Meet Otto. Press play to watch the video.
Your mission
Otto the otter lives in a pond that works like a mirror. He knows one of the oldest ciphers in the world, and it is one of the easiest too. Today he will teach it to you.
- Watch Otto's video.
- Open each tab, from left to right.
- Earn four badges to unlock The Pond's Secret.
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, and gentle enough to be a first cipher lesson. Plan on 30 to 40 minutes.
- Atbash has no key. The whole cipher is one chart: the alphabet with the backward alphabet under it.
- Students cannot click until they get it right. A missed question swaps in a new question, and three misses on a typed answer bring a new word.
- Words are picked at random, so neighbors will not have the same answers.
- The No chart tab has a math tie-in. A letter's place number and its partner's place number always add up to 27. A short example video at the bottom of that tab shows the steps, one letter at a time.
- The Pond's Secret is a team challenge with scratch paper. Smart teams write their own mirror chart first.
- The Mirror Machine, a free flipper, stays locked until the secret is found. That way nobody can use it to skip the practice.
- Offline extension: students write the chart on an index card, flip their spelling words, and trade with a partner.
- History on this page: Atbash is about 2,500 years old and was first used by scribes writing in Hebrew. The name comes from four Hebrew letters.
- Progress saves in the browser on this Chromebook only.
The mirror alphabet
Atbash is a cipher with no key and no wheel. You only need one chart.
Write the alphabet in a row. Then write it again underneath, backward. Now every letter has a partner. A swaps with Z. B swaps with Y. C swaps with X.
Click any letter in the top row to see its partner.
Swap, and swap back
To hide a word, swap each letter for its partner.
Here is the best part. To read the code, you do the very same thing. Swap every letter for its partner again, and the word flips right back.
Count from both ends
You can find a partner without the chart. Count how far the letter is from the start of the alphabet. Its partner is that same number of steps from the end.
C is the 3rd letter. Its partner is the 3rd letter from the end. Count back: Z, Y, X. So C swaps with X.
The two letters in the very middle, M and N, are next-door neighbors. They swap with each other.
Very old, very easy, not very secret
Atbash is about 2,500 years old. Scribes who wrote in Hebrew used it first.
The name is a clue to how it works. In the Hebrew alphabet, the first letter is Aleph and the last letter is Tav. The second letter is Bet and the second to last is Shin. Aleph swaps with Tav, and Bet swaps with Shin. A, T, B, SH. Atbash!
Atbash is quick, but it is not strong. There is only one way to do it, so there is no secret key. Any snoop who knows the mirror trick can read your note.
Put the chart away
A real code maker does not always have a chart. This time you only get the plain alphabet with its place numbers.
Use the counting trick. Or use this number trick: a letter's place number plus its partner's place number always adds up to 27. E is number 5. 27 take away 5 is 22, and letter 22 is V.
Watch Otto solve oneA quick example, one letter at a time.
Stuck? Press play to watch Otto solve one.
Words with a secret twin
Most words flip into a jumble that nobody can say. CAT turns into XZG.
But a few rare words flip into another real word. Code fans love to hunt for them. Can you find some?