27/12/2014
Techniques of Stenography
Physical
Steganography has been widely used, including in recent historical times and the present day. Known examples include:
Hidden messages within wax tablets — in ancient Greece, people wrote messages on the wood, then covered it with wax upon which an innocent covering message was written.
Hidden messages on messenger's body — also used in ancient Greece. Herodotus tells the story of a message tattooed on the shaved head of a slave of Histiaeus, hidden by the hair that afterwards grew over it, and exposed by shaving the head again. The message allegedly carried a warning to Greece about Persian invasion plans. This method has obvious drawbacks, such as delayed transmission while waiting for the slave's hair to grow, and the restrictions on the number and size of messages that can be encoded on one person's scalp.
During World War II, the French Resistance sent some messages written on the backs of couriers using invisible ink.
Hidden messages on paper written in secret inks, under other messages or on the blank parts of other messages.
Messages written in Morse code on knitting yarn and then knitted into a piece of clothing worn by a courier.
Messages written on envelopes in the area covered by postage stamps.
In the early days of the printing press, it was common to mix different typefaces on a printed page due to the printer not having enough copies of some letters otherwise. Because of this, a message could be hidden using 2 (or more) different typefaces, such as normal or italic.
During and after World War II, espionage agents used photographically produced microdots to send information back and forth. Microdots were typically minute (less than the size of the period produced by a typewriter). World War II microdots needed to be embedded in the paper and covered with an adhesive, such as collodion. This was reflective and thus detectable by viewing against glancing light. Alternative techniques included inserting microdots into slits cut into the edge of post cards.
During WWII, Velvalee Dickinson, a spy for Japan in New York City, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed the quantity and type of doll to ship. The stegotext was the doll orders, while the concealed "plaintext" was itself encoded and gave information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.
Jeremiah Denton repeatedly blinked his eyes in Morse Code during the 1966 televised press conference that he was forced into as an American POW by his North Vietnamese captors, spelling out the word, "T-O-R-T-U-R-E". This confirmed for the first time to the U.S. Military (naval intelligence) and Americans that American POWs were being tortured in North Vietnam.
Cold War counter-propaganda. In 1968, crew members of the USS Pueblo intelligence ship held as prisoners by North Korea, communicated in sign language during staged photo opportunities, informing the United States they were not defectors, but were captives of the North Koreans. In other photos presented to the US, crew members gave "the finger" to the unsuspecting North Koreans, in an attempt to discredit photos that showed them smiling and comfortable.
Digital messages
Image of a tree with a steganographically hidden image. The hidden image is revealed by removing all but the two least significant bits of each color component and a subsequent normalization.
Modern steganography entered the world in 1985 with the advent of personal computers being applied to classical steganography problems. Development following that was very slow, but has since taken off, going by the large number of steganography software available:
Concealing messages within the lowest bits of noisy images or sound files.
Concealing data within encrypted data or within random data.
Ref.wikipedia
