Polybius square decode
Polybius square decode, classical cipher, a puzzle not a protection, computed on your device. Output is uppercase letters only, since the row and column digits carry no case, spacing or punctuation.
Polybius square decode reads digit pairs off your text and looks each up as a row and column on the 5 by 5 grid built from key. An odd digit count is refused, and an out-of-range pair is refused by its position. Only uppercase letters come back, since a digit pair carries no case, spacing or punctuation.
Questions
What if I have an odd number of digits?
The tool refuses, since every letter needs exactly two digits, a row and a column.
What if a digit pair is out of range?
It is refused and named by its position; rows and columns are only 1 through 5.
Why is my output all uppercase with no spaces?
Because the grid only encodes the 25 letters, so case, spacing, digits and punctuation were never captured to decode back.
What key do I need?
The same one used to encode, or blank for the plain unkeyed square.
Can I use this from the command line?
Yes, it runs the same on the page, the CLI and MCP.