Evolution of Information-Hiding Technology
Authors: Lou Der-Chyuan , Liu Jiang-Lung , Tso Hao-Kuan 
Published: May 2011

Information-hiding technology is an ancient art and has existed for several centuries. In the past, messages could easily be intercepted because there was no technology of secret communication. Hence, a third party was able to read the message easily. This was all changed during 440 B.C., that is, the Greek Herod’s era. The Greek historian Herodotus in his writing of histories stated that Demaratus was the first person who used the technique of information hiding. Demaratus, a Greek who lived in Persia, smuggled a secret message to Sparta under the cover of wax. The main intent was to warn Sparta that Xerxes, king of Persia, was planning an invasion on Greece by using his great naval fleet. He knew it would be very difficult to send the message to Sparta without it being intercepted. Hence, he came up with the idea of using a wax tablet to hide the secret message. In order to hide the secret message, he removed all the wax from the tablet, leaving only the wood underneath. He then wrote the secret message into the wood and recovered the tablet with the wax. The wax covered his message to make the wax tablet look like a blank one. Demaratus’ message was hidden and never discovered by the Persians. Hence, the secret message was sent to Sparta successfully. Greece was able to defeat the invading Persians by using the secret message. Another example of information hiding was employed by another Greek named Histaiaeus. Histaiaeus wanted to instigate a revolt against the Persian king and had to deliver a secret message about the revolt to Persia. He came up with the shaved-head technique. Histaiaeus decided to shave the head of his most trusted slave and then tattooed the secret message on his bald scalp. When the hair grew back, the secret message was covered, and then Histaiaeus ordered the slave to leave for Persia. When the slave reached his destination, his head was shaved, showing the secret message to the intended recipient. Around 100 A.D., transparent inks made it into the secret field of information hiding. Pliny discovered that the milk of the thithymallus plant could easily be used as transparent ink. If a message was written with the milk, it would soon evaporate and left no residue. It seemed that the message was completely erased. But once the completely dried milk was heated, it would begin to char and turned to a brown color. Hence, the secret message could be written on anything that was not too flammable. The reason it turned brown was because the milk was loaded with carbon, and when carbon was heated, it tended to char. Information hiding became downfallen and won no respect until World Wars I and II. Invisible inks, such as milk, vinegar, fruit juices, and urine, were extensively used during the wars. All of them would darken when they were heated. The technology was quite simple and noticeable. Furthermore, World War II also brought about two inventions of new technologies. The first one was the invention of the microdot technology. The microdot technology was invented by the Germans to convey secret messages to their allies. The microdot was basically a highly detailed picture shrunk to about the size of a period or dot, which permitted hiding large amounts of data into the little microdot. By using a microscope, the hidden message would be revealed. The Germans would put their dots into their letters, and they were almost undetectable to the naked eye. The other technology was the use of open-coded messages. For open-coded messages, certain letters of each word were used to spell out the secret message. Open-coded messages used normal words and messages to write the buffer text that hid the message. Because they seemed normal, they often passed the check of security. For example, the following message was a common example of open-coded messages and was actually sent by a German spy during World War II. Apparently neutral’s protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on by-products, ejecting suets and vegetable oils. By extracting the second letter in each word, the secret message was revealed: Pershing sails from NY June 1. This technique was effective because it could pass through the check of security and was easy for someone to decode (Johnson, Duric, & Jajodia, 2001; Katzenbeisser & Petitcolas, 2000; Schaefer, 2001). The technologies mentioned here are different ways of information hiding in different eras. With the development of computer technology, it is becoming hard for the third party to discover the secret message.