microdot

A photograph or negative reduced to about 1 mm² or less, unreadable without magnification.

Description based on Crypto Museum. The microdot is a technique of steganography: according to the International Spy Museum, steganography conceals the very existence of a message, for example by means of secret ink or microdots.^1,2

An example from the FBI files: William Sebold, recruited in 1939 by German intelligence but in reality working with the FBI, was given microphotographs with enciphering instructions in Hamburg before leaving for the United States in 1940. Under FBI control he then maintained communications with Germany, which led to the break-up of Fritz Duquesne's spy ring (33 people convicted).³