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Final Report

  1. 1. MPEG Video Watermarking Technologies: Techniques and Issues [Spring 2002] Deepak Sharma Email: Presented to: Dr. Chee-Hung Henry Chu Center for Advanced Computer Studies University of Louisiana at Lafayette
  2. 2. Contents 1. Introduction 3 2. Watermarking 3 3. Requirements for MPEG Video Watermarking 4 4. Watermarking Algorithms 5 4.1 Tamper Resistant Hardware 5 4.2 Video Watermark Embedment using PCA 6 4.3 The Zhao Koch Algorithm 7 4.4 The Fridrich Algorithm 8 5. Additional Observations 9 6. Conclusion 10 7. References 11 2
  3. 3. Abstract With the current growth rate of broadband networks, distribution of multimedia content over the Internet will become the norm. With such a growth the major concern for the content providers will be security. A lot of research is currently going on that combines public key cryptography, watermarking and other techniques. This report presents a brief overview of several watermarking techniques that may be used for protecting MPEG video files. It starts by presenting why watermarking is needed and then goes on to present some watermarking schemes. 1. Introduction Current research will lead to computers with 2GHz microprocessors, 1 GB memory and over 80 GB of hard disk space available for only a few hundred dollars. Such computers will be able to connect to the Internet via a 1 GB connection. With the amazing configuration of these machines and networks, many things that are barely imaginable today will become feasible. Online movies (VoD – Video on Demand), audio streaming, games, ebooks, etc will be more efficient than what we know them today. With such technology available and multimedia content flowing across networks, a major issue will be developing secure technologies to protect the multimedia content. With the expanse of the Internet, online multimedia distribution is the fastest and cheapest way for the content providers to distribute their content. However, with the benefits of the Internet comes its lack of security. Research is now being focused on public key cryptography/watermarking schemes to protect multimedia content. 2. Watermarking Digital watermarking is the enabling technology to prove ownership of copyrighted material, detect the originator of illegally made copies (also called fingerprinting), monitor the usage of copyrighted multimedia data and analyze the spread spectrum of the 3
  4. 4. data over networks and servers [2]. In the two types of watermarking the first one is for ownership where an identical watermark is embedded into every copy of the digital content. Hence, it cannot be used to distinguish who is the user that distributed the illegal copy. In the other type of watermarking, called fingerprinting, different watermarks are embedded into different copies of the digital content. Hence it can be used to trace the illegal users. The disadvantage of this scheme is that it is quite expensive to resist colluding attacks. In order to prevent any copyright forgery, misuse or violation, the key to the copyright labeling technique (watermarking) is to provide security and robustness of the embedded label against a variety of threats that include [2]: • Detecting embedding locations by comparing differently labeled versions of the same original material. • Finding or altering the embedded label through visual or statistical analysis. • “The IBM-attack”: Instead of introducing a new watermark with an own algorithm and claiming the authorship, a counterfeit original of the watermarked picture is produced by removing a watermark, thus claiming that the original of the real owner contains the watermark which we removed. • Damaging or removing the embedded label using common multimedia processing. MPEG compression itself performs multimedia processing like lossy compression and scaling, hence all necessary transformations on the frames may lead to a distortion of the embedded information and the owner cannot retrieve the label anymore. 3. Requirements for MPEG Video Watermarking MPEG compression algorithms employ Discrete Cosine Transform (DCT) coding techniques on image blocks of 8x8, prediction and motion compensation. The resulting output stream contains a sequence of I-, P- and B- frames [2]. The following requirements are considered important for MPEG video watermarking: 4
  5. 5. • Robustness against high compression rates of the DCT compression, motion compensation and prediction. • Robustness against scaling. • Labeling of every single video frame (I-, P- and B- frames) to provide continuous watermarking and avoid attacks of cutting single frames. • Ensuring correct decoding of the frame sequences without visual artifacts. • Runtime performance for streaming video or stored video. 4. Watermarking Algorithms 4.1 Tamper Resistant Hardware This scheme uses both public key encryption and symmetric key encryption. A tamper resistant hardware includes: Sn  A unique serial number SK_sn  Private key PK_sn  Public key WE_sn  Watermark embedding process M  Multimedia file K  Symmetric key PK_sn (K) K (M) SK_sn K M WE_sn Symmetric key decryption Tamper-resistant hardware with serial number sn Output Watermarked M 5
  6. 6. Here K is the symmetric key to encrypt the multimedia file M. K(M) is the cipher text of M with the secret key K. PK_sn(K) denotes the encryption of K with public key PK_sn. Each customer has one tamper-resistant hardware device which has a serial number (sn) embedded in it. The manufacturer of the hardware should never disclose the private key SK_sn. The manufacturer issues a certificate to prove the validity of PK_sn and bind PK_sn and sn together. To attain a multimedia file M, a customer may pay and send his PK_sn, sn and the certificate to the content provider. The content provider must verify whether PK_sn is a legal public key before encrypting K by PK_sn and sending the cipher text to the customer. Advantages: • In this scheme the multimedia files are encrypted with different keys, so even if one of the keys is compromised, the system does not get compromised. Since each device has a unique serial number, the illegal user can easily be traced and the device can be put into a revocation list. • The tamper resistant hardware devices are independent of the content providers; so all the content providers may use them. Hence the scheme provides flexibility. • The encryption is done by symmetric key cryptographic algorithm that is fast and inexpensive. Public key cryptosystem is used only for hiding the secret keys. 4.2 Video Watermark Embedment using Principal Component Analysis PCA has been used extensively in pattern recognition and in finding basis supports such as eigenfaces, feature space reduction in image databases and classifying video streams. This technique uses PCA along with clustering to identify two Regions of Interest (ROI) in a given video. Of the two ROIs one has a higher mean absolute motion activity (High Motion Class – HMC) and one has a lower mean absolute motion activity (Low Motion Class – LMC). The basic motivation behind having two motion-separated classes is to 6
  7. 7. counterattack statistical analysis by attackers. PCA leads an elegant way to combine a set of different features (motion, spatial coordinates, intensity values, etc) to give a robust and compact representation that serves as a strong ownership indicator of a given video frame. This scheme uses this representation only locally in time to prevent any localized attack such as frame dropping. Watermark Embedding Stage: After determining the principal direction of projection, a two class fuzzy c-mean clustering is done to determine the two binary support maps M1 and M2 for the two ROIs. Now inserting two watermarks W1 and W2 forms a new watermarked image: I’ = I + W1 * M1 + W2 * M2 Watermark Retrieval Stage: ROIs are extracted by applying PCA and clustering. Then a correlation based watermark tester is used to examine the presence of a watermark. This scheme allows any correlation-based watermark to be used without the storage of original video. Advantages: • In this scheme the dual watermark and their location provide a strong ownership stamp to a video frame without referring to the original sequence. • This scheme is robust against several forms of attacks. 4.3 The Zhao Koch Algorithm The Zhao Koch algorithm embeds the copyright label in the frequency domain. The luminance information Y in the spatial domain is discrete cosine transformed (DCT) into the frequency domain and then quantized. The algorithm then pseudo-randomly chooses three coefficients from the quantized DCT encoded block and manipulates them to store a 7
  8. 8. single bit of information of the copyright label using a secret key. For embedding the 1 or 0 bit, Zhao and Koch define different patterns with High, Middle and Low as the manipulation rules. If storing a bit of information requires a significant change in the coefficients of a block, then the coefficients are manipulated to form an invalid pattern to tell the retrieval there is no information embedded in that block [3]. During the extraction process, the same coefficients are pseudo randomly selected using the secret key and the relationship between the coefficients are analyzed. Depending on the relationship a 0 or 1 is extracted. Advantages: • The algorithm does not need the original image for retrieval. • Watermark information is embedded in the compressed domain and can be easily applied to compressed video with minimal operations. Disadvantages: • Every block is modified and artefacts are common especially in smooth blocks or in sharp edges. • The algorithm is not robust against scaling or rotation because the image dimension is used to generate an appropriate pseudo random sequence. 4.4 The Fridrich Algorithm This algorithm uses a watermarking method based on pattern overlaying with its power concentrated mostly in the lower frequencies. The pattern is created using a pseudo random number generator and a cellular automaton with voting rules. The watermark bit sequence is used for initializing a pseudo random generator to create a random black and white initial pattern of the same size as the image. A cellular automaton with voting rules is applied till a convergence to a fixed point is obtained. The voting rule coalesces 8
  9. 9. random patches into connected areas. A smoothing filter to move the main portion of the power to lower frequencies further filters the pattern. The gray levels of the final pattern are scaled to a small range and the pattern is finally added to the image. The watermarked image shows no visible degradation caused by the overlaying pattern and the pattern is embedded in a robust sense [3]. Advantages: • With this method since the pattern is formed in a sensitive way based on the watermark sequence, even if the watermark pattern shows in uniform areas, it is not possible to mount an attack. • The watermark is also resistant with respect to collusion attacks (averaging several watermarked images to remove the watermark). Disadvantages: • The retrieval process requires the original, un-watermarked image. This is not acceptable for videos since then the entire original video would be needed to prove the watermark. • The watermarking algorithm does not embed any detailed information about the author as the watermark. It just embeds a pattern created using a pseudo random generator and a cellular automaton with voting rules. 5. Additional Observations With reference to the H.263/H.236+ video encoder, a study was conducted to determine the best stage during the video encoding to embed the watermark information. Depending on the watermarking method used, it may be advantageous to embed the watermark after the quantization stage since very little watermarking information would be required to be embedded. This method could be efficient when small information (a 1 or 0 bit) needs to be embedded. However, the retrieval process gets more complicated by this method. 9
  10. 10. The other stage to embed the watermark is at the video in stage. The watermark information can be embedded frame by frame and encryption can also be performed here and after this the DCT is performed. Depending on the watermark method used, the watermark stages could be decided, say if a method can efficiently embed the watermark in the frequency domain then it should do that after the quantization stage (or before) in the video encoder. Otherwise it is easier to embed the watermark at the video in stage where each frame or alternate frames can be watermarked. 6. Conclusion This report has presented the need for watermarking, types of digital attacks, requirements for MPEG video watermarking and then the ongoing research in watermarking technology. Different algorithms for watermarking with their advantages and disadvantages have been presented. Also the robustness and scaling of each method has been mentioned. Further work could concentrate on finding the appropriate stage for watermark embedding with goals such as minimal processing and maximum security. 10
  11. 11. References (1) Feng Bao, “Multimedia Content Protection by Cryptography and Watermarking in Tamper-resistant Hardware”. (2) Roy Wang, Qiang Cheng, Thomas Huang, “Identify Regions of Interest (ROI) for Video Watermark Embedment with Principle Component Analysis”. (3) Jana Dittmann, Mark Stabenau, Ralf Steinmetz, “Robust MPEG Video Watermarking Technologies”. (4) C. T. Hsu and J. L. Wu, “Digital Watermarking for Video,” Proceedings of Digital Signal Processing 97. 11