Polybius Square
Encode and decode messages using the Polybius square cipher. Each letter becomes a pair of numbers.
Polybius Square
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C | D | E |
| 2 | F | G | H | I | K |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
Note: I and J share the same cell
What is the Polybius Square?
The Polybius square is an ancient cipher invented by the Greek historian Polybius around 150 BCE. It encodes each letter as a pair of numbers representing its position in a 5×5 grid, making it one of the earliest fractionating ciphers.
How It Works
The standard Polybius square arranges the 26-letter alphabet in a 5×5 grid (I and J share a cell):
| 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 |
Each letter is encoded as its row number followed by its column number:
- A = 11 (row 1, column 1)
- B = 12 (row 1, column 2)
- H = 23 (row 2, column 3)
- Z = 55 (row 5, column 5)
Using Keywords
A keyword can be used to create a mixed alphabet square, making the cipher more secure:
- Write the keyword first (removing duplicate letters)
- Fill in remaining letters in order
- This creates a scrambled grid that both parties must know
Example with keyword "CACHE":
- First row: C, A, H, E, B
- Remaining letters fill in: D, F, G, I/J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z
Polybius Square in Geocaching
This cipher is popular in geocaching puzzles because:
- Number-based: Can be hidden as coordinate digits
- Visual appeal: Grid can be part of puzzle design
- Tap code connection: Forms basis of prisoner communication
- Historical interest: Ancient Greek origins
Recognizing Polybius Cipher
Look for these patterns:
- Paired digits: Numbers appear in pairs (11, 23, 44...)
- Range 1-5: No digit larger than 5
- Even length: Total digit count is always even
- 5×5 grid hint: References to squares or grids
Related Ciphers
Tap Code
Based on the Polybius square, prisoners use taps or knocks to communicate letters. The row is knocked first, then the column after a pause.
ADFGX and ADFGVX
WWI German ciphers that use a Polybius-like grid labeled with letters instead of numbers, then apply additional transposition.
Bifid Cipher
Uses the Polybius square but combines row and column numbers in a more complex pattern for additional security.
I/J Handling
Since the 26-letter alphabet doesn't fit in a 5×5 grid (25 cells), I and J traditionally share a cell. When decoding, context usually makes the correct letter clear. Some variants instead combine Q/Z or remove Q entirely.