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Internet has become the most efficient, effective and fastest communication media. The rapidly decreasing processing cost, bandwidth and storage have made the digital media more popular over the analog media. The information that is communicated comes in various and is used in numerous applications. In almost all applications it is preferred to transmit this information secretly. Such secret communication varies from cases of corporate communication, credit card purchases, and bank transfers to everyday emails. It is vulnerable to many problems like hacking, eavesdropping, copyright which increases the scope for secret communication.
The first aim is to research the fields of Steganography and Steganalysis, such that a variety of techniques can be presented for hiding a secret message within an image. The second aim of the project is to implement a system using the techniques provided and effectively hide a message in an image.
Purpose of Study:
The main purpose of the study is to propose a security system for transmitting secret data effectively through different images. To achieve this many steganographic methods and their advantages, disadvantages are studied and a better solution is proposed. Encryption and Decryption algorithms are used for hide secret data. Steganography technique is chosen among other techniques like Cryptography and Watermarking to overcome the drawbacks existing in these systems. The other purpose of the study is to discuss all the important techniques in detail and its importance in different applications. So the system hides data in a image file and sends it to the sender involving encryption and decryption.
Need for Steganography:
Cryptography and Steganography are the two techniques available for data hiding. Unlike Cryptography and Watermarking which can overlap usages in the data hiding process, Steganography hides the knowledge of the presence of information in the cover medium whereas cryptography reveals the presence of information but it is difficult to decode the data. Watermarking is different from both steganography having its main aim to protect the cover medium from modifications. Many types of media like audio, video and text are transferred. Cryptography has been the reliable means for secure communication for many decades. However the transfer of data through this technique may help the outsiders to capture the data. Consequently Steganography has been introduced to boost the security by inserting secret message into the digital image. Various digital steganographic techniques are available which are able to produce secret image that is identical from the original image (Amirtharajan, Akila, & Deepikachowdavarapu, 2010).
History of Steganography:
According to (Zaidan & Zaidan, 2009), "Cryptography is the art and science of protecting data, which provides ways for converting data into unreadable form using a special key (encrypt it), so that only those who possess a secret key can decipher (or decrypt) the message into plain text". It provides security to the data that is transmitted by verifying the identity of information, and ensuring that data is reached to the communication parties. The major drawback of data hiding using cryptography is that the encrypted messages can sometimes be broken which results in the loss of data in the communication media. This led to the development of Steganography in which the data is embedded with in another file seemingly an innocent file like an image.
According to (Johnson & Jajodia, 1998) Steganography is defined as "Steganography is the art and science of communicating in a way that prevent the detection of hidden messages." The term steganography is derived from Greek which means covered writing. This method conceals the existence of the original image through array of methods of secret communications (Halibozek, Kovacich, & Jones, 2008). Steganography is the process of hiding information by embedding a file inside another file. It is used mostly with graphics, sound, text, html and pdf files (Blyth & Kovacich, 2001). For example an audio file can be hidden inside a jpeg image which is not definitely possible in traditional encryption techniques. Also even if someone knows that a file is hidden inside another it is not easy to find out where it is hidden and how. It requires special software's to extract the hidden image which serves as the best way of data protection (Mansfield & Antonakos, 2010). Message hiding using steganographic methods minimizes the chance of detecting the message. It can be used to protect intellectual property or trade secrets hence maintaining confidentiality of the most valuble information by giving protection from thieves and unauthorized viewers. Further advancements in Steganography has been taken place by hiding data in the printed images which can read by only particular mobile phone cameras having a particular Java applet enable (Frith, 2007).
Steganography hides the message which finds it difficult for the attackers to know the presence of the hidden message. The information is hidden in carriers like text files, image files, audio and video files. The figure below shows the system of steganography.
The main steps involved in steganography process are:
To embed the secret message inside the cover media by using a stego-key.
To extract the same secret message from the cover media using the stego-key.
For the system to embed secret message, embedding algorithm is used and an extraction algorithm for extracting the message from the cover media. As seen from figure the inputs to the embedding algorithm are secret message, stego-key and cover media. Secret message is the actual message which is to be hidden; Cover media is the carrier file like the image, audio, video files in which the secret message is hidden; Stego-key is the additional data that is required in the process of hiding. It is used to hide and extract the secret message. Merging of the cover media and embedded message is stegomedia. Hence Steganography can be illustrated by the formula (Curran & Bailey, 2003)
Cover media + Embedded media + Stegokey = Stegomedia
Fig: Steganographic System
The message is then transmitted in a medium and reaches the other end of the communication point where the extraction is to be done. The recipient has to then extract the message from carrier image by using Stegokey. This can be accomplished through extraction algorithm where the original secret message is obtained.
For the steganographic communication to be more secure, the message can be compressed and encrypted prior to hiding of message in the cover media. The encryption of the message serves as another layer of security. The compressed message needs less space to be fit in the carrier when compared to the original message embedding in the carrier. It is also much easier to hide the compressed and encrypted message leading to high regularity.
The Steganographic method can be evaluated with respect to three quantities: Capacity, Security and Robustness. These three parameters affect Steganography and its usefulness. Capacity is the maximum amount of data bits that can be hidden in a given digital cover medium (Al-Husainy, 2009). The more the capacity for a given message size the less the bandwidth required to transmit. Robustness is the survivability factor of the hidden message after the media has undergone some changes like image blurring, image corruption, rotation, loss compression, noise corruption etc. Security is the measure of degree of difficulty in detecting a secret message with in a suspected media file (Al-Ani, Zaidan, Zaidan, & Alanazi, 2010). These three quantities should be balanced i.e. increasing the capacity reduces both robustness and security while reducing the capacity will increase security and robustness.
As mentioned earlier steganographic techniques use various cover medium to hide data which may be floppy disk, network packets, radio waves, and hard drive or computer files like images, audio, video and text. There are many regulations and restrictions for the use of steganography through threats from a variety of laws in diverse countries. There is a lack of secured information by copy right implementing agencies in organizations which require exact procedures to extract their information from the channel. Hence steganographic techniques are preferred which are very easy to embed the secret information at one end and extract the message from the other end (Abdul & Gutub, 2010). The basic types of Steganography are discussed below:
Now-a-days the communication between the people is through the use of computer, phone, fax which requires the communication channel to be secure. The variation in these types exists with the presence of secret keys and public keys which are used for embedding and extracting the hidden messages. Typically there exist three steganographic types (Al-Ani, Zaidan, Zaidan, & Alanazi, 2010).
Secret Key Steganography
Public Key Steganography
Pure Steganography is that system which does not require any information like stego-key for starting the communication process and no exchange of secret information is required. Thus the security of this type of system only depends on the secrecy in communication process.
Fig: Pure Steganography
This type of steganography can be defined as quadruple(C, M, D and E) where
C is the set of possible covers
M is the set of secret message with |C| â‰¥ |M|
E is the CÃ-Mâ†’C the embedding function
D is the Câ†’M of the extraction function with the property that
D is the (E(c, m)) = m for all m Ð„ M and c Ð„ C
Pure steganography needs no sharing of stego-key between the transmitter and receiver and hence it is used in most of the applications.
Secret Key Steganography:
A Secret key steganography system is one in which a cover is chosen by the sender and embedded in to the cover by using a secret key. The secret key should be known to the receiver and can then be extracted at the receiving end.
Fig: Secret Key Steganography
This type steganography can be defined as the quintuple(C, M, K, DK, EK) where
C is the set of possible covers.
M is the set of secret message.
K is the set of secret keys.
Ek is CÃ-MÃ-Kâ†’C
With the property that DK (EK(c, m, k), k) = m for all m Ð„ M, c Ð„ C and k Ð„ K
Public Key Steganography:
Public key steganography require two keys for transmitting information from sender end to the receiver end. First one is secret key and the next one is public key which is saved in the database. It is used for embedding of message in the process while the secret key is used to rebuild the hidden message. The information is encrypted by the sender with the public key of receiver and obtains a random-looking message by embedding it to known receiver channel.
At present the most common cover media used to hide secret messages are digital images due to the potential payload of the images. A typical digital image of 256 colors (8 bit) and 640Ã-480 pixel can hide about 300 KB of information. An image of high resolution, 24 bit color and 1024Ã-768 can hide about 2.3 MB of data on a whole. Digital images generally have a large size which is difficult for the transmission of image and the secret message over internet. So the image is compressed using different compression techniques and algorithms. Image steganography generally have four steps which are Image Compression, Message Embedding and Message Extraction (SitaramPrasad, Naganjaneyulu, GopiKrishna, & Nagaraju, 2009).
Image compression takes place in order to reduce the size of digital image and to avoid the difficulties posed in the transmission of the image along with the secret message. The compressed image is an exact replica of the original image. There exists a wide variety of compression algorithms like Joint Photographic expert group (JPEG), Graphic Interchange Format (GIF), Bit Plane Mapping (BMP). The last two techniques mentioned above are mainly used while employing a cover medium for message as it offers lossless compression. JPEG compression algorithm is calculated based on floating point for translating picture in to an array of integers. This way of compression may have an end result of rounding errors which may remove some portions of the image that are invisible to the human eye and can result in the destruction of secret message. These compression techniques use mathematical formulae in analyzing and reducing the image without affecting the original details of the image (Silman, 2001).
Two different types of compression techniques are presented lossy compression techniques and lossless compression techniques. The size of the compressed image in Lossy compression techniques is smaller which adds the provision for better transmission but increase the chances of losing the embedded data during the communication process as excess data in the image would be removed. Lossless compression technique does not compress the image to small size when compared to the size of lossy compression technique however the message that is transmitted can be retained without any loss of information.
Message Embedding and Extraction:
The secret information is embedded into the cover medium i.e. digital images which is then transmitted across the channel and received at the other end where the message should be extracted based on the stego-key. The embedding of data into an image can be achieved using two techniques. Image Domain tools and Transform tools. The Image Domain tools include Lease significant bit manipulation and Bit wise methods. Transform domain tools use algorithms like wavelet transformation or Discrete Cosine transformation for hiding data in the image. The Embedding techniques will be discussed in detail in the next section. The extraction of the message at the receiver end can be done based on the embedding algorithms which makes it easy for the receiver to extract the hidden message with the help of stego-key used at the transmitter end (SitaramPrasad, Naganjaneyulu, GopiKrishna, & Nagaraju, 2009).
Image Steganographic Techniques:
The steganographic systems can be categorized based on the covers used for secret communication. They can be grouped into six categories.
Transform Domain Systems
Spread Spectrum Technique
Cover Generation Technique