It isn’t just a spy movie trope: secret messages often show up as microdots. [The Thought Emporium] explores the history of microdots and even made a few, which turned out to be — to quote the video you can see below — “both easier than you might think, and yet also harder in other ways.”
If you want to hide a secret message, you really have two problems. The first is actually encoding the message so only the recipient can read it. However, in many cases, you also want the existence of the message to be secret. After all, if an enemy spy sees you with a folder of encrypted documents, your cover is blown even if they don’t know what the documents say.
Today, steganography techniques let you hide messages in innocent-looking images or data files. However, for many years, microdots were the gold standard for hiding secret messages and clandestine photographs. The microdots are typically no bigger than a millimeter to make them easy to hide in plain sight.
The idea behind microdots is simple. They are essentially tiny pieces of film that require magnification to read. After all, you can take a picture of the beach and shrink it down to a relatively small negative, so why not a document?
The example microdots use ISO 50 film to ensure a fine grain pattern, although microfilm made for the task might have been a better choice. Apparently, real spies used special film that uses aniline dyes to avoid problems with film grain.
However you do it, you need a way to take high-resolution images, put them on film, and then trim the film down, ready to hide. While microdots were put on pigeons as early as 1870, it was 1925 before technology allowed microdots to hold a page in only ten square microns. This was a two-step process, so between the film and the single-step processing, these homemade microdots won’t be that dense.
If all this is too much trouble, there’s always invisible ink. Or use a more modern technique.
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