvh lv xs lq wkh eurzq wuhh.”

In the next instalment, we will investigate the weakness of this approach.