Crack a substitution cipher
Frequency-analysis aid for monoalphabetic substitution ciphers. Auto-guess from letter frequency, then refine mappings manually until plaintext emerges.
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How frequency analysis works
In English, E is the most common letter, then T, A, O, I, N. Auto-guess maps cipher letters to English letters by frequency rank — often gets you 50-70% right; refine manually.
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Paste cipher
Any monoalphabetic substitution.
Bytes flow through
TextEncoder/TextDecoderfor predictable UTF-8 semantics — emoji, surrogate pairs, and combining marks all encode losslessly.inputUTF-8TextEncoderlosslessliveinput · raw bytescipher · UTF-8QEBOB FP …
loaded · UTF-8MIME · ready - 02
Auto-guess
Map cipher letters by frequency rank to English.
Hash and signature operations call
crypto.subtlefrom the Web Crypto API, so output matches OpenSSL / Node bit-for-bit.guessWeb Cryptosubtledeterministicliveguess · optionsfreq · liveQ → E
options applied · livelive - 03
Refine
Use bigrams (TH, HE) and trigrams (THE) to confirm.
Options re-run instantly through
useEffect+useMemo— switching URL-safe mode or padding doesn’t re-encode from scratch.refinememoisedoptionsinstantreactiverefine · encodedngram · readyTHE = 12
encoded · livelossless - 04
Read plaintext
Final plaintext shown above the mapping grid.
Copy uses the Async Clipboard API; downloads ship via
Blobwith the appropriate MIME (text/plain,image/png, …) so external tools recognise them.readclipboardblobMIME-correctclient-sideok · copiedready · doneThere is …
shipped · ready to shipsuccess
Encoding without surprises
Always UTF-8 first
Use TextEncoder / TextDecoder for bytes ↔ string conversion. btoa alone breaks on non-ASCII.
URL-safe vs standard Base64
Standard uses +, /, =; URL-safe swaps to -, _, no padding. Don’t mix them.
Hashes aren’t passwords
SHA-256 is fast; that’s good for fingerprints, bad for passwords. Use bcrypt / argon2 for credentials.
Watch padding rules
Some decoders are strict about = padding, others auto-pad. If round-trips fail at exactly 1 or 2 bytes, check padding.
Use Web Crypto for keys
Generate cryptographic keys via crypto.subtle, never Math.random. The latter is predictable.
JWT decode ≠ verify
Decoding a JWT just splits and base64-decodes. Verification needs the issuer’s public key / shared secret.
When to break a cipher
Puzzles, classroom, history.
Cryptograms
Newspaper-style word puzzles. Output round-trips losslessly with the matching decoder — both run client-side, so end-to-end testing happens in a single tab. Encoding flows through TextEncoder for predictable UTF-8 semantics.
RFC-compliantClassroom
Teach frequency analysis hands-on. Web Crypto's subtle.digest produces output that matches OpenSSL, GPG, and Node's crypto module byte-for-byte. That makes the result trustworthy as a content fingerprint, ETag, or integrity check.
OpenSSL-matchedHistorical ciphers
Crack 18th-century diplomatic codes. Pairs cleanly with HMAC, JWT, and signed-URL workflows where bit-level correctness matters. The browser's crypto stack is FIPS 140-validated in Chrome and Firefox, so it's production-grade.
losslessCTF challenges
Quick win for beginner crypto puzzles. Output uses canonical RFC 4648 / 7515 alphabets so curl, openssl, and any standards-compliant decoder accept it as input. URL-safe variants drop padding for tokens that travel in query strings.
byte-identicalDemo
Show why substitution is weak. Heavy hashing runs through useMemo so large inputs don't block typing. PBKDF2, bcrypt, and Argon2 expose configurable cost factors — tune for your threat model, not the default.
spec-gradeEscape rooms
Online tool for cipher-based escape games. Same input plus same options always yields identical bytes. That's the foundation for content-addressed storage (IPFS, Git, S3 ETags) and means the output is safe to commit to fixtures.
production-readyFrequently asked questions
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