Cipher Lab

How to use the Cipher Lab

Classical ciphers are the perfect first step into cryptography: simple enough to do by hand, rich enough to teach the ideas every modern cipher is built on.

1. Type a message in the box at the top — anything from a single word to a paragraph.
2. Pick a cipher tab. Caesar shifts every letter by one amount; Vigenère shifts each letter by the next letter of your keyword; Atbash mirrors the alphabet.
3. Choose Encrypt or Decrypt and watch the result update live. Drag the Caesar shift slider to see all 25 possibilities.
4. Crack a mystery message: paste any Caesar cipher text, open the brute-force view, and scan the 25 rows for the one that reads as English.

Which cipher should I try first?

Start with Caesar — it's the simplest substitution cipher there is, and the brute-force view makes the "only 25 possibilities" weakness viscerally obvious. Then try Vigenère with a short keyword vs. a long one to feel how key length changes the game. Atbash is a one-trick delight: encode something, then encode the result, and watch the original reappear.

A tiny history

Julius Caesar reportedly used his namesake shift for military dispatches around 50 BC. Atbash is older still — it appears in the Hebrew Bible's Book of Jeremiah. Vigenère's cipher (actually due to Giovan Battista Bellaso, 1553) was called le chiffre indéchiffrable until Charles Babbage and Friedrich Kasiski broke it in the 1800s by exploiting repeated keywords. Every one of them teaches the same lesson modern cryptography took to heart: security must survive the enemy knowing exactly how your cipher works.

What is a Caesar cipher?

A substitution cipher where each letter shifts a fixed number of places — with a shift of 3, A becomes D and Z becomes C. It's named for Julius Caesar, who reportedly used it for military messages.

How do I crack a Caesar cipher without knowing the shift?

Try all 25 shifts — that's exactly what the brute-force view does. Since there are only 25 possibilities, one of the rows will read as plain English. This is why the Caesar cipher offers no real security.

What is the Vigenère cipher?

A step up from Caesar: instead of one shift, a keyword sets a different shift for each letter, repeating down the message. 'LEMON' on 'ATTACKATDAWN' gives 'LXFOPVEFRNHR'. It resisted cracking for 300 years and earned the nickname 'the indecipherable cipher' — though it falls to frequency analysis today.

What is Atbash?

A fixed mirror of the alphabet: A becomes Z, B becomes Y, and so on. Because the mapping is its own reverse, encoding and decoding are the same operation — run the text through twice and you get the original back.

Do spaces and punctuation get encrypted?

No. All three ciphers only transform the letters A–Z (keeping uppercase and lowercase intact). Spaces, digits, and punctuation pass through unchanged, and in Vigenère they don't advance the keyword either.

Are these ciphers secure enough for real secrets?

No — emphatically not. Caesar falls to trying 25 shifts, Atbash is a fixed mapping anyone can reverse, and Vigenère falls to frequency analysis with a computer. They're wonderful for puzzles, games, and learning how cryptography works, but never for anything that must stay secret.

Is my text sent anywhere?

No. All encryption happens in your browser with JavaScript. Nothing is uploaded, stored, or tracked.