Steganography is a technique of hiding communication by concealing the secret message into a fake message. The term Steganography has Greek influences which means “covered writing”. The main idea behind the Steganography is to prevent suspicion about the existence of the information.

• Pure Steganography does not require the exchange of a cipher such as a stego-key. It assumes that no other party is aware of the communication.

• Secret key Steganography where the secret (stego) key is exchanged prior to communication. This is most susceptible to interception. Secret Key Steganography takes a cover message and embeds the secret message inside of it by using a secret key (stego-key). Only the parties who know the secret key can reverse the process and read the secret message.

• Public key Steganography where a public key and a private key is used for secure Communication. The sender will use the public key during the encoding process and only the private key, which has a direct mathematical relationship with the public key, can decipher the secret message.

Types of Steganography

Image Steganography

• The image Steganography is used to hide a secret message inside an image. The most widely used technique to hide secret bit inside the LSB of the cover image. Because this method uses bits of each pixel in the image, it is necessary to use a lossless compression format, otherwise the hidden information will get lost in the transformations of a lossy compression algorithm.

• When using a 24 bit color image, a bit of each of the red, green and blue color components can be used, so a total of 3 bits can be used for each pixel, in this way we can use more secret bit to hide data in it.

Audio Steganography

• Audio stenography can conceal the secret message in the audio file with the help of its digital representation. It can be achieved easily as a typical 16-bit file has 216 sound levels, and a few levels difference could not be detectable by the human ear.

• The sender embeds secret data of any type using a key in a digital cover file to produce a stego file, in such a way that an observer cannot detect the existence of the hidden message. In many schemes a method of audio Steganography based on modification of least significant bits (LSB) the audio samples in the temporal domain or transform domain have been proposed.

Video Steganography

• Video Steganography brings more possibilities of disguising a large amount of data because it is a combination of image and sound. Therefore, image and audio Steganography techniques can also be employed on the video.

• Video files are generally a collection of images and sounds, so most of the presented techniques on images and - audio can be applied to video files too.

• The great advantage 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.

• The Video Steganography is nothing but a combination of Image Steganography and Audio Steganography.

Text Steganography:

• Steganography can be applied to different types of media including text, audio, image and video etc. However, text Steganography is considered to be the most difficult kind of Steganography due to lack of redundancy in text as compared to image or audio but still has smaller memory occupation and simpler communication.

• The method that could be used for text Steganography is data compression. Data compression encodes information in one representation into another representation. The new representation of data is smaller in size.

• One of the possible schemes to achieve data compression is Huffman coding. Huffman coding assigns smaller length code words to more frequently occurring source symbols and longer length code-words to less frequently occurring source symbols.