2. The Caesar shift

About 5 minutes · You need: paper, a pencil, a partner

Try it

Write the alphabet, then write it again underneath, shifted three places:

plain:   A B C D E F G H I J K L M
secret:  D E F G H I J K L M N O P

plain:   N O P Q R S T U V W X Y Z
secret:  Q R S T U V W X Y Z A B C

To hide a message, swap each letter for the one under it. HELLO MARS becomes KHOOR PDUV.

To read a secret message, go the other way, from the bottom row to the top.

Now decode this one: WKLV LV IXQ

Answer

THIS IS FUN. Each letter moves back three places: W→T, K→H, L→I...

What's happening

Julius Caesar used this code about 2,000 years ago to send orders to his generals. The key is the number of places you shift: here, 3. Your partner needs to know the key to read your message.

Make a code wheel: cut two paper circles, one a little smaller, write the alphabet around the edge of each, and pin them together in the middle. Turn the inner one to choose a key.

There are only 25 useful keys. Is that a problem?

Yes. Someone could just try all 25 in a few minutes. That is the subject of the next lesson.

Next: 3. Cracking Caesar · Going further: Book, chapter 2