Until then, you're invited to tag along as we review what it was like to be a spy in the 20th Century commencing in the mid-to-late 1940s.
The image at the top is actually from the American War of Independence. It used a technique known then as the 'Culper code' or sympathetic staining. The hidden writing was done between the lines of a normal letter, which has led to the phrase 'reading between the lines'.
Today's focus is on the use of another kind of secret ink, this particular kind of ink though wasn't developed until the 1970s and it was created by the Soviets.
You will probably seen it in movies: a spy scribbling a message in invisible ink, only for it to be revealed with a special chemical or heat source. But the world of secret writing is far more intricate than what Hollywood or Bollywood portrays. In this post you will read of a sophisticated method that combines chemistry and clever deception to conceal high value information.
Imagine for a moment that you receive a letter that seems perfectly ordinary at first glance. It's a friendly note from a colleague, or perhaps a relative or friend. What you don't realize is that hidden between and maybe over the lines, quite literally, is another message altogether. This is where the technique described in a fascinating ScienceDaily article (Nov. 8, 2006) comes into play.
Researchers discovered a method where secret writing is created using a sheet of paper impregnated with a specific chemical, cerium oxalate, This treated sheet isn't written on directly. Instead, it's placed between two other sheets of paper. The act of writing on the top sheet transfers the chemical to the bottom sheet, creating an invisible message.
The remarkable part is how this invisible message is revealed. By applying a developing agent (manganese sulfate, hydrogen peroxide, and other chemicals that react with the impregnated substance), hidden writing magically appears. This method avoids the obvious signs of alteration that come with traditional invisible inks applied directly to a document.
Here's where it gets more interesting. Even if the chemical magic remained undetected, one might think that the indentations left by the pen on the top sheet and transferred through to the bottom sheet would give the game away. A careful observer using oblique lighting or an electrostatic detection device (ESDA), might discern faint impressions on the underlying pages.
However, the genius of this layered technique lies in the fact that the ‘letter’ sheet already has legitimate writing on it! The indentations created from the normal letter serve as camouflage, masking the subtle indentations left by the secret ink message. It's like hiding a pebble on a beach – the surrounding environment helps it blend in.
As a matter of interest, some manuals I have read point to the use of a hot flat iron to remove indentations on book or notepad pages. Others suggest using a sheet of tin or plastic beneath the message page.
This method highlights the delicate balance between concealment and detection. While the chemical impregnation and layered approach offer a high degree of secrecy, the risk of detection through indentation analysis remains. This is where the skill of the operative comes into play:
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Applying even pressure to minimize indentation differences.
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Aligning the secret writing with existing text to further obscure the indentations.
: Choosing paper types that minimize indentation visibility.
While the ScienceDaily article presents a modern analysis, the concept of layered secret writing has historical roots. Throughout history, spies and secret agents have sought ever more ingenious ways to conceal their communications. From ancient Greece to the Cold War, the ability to transmit information undetected has been a critical advantage.
The Voynich Manuscript with its apparent cipher rings is intriguing but what was its true purpose? Some other time maybe :)
Since spying began, the effectiveness of any non-digital secret writing or clandestine communication technique relies on the human element or ‘Humint’ as it is more commonly known. The skill of the writer, the knowledge of the recipient, and the vigilance of the adversary all play crucial roles. As technology advances, so too does the art of deception and concealment and the methods used to uncover them.
ScienceDaily. "Secret Writing: New Way To Create Invisible Messages." ScienceDaily. ScienceDaily, 8 November 2006. www.sciencedaily.com/releases/2006/11/061108103815.htm