Polybius cipher online – encoder/decoder
Polybius cipher encoder and decoder online: turn letters into 5 × 5 grid coordinates. Includes the shared I/J cell and a worked example.
How do you send a message with torches?
What is the Polybius square?
The Polybius square turns every letter into a pair of digits: the row number and the column number in a 5 × 5 grid. Instead of letters you get a sequence of coordinates, which is why the method is also called a coordinate cipher.
How is the 5 × 5 grid built?
The Latin alphabet has 26 letters and the grid holds 25 cells, so I and J share one cell. Letters are filled in row by row: the first row is A B C D E, the second F G H I/J K, and so on until Z lands in cell 55.
Example with the word REBUS
The message “REBUS” is written as “42 15 12 45 43”: R sits in row 4 column 2, E in row 1 column 5, B in row 1 column 2, U in row 4 column 5 and S in row 4 column 3. You can reproduce this example with the working tool above.
Complete coordinate table
The top row shows the letter and the bottom row the pair of digits that replaces it: the first digit is the row, the second the column. Cell 24 serves both I and J.
| Letter | 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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Coordinates | 11 | 12 | 13 | 14 | 15 | 21 | 22 | 23 | 24 | 25 | 31 | 32 | 33 | 34 | 35 | 41 | 42 | 43 | 44 | 45 | 51 | 52 | 53 | 54 | 55 |
How do you decrypt the text?
Split the ciphertext into pairs of digits and read each pair as a row and a column. A slash marks a space, and characters outside the grid are kept in square brackets by the tool. When you reach cell 24 you decide yourself whether the word needs I or J.
Why turn letters into numbers?
Coordinates travel through channels that know nothing about letters: torches, knocks on a wall or light signals. Two digits per letter also serve as a building block for stronger ciphers — ADFGVX and the Nihilist cipher first split letters into coordinates and only then scramble them.
How can the Polybius square be broken?
The ciphertext consists only of digits from 1 to 5 arranged in pairs, which already reveals the method. Because every letter always produces the same pair, ordinary frequency analysis is enough: the most common pair matches the most common letter of the language. An unknown letter arrangement in the grid slows the work down but does not stop it.
Why is the Polybius square insecure?
It is still a substitution: letter order is preserved and every letter has a fixed counterpart. The ciphertext is twice as long as the message yet hides none of its structure. Without an added transposition layer the method does not protect confidential content.
See also: Rail fence cipher
History of the Polybius square
The Greek historian Polybius described the system in the second century BCE as a way of sending messages over a distance: two groups of torches signalled the row and the column. The grid returned in modern cryptography as a component of field ciphers, and in the prisoners’ tap code it carries conversations through a wall.
Printable Polybius square
An A4 worksheet with the current square layout, coordinate instructions, a worked example and a puzzle for students.