Ciphertext Frequency Analysis & Chi-Square Tool
Letter statistics, index of coincidence, chi-square.
About this tool
Substitution ciphers preserve letter statistics, and that is how they die: English text has a fingerprint (E≈12.7%, T≈9.1%…) that survives any monoalphabetic substitution. This tool charts your text's letter frequencies against English, computes the index of coincidence (≈0.067 for English, ≈0.038 for random), and gives the chi-square distance — the numbers that tell you what kind of cipher you're facing.
Like everything on encrypt.tools, this runs entirely in your browser — nothing you enter is transmitted or stored.
Frequently asked questions about Frequency Analysis
What is the index of coincidence?+
A statistic measuring how likely two random letters in the text match. English is about 0.067; random text about 0.038. It helps distinguish monoalphabetic from polyalphabetic ciphers and estimate key length.
How does chi-squared help?+
It measures how far the text's letter frequencies are from English. A low value suggests plain or monoalphabetically-substituted English; a high value suggests a different distribution.
What cipher does my text use?+
An index of coincidence near 0.067 points to a substitution cipher or plaintext; near 0.04 points to a polyalphabetic cipher like Vigenère. The tool reports both statistics.
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