The Tap Code is a manual cipher system used for encoding messages through a series of taps, knocks, or other simple signals, and was popularized for covert communication among prisoners of war in the 20th century, particularly during World War II and the Vietnam War. It is essentially a numeric substitution cipher based on a 5×5 Polybius square in which letters are assigned coordinates, with C and K sharing a single position. The cipher allows messages to be transmitted through simple auditory, tactile, or visual signals, making it useful when writing or speaking is impossible or monitored.
1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
1 | A | B | C/K | D | E |
2 | F | G | H | I | J |
3 | L | M | N | O | P |
4 | Q | R | S | T | U |
5 | V | W | X | Y | Z |
In the Tap Code, each letter is represented by two numbers corresponding to its row and column in the Polybius square. Each number is communicated by a sequence of taps: the first set of taps indicates the row, and the second set indicates the column. For example, to encrypt the word HELLO, letters might map as H=23, E=15, L=31, L=31, O=34. In practice, the letter H would be tapped twice, pause, then three taps to indicate 23, E tapped once, pause, then five taps for 15, and so on. Decryption is performed by counting taps and mapping the coordinates back to letters in the same Polybius square.
The Tap Code is valued for its simplicity and practicality rather than cryptographic strength. Its security is minimal against anyone who can observe or count the taps, but it excels in constrained environments where standard writing or telegraphy is impossible. The method emphasizes covert communication, operational discipline, and memory of the Polybius square.
Historically, the Tap Code was famously employed by prisoners in German POW camps during World War II and later by U.S. prisoners of war in Vietnam to communicate across cell walls. It illustrates a human-adapted form of cryptography, relying on tactile or auditory signals to overcome surveillance and isolation. Encrypting a simple word like HELLO produces a sequence of paired taps corresponding to 23 15 31 31 34, demonstrating the cipher’s core principle of coordinate-based numeric representation transmitted through minimalistic signaling.
The Tap Code remains a classic example of improvised cryptography that balances simplicity and practicality, highlighting how numeric substitution can be adapted for constrained or covert communication, and providing insight into the resourcefulness of historical cipher users operating under restrictive conditions.