Video Encryption Based on Special Huffman Coding and Rabbit Stream Cipher

Transcription

1 2011 Developments in E-systems Engineering Video Encryption Based on Special Huffman Coding and Rabbit Stream Cipher Sufyan T. Faraj Al-Janabi College of Computer University of Anbar Ramadi, Iraq Khalida Shaaban Rijab Dept. of Computer Engineering University of Technology Baghdad, Iraq Ali Makki Sagheer College of Computer University of Anbar Ramadi, Iraq Abstract In this paper a two Special Huffman Tree () algorithms have been designed and implemented to be encoded with an MPEG video file instead of the standard Huffman tree algorithm. The first has been built with 89-entries, and the other with 100-entries. The s are encrypted using part of the key-stream generated by Rabbit algorithm. The other part of the key-stream is used in insertion operation. In this latter operation, a number of bits of the key-stream are inserted in the coded desired file. The encrypted s with the encoded desired file are sent to the receiver, so no one knows the tree used to encode the file except the sender and the receiver. Many types of tests and measurements (such as efficiency, compression, speed, and security measurements) have been performed to evaluate the performance of this algorithm. Calculation of the compression efficiency of s has been found to be higher than that of the standard Huffman tree. Also, it has been shown that the increasing ratio in size of the generated file to that of the original file is very small and can be neglected. Keywords-multimedia compression; cryptography; video encryption; Huffman coding; stream cipher I. INTRODUCTION The growth of the Internet accompanied by cheaper access to increasingly high bandwidth has led to the development of a large number of applications which deliver digital multimedia content to users. These applications include audio streaming, video-on-demand, video conferencing, etc. Such applications often involve the transmission of large volumes of data from a few sender nodes to a large number of receiver nodes. In order to provide secure data transmission for these applications, it is necessary to design special encryption algorithms for multimedia data because of their special characteristics, such as their coding structure, large amount of data, and real-time constraints [1]. The MPEG video encryption algorithms should aim towards efficient and real-time processing. This is crucial for them to become an integral part of the video delivery process and at the same time preserve the highest security level and compression ratio. There are already some encryption algorithms to secure MPEG video. Each of them has its strength and weakness in terms of the security level, speed, resulting stream size metrics, and the combination of these metrics [1], [2]. It is basic to notice that conventional cryptographic algorithms are mainly developed for protecting alphanumeric data. In spite of that encrypting the whole audiovisual data stream with conventional cryptographic ciphers is one way to achieve multimedia security, the large size of audiovisual data requires a considerable amount of computational power with this approach. Since audiovisual data usually contain a much lower information density than text data, enciphering schemes with a significantly lower computational cost are feasible and desirable. Indeed, the encryption/decryption speed is often critical to multimedia contents because real-time processing is required in some applications [3]. Instead of using the naive algorithm approach that encrypts the entire MPEG stream using standard encryption methods (such as DES or AES), it is much more convenient now to follow an adaptive content-based multimedia encryption approach. In this approach, certain encryption algorithms are applied to specific portions of the content. These portions are selected based on quality, semantics or even objects of the multimedia scene. Such partial encrypting of MPEG bit-stream offers faster speed to secure real-time MPEG transmission, which reduces the computation complexity [4]. The remaining of this paper is organized as follows: Section 2 outlines some preliminaries and basic concepts. The proposed video encryption technique is presented in Section 3. Next, some measurements and experimental results of the system implementation are discussed in Section 4. Finally, Section 5 concludes the paper. II. BASIC CONCEPTS AND PRELIMINARIES A. Encryption with Entropy Coding Huffman coding is a compression algorithm that achieves data compression by encoding data based on its frequency. It uses a binary tree to handle the data. The basic idea is to have items of high frequency in higher levels of the tree. Hence, they will have a shorter code. The Huffman coder is very popular entropy coder that is used at the last stage of many modern audiovisual compression systems, such as MPEG video, MPEG audio and JPEG image compression. It has a very simple statistical model that is often can be represented as a fixed-size non-adaptive binary tree. Moreover, it is possible to notice that the entropy coder and the encryption cipher can have some kind of similarity as both of them turn the original data into redundancy-free bit streams. Indeed, these resultant streams cannot be decoded without certain information [3], [5]. For encryption, the required information is the key. On the other hand, for entropy coding, this information is the statistical /11 $ IEEE DOI /DeSE

2 model. Thus, it is important to explore whether hiding this model could effectively prevent decoding of the compressed bit stream [3], [6]. B. Rabbit Stream cipher The Rabbit algorithm takes a 128-bit secret key as input and generates an output block of 128 pseudo-random bits from a combination of the internal state bits (in each iteration). The encryption/decryption is carried out by XORing the pseudorandom data with the plaintext/ciphertext. The size of the internal state is 513 bits. It is divided between eight 32-bit state variables, eight 32-bit counters and one counter carry bit. The eight state variables are updated by eight coupled non-linear integer valued functions. The reason behind using the counters is to secure a lower bound on the period length for the state variables. Rabbit was designed to achieve two main goals: security and speed (especially in software implementations) [7]. Rabbit consists of the following main schemes [8], [9]: Key Setup Scheme: The algorithm is initialized by expanding the 128-bit key (which is divided into eight subkeys) into both the eight state variables and the eight counters such that there is a one-to-one correspondence between the key and the initial state variables and the initial counters. Then, the system is iterated four times, according to the next-state function, to diminish correlations between bits in the key and bits in the internal state variables. Finally, the counter variables are modified to prevent key recovery. Initialization Vector (IV) Scheme: The IV setup scheme modifies the counter state as function of the IV. Next, the system is iterated four times according to the next-state function, to make all state bits non-linearly dependent on all IV bits. The modification of the counter by the IV was designed such that to guarantee all the 2 64 different IVs lead to unique key-streams. Next-State Function: This is involved in both key setup and key-stream generation. It takes eight counter variables as input and produces a 128 bit key-stream block as output. It works by going through system iteration, counter modification, and iteration of the g-function. It has very good diffusion and nonlinearity properties. C. Measurements Entropy can be defined as the theoretical minimum average number of bits that are required to transmit a particular source stream. Entropy is a very important measurement in image and video compression and can be computed using the following formula attributed to Shannon [10]: n H log 2 i 1 P i P i where H is the entropy, n is the number of different symbols in the source stream, and P i is the probability of occurrence of each symbol i. The efficiency of a particular encoding scheme can be computed as a ratio of the entropy of the source to the average (1) number of bits per codeword that are required with the scheme. This can be computed using the following equation: L avg n i 1 N where L avg is the average number of bits per codeword, N i is the number of iterations, and P i is the probability of occurrence of symbol i. III. THE PROPOSED VIDEO ENCRYPTION TECHNIQUE The proposed algorithm depends on generating a Huffman tree which is special for the desired file to be encoded instead of using standard Huffman tree. The special Huffman tree () is then encrypted using stream cipher method, merged with the encoded desired file, and then sent to the receiver. In this way no one knows the tree used to encode the file except the sender and the receiver who know the key used in the stream cipher. Some bits of the key-stream are inserted in the encoded desired file. This increases the security of the file since it prevents the opponent from expecting the tree used in encoding the file. But it does not increase the size of the file too much. Sometimes it might even reduce it. Indeed, the execution of this method takes a reasonable time which does not affect the computational speed. The input video is compressed to the desired bit rate by using (actually two types of have been used in this work) and encrypted by applying the Rabbit stream cipher, at the transmitter side. At the receiver side, inverse operations are performed to obtain the reconstructed video signal for display. This is to be described in more details in the following subsections. The operations in the sender side can be partitioned into the following stages: Building, decoding/encoding, generating the key-stream, Huffman tree encryption, insertion stage, and reconstructing the encrypted MPEG file. A. Building Special Huffman Tree The is built to define the number of zero coefficients preceding each symbol denoted by acz and the absolute nonzero coefficient of ac-coefficients block denoted by acsyms. For a specific MPEG video file "Test", two different s have been generated. The first is with 89 entries by coding the elements with probability larger than or equal to 13. The second is with 100 entries by coding the elements with probability larger than or equal to 8. The other elements are not encoded, and the sum of their probabilities is used as probability to the escape element which is added to other elements of the tree. The algorithmic steps of building are demonstrated in Fig. 1. B. Decoding/Encoding Stage This stage is implemented on each slice related to intra frame, after determining the start and the end of each one. This implementation can be partitioned into three other sub-stages, which are iterated on each block in this slice sequentially. These sub-stages are: Decoding with Standard Huffman Coding: In this sub-stage the video bit-stream which represents the VLC of accoefficients for blocks in each slice related to intra frame i P i (2)

3 (I,B or P), is decoded by using the standard Huffman tree to get pairs of (run-level) for each block. Figure 1. The steps of building. Encoding with : Encoding the runs and levels for accoefficients to get the bit-stream (VLC) using one of the s. This operation is performed according to the results of the previous sub-stage. Reconstructing the Encoded Slices: This sub-stage reconstructs the encoded slices in each intra frame by merging each dc-coefficient with the encoded ac-block related to it and saving the result in slice buffer. C. Generating the Key-stream The key-stream is generated using Rabbit stream cipher which produces the required random bits (128 bits each iteration) and uses a key with a length of 128 bits. The keystream is saved in stream buffer. This method justifies a key size of (128 bits) for encrypting up to (2 64 bytes) of plaintext. A complete description of the algorithm aspects can be found in [7], [9]. The total key-stream which is required to be produced is named total_keystream such that: total_keystream = t_keystream + b_keystream (3) where: t_keystream is the key-stream required to encrypt the, and b_keystream is the key-stream used for insertion operation. D. Huffman Tree Encryption The s are encrypted using the t_keystream generated by the Rabbit i.e. the encrypted s are generated by XORing each entry in s with the random number in the stream buffer. This operation is illustrated in the algorithm shown in Fig. 2. E. Insertion Stage This stage is important to increase the security of encrypted slices. In some previous works this method was suggested with a limitation that the distance between two insertions should be larger than 50-bit so as to prevent the heavy increase in the size of the cipher plain text. In this work the technique has been applied with a different limitation that the distance between any two insertions should be less than 8- bit. This condition ensures the scrambling the DC-confections since each DC- coefficients is 8-bit. It also does not result in a heavy increase in the size of the cipher plain text since the tree used to encode the slices is specific for each video. Thus there is a compromise between the increasing of size due to insertion and the decreasing of size due to specific. The insertion operation is implemented as shown below: 1- Select random bits of b-keystream which is presented as vector Z=(z 0, z 1,, z i-1 ) each z i is a 1-bit integer. 2- Add one random bit to the ( w i ) the bit in the encrypted bit stream where w is constant which represents the distance between any two insertions. With two values of w this algorithm is implemented. For the bit stream X of the length lx bits, which represents the encoded slices, the total number of random bits in b_keystream required to perform insertion algorithm is lx/w. This means that the number of iterations is ( lx/ w)*128. F. Reconstruct the Encrypted MPEG File This stage replaces the slices in each intra frame (I, B or P) in the original MPEG file with encrypted slices. Then the resultant file is merged with encrypted Huffman tree to be sent as a single file. We use one of the reserved headers that are not typically used, as an indicator to the beginning of encrypted Huffman tree. The selected header is: b0 in Hex (See Fig. 3). Figure 2. The encryption algorithm

4 decoding is done using, while encoding is done using the standard tree. IV. EXPERIMENTAL RESULTS AND MEASUREMENTS The proposed algorithm incorporates encryption /decryption with MPEG video compression/decompression in same step. The primary goal of this algorithm is to get a key space which is large enough against well known attacks, save the encryption computation time by taking the advantage of combining MPEG compression and data encryption, and avoid affecting the video compression ratio. The proposed system has been implemented in software as two peer entities. The first represents the sender side part, while the second represents the receiver side part. Each part includes a number of basic components, which consist of different headers. The output row from each basic component points to the result produced by that component. All these components and their headers have been implemented by using Visual C++. To evaluate the performance of the proposed system, several types of tests and measurements have been performed. These tests and measurements are implemented on a sample "Test" MPEG video file. A. Efficiency Measurements One way to evaluate the efficiency of a coding scheme is to determine its efficiency with respect to the lower bound, i.e., entropy. The efficiency η is defined as follows: H (4) L avg Figure 3. Reconstructing the encrypted MPEG file. G. Receiver Side Operations Almost the inverse stages are needed at the receiver side, which are generating the key-stream, separation stage, removing (extracting) the insertion bits, Huffman tree decryption, and finally the decoding/encoding stage. The first stage "Generating the key-stream" is exactly the same as "Generating the key-stream" on the sender side. It must give the same key-stream that is obtained at the sender side since the sender and the receiver have the same key and apply the same algorithm (Rabbit). On the other hand, "Separation", "Removing the insertion bits", and "Huffman tree decryption" stages are exactly opposite to "Reconstruct the encrypted MPEG file", "Insertion", and "Huffman tree encryption" stages on the sender side respectively. From the previous stages the receiver gets the encrypted Huffman tree generated by the sender and the slices encoded by this tree. Decoding/Encoding stage is the same as Decoding/Encoding stage at the sender side except that where H is the average information content per symbol of the source Entropy, and L avg the average length of the code words in the code. An efficiency comparison has been made between the s which were generated and the standard Huffman Tree. These comparisons are performed after calculating the entropy H and the average length of the code L avg. Table I represents a typical comparison between the values of efficiency, entropy and average length of s with that of the original Huffman tree. From this table, it can be noticed that the original Huffman tree efficiency is smaller than the efficiency related to generated s. This is because standard Huffman tree is generated from a different set of videos, while the s are delivered from the specific video file. TABLE I. COMPARISON BETWEEN THE EFFICIENCIES OF STANDARD HUFFMAN TREE AND S WITH 89-ENTRY AND 100-ENTRY. Standard Huffman Tree 89-entry 100-entry H Bit/symbol L avg Bit/symbol η efficacy 77% 94.7% 95%

5 It was expected that the with 100-entries (and escape represents the probabilities < 8) has efficiency better than that of 89-entries (escape represents the probabilities < 13) but this difference has not be noticed because the summation of probabilities (< 8 and < 13 respectively) is very small. B. Compression Measurements The s are encrypted using the t_keystream generated by the Rabbit. MPEG compression ratio depends on the Huffman codeword list that may decrease if an arbitrarily Huffman codeword list is used to encode the MPEG video. To avoid affecting compression ratio, the has been used since it is generated from different sets of training slices related to intra frame specific for the desired video. The size of the original file "test" which depends on the standard Huffman tree in encoding process is bytes. Table II illustrates several sizes of the generated files using the proposed algorithm with their increasing ratio to the original file. These are used to compare the compression performance of with that of the standard Huffman tree. In this Table each raw represents one of the s, while the columns are divided to three fields. Each field represents a specific distance w (which is a distance between two inserted bits), the field w= represents the values of generated files which were encrypted without insertion operation. So the increasing ratios for these files are in negative which means that the sizes of these are smaller than that of the original one as expected. It can be noticed that there is a small increment in the size of the files when w=5 and w=7. However, this increment is larger when w=5 with a security better than that of w=7. TABLE II. TOTAL SIZE OF ENCRYPTED FILES USING THE PROPOSED ALGORITHM WITH THEIR INCREASING RATIO TO THE ORIGINAL FILE WITH SIZE BYTES. C. Key-Setup Speed Measurement This type of measurements includes all the speeds of operations prior to actual encryption of the first bit in the plaintext. These operations are key-stream generation and generation. The numbers of iterations required for generating t_keystream and b_keystream are as shown in Table III. Each row in this table represents one of the generated s (with 89-entry and 100-entry). The required time to generate each is computed and found to be very small as demonstrated in Table IV. The time required for key-setup is the summation of the time required to generate and the time of the keystream. From this table, it is obvious that the time required to generate is very small compared to the time required to generate the key-stream so it can be neglected. TABLE III. THE NUMBER OF ITERATIONS REQUIRED FOR GENERATING T_KEYSTREAM, B_KEYSTREAM AND KEYSTREAM. t_keystream Iterations Number b_keystream Iterations Number Key-stream Iterations Number W=5 W=7 W=5 W=7 89-entry entry TABLE IV. THE PERFORMANCE OF KEY-SETUP OPERATION WHICH INCLUDES GENERATING AND KEYSTREAM Generation Time (ns) Key-stream Generation Time (μs) key-setup Generation Time (μs) w=5 w=7 w=5 w=7 89-entry entry D. Encryption Speed The additional encryption time of the proposed method "-encryption" is only the insertion time which is demonstrated in Table V. The last column in this table represents the percent increment in encryption time to the total duration time of the video which is 3sec. The maximum and average values of the insertion time of four -Rabbit encrypted MPEG files (etest89-ins7.mpg, etest89-ins5.mpg, etest100-ins7.mpg, and etest100-ins5.mpg) with 89 entriesinsertion distance of 7, 89 entries-insertion distance of 5, 100 entries-insertion distance of 7, and 100 entries-insertion distance of 5 respectively are shown in Table VI. It is shown that the maximum value of the insertion time for all figures is in the second slice. E. Security Estimation The security of previously proposed MPEG video encryption algorithms, which are based on standard Huffman tables, can be not high enough against some known attacks. Thus, some more powerful techniques have been introduced in the proposed algorithm for MPEG video encryption. These techniques are insertion operation and generating. The insertion technique is to increase the security and the resistance against plaintext attack. The distance between two inserted bits (w) must be less than 8-bit (1-7), since each DC-coefficient is 8-bit. This condition further increases the security and the resistance against plaintext attack. Generation of is specific for the desired video, encrypting it and then sending it to the receiver within the encrypted file. All these steps increase the security and the key space since the tree and its position in the file are unknown except by the sender and the receiver

6 TABLE V. THE INSERTION TIME AND PERCENT INCREMENT IN ENCRYPTION TIME TO THE TOTAL DURATION TIME OF THE VIDEO. Insertion Time (ms) 89/100-entry Time Increase (%) w= w= TABLE VI. THE MAXIMUM VALUES OF THE INSERTION TIME AND THE AVERAGE VALUES TO THE TOTAL DURATION TIME OF THE VIDEO. Encrypted MPEG file Total Insertion Time (ms) Maximum Insertion Time Value (μs) Slice index Average Values of the Insertion Time(μs) etest89-ins etest89-ins etest100-ins etest100-ins The simplest cipher text-only attack is to determine the complexity of brute force attack on this algorithm. Of course, the key space must be known. It can be calculated that the key space size of the proposed encryption algorithm is equal to the total iterations required to generate the brute force attack on this algorithm which is (2 128 *size of the encrypted file in byte *7). Hence, this attack is infeasible on this algorithm. In addition, it was noticed previously that Rabbit algorithm has high resistance against the standard attacks on stream ciphers. It is found that algebraic, Guess-and-Verify, Guessand-Determine, correlation, differential attacks are infeasible on Rabbit. Thus, it is quite justified to expect our proposed algorithm to be of a high resistance against these attacks. V. CONCLUSIONS From the results which have been obtained previously and from calculations and measurements that have been performed to evaluate the proposed algorithm, we can conclude that the efficiency of the generated is better than that of the standard Huffman tree. It has been also found that the efficiency of 100-entry and 89-entry has approximately the same value, with a little increment in the efficiency of with 100-entry. The overall compression performance of with 89-entry and 100-entry is approximately the same as that of the standard Huffman tree. Compression performance is improved in case of encryption without insertion. The effect of the insertion operation on this parameter is very little and can be neglected. The proposed algorithms that uses insertion operation has high resistance against plaintext attack. The brute force attack is infeasible on the proposed algorithm. Indeed, the security of the encrypted file with insertion is better than that without insertion, and the encrypted file with insertion of w=5 has better security than that of w=7. REFERENCES [1] L. Qiao, "Multimedia Security and Copyright Protection", Ph.D. Thesis, University of Illinois at Urbana-Champaign, October [2] L. Qiao and K. Nahrstedt, "Comparison of MPEG Encryption Algorithms", International Journal on Computers & Graphics, Special Issue on Data Security in Image Communication and Network, vol. 22, num. 3, Permagon Publisher, [3] C.-P. Wu and C.-C. J. Kuo, "Design Of Integrated Multimedia Compression And Encryption Systems", accepted by IEEE Trans. Multimedia,Vol.7,No.5,October 2005, [4] Habib Mir M. Hosseini, Pong Mee Tan, Encryption of MPEG Video Streams, IEEE /06/$ [5] C.-P. Wu and C.-C. J. Kuo, "Efficient Multimedia Encryption Via Entropy Codec Design", In Proceedings of SPIE International Symposium on Electronic Imaging, Vol. 4314, pp , [6] C.-P. Wu and C.-C. J. Kuo, "Fast Encryption Methods For Audiovisual Data Confidentiality", In Proceedings of SPIE International Symposium on Electronic Imaging, Vol. 4209, pp , [7] M. Boesgaard, M. Vesterager, T. Pedersen, J. Christiansen and O. Scavenius, "Rabbit : A New High-Performance Stream Cipher", In Fast Software Encryption 2003, Lecture Notes in Computer Science vol. 2887, Springer-Verlag, 2003, [8] Ruhma Tahir, Muhammad Y. Javed, Attiq Ahmad and Raja Iqbal, SCUR: Secure Communications in Wireless Sensor Networks using Rabbit, Proceedings of the World Congress on Engineering 2008 Vol I, WCE 2008, July 2-4, 2008, London, U.K. [9] Martin Boesgaard, Mette Vesterager, Thomas Christensen, Erik Zenner The Stream Cipher Rabbit ECRYPT Stream Cipher Project Report 2005/006. [10] F. Halsall, "Multimedia Communicatios", Addison-Wesley,