CS Unplugged activity

Caesar Cipher: lesson plan

A short teacher-led plan for Caesar Cipher: hide a message by shifting every letter, then crack one with no key. About 50 minutes. Leaving it for a sub? Use the box below.

For a sub: the 2-minute version

Skip everything below this box. Nothing to set up.

Optional, if you can project: type https://learn.porttack.com/go/caesar into the browser’s address bar (not a search box), press Enter, then use the right arrow key to step through. Stop at the slide titled Decode, Then Encode, then hand out the sheets. Unsure about a slide? Skip the slides: the sheet explains everything.

Hand out the worksheet, one per student. Names on top.
Say: “Today you’re spies. Fill in your shift table, then decode the secret messages. Every letter moves the same number of places.”
If the teacher left more puzzles, hand the next one to each student who finishes: a new set of coded messages.
Restless room? Let neighbors work in pairs. Talking about the puzzle is fine.
Collect every sheet, finished or not.

Stuck on Crack it? Tell students to try key 1, then key 2, and so on until real words appear. That really is the method.

Before the sub day (teacher, or whoever preps the sub folder): print one copy of the worksheet per student, plus keep-going sheets from the New set page: set Copies to your class size and Sets per student to 2 (front and back). That covers a full period even for your fastest students.

Students will be able to

Encrypt and decrypt a message with a shift (Caesar) cipher and a key.
Crack a Caesar cipher by trying every key.
Explain why a cipher with only 25 keys isn’t safe.

Before class

Print one worksheet per student.
Print keep-going sheets from the New set page: Copies = class size, Sets per student = 2 (front and back).
Optional: the slides and the cipher wheel (needs scissors and a brad).

The lesson

1. Hook (5 minutes)

Show the slides through HELLO becomes KHOOR. Ask a student to decode KHOOR out loud with key 3.

2. The sheet (20 minutes)

Students fill in their shift table, decode the messages, then encode one of their own. The most common mistake is shifting the wrong direction when decoding: decoding moves letters back.

3. Crack it (10 minutes)

Show the No Key at All? slide and let students guess how to crack a message before they start the last part of the sheet.

4. Keep going (as students finish)

Partners trade encoded messages, then keep-going sheets.

5. Wrap-up (5 minutes)

Ask: “How long would cracking a Caesar cipher take a computer?” (Less than a blink: only 25 keys.) “So how do real ciphers stay safe?” (So many keys that trying them all would take longer than the age of the universe.)

Going further

Programming students: the CS50 Caesar problem set has them write this cipher in Python, with the key typed on the command line. It works best right after the paper version, because students already know exactly what the program should do. It’s online, not printed, and checks itself with check50.
Cracking it a smarter way: Caesar Cipher: cracking it with letter frequency finds the key without trying all 25.
A cipher that’s much harder to crack: Vigenère Cipher uses a keyword, so each letter shifts by a different amount.