Tap code
Knock-knock, spelled in a 5×5 grid.
Where it came from
Tap code is a knock-based cipher built on the Polybius square: a 5×5 grid of letters addressed by row and column. It became famous among American POWs in Vietnam, who tapped messages through prison walls.
Because you only need to count taps, it works without sight or sound equipment — a lifeline of communication under isolation.
How it works
Each letter is two numbers (row, column) in a 5×5 grid (I and J share a cell). You tap the row count, pause, then the column count. "E" (row1,col1) is one tap, a pause, one tap.
UWC encodes and decodes Tap code in your browser — a quiet nod to one of history's most human ciphers.