buttcrack. Cipher Wiki

Keyword substitution

From the Buttcrack Cipher Wiki — the free field guide to classical ciphers

Keyword substitution — The mixed alphabet is generated from a keyword followed by the unused letters in order, which makes it memorable.

Mixed alphabet built from a keyword, then the remaining letters in order.

A worked example§

Encrypting a sample with the cipher itself, at build time:

MEET ME BY THE OLD CLOCK TOWER AT DAWN

Key: CIPHER

produces the ciphertext

KEETKEIYTBEMJHPJMPGTMWEQCTHCWL

This example is generated by running the cipher when the page is built, and the round trip is checked, so it always matches what the solver does.

How it is broken§

Exactly as hard as a random mixed alphabet to break by statistics, but far easier to guess at, because the tail of the alphabet stays in order and betrays the keyword's length.

History and context§

The practical compromise of the pencil-and-paper era: a full random alphabet is stronger but nobody can carry one in their head.

See also§

Simple substitution

Browse every cipher in the Monoalphabetic substitution family, read the history of codebreaking, or return to the wiki main page.

Frequently asked questions§

How is the Keyword substitution broken?

Exactly as hard as a random mixed alphabet to break by statistics, but far easier to guess at, because the tail of the alphabet stays in order and betrays the keyword's length.

What key does the Keyword substitution use?

keyword. The keyspace is unbounded in practice.

How much ciphertext do I need?

At least 40 characters for this solver to attempt it; short messages can be readable and still not be proof.

Can I break a Keyword substitution on this page?

Not with the browser build, which targets the common puzzle families. The desktop version searches it: see the Windows app page.

Categories: Monoalphabetic substitution | Full-version ciphers | Cipher wiki
The facts in this article are generated from buttcrack's cipher registry when the site is built, so they describe the implementation you can run on this page. Registry-generated content is checked against the code; the history is editorial.

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It tries: Caesar, Atbash, ROT13, affine, Trithemius, rail fence, single-byte XOR, periodic Vigenère-family ciphers, autokey, selected encoding layers (such as Base64, hex, binary, decimal ASCII, Morse, and reverse), and—only with 60+ A–Z letters—statistical substitution.

It does not: prove a decryption, cover every classical cipher, or break modern encryption such as AES or RSA. Its ranking model is tuned for English, so a high score is a lead to verify with the method, key, and source context—not a guarantee.

Short text, non-English plaintext, non-Latin or symbol alphabets, missing keys, and unsupported formats can all leave no high-confidence answer. When that happens, the result includes input-specific observations and suggested next checks; those observations are not a claim to know the exact cause.

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