Identify a classical cipher
Ranks guesses about which classical cipher (if any) produced a piece of text, from index of coincidence, a Caesar-shift chi-squared fit and digram statistics: a lead, never a certain answer.
Identify a classical cipher scores five guesses about your text: a Caesar shift by chi-squared fit, a substitution, a polyalphabetic cipher, a transposition read off digram rate, and plain English. Each candidate gets a 0 to 100 fit score, sorted, never one verdict. Under 20 letters gets a request for more text, and under 200 the ranking is flagged as weak evidence.
Questions
What candidates does it rank?
Five: a Caesar shift, a monoalphabetic substitution, a polyalphabetic (Vigenere-like) cipher, a transposition, and plain English, each scored 0 to 100.
How does it score a Caesar shift?
It tries all 26 shifts and picks the one whose letter frequencies fit English best by chi-squared, then scores that fit.
What is the minimum input length?
Twenty letters. Below that the tool returns a message asking for more text rather than a ranking.
Why does it warn about short text?
Below 200 letters, index of coincidence is noisy enough that the substitution and polyalphabetic guesses can rank incorrectly, so the tool adds a note saying the ranking is weak evidence.
Does it decrypt the cipher?
No. It only ranks guesses about which cipher was used; it never tries to recover the plaintext.
Can I use this from the command line?
Yes, the same code runs on the page, the CLI and MCP.