Steganography has been derived from Greek word Stego which means Covered and Graphia which means writing. Steganography is an ancient technique of covert communication. The earliest form of Steganography has been reported by the Chinese. The secret message was written in very fine silk or paper, and then it was rolled into a ball and covered with wax. The messenger would either swallow the ball or hide it in his nether parts. Herodotus has mentioned in one of his seminal works of history, Histories during the 400B.C about the tradition of secret writing. In his writings he has mentioned about the conflicts between Greece and Persia. A king named Histiaeus encouraged the Aristagoras of Miletus to revolt against the Persian king. He used to tonsure the head of his most trusted servants and tattooed the scalps with secret message and waited for the hair to grow. The servant used to travel between the borders without carrying anything contentious freely. At the reception end his head would be tonsured again and the message will be conveyed. Similarly during the World War II, the Germans invented the use of microdots. Image containing great details were scaled down to the size of microdots. The German usage of secret sharing is regarded as the recent evolution of Steganography. Another example of Steganography is during the Vietman War were the captured US armed force showed hand gestures during a photo sessions to convey some military secrets. The field of Steganography is limitless and any kind of cover media can be used for carrying secret messages. Cover medias can be text, images (grey, binary, color), audio, video etc. An example is illustrated as follows.
Siva and Adam dined happily.
The following message is not simply a plain text but it carries a secret message within it. By extracting every second letter from each word we derive a secret message as follows.
Other methods of non technical Steganography is using lime and other citrus liquids to write secret data. The message will be imperceptible to the eye. When it is shown over a flame the writings will become perceptible.
Steganography is method of information hiding. Steganography can be considered as an art as well as science for secret communication.
The best way to explain the purpose of using Steganography is the Prisoner Problem. Now let us consider two prisoners Alice and Bob, and Wendy to be the warden. Now if Alice and Bob want to devise a plan for escaping then they got to communicate with each other. If Wendy is a passive warden then he won’t interfere in there communication but if he is an active warden then he will interfere and extract the secret message sent. Hence Alice and Bob got to communicate in such a way that Wendy is not able to decipher there plan. This can be done by the technique of Steganography.
Steganography is been used in order to enable secret communication. It is also being used by terrorist for covert communication which is potential for endangering our national as well as world security. Despite the ill effect there are positive sides of Steganography. For example a photographer can store the aperture size, future references etc while taking a picture. Steganography has a wide application in medical imaging were the details of the patients are embedded within the medical image. Similarly Steganography can be used for different friendly applications. It is also used for copy right protection using it along with Watermarking.
The use of Steganography also has an important role in strengthening national security. Steganography is defined as secret hiding. It can also be regarded as secret sharing since messages can be shared secretly without being hacked or corrupted.
COMPARISON BETWEEN CRYPTOGRAPHY, STEGANOGRAPHY AND WATERMARKING
The other forms of data hiding are cryptography and watermarking. Cryptography is a technique in which the secret message is encrypted and sent in an unintelligent format. The basic difference between Steganography and cryptography is that Cryptography scrambles the data whereas Steganography just hides the data. Cryptography scrambles the secret data in such a way that it appears to be gibberish to any unintended user. The secret data to be communicated is a mixture of permutation and substitution and hence any unintended user can’t receive the message.
Steganography is different from Cryptography. Steganography rather hides the data whereas Cryptography encrypts the data. Steganography provides much more security when compared to cryptography because there is no chance of any unintended user to know that a message is being sent whereas in Cryptography there will always be a suspicion that a message is being sent. Hence these are more prone to be hacked or suppressed.
Watermarking is generally used for authentication and copyrights protection. Watermarking can be used for creating a image in order that it is recognizable. It can also be used to mark a digital file so that it is intended to be visible (visible watermarking) or visible to its creator (invisible marking). The main purpose of watermarking is to prevent the illegal copying or claim of ownership of digital media.
CHARACTERISTICS OF STEGANOGRAPHY, CRYPTOGRAPHY AND WATERMARKING
The common characteristic between Steganography, Cryptography and Watermarking is that they transmit the secret information in such a way that only the receiver is able to decrypt the data. These techniques had been prevalent during the ancient times have been transported to the digital world. It has become nearly impossible to extract or detect the secret messages. In digital domain Steganography and watermarking have a tie and it is majorly used in digital images. These have other uses as well. Both cannot exist by themselves and hence they require cover objects. Steganography requires a cover media to carry the secret information and watermarking requires a carrier object which it is intended to protect. These similarities create a link within them and hence some modifications can lead the transportation from one technique to another. Due the similarities present between these it is difficult to distinguish between the both but actually there is a remarkable difference between them. Cryptography encrypts data in two methods secure or unbreakable (e.g. One-time pad) systems and breakable (e.g. RSA) systems. Through both the systems communication carried out is known to all. But it is time consuming and often fruitless to crack a code. The robustness of the code lies upon the difficulties faced while reversing the code in different permutations and combinations. Due to its robustness it is used for security purposes. For example Cryptography is used for online shopping, banking etc. The credit card number, expiration etc and other crucial information’s are encrypted and sent so that an unintended user can’t access the details.
Steganography offers high carrier capacity keeping embedded message invisible and maintaining the fidelity of the cover media. The efficiency of the Steganographic method is that one shouldn’t know that a media file has been altered in order for embedding. If the malicious user knows if there is some alteration the Steganographic method is defeated and less efficient. The embedded message is very fragile and hence if any modification is done to the stego image th e whole secret message is corrupted. The effectiveness lies on the ability to fool an unintended user. The layers of communication can be more than one layer. A secret message can be embedded with a digital image which in turn can be embedded within another digital media or video clippings.
Watermarking is required in order for authentication and copyright protection of digital files. The embedded watermarking is required in an object to make it impossible to remove completely. If the embedded watermarking is removed, then the marked object is either distorted or destroyed making it useless for anyone. This is the reason why watermarking is more robust when compared to the other image processing techniques, such as compression, cropping, rotation etc. hence even if a tiny bit of information is extracted by modification and tempering the rightful owner can still claim ownership. If the owners name is embedded in the digital image and the particular image then the original information is tampered and destroyed. Unlike Steganography, it is acceptable for everyone one to see the watermark embedded in it including the invisible ones.
COMBINED EFFORTS OF STEGANOGRAPHY, CRYPTOGRAPHY AND WATERMARKING
Cryptography is used as a paisano of the other two data hiding techniques. Data is encrypted in both the techniques in order to increase the randomness of the Steganography to avoid the statistics-based attacks and to protect the hidden data in watermarking. Commonly it is a practice to first encrypt the data and then use any one of the embedded techniques.
The importance of watermarking can be stated as follows. Suppose Rs 100 bill is introduced in December 2009 then watermarking is required in order to prevent illegal copies. If the bill is shown in light a small image will appear within the large image. The watermarking is actually a part of the large paper and is visible on both sides. Hence it becomes difficult to produce a paper with such features. In addition to these features some tiny writings which are invisible to the human eyes are present in the paper.
A banker having the necessary equipments (magnifying glass) can tell the difference between the original bill and the fake bill. Steganography makes its play here. The tiny printing done on the bill represents Steganography. It is implemented in order to achieve high security. The tiny printing done in the paper cannot be copied since any commercial printer is incapable to print such a fine and thin print leading to black spots. These are the reasons why Steganography is used for high security.
Cryptography is actually implemented in the bill. A serial number is been printed in the bill which may contain information about the location and date of bill is printed or any other confidential information. The unique serial number for each bill can be used for tracking purposes. Using Steganography, Cryptography and Watermarking it becomes impossible to reproduce Rs100 bill. It must be kept in mind that all three are different and have different functionality.
REQUIREMENTS OF STEGANOGRAPHY
Any digital media can be used as a cover media. The cover media can be a text, image (color, gray), audio or video etc. Cover media is required in order to hide and carry the information. Usually digital images are required in order to hide secret messages. The secret message is hidden within the digital image. After the message is embedded within the cover media
An innocuous image consisting of scenery, people and other objects are the nominees for cover media. The availability of natural images is not a problem due to the affordable price of digital cameras nowadays. Natural images are the best candidates for cover image since they have higher resolution providing flexibility and other necessary need.
Images are selected because the cover media is selected depending upon how the human visual system works. The gray image consists of pixels with intensity value from 0 to 255. The human eye cannot differentiate between a pixel intensity of 244 and 245. It will appear both the pixel intensities to be same. Also natural image s contains lots of noise and hence there is lots of pixel value intensity difference especially in the edges. This variation in the pixel intensity is known as randomness and can be exploited to embed the data without providing any suspicion to human eye. The human visual system won’t be able to distinguish between the image before modulation and the image after modulation.
The secret data is embedded within the cover image. The resulting image is called a stego-image. The stego-image will appear to be the same as the original image. The human eye cannot usually distinguish between the stego-image and the original image except there is a significant change between the images. In that case the Steganographic method employed is ineffective. Hence there are certain Stego-system criteria which we have to follow while doing a Stegagraphic implementation. They are stated as follows.
The major criteria of Steganography that the cover media shouldn’t be significantly altered or modified.
The embedded data should be directly encoded in the cover and not in the header or wrapper.
The embedded data should be immune to modifications of the cover. That is if the cover media under goes any change then data embedded within it should be unaltered.
During transmission there are chances of noise to occur and hence distortion due to noise cannot be eliminated. Due to this issue error coding algorithms should be included where ever it is required.
It is advisable to avoid the suspicious portions while embedding the message in the cover. This because after embedding in these regions they become open to suspicion. For example by embedding in smooth portions of the image, the resultant image becomes distorted and open to suspicion. Hence such portions are often avoided and portions where there is variation in the pixel intensity are preferred.
The embedding efficiency must be improved. Efficiency of the embedding capacity depends upon the number of pixels in the image used for embedding purposes.
The stego-image should be robust to steganalysis. Steganalysis is a method of identifying the difference between the original image and stego-image. Using steganalysis one can derive the information about the secret message.
The stego-image should reassemble the original image and there should be any suspicion.
The cover image should have high carrier capacity. That is the embedding capacity should be large. The amount of information present in the image should be resistant to distortion and should be comparable to the number of pixels present in the image.
The three basic requirements of Steganography are robustness, capacity and imperceptibility. All these requirements should be satisfied for having an effective Steganographic method.
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