Cipher Decoder

Paste the ciphertext and the key works itself out. For a Caesar cipher the tool tries every alphabet shift; for a Vigenère cipher it builds the key letter by letter; for Atbash a single variant is enough. The version that reads most like coherent text rises to the top, and the rest stay in a list beside it.

The score rests on three signs: how often each letter appears, the share of common letter pairs such as “st” and “no”, and the share of recognizable words. The Russian alphabet gets analyzed alongside the Latin one, and the language is detected on its own.

How much text you need

A Caesar cipher breaks with as few as 20–30 letters; often one phrase is enough. Vigenère needs more: each column of the keystream should get at least two or three dozen characters, so a five-letter key calls for a hundred letters of text.

On short strings the tool says the guess is unreliable and does not pass off a random answer as the solution. The analysis under the decryption shows how many letters it found, what the index of coincidence came to, and which key length it points to.

When decryption fails

The cipher may not be one of these three: a simple letter substitution, a transposition, Base64 encoding. Frequency analysis is not built for those, and the suggested versions will read as nonsense.

A manual hint helps too. Pick the language when the text is mixed: on a blend of alphabets the frequencies drift off. Turn “ё” into a separate letter if the text was enciphered with a 33-letter alphabet. The reverse task, encrypting text yourself, is handled by the Caesar, Vigenère and Atbash ciphers.