ISSN: 2229371X
Lavanya.S, Anuradha.C

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This work proposes a Secure and authenticated discrete reversible data hiding in cipher images deals with security and authentication. In the first phase, a content owner encrypts the original uncompressed image using an encryption key. Then , a data hider may compress the least significant bits of the encrypted image using a data hiding key to create a sparse space to accommodate some additional data. With an encrypted image containing additional data, if a receiver has the data hiding key, receiver can extract the additional data though receiver does not know the image content. If the receiver has encryption key, can decrypt the received data to obtain an image similar to the original one , but cannot extract the additional data. If the receiver has both the data hiding key and the encryption key, can extract the additional data and recover the original content without any error by exploiting the spatial correlation in natural image when the amount of additional data is not too large.
Keywords 
data hiding, data embedding, data extraction, image encryption, reversible data hiding. 
INTRODUCTION 
Military and medical images are media having some distortion is unacceptable. Hence for data hiding we have technique using which we can extract data correctly and after that original cover content can be perfectly recovered. This technique so known as reversible data hiding [4]. There are many reversible techniques such as expansion method and histogram shift method. Another kind of method makes use of redundancy n a cover by performing lossless compression to create a spare space for data embedding [1]. 
Encryption is well known for privacy protection. For securely transmission of image content owner encrypt it be for etransmit it to other person. In some application scenarios, an inferior assistant or a channel administrator hopes to append some additional message, such as the origin information, image notation or authentication data, within the encrypted image though he does not know the original image content. For example, when medical images have been encrypted for protecting the patient privacy, a database administrator may aim to embed the personal information into the corresponding encrypted images. It may be also hopeful that the original content can be recovered without any error after decryption and retrieve of additional message at receiver side. That means a reversible data hiding scheme for encrypted image is desirable [1]. But, in some applications, an inferior assistant or a channel administrator hopes to append some additional message, such as the origin information, image notation or authentication data, within the encrypted image though he does not know the original image content. And it is also hopeful that the original content should be recovered without any error after image decryption and message extraction at receiver side. A content owner encrypts the original image using an encryption key, and a datahider can embed additional data in to the encrypted image using a datahiding key though he does not know the original content. With an encrypted image containing additional data, are ceiver may first decrypt it according to the encryption key, and then extract the embedded data and recover the original image according to the datahiding key. In the scheme, the data extraction is not separable from the content decryption. In other words, the additional data must be extracted from the decrypted image, so that the principal content of original image is revealed before data extraction, and, if someone has the datahiding key but not the encryption key, he cannot extract any information from the encrypted image containing additional data. 
This paper proposed new scheme of data hiding it is also known as separable reversible data hiding. Content owner can encrypt this image be for etransmission by using encryption key. And additional data can be added using the data hiding key. At the receiver side if receiver has only data hide key, he can only extract the data from image. If receiver has encryption key then he can decrypt the image. But if receiver has both, data hiding and encryption key then he can extract hided data and as well as can recover image [2]. 
PROPOSED SYSTEM 
Proposed system has three main phases: 
A. Image Encryption: 
B. Data embedding: 
C. Data extraction and image recovery: 
Using encryption key content owner encrypt image. Then data hider creates parse space for hiding data by replacing least significant bits (LSB) in encrypt image using data hiding key. At the receiver side embedded data at sparse space can retrieved from encrypted image using data hiding key. As data hiding affects only on LSB, hence after decryption original image can be correctly retrieved. When using both of the encryption and datahiding keys, the embedded data can be successfully extracted and the original image can be perfectly recovered by exploiting the spatial correlation in natural image.Fig.2 shows the three cases at the receiver side. 
Image Encryption [4], [2]: 
Assume the original image is in un compressed format and each pixel with gray value fall in gin to[0,255]is represented by8bits.Denotethebitsofapixelasbi,j,0,bi,j,1,……,bi,j,7 where (i,,j) in dicates the pixel position, and the gray valueas pi,j.Thus 
Where ri,j,k are determined by an encryption key using a standard stream cipher. Then, Bi,j,ka reconatenated orderly as the encrypted data. A number of secure stream cipher method scan be used to ensure that anyone without the encryption key, such as a potential attacker or the data hider, cannot obtain any information about original content from the encrypted data. 
Data Embedding [4], [2]: 
With the encrypted data, although a datahider does not know the original image content, he can embed additional message into the image by modifying a small proportion of encrypted data. Firstly, the datahider segments then crypted image into a number of nonover lapping blocks size dbys×s. In other words, the encrypted bits bi ,j, satisfying (m1)+s +1≤ I≤ m+s,(n1) +s+1≤ j≤ n +s and0≤ k ≤7( mand nare positive integers) are within as am block. Then, each block will be used to carry one additional bit. For each block, pseudorandom ly divide the s2 pixels into two sets S0 and S1 according to a datahiding key. Here, the probability that a pixel belongs to S0 or S1 is1/2. If the additional bit to beembeddedis0, flip the3 least sign if I can tb its (LSB) of each encrypted pixel in S0, 
B’i,j,k= B’i,j,k’ (i,j)ÃÂS0and = 0,1, 2 (4) 
If the additional bit is1,flip the 3 encrypted LSB of pixel sinS1, 
B’i,j,k= B’i,j,k’ ( i,j)€S1 andk= 0, 1, 2. (5) 
The other encrypted data are not changed. 
Data Extraction and Image Recovery [4], [2]: 
When having an encrypted image contain in gem bedded data, are ceiver first ly generates ri,j,k according to then cryption key, and calculates the exclusiveor of the received data ri,j,k and to decrypt the image. We denote the decrypted as. Clearly, the original five most significant bits (MSB) are retrieved correctly. For a certain pixel, if the embedded it in the block including the pixel is zero and the pixel belongs to S1, or the embedded it is1 and the pixel belongs to S0, the datahiding does not affect any encrypted bits of the pixel. So, the three decrypted LSB must be same as the original LSB, implying that the decrypted gray value of the pixel is correct. On the other hand, if the embedded bit in the pixel’sblockis0 and the pixel be longs to S0, or the embedded bit is 1 and the pixel belongs to S1, the decrypted LSB That 
Then, the receiver will extract he embedded bits and recover the original content from then cryptic image. According to the datahiding key, he may segment the decrypted image into blocks and divide the pixels in each block into two sets in as Amway. For each decrypted block, the receiver flips all the three LSB of pixel sin S0 to form a new block, and flips all the three LSB of pixels in S1 to form another new block. We denote the two new blocks asH0 and H1.There must be that either H0 orH1 is the original block, and another one is more seriously interfered due to the LSB flip operation. For the two blocks sized by S×S, define a function to measure the fluctuation in them and de note the values of fluctuation function no fH0 and H1 as f0 and f1respectively.Because of spatial correlation in natural image, the fluctuation function of original block is generally lower than that of seriously interfered version. So, the receiver can perform data extraction and image recovery by comparing f0 and f1. If f0 <f1, regard H0 as the original content of the block and let the extracted it be 0. Otherwise, regard H1 as the original content of this block and extract a bit1. Finally, concatenate the extracted bits to retrieve the additional message and collect the recovered blocks to form the original image [4], [2]. 
CONCLUSION 
When the receiver has both of the keys, he can extract the additional data and recover the original content without any error by exploiting the spatial correlation in natural image if the amount of additional data is not too large. If the loss less compression method in [6]or[7] is used for the en crypted image containing embedded data, the additional data can be still extracted and the original content can be also recovered since the loss less compression does not change the content of the encrypted image containing embedded data [2]. However, the lossy compression method in [5] compatible with encrypted images generated by pixel permutation is not suitable here since the encryption is performed by bitXOR operation [2].So, in future a complete mixture of image encryption and data hiding compatible with lossy compression demands further research. 
References 
