Microdot encryption, a spy favorite, refers the reduction of a text or an image to the size of a small disc, often the size and shape of a typographical dot, such as a period or the tittle of a lower-case j or i. This encryption technique was primarily used to prevent detection by unintended recipients, particularly when conveying sensitive or classified materials. In Germany, after the Berlin Wall was erected, special cameras were used to generate microdots which were then adhered to letters and sent via normal means. These microdots often went unnoticed by inspectors, and information could be read by the intended recipient using a microscope. Because of the minuscule size of this transmission technique, compressed information could be hidden inside of hollowed out coins, rings, and other small items.
Microdotting is different from both the kinds of secret writing that came before it and those that came after it: in earlier analog modes, a secret message was physically covered by other material, while in more recent digital steganography, information is dispersed imperceptibly within the insignificant bits of the code of an image file. Take as an example these two sets of images:
In the first set of images, the code for this image of a cat is dispersed and stored within the least significant bits of the code for this image of a tree. In the second set, from the FBI’s work on microdot prototypes, this text is compressed and affixed within this telegram containing text about the different colors that comprise white light. So whereas the cat image can only be recovered and reconstituted through digital decryption, the microdot remains physically present, materially and textually intact, and yet it’s effectively indiscernible to the naked eye[…]It’s unnoticeable, but not invisible—on the contrary, it becomes a new part of the materiality of the page.
[…]In fact, whoever worked on the prototype documents for the FBI had a shrewd sense of humor in embedding a microphotograph produced by light within the text of a seemingly arbitrary passage about light wavelengths—there’s a kind of self-conscious proto-spam quality that underscores how microdotting itself twisted and bent what it meant to write and see through light, how it offered a new formation of the materiality of writing. Thus the salient questions here are ones of the compression and compaction of printed text rather than its algorithmic dispersion: in the decades prior to the rise of digital computing after World War II, at the Borgesian extreme end of print inscription, secrecy becomes a kind of endgame of paper writing, a profoundly, microscopically embodied question of the touch of the finger, of the texture of paper, of information that can be touched and seen, but not read.
Read more about the history of microdot encryption, compression techniques, and information [im]materiality here.