1 Multimedia Encryption Sistem Multimedia

2 Multimedia Encryption  Special application of general encryption to multimedia such that the content cannot be rendered intelligibly or to an acceptable perceptual quality.  Have a number of unique requirements and desirable features that a general cryptosystem lacks.  Different applications may have a different list of requirements and a different order of priorities. Trade-off may be necessary

3 Applications  Confidential videoconferences  Confidential facsimile transmissions  Medical image transmission and storage  Streaming media  DVD content protection  Pay-TV  Digital transmission through IEEE 1394 interface

4 Characteristics of Multimedia Applications  Characteristics  High data rate  Power hungry  Real-time constraint  Continuous  Synchronous  Loss-tolerant  Prioritized components  Different values of content Different security requirements  Different distribution channels DVD, Satellite TV, Internet, wireless

5 Box Office Revenues vsTime

6 Major Requirements and Desirable Features  Complexity is an important consideration Real-time applications, low-power device  Content leakage (or perceptibility) Content degradation vs. secrecy  Compression efficiency overhead Due to change of compression parameters/procedure, change of data statistics, additional header etc.  Error resilience. Error confinement in lossynetwork, synchronization  Adaptability and scalability Dynamic bandwidth/resources, Encryption be transparent to an adaptation process

7 Major Requirements and Desirable Features(cont.)  Multi-level Encryption Enable multiple accesses: resolution, quality, size, frame rate “what you see is what you pay “  Syntax compliance Transparent, “backward”compatibility, inherit other nice properties of compression standards.  Content agnostic Encryption does not depend on content types or the specific coding technology E.g., Windows Media Rights Manager, OMA’sDRM  Random access, transparency, scene change detection without decryption

8 Encryption and Compression

9 Security Break of Multimedia Encryption  Complete break Recover full plain bitstreamby finding the key etc,  Perceptual break Render acceptable perceptual quality or recover certain content information without a key  Local break Deduce a local plain bitstream/content information  Information deduction Gain certain information, less severe break

10 Attacks on Multimedia Encryption  Traditional attacks  Additional attacks that exploit the unique features of multimedia data Statistical attack  Exploit correlation between different portions of multimedia data  Especially for selective encryption  Compression makes the attack difficult, fortunately Error-concealment based attack  Perceptual redundancy exists in compressed media  Perceptual break is possible, i.e. conceal encrypted data

11 Multimedia Encryption Approaches  Conventional/Naïve approach Encrypt a compressed codestreamas a whole  Full Encryption  Selective Encryption  Joint Compression and Encryption  Syntax-Compliant Encryption  Scalable Encryption and Multi-Access Encryption

12 Conventional Approaches  Directly distort visual data in spatial domain Difficult to compress, potentially high complexity Vulnerable to correlation attacks  Encrypt compressed data using DES etc. Significant processing overhead  Difficulty in some real-time application with low-power device Plain text attack using known syntax Not secure for adaptation at intermediate nodes  require key to decompress/decrypt/re-code/re-encrypt Little transparency

13 Fast Encryption  Encrypt half of the compressed bitstream( Qiao& Nahrstedt’97 ) Using XOR + DES  Encrypt (A, B) as (DES(A), (A XOR B) ) Secure, speedup by a factor of two

14 Full Encryption  Approach Partition and packetizecompressed bitstreaminto structured data packets with header and data field Apply encryption to the data field and leave headers unencrypted Decryption info inserted into headers Usually works with a multimedia format that supports encryption,e.g., Microsoft’s ASF  Strength Allow parsing and extracting basic info without decryption Highest security, small overhead for decryption info Content agnostic  Limitation: complexity, limited flexibility

15 Selective Encryption  Only I-frame/blocks encrypted (Maples & Spanos’95, Meyer & Gadegast’95 ) Reduce processing overhead/delay Not sufficient security Plain text attack using known syntax Not very secure for trans-coding Little transparency  Sign bits, MVs(Shi & Bhargava’98, Zeng & Lei’99, Wen et al’01)  Privacy/security low due to information leakage Useful for apps focusing on introducing quality degradation

16 Joint Scrambling and Compression  Shuffle DCT coefficients within 8x8 block (Tang 96) Randomize 8x8 DCT coefficient scan order  Simple Some level of security  Local scrambling -> spatial energy distribution unchanged - > less effective scrambling Significantly reduce compression efficiency (up to 50%) –destroy run-length statistics  Shuffle lines of wavelet coefficients ( Macq& Quisquater’94 ) Change 2-D statistical property, Reduce compression efficiency

17 Joint Scrambling and Compression  Selective scrambling in transform domain, prior to compression (Zeng & Lei’99)  Advantages Simple and efficient. Provides different levels of security, Allows more flexible selective encryption  easier for locating what data to be selected Limited adverse impact on compression efficiency, Allow transparency Allow trans-coding without decryption Allow other useful features without decryption

18 Overview

19 Wavelet Based Systems A 3-level subbanddecomposition Allow some level of transparency e.g, free access to low resolution require key for high definition TV

20 Wavelet Based Systems  Goal: Scrambling/shuffling that does not destroy statistical properties of each subband  Selective bit scrambling Sign encryption  sign bits: “uncompressible”, but critical to image quality  Block shuffling Divide each subandinto kblocks Shuffle the blocks within a subband  retain local2-D statistics Different shuffling tables for different subbands

21 Wavelet Based Systems  Block rotation Rotate each block Special case of shuffling coefficients within block

22 Security Analysis  Sign encryption M: # of non zero coefficients 2 M trials (including inverse transform) for complete recovery example: M= > trials  Block shuffling kblocks, nzero blocks # of different permutation: k!/n! example: k=64, n= > K!/n!=10 28 each permutation requires an inverse wavelet transform  Block rotation (+shuffling) # of configuration: (8*k)!/(8*n)! >>K!/n!  Other attacks? Your exercises!

23 Wavelet-based System

24 Wavelet-based System PSNR Table 1: Impact of different scrambling techniques on compression efficiency. Image sizes are 512x512, 5-level decomposition, 64 blocks each band.

25 DCT Based Systems  JPEG/MPEG/H.26x  Video compression GOP (I BBPBBP…)  I: intra-frame  P, Bpredictive-coded frames block: 8x8, for DCT coding,  zigzagordering of DCT coefficients Macroblock(MB): 4 lum. blocks + 2 chrom Blocks  unit for motion compensation  intra-coded vs. predictive coded Slice: a horizontal strip of MBs

26 DCT Based Systems  DCT coefficient scrambling Sign encryption Coefficient shuffling within each slice  shuffle coefficients of sameband  little impact on compression efficiency  each band has a different shuffling tables  Motion vector scrambling for P, B frames Sign flipping MV shuffling within each slice Important for distorting motion information  Dynamic-keys for more secure video transmission

27 I-Frames of DCT-based System

28 I-Frames of DCT-based System Table 2: Impact of different scrambling techniques on compression efficiency for one I frame of “carphone”sequence.

29 DCT-based System (Sequence) Table 3: Impact of different scrambling techniques on compression efficiency for 41 (one I frame followed by 40 P frames) frames of “carphone”sequence

30 Video Demo

31 References  T. Maples and G. Spanos, “Performance study of a selective encryption scheme for the security of networked, real-time video," Proc. 4th Inter. Conf. Computer Communications and Networks, Las Vegas, Nevada, Sept  J. Meyer and F. Gadegast, “Security mechanisms for multimedia data with the example MPEG-1 video,”  C. Shi and B. Bhargava, “A fast MPEG video encryption algorithm,”Proc. ACM Multimedia, pp ,  L. Tang, “Methods for encrypting and decrypting MPEG video data efficiently,”Proc. ACM Multimedia,  W. Zeng and S. Lei, “Efficient frequency domain selective scrambling of digital video”, IEEE Tran. Multimedia,vol. 5, no. 1, pp , March A preliminary version also in Proc. ACM Multimedia, Nov  Bin Zhu, “Multimedia encryption, “book chapter in Zeng, Yu, and Lin (Eds), Multimedia Security Technologies for Digital Rights Management, ISBN: , Elsevier, July 2006.