Best guesses
How it decides
First it looks at what the message is made of. Dots and dashes, braille cells, digit pairs, groups of three numbers and NATO words each belong to one code or two, so it tries the decode and shows you what comes out.
Letters take more work. It checks whether they are English letters in the wrong order, which is what a rail fence does. It tries all 25 Caesar shifts and the Atbash swap and keeps whichever reads most like English. If none does, it measures how often letters repeat. One scrambled alphabet repeats letters as often as English does; a Vigenère cipher spreads them out, and then it hunts for the keyword the textbook way, one column of letters at a time.
Two things it can't do. A keyword cipher and a random substitution are both one scrambled alphabet, so nothing in the message tells them apart; the cryptogram solver is the way into either. And a message of a dozen letters is too short for statistics, so it says so rather than guess.
The codes it knows
Morse code, NATO alphabet, Caesar shift, ROT13, Atbash, Tap code, Pigpen, Braille, Polybius square, Flag semaphore, Keyword cipher, Vigenère, Rail fence, Substitution, Book cipher, plus plain letter numbers (A=1 to Z=26).
Learn to crack them yourself
Every code here is one that Covert Mail, our free app, teaches. It takes you through each one a few characters at a time, plans practice around the ones you miss, and keeps an archive of famous real messages to unlock. It is free on the App Store.
Download on the App Store▶ Coming soon to Google Play
Also for puzzles: the cryptogram solver and the secret message reader. Every code, with its translator and chart:
Frequently asked questions
How do I identify a cipher?
Start with what the message is made of. Dots and dashes are Morse code. Pairs of digits from 1 to 5 are a Polybius square or tap code. Groups of three numbers are usually a book cipher. Grids with dots are pigpen. If it is all letters, count them. When a few letters come up far more than the rest, the way E, T and A do in English, it has one alphabet, like a Caesar or substitution cipher. When the letters come up evenly, it has a changing one, like Vigenère.
What cipher uses numbers?
Several. Digit pairs such as 23 15 31 are a Polybius square or tap code: a row and a column in a 5 by 5 grid. Numbers from 1 to 26 are usually just letter numbers, A=1 to Z=26. Groups of three, like 12.4.7, are a book cipher: page, line and word in a book both sides own, and without the book nobody can read it.
How can you tell if it's a Caesar cipher?
Try all 25 shifts. Exactly one of them turns a Caesar cipher into English, so a computer can check them all in an instant, which is what this page does. The letter counts give it away too: the commonest letter is usually E moved along by the shift.
What is the index of coincidence?
The chance that two letters picked from the message are the same letter. English text scores about 0.066, and so does any cipher that swaps each letter for one other letter, since swapping doesn't change how often letters repeat. Random letters score about 0.038. A Vigenère cipher lands in between, which is how it gives itself away.
Can this identify any cipher?
No. It knows the fifteen codes Covert Mail teaches, plus letter numbers. Modern encryption, the kind that protects your bank, can't be identified or broken by looking at it, and isn't meant to be.
Is what I paste sent anywhere?
No. The guessing runs in your browser, and nothing you type is sent to us or anyone else.