Kristie Macrakis is a professor of history, technology, and society@the Georgia Institute of Technology. She received her Ph.D. in the history of science from Harvard University, and also specializes in the history of espionage. She is the author or editor of five books, including Seduced by Secrets: Inside the Stasi's Spy-Tech World (Cambridge University Press, 2008). Her latest book is Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda (Yale University Press, 2014), from which this article is adapted. Internet: http://www.kristiemacrakis.com