https://en.wikipedia.org/wiki/Shave_and_a_Haircut

The former prisoner of war and U.S. Navy seaman Doug Hegdahl reports fellow U.S. captives in the Vietnam War would authenticate a new prisoner's U.S. identity by using "Shave and a Haircut" as a shibboleth, tapping the first five notes against a cell wall and waiting for the appropriate response. U.S. POWs were then able to communicate securely with one another via the quadratic alphabet code.

https://en.wikipedia.org/wiki/Tap_code

knock code, is a way to encode text messages on a letter-by-letter basis in a very simple way. The message is transmitted using a series of tap sounds, hence its name.

The tap code has been commonly used by prisoners to communicate with each other. The method of communicating is usually by tapping either the metal bars, pipes or the walls inside a cell.

The tap code is based on a Polybius square using a 5×5 grid of letters representing all the letters of the Latin alphabet, except for K, which is represented by C.

Each letter is communicated by tapping two numbers, the first designating the row and the second (after a pause) designating the column. For example, to specify the letter "B", one taps once, pauses, and then taps twice. The listener only needs to discriminate the timing of the taps to isolate letters.