Answer: Postal Chess

Correspondence chess, or “postal chess”, is chess played by assigning alphanumeric values to the individual squares of the chess board and then using the coordinates, like “A7” and “B3” as well as absolute numbers—seen in the postal chess postcard example here—to communicate chess moves to an opponent by mail. The practice first appeared in Germany in the late 1920s and spread around the globe.

During World War II, however, the United States banned postal chess—by censoring out any correspondence with chess coordinates or board diagrams in it—over concerns that the game of chess could be subverted into a code system to send communication between spies.

Although there are no known examples of spies actually using the coordinates of a game of chess to communicate secret messages, a pair of researchers in 2009, Abdelrahman Desoky and Mohamed Younis, outlined how the coordinates of the postal chess system could, using an algebraic system (as well as extraneous chess data like listings of player rankings on the Internet), be used to hide a message that had been translated from its original language into binary. The system, while a fascinating exercise, requires a lengthy game of chess to conceal even a modest message and, if carried out by actual postal delivery, would take months to transmit.

Nonetheless, the very premise of spies using chess as a method of communication is a fascinating one because it would be devilishly difficult to detect. Because chess games take time and each move would be mailed individually, not only would the method of communication both unfold over a lengthy span of time and be relatively obfuscated to begin with, but the majority of people are either not familiar with the rules of chess or only possess the basic kind of familiarity with the game that allows them to play (albeit not proficiently). Only a postal censor or other counter-spy who both had access to the entire stream of mail-based communication over time and who had an in-depth understanding of the game would even begin to notice any sort of irregularities in the chess game unfolding under their nose.

So with that in mind, although there are no known examples of spies using chess board coordinates like a cipher, the reality is that detecting such a cipher would be so difficult that it would likely remain totally undetected. While the United States may have seemed paranoid for censoring the postcards and letters international chess players used to communicate with each other, that paranoia may have been well justified.