What cipher is this?
Paste ciphertext and this tool measures its statistics to suggest which classical cipher produced it, and its likely key length. Nothing is sent to a server.
Bars: this text. Marks: typical English.
| Measure | Value | What it tells you |
|---|
How identification works
The index of coincidence measures how often two letters drawn from the text match. English text scores about 1.73 (normalized), random text about 1.0. A monoalphabetic cipher — Caesar or substitution — keeps the English score because it only relabels letters. A repeating-key cipher flattens the score toward 1.0, but the score rebounds when the text is split into columns at the key length, which is how the key length is found.
Autokey ciphers, where the message itself extends the key, defeat that column trick because the key never repeats. A ciphertext autokey still leaves a tell: subtracting each letter from the one a primer-length earlier reconstructs the plaintext, so that difference reads as English. Plaintext autokey hides even this, and is the hardest of the family to spot.