Microdot

Microdots are text or photographic images that are reduced in size to prevent their viewing by unintended recipients. A microdot is usually shrunk down to the size and shape of a period or the dot of an i or j, and was usually sent by writing an innocent-looking letter containing periods, i's, or j's, as most messages do. This technique was invented by the Germans during World War II and then later used by many countries to pass messages through insecure channels. After the erection of the Berlin Wall, special cameras were used to generate microdots which were then adhered to letters and sent via normal means. Owing to the extremely small size of the microdot, these messages typically went unnoticed by inspectors and information could then be read by the intended recipient via microscope.

See also: List of cryptology topics, Steganography, A Scanner Darkly

References

• White, William. The Microdot: History and Application. Williamstown, NJ: Phillips Publications, 1992.