Vigenere cipher solver
Recover the key and decrypt Vigenere-enciphered text without knowing the key in advance.
Vigenere cipher solver
Paste Vigenere-enciphered text and this tool works out the key on its own, then decrypts the message. It combines three classic techniques from classical cryptanalysis: Kasiski examination finds repeated letter sequences in the ciphertext and the distances between them, which point at likely key lengths; the index of coincidence scores every candidate length from 1 up to the limit you set and favors the one whose letter distribution looks least like random noise; and frequency analysis then treats each position of the key as its own shifted alphabet, comparing its letter counts against a reference language to find the most probable shift.
Set the key length yourself if you already know it, or leave it at 0 and let the tool auto-detect it — useful when you only have a snippet of ciphertext and no other clues. The reference language (English, Slovak, Czech or German) determines which letter-frequency table the analysis compares against, so pick the one the plaintext is likely written in. Switch the "Show" option to the full analysis report to see the repeated sequences Kasiski examination found, the index of coincidence for every tested key length, and the top-scoring key candidates side by side with their fit score — handy for checking the tool's reasoning or trying a close second guess by hand.
Statistical key recovery needs enough ciphertext to work with: a few hundred letters is usually plenty for a short key, while a one-line message may need you to supply the key length manually. Only the letters A–Z participate in the cipher; digits, spaces, punctuation and accented characters pass through unchanged, and the decrypted output keeps the original capitalization letter for letter.
Everything runs locally in your browser — the ciphertext is never uploaded anywhere, which matters for puzzle solutions, exercises or anything else you'd rather not paste into a third-party service. Copy the result, download it as a .txt file, or send it straight into another tool to continue working on it.