|The modified tap code table used during Operation Intrude F014.|
The tap code is a cipher, commonly used by prisoners to communicate with one another. The method of communicating is usually by "tapping" either the metal bars or the walls inside the cell, hence its name. It is a very simple cipher, not meant to avoid interception, since the messages are sent in cleartext.
United States prisoners of war in North Vietnam during the Vietnam War are most known for having used the tap code, although its use dated back to as early as the Korean War
The original tap code is based on a 5×5 grid of letters, representing all the letters of the Latin alphabet except K (C is used to represent K). Each letter was communicated by tapping two numbers: the first designated the row (horizontal) and the second designated the column (vertical). The letter "X" was used to break up sentences. The tap code requires the listener to only discriminate the timing of the taps to isolate letters. For example, to specify the letter "A", one would tap once, pause, and then tap once again.
Or to communicate the word "WATER" the cipher would be the following (the time between each pair of numbers is smaller than the one between two different letters):
..... .. . . .... .... . ..... .... .. (5,2) (1,1) (4,4) (1,5) (4,2) W A T E R
Because of the difficulty and length of time required for specifying a single letter, prisoners often devise abbreviations and acronyms for common items or phrases, such as "GN" for Good Night, or "GBU" for God Bless You.
By comparison, Morse code is harder to send by tapping or banging because Morse requires the ability to create two different sounding taps. A Morse novice would also need to keep a cheat sheet until he remembers every letters code, which his captors would likely confiscate. Tap code can be easily decoded in one's head by mentally using the table.
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During Operation Intrude F014, Dr. Drago Pettrovich Madnar used a modified version of the tap code to communicate his radio frequency to Solid Snake from his prison cell. Colonel Campbell also used the same version to communicate his third frequency to Snake. This version used a 5×7 grid in order to accommodate numbers.
Behind the scenes
The tap code was included in the original MSX2 manual for Metal Gear 2: Solid Snake. Its use in the game was an attempt at preventing video game piracy, since one must own the original game to have a copy of the manual. Of the two occurrences of tap code in Metal Gear 2, only Dr. Madnar's radio frequency is required for completing the game.
The condensed English manual for Metal Gear Solid 3: Subsistence, in which Metal Gear 2 was re-released, provides Madnar's frequency, but does not give the tap code table itself, nor the second frequency for Colonel Campbell. Konami instead posted the missing information in a FAQ page on their official website, which has since been taken down. A more comprehensive manual for Subsistence, which included the tap code table, was later released with the Metal Gear Solid: HD Collection, in digital form.