The best known way to transfer building plans for the latest British warship to your German cohorts.
Called "the enemy's masterpiece of espionage" by J. Edgar Hoover, the microdot is a photographic image reduced to the size of the period at the end of this sentence that can contain an entire page of typewritten text with perfect clarity. The Nazis developed this method of steganography during World War II, and used it to secretly transmit information through common postal channels. Typically, an innocent-looking letter was sent containing periods, or dots on the letters i and j, onto which the information would be encrypted. The first microdot was detected in 1941 concealed within a period on a typed envelope being carried by a German agent.
The only problem with using microdots was that the special ink they were written with was very shiny; a letter suspected of containing microdots could be held up to a light and viewed at eye-level, looking across the page. Held in this way, the microdot ink would shine while normal ink would not.
Although the specific inventor of microdot steganography in unknown, it is commonly credited to someone named Professor Zapp. Thus, World War II microdot kits were often called Zapp outfits by British intelligence.
So you wanna make a microdot...
- Photograph your message using a specialized microdot camera or a normal 35mm camera. Use high-resolution film.
- Develop the film and mount the negative with a light behind it.
- Photograph this.
- Now take the negative of this second picture and cut out the part of the picture containing the negative of the original image. The result will be extremely tiny (less than a quarter inch across) and you've just created a microdot.
Reading a microdot
There are special viewers specifically designed for this purpose, some of them tiny enough to be easily concealed on your body. However, if you're not a double agent with a closet full of cool spy toys, you can just use a normal microscope.