Invisible inks are perhaps the most popular form of steganography, and have been known about since at least 1 AD when Pliny the Elder wrote about them. Ovid mentioned them as a method to correspond secretly with a lover. Giovanni Porta of polyalphabetic cipher fame wrote about invisble inks during the Rennaisance when they were a preferred form of steganography. And in the 20th century invisible inks became the concealment of choice for spies trying to sneak letters past censors.
There are two major types of invisible inks: those which require heat to be seen and those which require another chemical to be seen. The former category includes perennial favorites like milk, vinegar, fruit juices, and in a snitch - even urine will work as an invisible ink. However, the security of these inks was not very great. Anyone who heated the message would be able to see it. In Edgar Allen Poe's famous short story 'The Gold Bug', a map to secret treasure written with a heat sensitive ink is revealed accidentally by the narrator by just holding it near a fire to get better light on the parchment. In this case, the invisible ink failed. While it did not alert the narrator that a message was there, it was too easily revealed. The invisible inks revealed by heat are much like a simple cryptography system: it does its job to prevent a few fools from finding the information but does nothing for the rest of us. And the American black chamber censors were no fools.
The black chamber subjected many messages to heat to reveal hidden messages. Spies could therefore not rely on heat-revealed secret inks being a safe method of communication. And so for the most part they employed the second major category of secret inks that require a second complementary chemical to reveal the hidden message. There are literally thousands of chemical combinations that will reveal latent images. The best, though, were those chemicals that could only be revealed by very few others. In fact, a 'war of invisible inks' was raged during the World Wars and Cold Wars while Russian and German scientists battled to find more exclusive inks and the Americans attempted to find more general revealing agents.
The Americans thought that they had won the battle after creating an iodine vapor test that revealed nearly any cipher. The iodine vapor test worked well because it didn't work directly on the invisible ink, unlike other revealing agents. Instead, it noticed the chemical transformation that occurred to the fibers of the paper when ink was applied to them. The iodine test was not unbeatable, however, as spies soon learned to write their messages on a separate piece of paper with invisible ink and then press the paper tightly against the real letter. This method would not leave the marks in the paper fibers that the iodine test depended on. Another method was to split a piece of paper in two, directly through the edge. A spy could then write his message on one of the split pages and then reattach the two pieces. It was a very difficult procedure, and also nearly undetectable until the black chamber began to look for it.
The invisible ink wars ended in a draw as the spies began to rely more on microfilm which could transmit more information. Today, though, the spies probably rely on digital steganography, which hides the information within innocuous looking data. And so, the only ones using the invisible inks today are the millions of children playing with the secret ink pens and Crayola markers that change color when in the presence of a special 'clear' marker. What was the highest form of communication in the Renaisance is now, quite literally, child's play.
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