About this tool
The Cipher Identifier analyzes an unknown ciphertext and suggests which classical cipher most likely produced it. Rather than decrypting, it computes statistical fingerprints and matches them against the known signatures of common ciphers, giving you a starting point for the rest of the toolkit. Everything runs locally in your browser, so pasted text stays private.
The core measure is the index of coincidence (IC = Σ nᵢ(nᵢ−1) / N(N−1)), the probability that two randomly chosen letters match. English plaintext and monoalphabetic ciphers (Caesar, Atbash, Affine, Keyword) sit near IC ≈ 0.066, while polyalphabetic ciphers (Vigenère, Beaufort) flatten the distribution toward IC ≈ 0.038. The tool pairs this with a chi-square test against English letter frequencies: a monoalphabetic-looking IC combined with a low chi-square means the letter frequencies still match English, pointing to a transposition (Rail Fence, Columnar) that only reordered letters; a high chi-square points to a substitution that remapped them.
It also inspects the character set (only A/B suggests Baconian, only 0/1 binary, only digits an A1Z26 or Polybius scheme) and structural clues such as text length parity, absence of doubled letters, and no J, which together raise the likelihood of Playfair. The result is a ranked list of candidate ciphers with confidence notes alongside the raw statistics. It offers suggestions, not certainty — short texts especially make the numbers unreliable.