Ciphers · CaesarCaesar cipher decoder A→D
The Caesar cipher replaces each letter with the one a few places further along the alphabet: with a shift of 3, “a” becomes “d”. Paste the ciphertext — we try every shift and show the meaningful one first, so you can decode without the key, in English or in Russian with or without Ё.
Everything runs in your browser — the text goes nowhere
Encode
All shifts
best on top · press a row to pick itCipher wheel
print it, cut out two discs — and encrypt without a computerAssembled wheel · shift 3: A → D
The outer disc is plain text, the inner one is the cipher. Change the shift above and the disc turns.
A4 sheet, black and white. Two discs with dashed cut lines and a marked centre — pierce and fix with a pin or a paper fastener.
How it works
Shift around a circle
a → d · b → e · z → cEach letter is replaced by the one n places further on. After “z” the alphabet starts again.
Decrypting goes back
d → a (shift −3)The same shift the other way. A shift of 23 with 26 letters is the same as −3.
Why it is easy to break
25 optionsThere are few shifts: trying them all and finding meaningful text takes a fraction of a second — that is what the table above does.
Worked exercises
for a computing lessonEncrypt the word “hello” with the Caesar cipher, shift 3.
h → k e → h l → o l → o o → r
Answer: khoor
Decrypt “Uryyb, Jbeyq” when the shift is unknown.
try shifts 1…25 shift 13: U → H, r → e, y → l, b → o
Answer: Hello, World
Why does Russian “делу время” with shift 1 encrypt in two ways?
with Ё: е → ё → еёмф гсёна no Ё: е → ж → ежмф гсжна
Answer: the letter after “е”
Send the class a link with the exercise — “Share” keeps the text and the shift.
Frequently asked questions
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