On the Challenges of Applying Selective Encryption on Region-of-Interest in H.264 Video Coding

Abstract

Selective encryption of video data is not a new idea and various techniques proposed in previous literatures can effectively protect sensitive data from unauthorized access. After the initial appearance of selective encryption, some principles have become well known and widely adopted. One of the principles is the standard format compliance, which means that unauthorized users can decode and process the encrypted video without knowing the content. In the context of ROI encryption, this allows unauthorized users to decode and view the video except for the encrypted region containing sensitive information. However, there are some challenges for providing format compliance in H.264 video coding when applying encryption on certain regions and it is very important to address this issue in some applications such as privacy masking in video surveillance system. In this work, we describe the challenges and introduce some approaches to address these. Due to the bulky size, applying standard ciphers to multimedia data tends to be inefficient for real-time processing scenarios. Therefore, selective encryption initially appeared as means to relieve computational cost and some principles for designing selective encryption scheme have become widely adopted. Those principles include security, time efficiency, format compliance, compression performance and error robustness. The format compliance means that the encryption process does not change the encoded bitstream's data format in order to support such direct operations as browsing, playing, cutting, copying and so on. In the past decade, several selective encryption algorithms supporting format compliance have been reported, most of which are based on MPEG and H.264 video codec. In the context of ROI encryption, format compliant encryption algorithm allows unauthorized users to decode and view the video except for the encrypted region containing sensitive information. In the application of video surveillance system, the problem of privacy invasion can be addressed by encrypting only a privacy sensitive region and, if the encryption algorithm guarantees format compliance, a person with a lower level of security clearance can only get the non- sensitive information, and all the privacy information can be entirely unperceivable to him/her. Although ROI encryption supporting format compliance is simply the application of established selective encryption algorithm on a given region, ROI encryption in currently prevailing video codec, such as H.264 and MPEG4, has a number of unique challenges. The challenges are all about confining the degradation of visual quality to the specified region and keeping the other parts intact. However, most of the consisting elements in the encoded video do have little meaning in itself and can reconstruct a valid data only when combined with other elements. When a consisting element outside a ROI is dependent upon the element inside a ROI and the related element is encrypted, the reconstructed value outside a ROI can be unexpected one and can introduce artifacts in decoded video. As every video codec specifies different encoding algorithm, the challenges are dependent on the employed codec. In case of MPEG4, two challenges are identified and the solutions are described in (1). Coding schemes for independent ROI encryption in scalable video coding are presented in (2).