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Computer Science 654 Lecture 6: Steganography Professor Wayne Patterson Howard University Spring 2009 Adapted from Neil F. Johnson, George Mason University

Dec 18, 2015



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  • Computer Science 654 Lecture 6: Steganography Professor Wayne Patterson Howard University Spring 2009 Adapted from Neil F. Johnson, George Mason University
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  • Introduction 1. Introduction Steganography conceals the fact that a message is being sent. It is a method akin to covert channels, spread spectrum communication and invisible inks which adds another step in security. A message in ciphertext may arouse suspicion while an invisible message will not.
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  • 1.1 Purpose of Paper This paper introduces steganography by explaining what it is, providing a brief history with illustrations of some methods for implementing steganography, and comparing available software providing steganographic services. Though the forms are many, the focus of the software evaluation in this paper is on the use of images in steganography.
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  • 1.2 Structure of Paper Section 2 will define steganography, provide a brief history, and explain various methods of steganography. Section 3 will review several software applications that provide steganographic services and mention the approaches taken. Section 4 will conclude with a brief discussion of the implications of stegaonographic technology. Section 5 will list the resources used in researching this topic and additional readings for those interested in more in-depth understanding of steganography.
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  • 2. Steganography 2.1 Definition The word steganography literally means covered writing as derived from Greek. It includes a vast array of methods of secret communications that conceal the very existence of the message. Among these methods are invisible inks, microdots, character arrangement (other than the cryptographic methods of permutation and substitution), digital signatures, covert channels and spread-spectrum communications. Steganography is the art of concealing the existence of information within seemingly innocuous carriers.
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  • Steganography can be viewed as akin to cryptography. Both have been used throughout recorded history as means to protect information. At times these two technologies seem to converge while the objectives of the two differ. Cryptographic techniques "scramble" messages so if intercepted, the messages cannot be understood. Steganography, in an essence, "camouflages" a message to hide its existence and make it seem "invisible" thus concealing the fact that a message is being sent altogether. An encrypted message may draw suspicion while an invisible message will not [JDJ01].
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  • David Kahn places steganography and cryptography in a table to differentiate against the types and counter methods used. Here security is defined as methods of "protecting" information where intelligence is defined as methods of "retrieving" information [Kahn67]: Steganography has its place in security. It is not intended to replace cryptography but supplement it. Hiding a message with steganography methods reduces the chance of a message being detected. However, if that message is also encrypted, if discovered, it must also be cracked (yet another layer of protection).
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  • Kahns Security Table Signal Security Communication Security Steganography (invisible inks, open codes, messages in hollow heels) and Transmission Security (spurt radio and spread spectrum systems) Cryptography(codes and ciphers Traffic security(call-sign changes, dummy messages, radio silence) Electronic Security Emission Security (shifting of radar frequencies, spread spectrum) Counter-Countermeasures "looking through" (jammed radar)
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  • Kahns Security Table (more) Signal Intelligence Communication Intelligence Interception and direction-finding Cryptanalysis Traffic analysis (direction-finding, message-flow studies, radio finger printing) Electronic Intelligence Electronic Reconnaissance (eaves-dropping on radar emissions) Countermeasures (jamming radar and false radar echoes
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  • History and Steganography Throughout history, a multitude of methods and variations have been used to hide information. David Kahn's The Codebreakers provides an excellent accounting of this history [Kahn67]. Bruce Norman recounts numerous tales of cryptography and steganography during times of war in Secret Warfare: The Battle of Codes and Ciphers. One of the first documents describing steganography is from the Histories of Herodotus. In ancient Greece, text was written on wax covered tablets. In one story Demeratus wanted to notify Sparta that Xerxes intended to invade Greece. To avoid capture, he scraped the wax off of the tablets and wrote a message on the underlying wood. He then covered the tablets with wax again. The tablets appeared to be blank and unused so they passed inspection by sentries without question.
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  • History and Steganography Another ingenious method was to shave the head of a messenger and tattoo a message or image on the messengers head. After allowing his hair to grow, the message would be undetected until the head was shaved again. 1011011010
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  • History and Steganography Another common form of invisible writing is through the use of Invisible inks. Such inks were used with much success as recently as WWII. An innocent letter may contain a very different message written between the lines [Zim48]. Early in WWII steganographic technology consisted almost exclusively of invisible inks [Kahn67]. Common sources for invisible inks are milk, vinegar, fruit juices and urine. All of these darken when heated.
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  • History and Steganography With the improvement of technology and the ease as to the decoding of these invisible inks, more sophisticated inks were developed which react to various chemicals. Some messages had to be "developed" much as photographs are developed with a number of chemicals in processing labs. Null ciphers (unencrypted messages) were also used. The real message is "camouflaged" in an innocent sounding message. Due to the "sound" of many open coded messages, the suspect communications were detected by mail filters. However "innocent" messages were allowed to flow through.
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  • An example of a message containing such a null cipher from [JDJ01] is: Fishing freshwater bends and saltwater coasts rewards anyone feeling stressed. Resourceful anglers usually find masterful leapers fun and admit swordfish rank overwhelming anyday. By taking the third letter in each word, the following message emerges [Zevon]: Send Lawyers, Guns, and Money.
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  • The following message was actually sent by a German Spy in WWII [Kahn67]: 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. Taking the second letter in each word the following message emerges: Pershing sails from NY June 1.
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  • As message detection improved, new technologies were developed which could pass more information and be even less conspicuous. The Germans developed microdot technology which FBI Director J. Edgar Hoover referred to as "the enemy's masterpiece of espionage." Microdots are photographs the size of a printed period having the clarity of standard-sized 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 not hidden, nor encrypted. It was just so small as to not draw attention to itself (for a while). Besides being so small, microdots permitted the transmission of large amounts of data including drawings and photographs [Kahn67].
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  • Crosswords and Report Cards With many methods being discovered and intercepted, the Office of Censorship took extreme actions such as banning flower deliveries which contained delivery dates, crossword puzzles and even report cards as they can all contain secret messages. Censors even went as far as rewording letters and replacing stamps on envelopes. With every discovery of a message hidden using an existing application, a new steganographic application is being devised. There are even new twists to old methods. Drawings have often been used to conceal or reveal information. It is simple to encode a message by varying lines, colors or other elements in pictures. Computers take such a method to new dimensions as we will see later.
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  • Document Layout Even the layout of a document can provide information about that document. Brassil et al authored a series of publications dealing with document identification and marking by modulating the position of lines and words [Brassil-Infocom94, Brassil- Infocom94, Brassil-CISS95]. Similar techniques can also be used to provide some other "covert" information just as 0 and 1 are informational bits for a computer. As in one of their examples, word-shifting can be used to help identify an original document [Brassil-CISS95]. Though not applied as discussed in the series by Brassil et al, a similar method can be applied to display an entirely different message.
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  • Take the following sentence (S0): We explore new steganographic and cryptographic algorithms and techniques throughout the world to produce wide variety and security in the electronic web called the Internet. and apply some word shifting algorithm (this is sentence S1). We explore new steganographic and cryptographic algorithms and techniques throughout the