Two thousand years ago, Julius Caesar shifted every letter of his messages by three. Today you build his cipher, use it, and break it in under a minute — and learn the one idea every modern cipher still depends on.
Ten minutes. Read it before class; we start from it. Enrolled in our Runestone course? Open it from there so your progress counts.
Security. Packets travel across machines you don’t own (Unit 6). The oldest answer to that: make the message unreadable to anyone without the key.
Two minutes. No key. It’s real English and it’s good news. You’ll be able to read it by the end of section 4.
Encryption turns a readable message into an unreadable one; decryption turns it back. Four words name what you’re doing:
| Word | Meaning | Caesar example |
|---|---|---|
| plaintext | the readable message | HELLO |
| ciphertext | the scrambled message | KHOOR |
| cipher (algorithm) | the method | shift every letter forward, wrapping Z around to A |
| key | the secret setting the method uses | the shift: 3 |
A Caesar cipher has only 25 useful keys (shifts 1 through 25). An attacker can simply try them all — that’s a brute force attack. With 25 keys it takes a minute by hand. Modern ciphers use keys so long that trying them all would take longer than the age of the universe.
Part 1. Encrypt or decrypt with any shift. Trade a message with a partner.
Part 2. Someone sent this. You don’t have the key. Try all 25.
Now go back to the hook: the note was shifted by 5.
Five questions. Two need a wheel — or your fingers.
Companies sometimes advertise “our own secret encryption.” That’s a warning sign, not a feature. A method nobody outside has tested may have holes nobody has found yet. Trusted ciphers are public, studied by experts for years, and still unbroken — because their keys are huge.