Tap code
Simple code that sends letters as knocks on a 5×5 grid, and that prisoners of war in Vietnam used to talk through cell walls.
The tap code is a Polybius-style square with a 5×5 grid where C and K share a cell. Each letter is sent as two groups of taps: first the row, then the column, with a short pause in between. It needs no key and no equipment, only a wall and a pair of hands. Its security is practically nil; its point is that it is easy to learn under stress.
American prisoners of war in North Vietnam used it during the 1960s to communicate between cells. The method is slow and audible, but to anyone who does not know the code the messages sound like noise. Strictly speaking it is an encoding rather than a cipher.