I received a complimentary review copy of this book from the publisher.
Prisoners, Lovers, & Spies, a book by Kristie Macrakis, provides an interesting look into the history of invisible ink and other forms of secret writing. Much has been written about cryptography, including such classics as David Kahn’s The Codebreakers and Bruce Schneier’s Applied Cryptography, but relatively little had been published about invisible writing.
Prof. Macrakis is a professor of history, technology, and society at Georgia Tech. She’s written a number of other books on espionage-related topics, so it makes sense that she would turn her attention to invisible writing.
More Complicated than You'd Think
The problem with this sort of book is that everyone thinks invisible ink is a simple topic. Everyone who has ever owned a beginning magic book or a chemistry set knows that you can use lemon juice and a toothpick to inscribe a message on a piece of paper that only appears when the paper is heated over a flame or a lightbulb. For many years, the science of invisible writing was in fact limited to a number of easily obtained substances and the use of heat or simple developing fluids that reacted with the ink.
The study of natural magic, instituted in the Middle Ages and a precursor to the Scientific Revolution, led to number of discoveries that were of use to the prisoners, lovers, and spies named in the book’s title. During the late 16th century, the partisans fighting on behalf of Queen Elizabeth and Queen Mary, and even Queen Mary herself, used invisible ink in their attempts to communicate secretly with their supporters.
Skipping forward a few centuries, invisible ink played a huge role in every war from the American Revolution to World War II and beyond into the Cold War. The scope and breadth of mail censorship and interception, all with the goal of discovering both indiscreet and discreet communications, was staggering. Even with the tens of thousands of letters going through the British Imperial Censorship office in Bermuda and the American stations in Miami and Puerto Rico, a total of 339 letters with secret writing were intercepted.
During World War II, the Germans instituted a new means of secret communication: the microdot. These tiny circles, which could be hidden in a book as the dot on an “i" or a period, could contain a substantial amount of information for the time. As World War II ground to a close and the Cold War started, microdots played a significant role in covert communication. That’s not to say that invisible ink and secret writing went away. In fact, the author leads off the book with the story of how she came to acquire a carefully hidden East German Stasi formula for invisible ink through an archive request at the German Cold War library collection. It was a story worth waiting for.
Macrakis covers specific historical periods in each chapter. She states in the introduction that she wrote the book so that anyone could dip into it and read about the time they were interested in. That choice, which is eminently reasonable, means that there is some noticeable repetition when you read the book in one go, but it’s not too distracting.
What I find particularly interesting, in addition to the art and science of the writing itself, are descriptions of the organizations put in place to detect, develop, and exploit information from secret writing. The scope of the mail interception effort during World War II is impressive. Although the author doesn’t make this comparison explicit, I can’t help but wonder what the level of effort would be in relation to current National Security Agency efforts to intercept secret communication.
The last chapter of the main part of the book gives a brief overview of steganography, which is the process of hiding a message within another file. For example, one could use the least significant bits of an image file to encode a message without changing the image’s appearance to the casual observer. Of course there are tools to detect steganographic writing, but experts in the field are extremely reluctant to talk about what they do. That means the chapter on steganography is a bit disappointing, but it’s hard to blame the author for her sources’ lack of forthrightness.
The appendix contains a number of formulas that can be used to create and reveal invisible ink. Some of the substances can be harmful to humans, so creating any of the inks or developing agents would be done strictly at your own risk. I’m glad the publisher didn’t shy away from providing these recipes, though—they’re an important part of the subject’s history and the book would be incomplete without them.
Recommendations and Conclusions
Prisoners, Lovers, & Spies is a worthwhile book for anyone with an interest in espionage tradecraft or who just thinks that secret writing is a fun and interesting subject. I fall into both camps, so I enjoyed Prof. Macrakis' work. Recommended.
Curtis Frye is the editor of Technology and Society Book Reviews. He is the author of more than 30 books, including Improspectives, his look at applying the principles of improv comedy to business and life. His list includes more than 20 books for Microsoft Press and O'Reilly Media; he has also created more than 20 online training courses for lynda.com. In addition to his writing, Curt is a keynote speaker and entertainer. You can find more information about him at www.curtisfrye.com and follow him as @curtisfrye on Twitter.