Which number is the row?

The first digit is the row and the second is the column. With one-based numbering, 11 is the upper-left cell and 55 the lower-right. Choose numbering from zero if the original uses 00 to 44.

Can I decode a keyed Polybius square?

Yes, when you supply its keyword and match the numbering convention. The tool builds that square and decodes exactly; it does not recover unknown keyed squares.

Why do I get I where I expected J?

The standard 5x5 square merges I and J. The 6x6 workflow keeps J separate and adds digits. A 3x3 or letter-coordinate variant is not implemented.

Polybius Square Cipher Decoder and Encoder

This workflow opens the standard 5x5 Polybius square with one-based row-column coordinates. Decode 24 11 32 to IAM: I and J share one cell, so JAM cannot be recovered distinctly. Use an optional square keyword to change the layout, match the sender's numbering base, and inspect the square and first calculations. Use spaces between pairs and / between message words.

Cipher text8 characters

Plain textPolybius 5 x 5, I and J share cell 24

IAM

decode row then column, numbered 1 to 5; letters become two-digit pairs and / marks a space between words; I and J share one cell, so J decodes as I; case is not kept; digits, punctuation and letters outside A to Z are dropped

	1	2	3	4	5
1	A	B	C	D	E
2	F	G	H	I/J	K
3	L	M	N	O	P
4	Q	R	S	T	U
5	V	W	X	Y	Z

24row 2, column 4, I (J shares this cell)
11row 1, column 1, A
32row 3, column 2, M

encode encode each letter as its row and column

Square

Numbered from

Square keyword

Method: Polybius 5 x 5, I and J share cell 24

Pairs read: 3

Encode: encode each letter as its row and column

Each letter becomes its row and column in a square: 5 x 5 with I and J sharing a cell, or 6 x 6 with the digits 0 to 9 added.

Textbook checks, one per method

Each row is worked by the same code as the stage above. Load puts it in the workbench with its keys.

Caesar, shift 3ABCXYZ
DEFABC
Keyword KRYPTOSKNOWLEDGE IS POWER
DGHVETPST BM IHVTL
Polybius 5 x 5, I and J share cell 24JAM
24 11 32, which decodes as IAM
A1Z26, numbers separated by spaces1 26
AZ
Nihilist, square ZEBRAS, key RUSSIANDYNAMITE WINTER PALACE
37 106 62 36 67 47 86 26 / 104 53 62 77 27 55 / 57 66 55 36 54 27
Book cipher, a 40-word reference text3 17 52
error: word 52 is outside the 40-word reference, and nothing is guessed
One-time pad, key XMCKLHELLO
EQNVZ

Puzzles and teaching, not secrets. These are historical and puzzle ciphers. Caesar has only 25 shifts on A to Z, so the ranked list that Decode shows reads most messages without the key, and keyword, Polybius and A1Z26 text gives way to letter counting. The one-time pad mode shows the arithmetic of a key you type and nothing more: this tool does not make keys and makes no claim that a message stays secret.

Accuracy. Each method follows the convention shown beside it: Caesar shifts count from 0 at the first letter of the alphabet and wrap after the last, the 5 x 5 Polybius and Nihilist squares give I and J one cell, A1Z26 reads 1 26 as AZ, and a book number outside your reference is an error, never a guess. The ranked list of Caesar shifts is an estimate from English letter frequency. These are historical and puzzle ciphers, not modern secure encryption, and the one-time pad mode only shows the arithmetic of the key you type, with no claim that it keeps a message secret.

Common questions

Which number is the row?: The first digit is the row and the second is the column. With one-based numbering, 11 is the upper-left cell and 55 the lower-right. Choose numbering from zero if the original uses 00 to 44.
Can I decode a keyed Polybius square?: Yes, when you supply its keyword and match the numbering convention. The tool builds that square and decodes exactly; it does not recover unknown keyed squares.
Why do I get I where I expected J?: The standard 5x5 square merges I and J. The 6x6 workflow keeps J separate and adds digits. A 3x3 or letter-coordinate variant is not implemented.

Each method follows the convention shown beside it: Caesar shifts count from 0 at the first letter of the alphabet and wrap after the last, the 5 x 5 Polybius and Nihilist squares give I and J one cell, A1Z26 reads 1 26 as AZ, and a book number outside your reference is an error, never a guess. The ranked list of Caesar shifts is an estimate from English letter frequency. These are historical and puzzle ciphers, not modern secure encryption, and the one-time pad mode only shows the arithmetic of the key you type, with no claim that it keeps a message secret.