Steganography (hiding/covering information)

Stego means cover, graphia means text/writing

A plaintext message may be hidden in one of two ways. The methods of steganography conceals the existence of the message, whereas the methods of cryptography render the message unintelligible to outsiders by various transformations of the text.

Steganography is data hidden within data. It is an encryption technique that can be used along with cryptography as an extra-secure method to protect data.

Thus,the technique of hiding information in any other information & also hiding the fact that communication is taking place is known as steganography.

The main aim of steganography is for a sender to transfer a plaintext to a receiver in such a way that only the receiver can extract the plaintext because only the receiver knows the hidden plaintext exists in the first place & how to look for it.

Steganography techniques can be applied to images, a video file or an audio file. Typically, however, steganography is written in characters including hash marking, but its usage within images is also common. At any rate, steganography protects from pirating copyrighted materials as well as aiding in unauthorized viewing.

How it works

Steganography replaces unneeded or unused bits in regular computer files (Graphics, sound, text) with bits of different and invisible information. Hidden information can be any other regular computer file or encrypted data.

Types of steganography are:

There are different ways to hide the message in another, well known are Least Significant bytes and Injection.

• When a file or an image is created there are few bytes in the file or image which are not necessary or least important. These types of bytes can be replaced with a message without damaging or replacing the original message, by which the secret message is hidden in the file or image.

• Another way is a message can be directly injected into a file or image. But in this way, the size of the file would be increased accordingly depending on the secrete message

Least Significant Bit (LSB) insertion is the most widely known algorithm for image steganography, it involves the modification of the LSB layer of the image. In this technique, the message is stored in the LSB of the pixels which could be considered as random noise. Thus, altering them does not have any obvious effect on the image.

Techniques for steganography

The most common in digital steganography are:

─ Image-based techniques: the carrier is an image. Usually, one hides messages in the noise component of a given image.

─ Sound-based techniques: the carrier is sound.

─ Text-based techniques: can consist of typos, spacing schemas, rendering mistakes et.al. in order to convey messages

─ Network packets: hiding is performed in the unused headers of an TCP/IP packet

─ OS-hiding: embedding in unused portions of the hard-drive.

Classification of Steganography Techniques:

1. Spatial Domain Techniques: These techniques use the pixel gray levels and their color values directly for encoding the message bits. These techniques are some of the simplest schemes in terms of embedding and extraction complexity. The most common algorithm belonging to this class of techniques is the Least Significant Bit (LSB) Replacement technique in which the LSB of the binary representation of the pixel gray levels is used to represent the message bit.

2. Transform Domain Techniques: These techniques try to encode message bits in the transform domain coefficients of the image. Data embedding performed in the transform domain is widely used for robust watermarking. Similar techniques can also realize large capacity embedding for Steganography. Candidate transforms include discrete cosine transform (DCT), discrete wavelet transform (DWT), and discrete Fourier transform (DFT).