Meet Tally. Press play to watch the video.
Your mission
Tally the raven lives on top of the castle tower. She has watched code makers and code breakers for years. Today you will learn both jobs.
- Watch Tally's video.
- Open each tab, from left to right.
- Earn four badges to unlock The Tower Vault.
Take your time. If you miss a question, you get a new one, so think before you click.
My badges
Teacher note
- Built for grades 3 to 5. Plan on two sessions of about 40 minutes. Crack a code is the long part.
- This page works best after the Caesar wheel lesson, because it starts from Caesar's weak spot.
- Crack a code teaches frequency analysis: counting letters to break a code with no key. The first note is coached step by step. The next two give two free clues each.
- Cracking is real productive struggle. Partners help a lot. Encourage students to read half-finished words out loud.
- Students cannot click until they get it right. A missed question swaps in a new question, and the code cracking check only says solved or not solved.
- Keys and messages are picked at random, so neighbors will not have the same answers.
- The Tower Vault is a team challenge. The Cipher Machine and Letter Counter stay locked until it is open, so nobody can use them to skip the practice.
- Offline extension: partners agree on a keyword, write short notes, and trade. A second pair tries to crack a longer note by tallying letters on paper.
- Progress saves in the browser on this Chromebook only.
Caesar's weak spot
The Caesar wheel slides the whole alphabet. Every letter moves the same number of steps.
That is easy to use, but it has a weak spot. There are only 25 ways to slide the alphabet. A snoop can try every one of them in a few minutes.
Code makers needed something much harder to guess.
Build a secret alphabet
A keyword cipher does not slide the alphabet. It scrambles it. You only need a secret word, called the keyword.
- Write the keyword under the start of the alphabet.
- If a letter shows up twice in the keyword, skip it the second time.
- Then write the rest of the alphabet in order. Leave out the letters you already used.
Swap the letters
Now you have a key with two rows. The top row is for message letters. The bottom row is for code letters.
To hide a message, find each letter in the top row and write the letter under it. To read a code, find each letter in the bottom row and write the letter above it.
With the keyword ZEBRA, the word CAT turns into BZS Click a letter in the top row to see its partner.
Too many keys to try
There are more than 400 septillion ways to scramble the alphabet. That is a 4 with 26 zeros. No snoop could ever try them all.
For hundreds of years, people thought this kind of cipher could not be broken. A queen named Mary used one for her secret letters. She was sure nobody could read them.
She was wrong. A code breaker named Thomas did not try keys at all. He counted how often each symbol showed up. In English, the letter E shows up more than any other letter. That one clue helped him read every word.
Count the letters
A keyword cipher has a hidden weakness. The same message letter always turns into the same code letter. So a letter that is common in English is still common in the code. It is only wearing a mask.
This chart shows the letters that show up most in English writing.
Three tricks code breakers use
- The most common code letter is probably E.
- A three-letter word that shows up again and again is probably THE.
- A word with only one letter is almost always A or I.