Stegnography

Most of us wondered why couldn't the FBI detect or get a clue about the September 11th attack? Though the terrorist communicated all their information's through the web it wasn't easy to tamper! Have you wondered why? It was because of a special technique called Steganography. To ensure the security of data, a technique called STEGANOGRAPHY is used.

"Steganography is the art and science of communicating in a way which hides the existence of the communication"

Steganography (Greek for 'covered writing') is a science that is quite different from cryptography. In the field of Steganography, some terminology is used. The term "cover" is used to describe the original, innocent message, data, audio, still, video and so on. When referring to audio signal steganography, the cover signal is sometimes called the "host" signal. The information to be hidden in the cover data is known as the "embedded" data. The "stego" data is the data containing both the cover signal and the "embedded" information. Logically, the processing of putting the hidden or embedded data, into the cover data, is sometimes known as embedding. Occasionally, especially when referring to image steganography, the cover image is known as the container.

Data hiding can be done in three different media: -

Text, Images, Audio signals
This is done based on Bender's specification.

Bender's Specification :

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 that 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.

Steganography in Text :

Various methods of text steganography are available. They are Line shift coding ,Word shift coding, Feature coding an e.g. for the text Steganography.

Consider a text :

Apparently neutrals protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on byproducts, ejecting suets and vegetable oil.

When we take only the second letter of each word in the above text we get the following message.

Perishing sails from NY June 1.

This shows that there is the message hidden behind the given text is highly secure. The original message does not arise any suspicion for the readers. The message reaches the correct person over a common channel.

Steganography in Images :

Image steganography has come in recent years with the development of powerful graphical computers, and steganographic software is now available over the Internet for everyday users.

Guidelines to Image Steganography :

Digital images are typically stored in either 24-bit or 8-bit per pixel files. 24-bit images are sometimes known as true colour images or gray scale images. Gray-scale images are very good because the shades gradually change from byte to byte. The importance is not whether the image is gray-scale or not, the importance is the degree to which the colors change between bit values.Once a suitable cover image has been selected, an image encoding technique needs to be chosen.

The most common approaches to information hiding in images are:

Least significant bit (LSB) insertion: The least significant bit insertion method is probably the most well-known image steganography technique. It is a common, simple approach to embedding information in a graphical image file.

Masking and filtering techniques: Masking and filtering techniques hide information by marking an image in a manner similar to paper watermarks. Because watermarking techniques are more integrated into the image, they may be applied without fear of image destruction from lossy compression. By covering, or masking a faint but perceptible signal with another to make the first non-perceptible, we exploit the fact that the human visual system cannot detect slight changes in certain temporal domains of the image.

Algorithms and transformations:

JPEG images are used across networks such as the Internet because of the high colour quality with good compression. JPEG images use the discrete cosine transform (DCT) to achieve compression.

Here is a sample image that hides the given message using the LSB technique. Here the container file is a "Shakespeare's" image. The message displayed is hidden behind the image.

Container File :



Message :

Steganography is the art and science of communicating in a way, which hides the existence of the communication.

Stego


Thus we find no change in the steg image.

Audio Steganography :

Methods of Data Hiding : There are three methods of data hiding

Low-bit Encoding:

Similarly to how data was stored in the least-significant bit of images, binary data can be stored in the least-significant bit of audio files. Ideally the channel capacity is 1kb per second per kilohertz; so for example, the channel capacity would be 44kbps in a 44 kHz sampled sequence.

Phase Coding:

The phase coding method works by substituting the phase of an initial audio segment with a reference phase that represents the data.

Echo data hiding:

Echo data hiding embeds data into a host signal by introducing an echo. Varying three parameters of the echo hides the data: initial amplitude, decay rate, and offset, or delay. As the offset between the original and the echo decreases, the two signals blend. At a certain point, the human ear cannot distinguish between the two signals, and the echo is merely heard as added resonance. This point depends on factors such as the quality of the original recording, the type of sound, and the listener.

Is Steganography only for such a destrcutive purpose?

Steganographic techniques can be used for a number of purposes other than covert communication or deniable data storage: information tracing and finger printing, tamper detection, and perhaps most importantly digital watermarking. A digital watermark (inserted into, say, an image as we see in our currencies) is invisible to the eye, undetectable without the appropriate secret key, but contains small ownership identification.

Writers, photographers, musicians, and artists are among those who have taken adantage of the worldwide publishing opportunities provided by the Internet, yet these same people are frequently taken advantage of by on-line pirates. A much wider field of steganography is digital watermarking allows copyright owners to incorporate into their work identifying information invisible to human eye, yet protecting against the dangers of copyright infringement.