Secret Writing
Forensic analysis often involves the examination of hand-written material, computer files, bank account and other account information, and other sources of data. Sometimes, the information needs merely to be retrieved for analysis, having been recorded in a straightforward and readily understandable way. However, information can be encrypted or even physically hidden so as to make its detection and deciphering challenging, even virtually impossible.
One such forensically relevant means of communication is known as secret writing. Secret writing is any means of written communication whereby written text can be concealed, whether it is enciphered/encoded or not. Codes and ciphers are sometimes mistakenly placed under the heading of secret writing, but this is accurate only if that expression is taken in its most general sense, as writings that are concealed in any way. Whereas codes and ciphers conceal the meaning of a message, secret writing conceals the actual message.
Techniques of secret writing include the use of invisible ink and carbon copies. Widely applied from ancient times until the early twentieth century, secret writing has been almost entirely eclipsed by more modern methods of concealing messages, such as microdots.
There has long been a desire to keep messages hidden from prying eyes. Indeed, secret writing dates from antiquity. Herodotus described a method of secret writing employed in the Persian Wars. As the Persian emperor Xerxes was preparing to march on the Greek city-states in 480 B.C., a Spartan expatriate name Demaratus learned of the plans and contrived to warn his compatriots. The problem was how to do so in such a way that the Persians themselves would not intercept the message, a challenge for which Demaratus contrived a clever solution.
As Herodotus recorded, Demaratus scraped the wax from a pair of wooden tablets, wrote his message on the wood beneath, then poured hot wax onto the tablets again. Of course the Spartans lacked the advantage of knowing that they were receiving a secret message, but according to Herodotus—who qualified his claim with the caveat "as I understand [it]"—Gorgo, the daughter of a citizen named Cleomenes, received a divine revelation. Thanks to the intervention of the gods, the Spartans realized that they had simply to scrape off the wax and read the message written on the wood beneath it. The Greeks thus began to prepare for the coming invasion, and routed Xerxes' navies at Salamis.
One form of secret writing known to many children from school projects involves invisible ink. This technique uses an acidic citrus juice; lemon juice is most often the preferred choice because it dries without leaving any evidence it has been applied. The juice takes the place of ink, and is applied using a fine stylus, or even an ordinary toothpick. After the juice dries, the acid remains on the paper, which it weakens. When the paper is gently heated, or examined under alternate illumination, the message becomes visible.
Other liquids for invisible ink include milk, which is mildly acidic, as well as white wine, vinegar, or apple juice. In the past, prisoners of war have used their own sweat, saliva, or even urine, all of which contain acidic secretions that adhere to the paper, weakening it, even after the water in those bodily fluids has evaporated. A slight variation on this technique is the use of a baking soda and water mixture as the invisible ink, and, after drying, applying grape juice concentrate with a paintbrush. The acid in the grape juice reacts with the baking soda (a base or alkali in chemical terms), exposing the message.
During the late nineteenth and early twentieth centuries, carbon copies provided a means of secret writing. This method involved a means not unlike the one still used today when signing a credit-card receipt. The back of the receipt is impregnated with graphite, a carbon allotrope (a version of a chemical element distinguished by molecular structure) also used in pencil lead. Therefore, when one signs the front of the receipt, the pressure transfers the graphite to the second page, leaving an impression as though one had written on it in pencil.
In a version utilized in the intelligence community, one piece of paper contains a special chemical that will be invisible when transferred to a second sheet. This makes it possible to inscribe secret writing on the back of an envelope, which can be mailed. Upon receipt, the message can be developed by exposure to water or heat.
The use of secret writing has declined since the middle of the twentieth century. Among the techniques that have become prominent is the microdot, or photographic image miniaturized to the size of a dot, which was actually developed in the mid-1800s.
Much more sophisticated is the technique of steganography, the concealment of information within other, apparently innocuous, data in a computer file.
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