Steganographic techniques

Modern steganographic techniques:

Modern steganography entered the world in 1985 with the advent of the Personal Computer applied to classical steganography problems. Development following that was slow, but has since taken off, based upon the number of 'stego' programs available.

* Concealing messages within the lowest bits of noisy images or sound files.
* Concealing data within encrypted data. The data to be concealed is first encrypted before being used to overwrite part of a much larger block of encrypted data.
* Chaffing and winnowing
* Invisible ink
* Null ciphers
* Concealed messages in tampered executable files, exploiting redundancy in the i386 instruction set
* Embedded pictures in video material (optionally played at slower or faster speed).
* A new steganographic technique involves injecting imperceptible delays to packets sent over the network from the keyboard. Delays in keypresses in some applications (telnet or remote desktop software) can mean a delay in packets, and the delays in the packets can be used to encode data.
* Content-Aware Steganography hides information in the semantics a human user assigns a datagram; these systems offer security against a non-human adversary/warden.
* BPCS-Steganography - a very large embedding capacity steganography.
* Blog-Steganography. Messages are fractionalyzed and the (encrypted) pieces are added as comments of orphaned web-logs (or pin boards on social network platforms). In this case the selection of blogs is the symmetric key that sender and recipient are using. The carrier of the hidden message is the whole blogosphere.

• Hidden messages in wax tablets: in ancient Greece, people wrote messages on the wood, then covered it with wax so that it looked like an ordinary, unused tablet.
• Hidden messages on messenger's body: also in ancient Greece. Herodotus tells the story of a message tattooed on a slave's shaved head, hidden by the growth of his hair, and exposed by shaving his head again. The message allegedly carried a warning to Greece about Persian invasion plans. This method has obvious drawbacks:

1. It is impossible to send a message as quickly as the slave can travel, because it takes months to grow hair.
2. A slave can only be used once for this purpose.
   

• Hidden messages on paper written in secret inks under other messages or on the blank parts of other messages.
• During and after World War II, espionage agents used photographically produced microdots to send information back and forth. Since the dots were typically extremely small—the size of a period produced by a typewriter or even smaller—the stegotext was whatever the dot was hidden within. If a letter or an address, it was some alphabetic characters. If under a postage stamp, it was the presence of the stamp. The problem with the WWII microdots was that they needed to be embedded in the paper, and covered with an adhesive (such as collodion), which could be detected by holding a suspected paper up to a light and viewing it almost edge on. The embedded microdot would reflect light differently than the paper.
• More obscurely, during World War II, a spy for the Japanese in New York City, Velvalee Dickinson, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed how many of this or that doll to ship. The stegotext in this case was the doll orders; the 'plaintext' being concealed was itself a codetext giving information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.
• Counter-propaganda: During the Pueblo Incident, US crew members of the USS Pueblo (AGER-2) research ship held as prisoners by North Korea communicated in sign language during staged photo ops to inform the United States that they had not defected, but had instead been captured by North Korea and were still loyal to the U.S. In other photos presented to the US, the crew members gave "the finger" to the unsuspecting North Koreans, in an attempt to discredit the pictures that showed them smiling and comfortable.
• The one-time pad is a theoretically unbreakable cipher that produces ciphertexts indistinguishable from random texts: only those who have the private key can distinguish these ciphertexts from any other perfectly random texts. Thus, any perfectly random data can be used as a covertext for a theoretically unbreakable steganography. A modern example of OTP: in most cryptosystems, private symmetric session keys are supposed to be perfectly random (that is, generated by a good Random Number Generator), even very weak ones . This means that users of weak cryptography (in countries where strong cryptography is forbidden) can safely hide OTP messages in their session keys.