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SteganographyÂ is the science of writing hidden or presenting messages in such a way that no one, apart from the sender and intended recipient, even suspects the existence of the message. It is a form ofÂ security through obscurity.
The word steganography is of Greek origin and means 'concealed writing'. It stems from the Greek word steganos meaning 'covered or protected' , and graphein meaning 'to write' .
Demaratus was a Greek citizen and he is the regarded as the first person who used Steganography. At a time when the Persians were going to attack on Greece. Demaratus got to know of this plot and he decided that he should warn Greece about the secret attack. He knew it would be very hard to send the message over to Greece without it being intercepted. This was the time when he came up with the idea of using a wax tablet to hide his message.
The procedure he applied to hide his message was that he scraped all the wax away from the tablet revealing the wood from underneath. He then scraped his message into the wood and when he finished, recovered the wood with wax. The wax covered and concealed his message and it looked like it was just a blank wax tablet. Demaratus's message was hidden and so he sent this to Greece. The hidden message was not discovered by the Persians and successfully made it to Greece and because of this message; Greece was able to defeat the invading Persian force.
This was the first known case of the use of Steganography and since then, the complexity of Steganography has increased and this technology has advanced by leaps and bounds.
1.2.4 Techniques of Ancient Steganography :
22.214.171.124 Chinese Technique:
The Ancient Chinese developed their own techniques of Steganography. They would write their secret message on a piece of fine silk. Once the message was complete, they would crunch the silk into a little ball. They would then cover the ball with wax and then the messenger would swallow the ball. When the intended recipient was reached , the messenger would excrete and after the revelation process was complete , the messages would be shown to the recipient . It did have its disadvantages . Nobody knows for sure how effective this technique was .
126.96.36.199 Transparent Ink Technique:
As early as 100 A.D, transparent inks made their way into the secrecy world. Pliny the Elder discovered that the "milk" of the thithymallus plant could be used as a transparent ink. If one wrote a message with the above mentioned milk, it soon evaporated and left virtually no residue. Visually it appeared as if the message had been completely erased. But once the milk had completely dried, and was heated, it would begin to char and turn a brownish colour. This message could be written on anything that was not too flammable. The reason it turned brown was because the milk of the thithymallus plant contains carbon and when carbon is heated, it tends to char.
188.8.131.52 Invisible Ink Technique:
Invisible ink was a technique which was used during the American Revolutionary war. The technique used during this war was quite simple and noticeable.The technique the generals and soldiers used was one where a letter was written and the letter itself had no real meaning. In between all the lines of letters were gaps and empty spaces. This was to distinguish a line from the next. This is the place where the secret message was hidden using invisible ink. The message was written in the gaps with the invisible ink and then when the ink dried up, it looked like a normal letter again.
The downfall of this system was the way in which the hidden message was retrieved. In order to read the message, all one had to do was hold it up to a light and the ink would become visible. So, if the enemy intercepted the letter and incidently happened to read the letter with a light behind it, the secret message would easily be revealed.
184.108.40.206 Grille system technique:
The grille system was another wartime technique. This technique involved strategically placing letters within a text which , to the ordinary observer , seemed ordinary . The secret message was sent and then the receiver was able to see the secret message by using a special grille. The grille was usually just a slab of wood that would fit over the message. The slab had strategically placed holes in it at the spots where the letters would have been carefully placed. The letters which would come under the holes would then spell out the secret message created at the sender side. This technique was quite effective due to the fact that the person trying to intercept the message would not be able to decode it unless they had the correct slab but on the other hand , this was also one of the major downfalls of this technique.
The design of the slab had to be the same at both the sender and the recipient's end. One design of the grille was not used very often and if the receiver's grille was lost or broken, the message would be unreadable and thus useless.
220.127.116.11 Microdot technology Technique:
The Microdot technology was invented by the Germans to convey secret messages to their allies. The microdot is basically a highly detailed picture shrunk technically to about the size of a dot. It was possible to fit an entire page of text or an entire picture into the little microdot, which is what made the microdot such a successful technique. The picture could again be seen either by simply using a microscope. The dot was almost undetectable to the naked eye.
18.104.22.168 Open coded message technique:
This technique is quite similar to the grille technique but this time no grille is needed. For open coded messages, certain letters of each word , which are strategically placed are used to spell out the secret message. Open coded messages use normal words and messages to write the buffer text that conceals the message and because they seem normal, they often pass the encountered security checks.
Here is a frequently used example of open coded messages. This message was actually sent by a German Spy during World War II :
"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."
The technique is that by taking the second letter of each word, the hidden secret message is revealed, which is : Pershing sails from NY June 1.
This technique is effective because it can pass through filter devices but can be easily decoded by someone if they know that a message is hidden in the text.
1.3 Modern Steganography :
With the invention of the computer , Steganography techniques have changed altogether. Computer technology has made it much easier to hide messages and also made it a lot harder to discover the hidden message.
Modern Steganography can be classified into three main categories :
1. Audio Steganography
2. Video Steganography
3. Image Steganography
1.3.1 Audio Steganography:
In a computer-based audio steganography system, one of the methods of embedding a secret message is by doing so in digital sound. The secret message is embedded by slightly altering the binary sequence of an audio file. Existing audio steganography software can embed messages in WAV, AU, and even MP3 sound files.
Even though embedding secret messages in digital sound is usually a more difficult process than embedding messages in other media, such as digital images yet , in order to conceal secret messages successfully, various methods for embedding information in digital audio have been introduced. These methods range from simple algorithms that insert information in the form of signal noise to more advanced methods that exploit sophisticated signal processing techniques to hide information. Commonly used audio steganography methods are listed and discussed below :
i. LSB coding
ii. Parity coding
iii. Phase coding
iv. Spread spectrum
v. Echo hiding
22.214.171.124 LSB Coding
This method is commonly used as modifications to the last significant bit usually do not create audible changes to sounds.
Another method involves encoding messages using frequencies that are inaudible to the human ear. Using any frequencies below 20 Hz or above 20000 Hz, messages can be hidden inside sound files and will not be detected by human checks.
126.96.36.199 Parity coding
In the parity coding technique,instead of breaking a signal down into individual samples , the signal is broken down into separate regions of samples and each bit from the secret message is encoded in a sample region's parity bit. If the parity bit of a selected region does not match the secret bit to be encoded, the process flips the LSB of one of the samples in the region , which as above discussed does no major harm to either the original file or the Stegofile . Thus, the sender has more choices in encoding the secret bit .
188.8.131.52 Phase coding
Phase coding addresses , to some extent , the disadvantages of noise inducing methods of audio steganography. It works on the principle that the phase components of sound are not as perceptible to the human ear as noise is and rather than introducing perturbations, the technique encodes message bits as phase shifts in the phase spectrum of a digital signal, achieving an inaudible encoding in terms of signal-to-perceived noise ratio.
184.108.40.206 Spread spectrum
Basic spread spectrum (SS) method attempts to spread secret information across the audio signal's frequency spectrum as much as possible. As a result, the final signal occupies a bandwidth in excess of what is actually required for transmission.
220.127.116.11 Echo hiding
In echo hiding, information is embedded in a sound file by introducing an echo into the original discrete signal. Like the spread spectrum method, it too provides advantages such that it provides a high data transmission rate and provides robustness when compared to the noise inducing methods. If only one echo is produced from the original signal, only one bit of information can be encoded. Therefore, the original signal is broken down into blocks before the encoding process can begin. Once the encoding process is completed, concatenation of the blocks back together is processed to create the final signal.
1.3.2 Video Steganography:
Video files are generally a collection of images and sounds, so most ofthe presented techniques on images and audio can be applied to video files too. The great advantages of video are the large amount of data that can be hidden inside and the fact that it is a moving stream of images and sounds. Therefore, any small but otherwise noticeable distortions might go by unobserved by humans because of the continuous flow of information.
1.3.3 Image Steganography:
Image Steganography can be classified into further two types
i. Spatial Domain Technique
ii. Transformed Domain Technique
18.104.22.168 Spatial Domain Technique
Spatial Domain techniques are the simplest techniques of data hiding in which we only play with the pixel values and embed our message in a digital picture.
Least significant bit (LSB) is the one of the most commonly used types of insertion schemes used currently in digital Steganography. As each pixel is made up of three bytes, consisting of either a 1 or 0. Suppose if we had the following pixel:
R G B
1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1 1 1 0 0 1 1 0 0
The LSB would be as follows:
R G B
1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1 1 1 0 0 1 1 0 0
The last bit in a byte is known as the least significant bit (LSB), which means that there is enough information in the seven bits preceding it to ensure that the correct color will be established. When the embedded data's bits are substituted into the least significant bits (LSB's) location it will have little to no effect on the appearance of the images to the naked human eye. The software used to embed the hidden file will usually neglect the LSB's. Once these bits are zeroed out ( neglected ) they are ready to be used by the steganography software to contain whatever secret data is going to be transmitted.
22.214.171.124 Transformed Domain Technique
Another class of techniques used involves embedding the message by modulating coefficients in a transform domain, such as the Discrete-Cosine Transform (DCT) (used in JPEG compression), Discrete Fourier Transform, or Wavelet Transform. Transform techniques can offer superior robustness against lossy compression because they are designed to resist or take advantage of the methods of popular lossy compression algorithms. An example of a transform-based steganographic system is the "Jpeg-Jsteg" software, which embeds the message by modulating DCT coefficients of the stego-image based upon bits of the message and the round-off error encountered during quantization. Transform-based Steganography also typically offers increased robustness to scaling, rotations or cropping, depending on the invariant properties of a particular transform.
1.7 Digital Steganography:
This is an Image of a tree. By removing all but the two least significantÂ bitsÂ of eachÂ colourÂ produces an almost completely black image and further processing that image 85 times brighter produces the image below.
An image of a cat extracted from above image.
Modern or digital steganography entered the world in 1985 with the advent of the personal computer.Â Development following that was slow, but has since taken off by leaps and bounds and a number of "stego" programs available in the market now.
Digital Steganography basically includes the concealment of information within computer files. In digital steganography, electronic communications may include steganographic coding on the inside of a transport layer, such as a document file, an image file, a program or a protocol. Media files are an ideal opportunity for steganographic transmission because of their large size.
Â Digital steganography techniques include:
Concealing messages within the lowest bits ofÂ images or sound files
Hiding data within encrypted data or within random data. The data to be hidden is first encrypted before being used to overwrite part of a much larger block of encrypted data .
Chaffing and winnowing techniques.
Mimic functionsÂ convert one file to have the statistical profile of another function. This can spoil statistical methods that help brute-force attacks identify the right solution in aÂ cipher text-only attack.
Concealing messages in tampered executable (.exe) files, exploiting redundancy in the i386 instruction set.
Pictures embedded in video (optionally played at slower or faster speed or frame rate).
Applying unnoticeable delays to packets sent over the network from the keyboard. Delays in key presses in some applications (telnet orÂ remote desktop software) can mean a mean delay in packets and the delays in the packets can be used to encode data.
Content-Aware steganography conceals information in the semantics a human user assigns to a datagram. These systems offer better security against a non-human adversary/warden.
In Blog-Steganography , messages areÂ fractionalizedÂ and the encrypted pieces are added as comments of orphaned web-logs (or pin boards on social network platforms). In this case the selection of blogs is the symmetric key that the sender and the recipient are using and the carrier of the hidden message is the wholeÂ blogosphere.
1.8 Network Steganography:
Information hiding techniques that may be used to exchange steganograms in telecommunication networks can be categorized as a form of network steganography.
This nomenclature was introduced by Krzysztof Szczypiorski in 2003. Network steganography utilizes communication protocols' control elements and their basic functionalites. Therefore , such methods are harder to detect and eliminate.
Network steganography methods involve modification of the properties of a single network protocol. These modifications can be applied to the PDU (Protocol Data Unit), to the time relations between the exchanged PDUs,Â or both (hybrid methods).
Moreover, it is feasible to make use of the relation between two or more different network protocols to enable secret communication. These applications are termed as inter-protocol steganography.
Network steganography covers a broad spectrum of techniques which include:
Steganophony - The concealment of messages inÂ VoIPÂ (Voice over IP) conversations. As an example, the employment of delayed or corrupted packets that would normally be ignored by the receiver could be used. This method is called LACK - Lost Audio Packets Steganography, or, alternatively, concealing information in unused header fields.
WLAN Steganography - The utilization of methods that may be exercised to transmit steganograms in Wireless Local Area Networks fall under this category. The HICCUPS system (Hidden Communication System for Corrupted Networks) is a practical example of WLAN Steganography .