Chaos-Based Encryption Algorithm. for Compressed ... - IEEE Xplore

TSINGHUA SCIENCE AND TECHNOLOGY ISSN 1007-0214 18/23 pp605 - 611 Vohm e 8, Nun ber 5, October 2003

Chaos-Based Encryption Algorithm. for Compressed Video * YUAN Chun (~

$), ZHONG Yuzhuo (~rp~ij() *

* ,

HE Yuw en CM~)()

Deparjn en t of Computer Science and Technology, Tsitghua Un iversity, Beij itg 100084, Ch na Abstract

Encryption for compressed video streams has attracted increasing attention with the exponential

grow th of dig ita 1m ultin ed ia de live ry and consun ption How eve r, m os t algorithm s proposed in the lite ra ture do not effectively address the peculiarities of security and perfonnance requirements.

This paper presents a

chaos-based encryption algorithm ca lled the chaotic se lective encryption of compressed video (CS ECV) which exploits the characteristics of the compressed video.

The encryption has three separate layers that can be

selected according to the security needs of the application and the processing capability of the client computet: The chaotic pseudo-random sequence gene ra tor used to gene ra te the key-sequence to random ize the in portant fields in the compressed video stream has its parameters encrypted by an asymmetric cipher and placed into the stream. The resulting stream is still a valid video stream. CSECV has significant advantages over existing algoritlms for security, decryption speed, inplementation flexibility, and error preservation

Key words:

chaos; chaotic encryption; se lective encryption; e rro rtp re se rving encryption; security; compressed video

In troduction W e have seen in recen t years an exp 10 sive grow th of digital m ultin edia app Iicat nns, such as digital TV, video-orr-dem and, video email, and video telephone However, m ultin edia data is vulnerab Ie to unauthorized copy and usage when tran sn itted over an open network or in storage Encryp t on is needed to protect the multinedia data Compared with text encryp tjon, m ultin edia encryp tjon has 80m e unique characteristics, such as the large size, high throughput, and real-tine processing An efficien t, secure, and ligh rw eigh t encryp t on algoReceived: 2002-02-27; revised: 2002-10-14 * Supported by the N atonal Key Research Techno logy Project ofM inistry of Infonn atnn Industry of China (No. 19991118) * * To whom correspondence should be addressed Email: zhong@media cs tsinghua edu en; Tel: 86-10-62786910

rithm is desirable to protect the compressed video. A video clip is generally compressed in a transfonn domainwith some type of entropy coding To protect a compressed video, encryp t on techniques can be applied to the original data, such as block sw app ing, or the data can be transfonn ed using DCT or wavelet coefficients, entropy-coded bit streams, or fonnat headers All existing algorithm s can be classified into either of the fo llow ing two types: 1) adapting a general encryp t nn algorithm, such as DES 0 r D EA, to the sp ecial propertiesofacompressedvideo stream; or2) applying a flexible cipher to encrypt a compressed video stream as if it were a general data stream. Zigzag-penn u tatjon ofDCT coefficients[l] is one of the early encryp t nn algorithm s used forM PEG video stream s The system uses a random penn utat on list to random ize theM PEG zigzag order of the DCT coefficien ts in an 8 X 8 block The method has the same computat onal complexity asMPEG zigzag ordering of DCT coefficients, but has reduced comp ressjon efficiency since the energy comp act on of a zigzag ordering ofDCT coefficients is greatly comp rom ised by a random ordering of these

606

coefficien ts The method is also vu lnerab Ie to frequency-based cryp tanalysis The video encryption algorithm (V EA) [2] is a selective encryption algorithm which encrypts only the sign bits S = "'SI '''S2 ···Sm ···Sm+ 1 '''Sm+ 2 '''S2m of the DCT coefficients in an M PEG compressed video, where s, is the sign bit of either the DC or the AC coefficient For I-frames, the sign bit of the DC coefficient is the sign of the resulting value from the differential coding A pseudo-random sequence B = ... bs...b: ... b; genera ted by a linear feedback sh ift reg ister (L F SR ) is app lied to the sign bitsw ith a bit-to-bit XOR operation Ek(S)= ... (hI 8 SI) ... (bm 8sm)··· (hI8Sm+ 1) ... (bm 8s2m). The m ajor advantage of the method is its low complexity, but L FSR as a random sequence generator is well known to be cryptographically insecure Furthermore, themethod has to repeatedly apply the same random sequence due to the large size of the content to be encrypted, which lowers the security A lattar and Gha ssan" proposed encrypting only parts of the compressed video, i e , the l'm acroblock and the headers ofa predictedmacroblock Three methods were proposed The first method applies the well known data encryption standard (DES) to every n-th Imacroblock, with the other blocks unencrypted, where n is detenn ined exp erim en tally A lthough only som e of the 1m acrob locks are encryp ted, the resulting video is useless to an unauthorized user due to the differentialencoding schen e used in M PEG to encode the DCcoefficients of blocks The second method proposed by A latter and Ghassan!" addresses the drawback in the firstmethod that the motion from one fram e to the nex t is still visib Ie by encryp ting the headers of all predicted m acroblocks Their th ird method is to encryp t every 0 ther 1m acroblock as well as the headers of predicted macrob locks The advantages of theirm ethods are layered encryp tion and strong security, bu t they are too comp lex for real-tin e app lications W u andMoo[4] exploited the special video coding structure of the en bedded conditional entropy coding ofwavelet (ECECOW) coefficients using a sequence cipher to encrypt the first several wavelet coefficien ts fo reach fram e In ECECOW, h igherorder conditional arithm etic coding is used to compressw avelet coefficients A rithm etic coding introduces long range dependency so that the coding of the current bit depends on all the previous bits Encryp tion of the first few wavelet

Tsinghua Science and Technology, October 2003, 8 (5): 605 - 611

coefficien ts of a fram e w ill render the who Ie fram e useless to a user w ithou t the righ t key However, the system also causes errors to propagate over a long range, which is an undesirab le feature in real app lications Rom eo et al [5] p ropo sed an encryp tion algo rithm for high throughput m ultin edia app lications The method uses an efficient asymmetric cipher to encrypt a session key, which is used to generate a pseudo-random sequence with a m ax in al-period L SFR. The random sequence is then app lied to the compressed video stream by a bit wise XOR operat nn Its complexity is very low, but the security is also low, due to its usage ofL SFR as the random sequence generato r. This paper proposes an encryp t on system for M PEG stream s called the chaotic selective encrypt on of compressed video (CSECV) which contains all the advantages of these previousm ethods Like p rev ou s rn ethods'