⌘ K
polybius · 5x5 grid · i/j merged · live

Encode with the Polybius square

Classical Greek cipher that maps each letter to a row-column pair on a 5x5 grid. I and J share a cell. Runs locally in your browser.

Polybius Square
Input
5 B
1 lines
Output
14 B
1 lines
Status
ready
live
Ratio
2.80×
out/in
input · text
output · polybius
23 15 31 31 34
Quick Start

How to use the Polybius square

Letters are placed on a 5x5 grid (I and J share one cell). Each letter becomes a two-digit code: the first digit is the row, the second is the column.

  1. 01

    Pick a mode

    Encode turns letters into digit pairs; decode reverses the process. The grid is fixed (I/J merged).

    Bytes flow through TextEncoder/TextDecoder for predictable UTF-8 semantics — emoji, surrogate pairs, and combining marks all encode losslessly.

    modeUTF-8TextEncoderlosslesslive
    mode · raw bytesencode · UTF-8
    encode | decode
    loaded · UTF-8MIME · ready
  2. 02

    Type or paste input

    When encoding, letters become row-column pairs. When decoding, paste digit pairs separated by spaces or newlines.

    Hash and signature operations call crypto.subtle from the Web Crypto API, so output matches OpenSSL / Node bit-for-bit.

    inputWeb Cryptosubtledeterministiclive
    input · optionstext · live
    HELLO
    options applied · livelive
  3. 03

    Grid lookup

    H sits in row 2, column 3 → 23. E sits in row 1, column 5 → 15, and so on.

    Options re-run instantly through useEffect + useMemo — switching URL-safe mode or padding doesn’t re-encode from scratch.

    gridmemoisedoptionsinstantreactive
    grid · encoded5x5 · ready
    A B C D E
F G H I/J K
L M N O P
Q R S T U
V W X Y Z
    encoded · livelossless
  4. 04

    Copy or download

    One click to clipboard. Download as .txt.

    Copy uses the Async Clipboard API; downloads ship via Blob with the appropriate MIME (text/plain, image/png, …) so external tools recognise them.

    exportclipboardblobMIME-correctclient-side
    out · copiedcode · done
    23 15 31 31 34
    shipped · ready to shipsuccess
Pro tips

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.

Related toolsBacon CipherSubstitution CipherA1Z26 CipherRail Fence CipherCaesar CipherAtbash Cipher
Use Cases

When to use the Polybius square

History, puzzles, knock-code reconstruction.

Crypto history

Ancient Greek cipher — useful for teaching pre-modern crypto. 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-compliant

ADFGVX foundation

Polybius is the substitution stage of ADFGVX — study them together. 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-matched

Knock codes

Tap codes used by prisoners and spies map directly onto the grid. 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.

lossless

CTF puzzles

Common decoding challenge in beginner CTFs. 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-identical

Escape rooms

Print a grid as a puzzle prop. 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-grade

Cross-tool checks

Verify other Polybius implementations match this layout. 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-ready
FAQ

Frequently asked questions

The classical 5x5 grid only fits 25 letters. Combining I and J keeps the standard English alphabet compatible with the grid.
Switch to decode mode and paste digit pairs. Spaces and non-digit characters are stripped automatically.
Yes — a keyed Polybius rearranges the letters using a keyword. This tool uses the standard alphabetical layout.
No. It is equivalent to a monoalphabetic substitution and falls to frequency analysis.
No. The cipher runs locally in your browser.
Suggest a tool

Something missing?

Tell us what you need. We ship the top requests every two weeks.

Sent straight to the maintainers · no account needed
47 in queue·8 shipped this month
Request queued. Thanks!