Chess code

From Memory Alpha, the free Star Trek reference.

A chess code is a system of using moves of chess pieces on a chessboard (be it 2- or 3-dimensional) as signs and countersigns in covert or insecure communications.

Kirk and his senior officers used a chess-based code as transporter clearance in 2268, when Garth of Izar planned to escape from the Elba II insane asylum. Chief Engineer Scott declined to beam Garth, disguised as Kirk, to the Enterprise, because Garth did not know the code response "queen to king's level 1" when Scott queried with the code phrase "queen to queen's level 3." (TOS: "Whom Gods Destroy")