Reversible data hiding in encrypted images (RDHEI) is an effective technique to embed data in the encrypted domain. An original image is encrypted with a secret key and during or after its transmission, it is possible to embed additional information in the encrypted image, without knowing the encryption key or the original content of the image. During the decoding process, the secret message can be extracted and the original image can be reconstructed. In the last few years, RDHEI has started to draw research interest. Indeed, with the development of cloud computing, data privacy has become a real issue. However, none of the existing methods allows us to hide a large amount of information in a reversible manner. In this paper, we propose a new reversible method based on MSB (most significant bit) prediction with a very high capacity. We present two approaches, these are: high capacity reversible data hiding approach with correction of prediction errors (CPE- HCRDH) and high capacity reversible data hiding approach with embedded prediction errors (EPE-HCRDH). With this method, regardless of the approach used, our results are better than those obtained with current state of the art methods, both in terms of reconstructed image quality and embedding capacity.
In this method, since we propose to embed the secret message by MSB substitution, the original MSB values are lost after the data hiding step. It is important, during the decoding phase, to be able to predict them without any errors. Indeed, in order to reconstruct the original image, we propose to use the previous pixels to predict the current pixel value. So, the first step consists of analyzing the original image content to detect all the possible prediction.
In order to make the original image I unreadable, we encrypt it by using an encryption key K e = (c,x 0 ), as shown in Fig. 3. The elements of this key are used as parameters of a chaotic generator, based on the Piecewise Linear Chaotic Map . By using this chaotic generator, a sequence of pseudo-random bytes s(i,j) is obtained and the encrypted pixels p e (i,j) can be calculated through exclusive-or (XOR) operation
In the data embedding phase, it is pos- sible to embed data in the encrypted image without knowing either the encryption key K e used during the previous step or the original content of the image. By using the data hiding key K w , the to-be-inserted message is first encrypted in order to prevent its detection after embedding in the marked encrypted image. Next, pixels of the encrypted image are scanned from left to right, then from top to bottom (scan line order) and the MSB of each available pixel is substituted by one bit b k , with 0 ≤ k < m × n, of the secret message
pre-process the original image to avoid all the prediction errors in order to be able to reconstruct the image during the decoding step. After this process, we can encrypt the pre-processed image without any problems. During the embedding phase, all the pixels of the encrypted image are marked with one bit of the message. Using this approach, we have a maximal payload, equal to 1 bpp. code.
After the prediction error detection phase, we propose to pre-process the original image I in order to obtain an image I 0 without any prediction errors. For each problematic pixel, we observe the amplitude of the error and we compute the value of the minimal pixel modification necessary to avoid this error. Eq. (9) shows the provision necessary to have no prediction errors during the decoding phase.
DIGITAL image security plays a significant role in all fields, especially in highly confidential areas like the military and medical worlds. With the development of cloud computing, the growth in information technology has led to serious security problems where confidentiality, authentication and integrity are constantly threatened, by illegal activities like hacking, copying or malicious use of information. The aim of encryption methods is to guarantee data privacy by fully or partially randomizing the content of original images . During the transmission or the archiving of encrypted images, it is often necessary to analyze or to process them without knowing the original content, or the secret key used during the encryption phase.
In particular, methods of reversible data hiding in the encrypted domain (RDHEI) have been designed for data en- richment and authentication in the encrypted domain, when the encryption phase is necessarily done in the first place as, for example, in a cloud computing scenario. Without knowing the original content of the image or the secret key used to encrypt the image, it is then possible to embed a secret message in the encrypted image. During the decoding phase, the original image must be perfectly recoverable and the secret message must be extracted without error. Therefore, there exists a trade-off between the embedding capacity and the quality of the reconstructed image. In recent years, many methods have been designed. The space to embed the message may be vacated after or before the encryption phase and, during the decoding phase, image reconstruction and data extraction can be processed at the same time
In this paper, we present a new high capacity reversible data hiding scheme for encrypted images based on MSB prediction. Due to the local correlation between a pixel and its neighbors in a clear image, two adjacent pixel values are very close. For this reason, it seems natural to predict a pixel value by using already decrypted previous ones, as in many methods of image coding and compression. However, in some cases, there are some errors. So, the first step of our method consists of identifying all the prediction errors in the original image and to store this information in an error location binary map (note that using overhead such an additional map is not necessary for our proposed method). After that, we propose two different approaches: the CPE- HCRDH (high-capacity reversible data hiding with correction of prediction errors) and the EPE-HCRDH (high-capacity reversible data hiding with embedded prediction errors).
CPE-HCRDH approach consists of correcting the prediction errors (CPE) before encryption. According to the error location map, the original image is pre-processed in order to avoid all the prediction errors and then, the pre-processed image is encrypted. In the EPE-HCRDH approach, the original image is directly encrypted, but after the encryption step, the location of the prediction errors is embedded (EPE). During the data hiding phase, in both approaches, the MSB of each available pixel is substituted in the encrypted image by a bit of the secret message. At the end of the process, the embedded data can be extracted without any errors and the clear image can be reconstructed losslessly by using MSB prediction.
In this work, we proposed an efficient method of reversible data hiding in encrypted images based on MSB prediction with a very high embedding capacity, which outperforms the last state-of-the-art methods. From our knowledge this is one of the first methods which proposes to use MSB instead of LSB for a RDHEI.
Due to the fact that MSB prediction is easier than LSB prediction in original domain and because image quality deterioration is not a problem in the encrypted domain, we are then able to have a very high capacity. By analyzing the original content of the image, the prediction errors are highlighted and an error location binary map is built. In the CPE-HCRDH approach, the original image is slightly modified in order to avoid all the prediction errors.