3. Cracking Caesar

About 5 minutes · You need: paper, a pencil

Try it

You found this secret message, but not the key:

PHHW PH DW WKH SDUN

Way 1: try every key. Shift each letter back by 1, then 2, then 3... Stop when you see real words. (It won't take long.)

Way 2: count letters. In English, E is the most common letter. Which letter appears most in the message? If H stands for E, how far is the shift?

Answer

H appears four times, more than any other letter. H is three places after E, so the key is 3: MEET ME AT THE PARK.

What's happening

Way 1 is called brute force: try every key. It works when there are only a few keys. Real codes today have more keys than there are grains of sand on Earth, so trying them all is impossible.

Way 2 is frequency analysis, invented by the Arab scholar al-Kindi more than 1,100 years ago. Letters in a language are not equally common, and a simple swap code keeps that pattern. For hundreds of years, it broke almost every code.

Why does counting work better on a long message than on a short one?

A long message shows the usual pattern clearly. A short one, like "OK", has too few letters to tell which is the most common.

Next: 4. A code nobody can break · Going further: History: Arab cryptanalysis