Steganography


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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.

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.

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.

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. 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].

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
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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2.2 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.

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.

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.

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. An example of a message containing such a null cipher 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.

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.

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].

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.

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. 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 world to
produce wide variety and security in the electronic web
called the Internet.

By overlapping S0 and S1, the following sentence is the result:

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.

This is achieved by expanding the space before explore, the, wide, and web by one point and condensing the space after explore, world, wide and web by one point in sentence S1. Independently, the sentences containing the shifted words appear harmless, but combining this with the original sentence produces a different message: explore the world wide web.

3. PC Software that Provide Steganographic Services
3.1 Background
Steganographic software is new and very effective. Such software enables information to be hidden in graphic, sound and apparently "blank" media. Charles Kurak and John McHugh discuss the implications of downgrading an image (security downgrading) when it may contain some other information [Kurak92]. Though not explicitly stated the author(s) of StegoDos mention embedding viruses in images [StegoDos].

In the computer, an image is an array of numbers that represent light intensities at various points (pixels1) in the image. A common image size is 640 by 480 and 256 colors (or 8 bits per pixel). Such an image could contain about 300 kilobits of data.

There are usually two type of files used when embedding data into an image. The innocent looking image which will hold the hidden information is a "container." A "message" is the information to be hidden. A message may be plain-text, ciphertext, other images or any thing that can be embedded in the least significant bits (LSB) of an image.

For example:

Suppose we have a 24-bit image 1024 x 768 (this is a common resolution for satellite images, electronic astral photographs and other high resolution graphics). This may produce a file over 2 megabytes in size (1024x768x24/8 = 2,359,296 bytes). All color variations are derived from three primary colors, Red, Green and Blue. Each primary color is represented by 1 byte (8 bits). 24-bit images use 3 bytes per pixel. If information is stored in the least significant bit (LSB) of each byte, 3 bits can be a stored in each pixel. The "container" image will look identical to the human eye, even if viewing the picture side by side with the original. Unfortunately, 24-bit images are uncommon (with exception of the formats mentioned earlier) and quite large. They would draw attention to themselves when being transmitted across a network. Compression would be beneficial if not necessary to transmit such a file. But file compression may interfere with the storage of information.


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