Steganography comes from the Greek and literally means, "Covered or secret writing". Although related to cryptography, they are not the same. Steganography’s intent is to hide the existence of the message, while cryptography scrambles a message so that it cannot be understood.

        Steganography is one of various data hiding techniques, which aims at transmitting a message on a channel where some other kind of information is already being transmitted. This distinguishes Steganography from covert channel techniques, which instead of trying to transmit data between two entities that were unconnected before.
The goal of Steganography is to hide messages inside other “harmless” messages in a way that does not allow any “enemy” to even detect that there is a second secret message present. The only missing information for the “enemy” is the short easily exchangeable random number sequence, the secret key, without the secret key, the “enemy” should not have the slightest chance of even becoming suspicious that on an observed communication channel, hidden communication might take place.

History of Steganography: 
Through out history Steganography has been used to secretly communicate information between people.

Some examples of use of Steganography in past times are:

  • During World War 2 invisible ink was used to write information on pieces of paper so that the paper appeared to the average person as just being blank pieces of paper. Liquids such as urine, milk, vinegar and fruit juices were used, because when each one of these substances are heated they darken and become visible to the human eye.
  • In Ancient Greece they used to select messengers and shave their head, they would then write a message on their head. Once the message had been written the hair was allowed to grow back. After the hair grew back the messenger was sent to deliver the message, the recipient would shave have off the messengers hair to see the secret message.
  • Another method used in Greece was where someone would peel wax off a tablet that was covered in wax, write a message underneath the wax then re-apply the wax. The recipient of the message would simply remove the wax from the tablet to view the message.
Steganography under Various Media:
In the following three sections we will try to show how steganography can and is being used through the media of text, images, and audio.

Often, although it is not necessary, the hidden messages will be encrypted. This meets a Requirement posed by the ``Kerckhoff principle'' in cryptography. This principle states that the Security of the system has to be based on the assumption that the enemy has full knowledge of the design and implementation details of the steganographic system. The only missing information for the enemy is a short, easily exchangeable random number sequence, the secret Key. Without this secret key, the enemy should not have the chance to even suspect that on an Observed communication channel, hidden communication is taking place. Most of the software that we will discuss later meets this principle.

When embedding data, it is important to remember the following restrictions and features:

  • The cover data should not be significantly degraded by the embedded data, and the embedded data should be as imperceptible as possible. (This does not mean the embedded data needs to be invisible; it is possible for the data to be hidden while it remains in plain sight.)
  • The embedded data should be directly encoded into the media, rather than into a header or wrapper, to maintain data consistency across formats.
  • The embedded data should be as immune as possible to modifications from intelligent attacks or anticipated manipulations such as filtering and resampling.
  • Some distortion or degradation of the embedded data can be expected when the cover Data is modified. To minimize this, error correcting codes should be used.

The embedded data should be self-clocking or arbitrarily re-entrant. This ensures that the embedded data can still be extracted when only portions of the cover data are available. For example, if only a part of image is available, the embedded data should still be recoverable.
Steganography in Text
The illegal distribution of documents through modern electronic means, such as electronic mail, means such as this allow infringers to make identical copies of documents without paying royalties or revenues to the original author. To counteract this possible wide-scale piracy, a method of marking printable documents with a unique codeword that is Indiscernible to readers, but can be used to identify the intended recipient of a document just by Examination of a recovered document

The techniques they propose are intended to be used in conjunction with standard security measures. For example, documents should still be encrypted prior to transmission across a network. Primarily, their techniques are intended for use after a document has been decrypted, once it is readable to all.

An added advantage of their system is that it is not prone to distortion by methods such as photocopying, and can thus be used to trace paper copies back to their source.

An additional application of text steganography suggested by Bender, et al. is annotation that is, checking that a document has not been tampered with. Hidden data in text could even by used by mail servers to check whether documents should be posted or not.

The marking techniques described are to be applied to either an image representation of a document or to a document format file, such as PostScript or Textiles. The idea is that a codeword (such as a binary number, for example) is embedded in the document by altering particular textual features. By applying each bit of the codeword to a particular document Feature, we can encode the codeword. It is the type of feature that identifies a particular encoding method. Three features are described in the following subsections:

This seminar provides an overview of steganalysis and introduced some characteristics of steganographic software that point signs of information hiding. This work is but a fraction of the steganalysis approach. To date general detection techniques as applied to steganography have not been devised and methods beyond visual analysis are being explored. Too many images exist to be reviewed manually for hidden messages so development of a tool to automate the process will be beneficial to analysts. The ease in use and abundant availability of steganography tools has law enforcement concerned in trafficking of illicit material via web page images, audio, and other transmissions over the Internet.

Methods of message detection and understanding the thresholds of current technology are under investigation. Success in steganographic secrecy results from selecting the proper mechanisms. However, a stegomedium which seems innocent enough may, upon further investigation, actually broadcast the existence of embedded information.

Development in the area of covert communications and steganography will continue. Research in building more robust methods that can survive image manipulation and attacks continues to grow. The more information is placed in the public's reach on the Internet, the more owners of such information need to protect themselves from theft and false representation. Systems to recover seemingly destroyed information and steganalysis techniques will be useful to law enforcement authorities in computer forensics and digital traffic analysis.

Most data-hiding systems take advantage of human perceptual weaknesses, but have weaknesses of their own. For now, it seems that no system of data-hiding is totally immune to attack. 

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