DT                   T a m á s  D é n e s mathematician                                       TD

free-lance expert

 

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Steganography is more than a tool for spies

 

 

As long as there has been written communication humans had the desire to conceal theit messages from the curious eyes of others. The method of hiding one piece of information within another is called steganography. The meaning of this Greek term is ‘covered writing’.

The first examples from ancient Greece were documented by Herodotus. In one story Demeratus wanted to notify Sparta that Xerxes intended to invade Greece. To avoid capture, he scraped the wax off the tablets and wrote a message on the underlying wood. Then he covered the tablets with wax again. The seemingly blank tablets passed inspection by the sentries without suspicion.

 

Method in ancient Greece

 

Another method used in ancient Greece was to shave the head of a messenger and tattoo a message on his head. After allowing his hair to grow, the message would be undetected until the head was shaved at the recipient’s place.

The most commom form of invisible writing from ancient times up to World War II was the use of invisible inks, which darken when heated. This allowed writing hidden messages between the lines of an innocent letter. Common sources for invisible inks were milk, juices and even urine. In World War II chemical inks came into use. They had to be developed similar to photographs in a processing lab.

 

Boom in World War II

 

Steganography was booming in World War II. Another method was to camouflage secret messages, which passed the enemy’s mail filters. Because these methods became more and more insecure due to increasing detection efforts, the Germans developed the microdot technology. Microdots are photographs the size of a printed period having the clarity of standardsized typewritten pages. The first microdots were discovered masquerading as a period on a typed envelope carried by a German agent in 1941. The message was neither hidden nor encrypted, but was too small to attract attention – at least for a while.

 

Steganography renaissance today

 

Since the end of the war steganography has led a hidden life and seemed to have fallen into oblivion. This has changed only recently when digital communication became dominant. There are three main reasons that led to a renaissance of steganography.

  1. Digital cryptography has become a focus of political discussion, and some governments have even banned it. For individuals and companies striving for confidentiality, steganography offers a useful complementary because it has one important advantage: A cryptographic message is always visible as such and arouses suspicion, but the very existence of a steganographic message is unknown to others. So combining cryptography and steganography can help avoid suspicion and protect privacy.
  2. Hidden digital watermarks enable producers to trace violations of intellectual property rights. Britain’s prime minister Margaret Thatcher demonstrated the use of hidden watermarks already in the 1980’s: After several press leaks of cabinet documents she had the word processors programmed to encode their identity in the word spacing, so that disloyal ministers could be traced.
  3. Steganographic software has become quite effective. With an increasing amount of communication taking place electronically, digital methods for hiding information in pictures, sound files and text files have gained importance.

 

Variety of methods

 

Today there is a vast number of steganographic methods and applications available. Generally, any so-called ’container’ can be used to carry the embedded, hidden information. Let us take the example of digital images. The easiest way for hiding information is to set the least significant bits of the image pixels to the bits of the embedded information. Information embedded in this way may be invisible to the human eye. However, it would be relatively easy for a suspecting third party to detect and remove the information. Through trivial filtering processes the value of many of the least significant bits could be changed. One counter-method would be redundancy, embedding the mark several times.

 

Limits of steganography

 

However refined the methods may be, some experts see serious drawbacks. Neil Johnson and Sushil Jajodia from the Center for Secure Information Systems of George Mason University in Fairfax, Virginia, state in a contribution on steganalysis that hidden data, like watermarked images, „are not as robust as is claimed”. The risk of detection and  destruction of the embedded message is considerable, especially if safe watermarking is advertised and potential ’crackers’ are made aware that there is some mark to remove. This happened some years ago to a well-known image software. What can generally be problematic is the balance between necessary bandwidth and the amount and robustness of hidden data.

History shows that the race between hide and seek is not always successful for the sender of the hidden information. A famous victim is Mary Queen of Scots, who conspired in 1586 to have Queen Elizabeth of England assassinated. However, the cipher she used was broken, and the English secret police caught the chief conspirator, to whom she wrote, which finally led to Mary’s arrest and execution.

Richard E. Smith, a US data security expert, said that he doesn’t „see many practical uses for steganography because it only works as long as nobody expects you to use it”. Others, like the steganography experts Ross J. Anderson and Fabian Petitcolas, think there are still a lot of oppurtunites for improvement. Their suggestion is to embed information in parity checks rather than in the data directly. This approach, they say, would allow to do public key steganography. The last word on steganography has not been spoken yet.

However, the risks of the Internet will increase the need for noteworthy tools, which help to reject various assaults on people’s privacy. The time for steganography might still be oncoming.