The Polybius square turns every letter into a pair of numbers — the row and column of a 5×5 grid — so a message becomes a stream of coordinates that says nothing without the grid.
Draw a 5×5 grid and fill it with the alphabet. There are only 24 letters after I and J share a cell, so both live in the same box.
Number the rows and columns 1–5. Every letter now has an address:
A is 11, B is 12, Z is 55.
To encrypt, replace each letter with its pair:
ATTACK becomes 11 44 44 11 13 25.
To decrypt, split the digit stream into pairs and look each one up in the grid — the pair is the row and column of the letter.
Each row follows one plaintext letter: which coordinates it becomes, and how the pair stream grows. Notice the shape: two digits per letter, nothing but the digits 1–5 — the whole fingerprint of this cipher.
Fill the grid with a Key first and the mapping changes
completely. With the keyword SPARTAN, the square starts
S P A R T A N… — the keyword's letters (deduplicated), then the rest
of the alphabet in order.
The pairs now point to different letters than the plain square would — an attacker who assumes the plain grid decodes gibberish. The I/J shared cell survives: the square always holds 25 letters.
Strip away the grid and nothing has moved: every plaintext letter maps
to one fixed symbol, always. The coordinates are just an alphabet
written as numbers — a Substitution, not a transposition. That
is why the ciphertext keeps a substitution's fingerprints: the most
frequent pair is the most frequent letter, E, wearing the number 15
of E's cell.
Open the tool, leave the square key empty and type HELLO. The
output is 23 15 31 31 34 — five letters in, five pairs out.
Watch the square under the controls: the highlighted cell jumps to the last letter of the output, with its row and column lit.
Switch to decrypt mode and paste the pairs back — HELLO returns.
The pairs are just addresses; the grid looks them up either way.
Type a square key, say SPARTAN, and encrypt the same HELLO. The
output changes completely — same pairs in the plain square, new
letters in the keyed one.
Clear the key and compare the two outputs pair by pair: only the letters that moved inside the grid changed their numbers.
Notice the square keeps exactly 25 letters whatever you type — duplicates in the key are skipped, and I/J still share a cell.
Encrypt MEET AT NOON with no key and write the pairs on paper in
rows of five pairs.
Cover the grid and try to read the message from the pairs alone — you will find yourself reconstructing the grid from memory, which is exactly the lookup table the cipher forces everyone to share.
Try the practice challenges: the decode exercises use the plain square, and the hardest one uses a keyed square — its keyword travels in the hints.
There is no cracker tab for this cipher — and that is not an oversight, it is the lesson.
Try it in the toolEvery plaintext letter maps to one fixed symbol, always: E is always
15 of the plain square, whatever comes before or after it. Strip the
pairs back to letters and you are holding an ordinary
Substitution — the very cipher whose guided breaking the
Keyword page teaches.
Count the pairs in the ciphertext. With the plain square, the most
frequent pair is E (15), then T (44), A (11), O (34) — the
usual English order wearing numbers.
Assign candidate letters to the frequent pairs, exactly like the Frequency analysis step of the keyword cracker.
When the square is keyed, the assignment is the square: recover enough letters and the grid fills itself in — then the keyword reveals itself the way it does in the keyword cracker.
Take a keyed-square Polybius ciphertext, paste it into the Keyword substitution cracker after rewriting each pair as its plain-square letter (or as any fixed placeholder), and the assignment board does the rest. The coordinates change the costume, not the weaknesses: a monoalphabetic cipher is a monoalphabetic cipher, whether its symbols are letters or numbers.
On its own the square is trivial. Chained into fractionation and transposition — ADFGVX — or given a periodic numeric key — the Nihilist cipher — the same 25 cells become the substrate of far tougher systems. That is why this page exists: to teach the coordinate idea once, cleanly, before the heavier ciphers borrow it.