Recently, several scrambling techniques have been proposed for video digitally archived. These methods realize efficient processing by partial encryption on MPEG compressed data while keeping compatibility to MPEG format. However, they have common drawbacks expanding the entire code. Also, they have not reported on local shuffling in a frame. In this work, we propose a new video scrambling scheme on MPEG compressed domain, which shuffles a part of DC and AC coefficients by DCT in a frame. Our scheme is applicable to local region without data expansion from the original MPEG file. 1. INTRODUCTION Scrambling is a well-known technique that makes image data meaningless visually. However, conventional methods for analog data cannot be used for video digitally archived, because most moving pictures are recently compressed in the form of MPEG that is the international standard for video coding. Therefore, a new approach directly applicable to MPEG compressed domain has been greatly needed for various applications such as video delivery on the internet. So far, several approaches have been proposed for MPEG video. Spanos and Maples [1] proposed to restrict MPEG encryption only to I-pictures that are the basis for predicting P and B-pictures. However, this scheme sometimes reveals visual information because of the presence of I-blocks in P and B-pictures. Meyer and Gadegast developed an encryption system called "SECM-PEG" (see [2] ), which provides four stages of encryption (only header encryption; encryption of header, DC and first AC coefficients of I-frames; all I-blocks encryption; full encryption). However, this is not compatible to MPEG because of special file format. Zeng and Lei [3] proposed a method to encrypt sign bits of motion vectors (MVs) with permutation of MVs. Although this scheme achieves fast shuffling, the encryption causes data expansion. In the similar period, Shi et al. [4] proposed to encrypt not only sign bits of MVs but also sign bits of DCT coefficients. Wen et al. [5] proposed a method to encrypt VLC (Variable Length Codes) table of MV. However, this scheme also causes small data expansion. Wang et al. [6] proposed a DCT-based transparent scrambling method, which is designed to show unauthorized users sufficiently degraded video rather than unrecognizable one. Since they perform scrambling by modifying quantized DCT cofficients before VLC in MPEG encoder, we cannot apply this scheme to video already achieved. Scrambling methods on MPEG compressed domain [3]- [5] are excellent in terms of efficient processing by partial encryption of MPEG compressed data while keeping compatibility to MPEG format. However, they have common drawbacks expanding the entire code. Also, they have not reported on local shuffling in a frame. From this point of view, in this work, we propose a new video scrambling scheme on MPEG [7] compressed domain, which shuffles a part of DC and AC coefficients by DCT in a frame. We consider the following conditions to design our scheme: (i) applicable to local region in a frame, (ii) no data expansion from the original MPEG file, (iii) compatible to general MPEG decoder (iv) 100% reversible to the original video. Through computer simulation, we verify the basic performance of the proposed scheme. In the following discussion, we describe MPEG-2 method as one of popular MPEG coding schemes. In MPEG-1 or MPEG-2, input frames are classified into three kinds of picture, i.e. I-picture, P-picture and B-picture. I-picture is independently encoded within itself by removing intra-frame redundancy. The entire frame is first divided into small blocks consisting of 8 × 8 pixels. Then each block is transformed to 8 × 8 frequency components by DCT, and the coefficients after quantization are encoded. DC components are separately encoded by predictive coding for deferential signals between neighbor blocks. 63 pieces of AC component (excluded DC coefficient) are encoded by using 2-dimensional Huffman codes in a certain scanning order (e.g. Zigzag scan). On the other hand, in P-picture and B-picture encoding, interframe redundancy is removed by one-way or both-way motion compensation between frames, respectively. The motion compensation is conducted for every macro block (MB) consisting of 16 × 16 pixels. Then a motion vector (MV) is searched for a minimum error between the current MB and the same size block in a specified region on a reference frame. If the minimum error E min is greater than a predefined threshold T , intra-coding is applied to MB. Otherwise, the differ-
Takayama et al. REF proposed a video masking scheme without data expansion.
A Video Scrambling Scheme Applicable to Local Region without Data Expansion
2006 IEEE International Conference on Multimedia and Expo
2006 IEEE International Conference on Multimedia and Expo
Computer Science