CIA File of the Day
SECRET WRITING
A short paper on recipes for secret writing. The first method is a solution of nitrate of soda and starch in water, which may be carried in handkerchiefs, starched collars, starched shirts, or anything else starched; the article is later put in water to obtain a solution for invisible writing. Iodide of potassium is named as the best means for developing.
Courtesy of the CIA FOIA Electronic Reading Room. Official page: CIA FOIA Electronic Reading Room. Document PDF: cia.gov reading room PDF. Copy: Internet Archive.