The Black Chamber

intercepted ciphertext —

letter frequency — observed vs English —

English profile this text

Index of coincidence — English ≈ 0.067, random ≈ 0.038. High ⇒ monoalphabetic (Caesar/substitution); flattened ⇒ polyalphabetic (Vigenère).

Key length (IoC) — For a Vigenère cipher, the period whose per-column IoC jumps to English.

Englishness of recovery — How English the recovered plaintext reads, by the trigram model. Climbs as the analysis resolves the key.

recovered plaintext —

The desk is quiet. Load a sample, or paste a cipher, and press Crack it.

awaiting ciphertext

A Black Chamber was a state's cipher-cracking office. This one is the workshop's analyst: give it ciphertext with no key, and it recovers the key and the message by classical cryptanalysis — frequency analysis, the index of coincidence, and a hill-climb on an English language model. It is the counterpart to the Volvelle and Scytale, which make the ciphers it breaks.

Load a sample cipher

Cipher type

Desk finish

How it reads. The bars compare this text's letter frequencies against English. A flat profile and a low IoC betray a Vigenère; an English-shaped profile means a Caesar or substitution. Watch the plaintext below resolve from gibberish to English as the hill-climb finds the key.