? Cipher identifier

How to use it

Paste a ciphertext into the box. Guesses appear at once, most likely first. Each comes with a confidence bar, the reason the tool thinks so, and a link to the tool that will finish the job. When a simple key fits, you also see how the text reads once decrypted.

The Examples buttons fill in samples. Each one is made by the same functions the other tools in the course use, so they are a handy way to see what Caesar, Vigenère, Base64 or an Enigma signal looks like.

How it works

First the tool looks at which characters the text is made of. Only dots and dashes: Morse code. Only zeros and ones: binary. Digits and the letters a–f in pairs: hex. Both cases, digits, “+” and “/”: Base64. Numbers up to 26: letter positions. Only A and B, in fives: Bacon’s cipher. It tries to decode each candidate and checks whether text comes out.

If the text is mostly letters, statistics take over.

• χ² compares the letter frequencies with those of English or Russian. If they match, the letters are the plaintext’s own, just moved around.
• The index of coincidence is the chance that two letters picked at random are the same. A language scores high (about 0.067 for English, 0.055 for Russian), random letters low (1/26 and 1/33). A simple substitution leaves it alone; a cipher with several alphabets, such as Vigenère, drags it towards random.
• If the index looks like a language, the tool tries every Caesar shift and the reversed alphabet of Atbash. If neither fits, it is a simple substitution with a shuffled alphabet.
• If the index is low, it tries Vigenère key lengths from 2 to 16 and takes the first one that makes the text read.

Latin letters in groups of five, with an index close to noise, are the mark of a cipher machine.

Questions

Why did it find nothing?

Frequency arguments need at least 20–30 letters, and a hundred is better. Take a short line apart by hand in frequency analysis. The tool also doesn’t know ciphers the course doesn’t cover: book ciphers, nomenclators, home-made systems.

What about modern ciphers?

The output of AES or any other modern cipher looks like random bytes. The tool will see hex or Base64 and say there is no text inside — and that really is all anyone can learn without the key.

Can I trust it?

Like a seasoned colleague glancing over your shoulder: it suggests where to start, but you check for yourself. How the codebreakers of the past did it is told in the famous ciphers lesson.

Is my text sent anywhere?

No. Everything runs in your browser.