How does a keyword change the Polybius square?

The keyword's unique letters are written into the grid first, in order, then the rest of the alphabet fills the remaining cells. This scrambles the standard layout so coordinates differ from a plain Polybius square.

Why is J missing from the grid?

A 5x5 square has only 25 cells but the alphabet has 26 letters, so J is traditionally merged into I. When decoding, readers treat any I as possibly J based on context.

How are letters encoded?

Each letter is replaced by its grid coordinates: the row number followed by the column number, both from 1 to 5. So a letter in row 3, column 2 becomes 32.

Is a keyed Polybius square more secure?

It is slightly stronger than a plain square because an attacker must guess the keyword too, but it is still a simple substitution and offers no real security against modern analysis.

Is my text uploaded anywhere?

No. The grid is built and the text encoded entirely in your browser, with nothing sent to any server.

Polybius Square with Keyword

The keyword Polybius square is a classic cipher that maps each letter to a pair of coordinates in a 5×5 grid. Adding a keyword scrambles the grid so the coordinates are not the same as a plain alphabetical Polybius square, making it a popular building block in puzzle hunts and historical cryptography. This tool builds the keyed grid and encodes text locally in your browser.

How it works

First the keyword’s letters are written into the grid left-to-right, top-to-bottom, skipping any letter that already appeared. Then the remaining letters of the alphabet (in order) fill the leftover cells. Because the square has only 25 cells, J is merged with I.

For the keyword GERA, the grid begins G E R A and then continues with B C D F H...:

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

To encode a letter you read its row then its column. A sits at row 1, column 4, so it becomes 14. The full message is a stream of two-digit pairs.

Tips and example

With the keyword GERA, the word HIDE encodes as:

H → row 2, col 4 → 24
I → row 2, col 5 → 25
D → row 2, col 2 → 22
E → row 1, col 2 → 12

Choose a memorable keyword and keep it secret — anyone with the keyword can rebuild the identical grid and decode instantly. Letters not in the grid (and the letter J, which maps to I) are normalised before lookup.