Decode substitution ciphers using frequency analysis. Interactive letter mapping with bigram/trigram hints and English pattern matching.
Paste ciphertext and the tool ranks each letter by how often it appears, then lets you assign plaintext letters one at a time until the text reads in English. The frequency ranking, bigram/trigram hints, and word-pattern matches are suggestions, not answers: the mapping stays manual, and the decoded text, hints, and candidates update as you confirm letters. The surprise: a few confirmed letters - often from one repeated short word - unlock more than the full frequency table. Frequency analysis assumes a fixed one-to-one letter substitution and reasonably long text; both must hold for the rankings to mean much. Everything runs in the browser, so the ciphertext never leaves your machine.
A concrete input and expected output from the current implementation.
Input
ZIT JXOEA WKGVF YGB PXDHL GCTK ZIT SQMN RGU
Expected output
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG - the fully decoded message, with the mapping confirmed (Z to T, I to H, T to E, J to Q, X to U, O to I, E to C, A to K, and so on).
This ciphertext was made with the classic keyboard substitution, whose cipher alphabet reads QWERTYUIOPASDFGHJKLZXCVBNM in keyboard order under the plaintext alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ. So ciphertext Z decodes to T, I to H, and T to E, making ZIT read THE, and because a substitution maps every letter the same way throughout, the rest of the pangram follows.
The tool counts every ciphertext letter and sorts the counts. In normal English prose E is the most frequent letter by a clear margin (around 13% of letters), followed by T, A, O, I, N, S, H, R, so the tool proposes the most common ciphertext letter as E. The proposal is a hypothesis the user accepts or reassigns; each accepted pair tightens every other suggestion.
Each ciphertext word is reduced to a shape - ABBA for a four-letter word whose first and fourth letters match - and compared against dictionary words with the same shape, so an ABBA-shaped word yields candidates like SEES or NOON. Bigram and trigram counters flag common sequences such as TH, HE, IN, THE and AND, which turns a repeated ZIT at both ends of a message into a confident THE.
The ranking is a guess, not a proof. In a typical puzzle of 200-300 letters the top few positions are usually right, but the middle of the ranking drifts, and a single wrong assignment corrupts every word that contains that letter. That is why the mapping stays manual and the hints recompute as you confirm letters.
Yes. A Caesar shift is a substitution cipher whose alphabet is rotated by a fixed amount, so it has exactly the fixed letter-to-letter map this tool handles and you will get the plaintext. It will not label the result a Caesar shift, though - if you already suspect one, rotating the alphabet by hand is faster.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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