MPEG-2 and ECC Security in DCT Domain

Transcription

1 MPEG-2 and ECC Security in DCT Domain Samia Bouaziz, Ramzi Hadaji and Abdellatif Mtibaa National School of Engineering of Monastir, Skannes 5000, Monastir, Tunisia. Abstract In this paper we give a new and robustness way to use the Elliptic Curve Cryptography for the security of a compressed MPEG-2 video. We encrypt the DC coefficient of the Index frame of the video, this allow our method to be faster than the other encryption scheme. We performed several tests and the experimental results have proved our method to be secure and efficient. Keywords: MPEG-2, encryption scheme, Elliptic curve cryptography (ECC), hardware, software MSC 2000 Subject Classification: 68U10. Introduction The development of technology and the use of multimedia for different purpose, digital TV, video conference, etc. Because data are often subject to malicious attacks, the security have become more and more popular and now have a widespread influence on our lives. Researchers are more and more concerned about encryption algorithms of compressed video based on the many applications of multimedia. Efficient methods for encrypting and decrypting video data have been proposed. However, there are large number drawbacks in the current schemes in respect of security and real-time performance. In his paper [1] Tang scrambled the discrete cosine transform (DCT) coefficients of every 8x8 video image block; Statistical characteristics of MPEG video were used by Qiao et al. [2] who have presented a new encryption algorithm. This algorithm can cut down 47% resources andtime, but it does not consider the correlative information of the video frames. Obtaining correlation on the data can allow hackers a success to decrypt video. One of other idea to encrypt video are a selective encryption technique that selects various I-frames and encrypts them. This method is secure and can resist the known-plaintext attack. Wu and Moo [3] presented a type of sequence encryption method based on the embedded conditional entropy coding of wavelet coefficients. This method combines an image compression algorithm with the embedded conditional entropy coding and achieves good security by encrypting part of the compression codes. There are other techniques where we encrypt only the head information of MPEG-2 video. But it is not effective because the head information only includes standard information and an attacker can decrypt it easily. Yen [4] use a NOR operation between the gray scale of every pixel of the video frame and a secret key which is generated by a chaotic map. Chaotic encryption scheme are also proposed by Chiaraluce et al. [5] with the encryption of the DC and AC coefficients of the I-frame, the sign of the AC coefficients ofthe P-frame and the motion vectors. It is secure and can easily be implemented in real-time. Yuan et al. [6] presented a chaotic selective encryption algorithm for compressed video. They use the chaos method as a pseudorandom sequence generator, and combined a dissymmetric key system with the sequence cryptography system. In a series of tests including cryptanalysis, algorithm evaluation and comparison with other algorithms, CSECV has shown significant advantages in security, real-time performance and flexibility of implementation. In this paper we propose a new and efficient encryption scheme algorithm for MPEG-2 video encoder based on the DCT domain. Our algorithm use the advantages presented by the elliptic curve cryptography (ECC) such as a good secure level for a very small key size. The benefits given by ECC allow a good way for software and hardware implementation. In our scheme reported here, we encrypt and decrypt the I- frames of the compressed video to create the same effect on the B-frames, P-frames, and entire video. This paper is organized as follows. In section 2, we describe the new algorithm based on the ECC to encrypt video. In section 3, we present the MPEG-2 video coding method, also we give a general architecture toward a software and hardware implementation. Section 4, contains the experimental results of a software implementation doing for a sample of images and video sequences. Finally, we present our conclusions based on the experiments in Section 5. The proposed ECC algorithm Basic of the algorithm Our algorithm is based on mixing blocks of DCT coefficients of I frames of an MPEG video using the classical and efficient protocol of Menezes-Vanstone, [7, 8]. The encryption and decryption are as follows: Let be an elliptic curve defined on a prime finite field. We have a with and E are public and remains secret. Encryption algorithm We choose a secret key and for all x x 1, x ) we define: e k ( x, k) y ( y0, y1, y2 ) Where y 0 k (

2 ( 1 2 c, c ) k The inputs of the unit are, which present the coordinates of each DCT coefficients, the secret key, the point, and the coefficient. The outputs of the unit are and. The coefficients and present the new coordinates of the DCT coefficients. By browsing all the pixels of the I frame, we obtain an encrypted image. Decryption algorithm Using its secret key we decrypts as follows: d k 1 1 ( y) ( y1 c1 mod p, y2 c2 mod p) Where a y ( c1, 2 ) 0 c Choice of the finite field One of the most important aspect towards an efficient hardware and software implementation of cryptosystem is the choice of the finite field. Finite field arithmetic is becoming increasingly important in today s computer systems, particularly for implementing cryptographic operations. Among the more common finite fields in cryptography are odd-characteristic finite field of degree 1 and evencharacteristic finite field of degree greater than 1. The latter is conventionally known as arithmetic or binary field arithmetic. arithmetic is further classified according to the choice of basis for representing elements of the finite field; two common choices are polynomial basis and normal basis, [9]. Fast implementation techniques for arithmetic have been studied intensively in the past twenty years. Among various arithmetic operations, multiplication has attracted most of the attention since it is a major building block for implementing elliptic curve cryptosystems. Depending on the choice of basis, the mathematical formula for a can be quite different, thus making major differences in practical implementation. Currently, it seems that normal basis implementation offer the best performance in hardware, while in software polynomial basis representation is more efficient. meet the requirements, and software simulation for encoding and decoding of both the program and the transport streams. Because MPEG-2 was designed as a transmission standard, it supports a variety of Packet formats and provides error correction capability that is suitable for transmission over cable TV and satellite links, for more details refer to [10]. For interlace, the temporal integrity of the chrominance samples must be maintained. Thus, MPEG-2 normally defines the first, third, etc. rows of 4:2:0 chrominance CbCr samples to be temporally the same as the first, third, etc. rows of luminance Y samples. The second, fourth, etc. rows of chrominance CbCr samples are temporally the same as the second, fourth, etc. rows of luminance Y samples. However, an override capability is available to indicate that the 4:2:0 chrominance samples are all temporally the same as the temporally first field of the frame. A compressed video sequence M consists of three parts: head information of the video stream, motion vector data stream and DCT data stream. There are three kinds of frames in the video stream M, namely I-frame, B-frame and P-frame. These frames are freely arranged by the encoder in the form: I P B B P B B, etc. Their relationship can be shown in Figure 1. The codes of the I-frames do not refer to any other frame and they provide the access points of the image code data. I-frames are also the beginning of decodes and are compressed based on common methods. P-frames are compressed effectively and can be predicted by the former I-frames and P-frames. B- frames are greatly compressed and cannot provide reference to the later frames. Obviously, I-frames as the beginning of decodes are more important than the other frames. The encryption of an Iframe can greatly influence the relevant B- frames, P-frames and entire video, so we scramble I-frames and encrypt them in our encryption scheme. The later tests prove this idea to be effective. Hardware Proposed Architecture In this section we propose a general architecture for software and hardware implementation of the algorithm that we described in the previous section. MPEG-2 video Coding The MPEG-2 standard was designed to provide the capability for compressing, coding, and transmitting high quality, multichannel, multimedia signals over broadband networks, for example using ATM (Asynchronous Transmission Mode) protocols. The MPEG-2 standard specifies the requirements for video coding, audio coding, systems coding for combining coded audio and video with user-defined private data streams, conformance testing to verify that bit streams and decoders Figure 1: The structure of an MPEG-2 video bitstream Block diagram of the ECC encoder As the purpose of our scheme is to address the problem of frame retrieval in encrypted domain while preserving the file size and format compliance for the frame. The most existing and efficient partial encryption techniques for video are mainly based on blocks shuffle, DCT coefficient permutation, and encrypting the signs of DCT coefficients. In our works here we have inserted an encryption unit just after the DCT block as shown in figure 2. The encryption unit will be explained in the following section. As a result we obtain scrambled DCT coefficient blocks just before the 6644

3 quantification stage. Finally the code stream obtained is entirely encrypted. Figure 2: ECC Encoder Encryption Unit The encryption unit as shown in figure 3 is based on our algorithm. The inputs of the unit are, which present the coordinates of each DCT coefficients, the secret key, the point, and the coefficient. The outputs of the unit are and. The coefficients and present the new coordinates of the DCT coefficients. By browsing all the pixels of the I frame, we obtain an encrypted image. Figure 3: Scrambling and encryption an I-frame We can detail the architecture of the encryption unit, in fact in our algorithm we need only to configure two arithmetic units the first one will look after the operations on the finite and the second unit is responsible for operations on the curve this is shown in figure 4, this is an advantage in hardware implementations because it reduces the size of cryptoprocessors. Experimental results The proposed ECC algorithm was programmed in MATLAB on a 64-bit Intel Core I7-4500U CPU 2. 4Ghz, 6GRAM and tested on four movies: The typical sequence 'Akiyo', a sequence of a film, a sequence of foot, and a sequence of cartoon. The choice of these sequences was based on the fact that the sequences of 'Akiyo' and the sequence of foot present low motions. However cartoon and film sequences present high motions. The first indicator of the effectiveness of the method is the time added to compression when encrypting sequences called 'Time Increase Encryption'. Table1: Calculation of 'Time Increase Encryption' Video s title (100 frames) Time increaseencryption Akiyo. avi 20% Film. avi 27% Foot. avi 23% Cartoon. avi 29% The experiments showed that the time of encryption does not exceed 30% (Table 1) of the compression time. So our encryption method is applicable in real-time applications, especially if we use FPGA implementations. The mixing of coefficient blocks keeps intra-block statistics so that our ECC algorithm maintains the compression ratio. In addition, this blocks mixing change the spatial frequency content of the video which is much harder to break compared to the intra-block mixing local used in other encryption algorithms. Figure5 and 6 shows the effects of scrambling of our algorithm on two fixed images and two others images taken from videos (Foot and 'Akiyo'). Note that the images are entirely scrambled in the case of fixed images, sequences with low motion ('Akiyo') and sequences with high motion (Sequence of Foot). Figure 4: Encryption Unit Figure 5: (a) Original image 'Lena', (b) encrypted image of' Lena ', (c) original image, (d) 6645

4 We can distinguish from this table that small ECC key size provides a very good level of security, this is a huge advantage for implementations of this method compared to hardware constraints such as memory size and real time. In our study we compared over time encryption for large library of video our method ECC to other that are most popular as AES, RSA, and the classical scrambling method of the 8x8 bloc of pixel. We obtained the following results in figure 8. Figure 6: (e) Image of a sequence of foot, (f) Image encrypted (f), (g) Image of the sequence 'Akiyo', (h) encrypted image In the following figure 7, we studied the effect on the color distribution in the histograms of an image before and after the encryption scheme. We can see that in the case of encryption image the distribution present a random and uniform noise which is essential to stop the various hacking methods. Figure 8 Figure 7: Histogram of a frame before and after encryption Conclusion This topic opens a new horizon for the use of algebraic curves in cryptography. We can think of applying the hyper elliptic curves to minimize the key size, of course with keeping a good level of security, which would also at the same time promote real-time applications in the field of FPGA implementations. In prospect we can also think of applying our protocol encryption on video sound to improve video security. To prove the efficiency of a new protocol, this one must be compared to the existing and see the advantages it offers compared to those. The table 2, below published by NIST (National Institute of Standards and Technology) compares different protocols for the same security levels relative to the sizes of keys. Table2: Comparison of key size References [1] Tang. L. Methods for encrypting and decrypting MPEG video data efficiently. In: Proceedings of the fourth ACM International Multimedia Conference. Boston, USA; (1996). [2] Qiao. Land Nahrstedl. K. A new algorithm for MPEG video encryption. In: Proceedings of the first international conference on imaging science, systems and technology. Vegas, Nevada; (1997). [3] Wu. XL and Moo. PW. Joint image/video compression and encryption via high-order conditional entropy coding of wavelet coefficient. In: Proceedings of IEEE international conference on multimedia computing and systems. Florence, Italy; Jul, (1999). [4] Yen. JCand Guo. JI. A new chaotic key-based design for image encryption and decryption. In: Proceedings 6646

5 of IEEE international symposium on circuits and systems. Geneva, Switzerland; (2000). [5] Chiaraluce. F, Ciccarelli. L, Gambi. E, and al. A new chaotic algorithm for video encryption. IEEE Trans Consumer Electron (2002). [6] Yuan. C, Zhong. YZand He. YW. Chaos based encryption algorithm for compressed video. Chinese J. Comput, [in Chinese ], (2004). [7] Menezes. A, Van Oorschot. P, and Vanstone. S, Handbook of Applied Cryptography, CRC Press, (1996). [8] Cohen. H, Frey. G, Avanzi. R, Doche. C, Lange. T, Nguyen. K and Vercauteren. F, Handbook of Elliptic and Hyperelliptic Curve Cryptography, Dis. Math. Its App. 1st Edition, (2005). [9] Deschamps-JP, Implement Finite-Field Arithmetic in Specific Hardware (FPGA and ASIC), McGrawHill, (2009). [10] Watkinson. J. MPEG-2. Focal Press; 1 edition, (1999). 6647