Mode: ENCODE

CAESAR CIPHER

The earliest known, and the simplest, use of a substitution cipher was by Julius Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet.

1. Choose a number between 1 and 25. This will be your “shift” value.

2. Write down the letters of the alphabet in order, from A to Z.

3. Shift each letter of the alphabet by the “shift” value. For example, if the shift value is 3, A would become D, B would become E, C would become F, and so on.

4. Encrypt your message by replacing each letter with the corresponding shifted letter. For example, if the shift value is 3, the word “hello” would become “khoor”.

5. To decrypt the message, simply reverse the process by shifting each letter back by the same amount. For example, if the shift value is 3, the encrypted message “khoor” would become “hello”.

plain: meet me after the toga party

cipher: PHHW PH DIWHU WKH WRJD SDUWB

Note that the alphabet is wrapped around, so that the letter following Z is A.

We can define the transformation by listing all possibilities, as follows:

plain: 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

cipher: d e f g h i j k l m n o p q r s T u v w x y z a b c