Performance Analysis of Data Hiding in MPEG-4 AAC Audio *

Transcription

1 TSINGHUA SCIENCE AND TECHNOLOGY ISSNll ll07/21llpp55-61 Volume 14, Number 1, February 2009 Performance Analyss of Data Hdng n MPEG-4 AAC Audo * XU Shuzheng ( ) **, ZHANG Peng ( ), WANG Pengjun ( ), YANG Huazhong ( ) Insttute of Crcuts and Systems, Department of Electronc Engneerng, Tsnghua Unversty, Bejng , Chna Abstract: A hgh capacty data hdng technque was developed for compressed dgtal audo. As perceptual audo codng has become the accepted technology for storage and transmsson of audo sgnals, compressed audo nformaton hdng enables robust, mperceptble transmsson of data wthn audo sgnals, thus allowng valuable nformaton to be attached to the content, such as the song ttle, lyrcs, composer s name, and artst or property rghts related data. Ths paper descrbes smultaneous low btrate encodng and nformaton hdng for hghly compressed audo sgnals. The nformaton hdng s mplemented n the quantzaton process of the audo content whch mproves robustness, sgnal qualty, and securty. The mperceptblty of the embedded data s ensured based on the maskng property of the human audtory system (HAS). The robustness and securty are evaluated by varous attackng algorthms. Tests wth an extended MPEG4 advanced audo codng (AAC) encoder confrm that the method s robust to the regular and sngular groups method (RS) and sample par analyss (SPA) attacks as well as other statstcal steganalyss method attacks. Key words: advanced audo codng (AAC); steganography; steganalyss; human audtory system (HAS); data hdng; nformaton hdng Introducton Data hdng, or steganography, s a challengng area of research whch has attracted much attenton for centures [1]. As an actve area of research, new technques are constantly emergng. In contrast to the actve research focusng on mage or vdeo schemes, there s lttle research on data hdng n audo schemes, especally n hgh compressed audo lke the MPEG-4 advanced audo codng (AAC). Human ears can detect even a small amount of embedded nose especally when the sgnal s weak. Receved: ; revsed: * Supported by the Chuanxn Foundaton (No ) and the Basc Research Foundaton of Tsnghua Natonal Laboratory for Informaton Scence and Technology (TNLst) ** To whom correspondence should be addressed. E-mal: xusz@mal.tsnghua.edu.cn; Tel: However, effectve data hdng n audo can create robust and mperceptble data wthn audo sgnals, thus allowng property rghts protecton or secret data exchange. Data hdng acts as an ntentonal nose nserted nto dgtal audo sgnals. Data hdng s a good method for preventng llct copyng and dssemnaton of copyrghted audo and can also be used to transfer secret nformaton n convert communcatons. A number of systems are currently avalable to transmt a watermark n a hdden channel wthn uncompressed audo sgnals. However, compressed audo, usng a perceptual audo encoder to save storage space as well as to mnmze the transmsson bandwdth and tme, s very popular and wdely used over the Internet and n wreless communcatons. Current research has manly focused on the pulse code modulaton (PCM) doman for AAC fles [2], where the hdden nformaton dsturbs the sgnal durng quantzaton n the audo

2 56 encoder. There s also some research that hdes data drectly nto the compressed audo [3]. The dsadvantage of ths method s that the hdden data treated as an addtonal dsturbance to the quantzaton nose, results n obvous degradaton of the audo qualty. Ths paper presents a method for data hdng durng the encodng process of the MPEG-4 AAC audo. The scheme has flexblty n hdng the data and obtans a good balance between robustness, securty, and capacty. 1 State of the Art 1.1 Advanced audo codng n MPEG-4 AAC, whch s ncluded n the MPEG-4 audo standard, s a compatble successor to the MPEG-2 audo codng [4]. As perceptual encoders, they explot knowledge of human percepton to shape the nose dstrbuton ntroduced by the rrelevancy reducton to acheve the best possble audo qualty. The basc structure of the perceptual encoder used n the MPEG audo famly s Tsnghua Scence and Technology, February 2009, 14(1): shown n Fg. 1. AAC encodng conssts of frequency transform, quantzaton, entropy codng, and btstream multplexng steps. AAC typcally employs modfed dscrete cosne transform (MDCT) wth 1024 samples per frame. The 1024 frequency samples n each tme frame are separated nto 49 frequency bands. Wthn the same frequency band, samples are consdered to have smlar perceptual effect to the human ear and, hence, share the same quantzaton step sze. Perceptual modelng s appled to the MDCT coeffcents to estmate the maxmum amount of dstorton that can be wthstood by each coeffcent. The quantzaton step sze s teratvely modfed untl the rate s below the target bt rate and the dstorton s below the maxmum acceptable value obtaned from the perceptual model. Huffman codng s used to encode the quantzed coeffcents and the quantzaton step sze. Fnally, the coded bts wth some sde nformaton are multplexed nto a sngle btstream to be transmtted or saved. Fg. 1 Basc structure of a perceptual encoder The man task of the perceptual encodng s to compress the dgtal audo as much as possble such that the sound qualty of the reconstructed sgnal s exactly the same as or as close as possble to the orgnal audo sgnal. There are also some other requrements lke low complexty and flexblty for a wde applcaton area. The modern hghly developed perceptual models and effcent redundancy reducton audo encodng systems offer a multtude of codng tools lke temporal nose shapng (TNS), jont stereo codng, and perceptual nose substtuton (PNS) to fulfll these requrements. 1.2 Data hdng Varous schemes are used for data hdng n audo, such as echo hdng [5], tme doman modfcaton [6], and spread spectrum technology [7]. Data hdng n audo must satsfy at least the three constrants of securty or mperceptblty, robustness, and capacty. These terms are commonly used to descrbe the propertes of dfferent data hdng schemes. Securty Data hdng n audo s also called naudblty. In most cases, securty, ncludng perceptual transparency of the hdden data, s consdered to be the most mportant ssue. In other words, the nose ntroduced by the hdden data should be almost naudble and should not degrade the audo qualty. The statstcal propertes of the embedded audo sgnal should be exactly the same as the orgnal audo to ensure that the hdden data s mperceptble and undetectable by thrd partes. Robustness The algorthm should be robust enough to wthstand unntentonal or ntentonal attempts such as removal or alteraton of the hdden data. Even wth unfavorable condtons such as bad wreless channels whch degrade the audo qualty, the hdden data should be recovered successfully. Capacty Often, the capacty of the hdden data s also a very demandng aspect. Capacty refers to the number of bts per second that can be transmtted by the data hdng system. Ths depends on the underlyng

3 XU Shuzheng ( ) et al Performance Analyss of Data Hdng n MPEG-4 AAC Audo 57 technology and the choce of parameters for the hdng scheme. At present, the data rates reach several hundred bts per second. Securty, robustness, and capacty have contradctory arguments so they cannot be adjusted ndependently. For nstance, ncreases of the data hdng capacty wll degrade the robustness and securty. These trade-offs form the trangle shown n Fg. 2 wth an approprate operatng pont wthn the lmts of the trangle chosen for dfferent applcatons. Fg. 2 Trade-off between securty, robustness, and capacty 2 Data Hdng Scheme 2.1 Basc structure The present scheme alters the quantzaton and codng process by modfyng the scale factor to produce more bts, wth the redundant bts replaced by hdden data. The basc structure of the system ncludng all of the relevant parts of the MPEG-4 AAC perceptual encoder s shown n Fg. 3. The encodng scheme s very effcent. The uncompressed nput audo sgnal s processed by the analyss flterbank wth parameters gathered from both the sgnal tself and the perceptual model. The data s frst compressed, encrypted, and then hdden n the AAC btstream. The hdng process takes place at the heart of the Layer III encodng process n the nner loop. The nner loop quantzes the nput data and reduces the quantzaton step sze untl the quantzed data can be coded wth the avalable number of bts. Another loop checks that the dstortons ntroduced by the quantzaton do not exceed the threshold defned by the psychoacoustc model. Fnally, the multplexer produces a btstream ncludng data and sde nformaton to complete the data hdng process. The adherence to the psychoacoustc model makes t very dffcult to hear the hdden data. For example, durng the tradtonal quantzaton and codng process, the compressed sgnal can be represented as 3 T 3T Xs 2 X 2 X (1) where T s the scale factor calculated by the perceptual model and X (=1,, 960) represents the quantzed data. For the same sgnal X s, the scale factor T s reduced, the quantzaton step 2 T becomes smaller, and X becomes larger, and has to be represented by more bnary bts. If the scale factor T s 4 whch means that the maskng threshold for psychologcal hearng s 2 T 16, and the sgnal X s s 128, then X s 8 (the bnary form s 1000). If the scale factor Fg. 3 Basc data hdng structure for the AAC perceptual encoder

4 58 T s reduced from 4 to 2, then X wll become 16 (10 000). If the least sgnfcant bt (LSB) of X s replaced by the hdden data, then the hdden data wll sound lke nose n the audo sgnal wth the ntroduced nose energy stll below the maskng threshold level so t wll not affect the audo qualty. Ths actve modfcaton method can flexbly ncrease the hdden data capacty by varyng the scale factors and the number of spectral subbands. The robustness and securty of the hdden data are acheved by the spread spectrum and the pseudo selecton of the subbands for data hdng. 2.2 Data extracton The extracton process should be mplemented before reconstructng the MDCT coeffcents because the nformaton s embedded n the quantzed coeffcent. Blnd extracton ntroduces two problems nto the data extracton process. A normal audo frame whch does not have any nformaton may be treated as a specal audo frame when the crcle redundant check (CRC) byte s passed wth a very low probablty. However, a specal audo frame may be treated as a normal audo frame when mstakes occur n the channel. These two problems are common n wreless communcatons. Thus, the nformaton should be changed to a specal data format wth a CRC byte nserted nto the nformaton for wreless transmssons. 2.3 Characterstcs Compared wth current data hdng schemes, the algorthm presented here has the followng advantages: (1) The scheme enables optmal coordnaton between the quantzaton strategy of the encoder and the data hdng process. All nformaton about the modfcaton of the quantzaton parameters s taken nto account by the data hdng process. (2) The scheme can flexbly vary the data hdng capacty by adjustng the number of coeffcents for the quantzaton, and thus ths scheme provdes adaptve data hdng. When the embedded data bt rate needs to be changed, the scheme can adaptvely change the quantzaton parameters. (3) The scheme assures hgh data hdng securty. The coeffcents whch carry the hdden data are pcked up by a pseudo random mechansm whch s unlkely to be ntercepted by a thrd party. Tsnghua Scence and Technology, February 2009, 14(1): Test Results 3.1 Audo qualty The qualty of the data hdng AAC sgnals was carefully assessed by subjectve lstenng tests. The AAC test tems represent a varety of typcal materals from dgtal rado broadcastng source sgnals [8] and the test used a MUSHRA lstenng test [9]. The qualty was evaluated by ten lsteners who were all experenced and famlar wth the set of test tems. Fgure 4 shows the mean values and 95% confdence level for the crtcal test tems. The results show that there s no statstcally sgnfcant degradaton between the orgnal AAC encoded tems and the modfed sgnals for all hdden data capactes vares from 10% to 100%. Fg. 4 Evaluated qualty of orgnal MPEG-4 AAC audo and modfed AAC audo Sgnal-to-nose rato (SNR) s a typcal objectve measure of audo qualty. In the compresson doman, the sgnal s defned as the orgnal uncompressed audo and the nose s defned as the dstorton between the decompressed audo and the uncompressed audo. The SNR of MPEG-4 AAC compresson s typcally db. The hdden data wll ntroduce some quantzaton nose whch wll slghtly reduce the SNR value. Statstcal calculatons wth large test sets show that there s only about db loss n the SNR when hdng messages at the maxmum bt rate. In addton, the use of the psychoacoustc model guarantees that the nose ntroduced by the data embeddng process does not exceed the threshold and only slghtly degrades the audo qualty. 3.2 Hdng data capacty Snce the actve modfcaton scheme requres more bts to represent the sgnal, the compressed audo wll be enlarged. Fgure 5 shows how the sze of the carrer

5 XU Shuzheng ( ) et al Performance Analyss of Data Hdng n MPEG-4 AAC Audo 59 of the AAC fle s ncreased as the hdden data ncreases, wth the ncreased sze of the AAC fle beng much smaller than that of the hdden data. For nstance, when the hdden data sze s ncreased by 60%, the rato of the hdden AAC fle to the orgnal fle sze s 132%, only ncreased by 32%. There are also some flat regons n the curve whch ndcate that the hdden data sze do not affect the AAC fle sze. Snce the strategy s to modfy the scale factors of some spectral subbands whose length are fxed, dfferent hdden data capactes may requre the same number of modfed subband scale factors. Fg. 6 Frequency doman waveform of the MPEG-4 AAC and converted AAC audo streams Fg. 5 Fle szes of the orgnal MPEG-4 AAC and converted AAC audo fles 3.3 Tme and spectrum waves Tests were also run to evaluate whether the hdden data n an AAC audo can be detected through more qualtatve methods, and the characterstcs of the changes. Further research could yeld a set of analytcal formulas descrbng the relatonshps between the hdden data, the voce or carrer data, the measurements, and the results. Ths research only analyzed the sound waveforms n the tme and frequency domans. CoolEdt2000 was used for the tme and spectral analyses. The orgnal AAC audo waveforms are compared wth the recordng contanng the encrypted hdden data. The resultng waveforms n the tme and frequency doman are shown n Fgs. 6 and 7. Both the tme and frequency analyses do not show any dstngushng dfferences between the orgnal recordng and the recordng wth the hdden data. However, there seems to be a slght amount of sgnal loss and a slght ncrease n dstorton due to the hdden data, but the overall measurement scores are Fg. 7 Tme doman waveform of the MPEG-4 AAC and converted AAC audo streams very good. The very mnor alteratons of the orgnal recordng when the data s hdden are so small as to not be perceptble. 3.4 RS and SPA detecton The regular and sngular groups method (RS) ntroduced by Frdrch et al. [10,11] and the sample par analyss (SPA) ntroduced by Dumtrescu et al. [12] have proven to be the most useful tools for detectng least sgnfcant bt (LSB) embeddng [13]. The RS estmates the length of the embedded data by countng the length varaton of the regular set and the sngular set. When the data s embedded n a random order, the length of the embedded data can be accurately estmated by ths

6 60 method. The SPA accurately estmates the length of the embedded data when more than 3% of the data s embedded n LSB. Andrew [13] verfed the credblty of the RS and the SPA n many tests. The present scheme s an LSB embedded method. However, the scheme can effectvely resst the RS and SPA detectons snce the embedded process works n the frequency and compresson doman. The AAC V2 correlaton s much hgher than that of LSB by RS detecton algorthm, and the correlaton becomes better as the hdng data ncreased. Fgures 8 and 9 show the results of usng RS and SPA to analyze sequence sets embedded data by a normal LSB scheme and the current AAC V2 scheme, whch ndcate that the later has better performance of securty. Tsnghua Scence and Technology, February 2009, 14(1): to be classfed wth the condton that the carrer and the stego are both blnd to the classfer. A judgment result very close to 50% ndcates hgh securty wth an absolutely secure system havng a result of 50%. The classfer structure s shown n Fg. 10. The dstance measurement module calculates the characterstc dfference between the converted and the orgnal AAC fles. Snce the analyss does not know what knds of characterstcs have been changed durng the data hdng process and to assure the generalty of the analyss, the system analyzes as many characterstcs as possble. Fg. 10 Basc classfer structure Fg. 8 RS analyss results for detectng hdden nformaton Fg. 9 SPA analyss results for detectng hdden nformaton 3.5 Detecton analyss Steganalyss can be charaterzed as classfcaton of two elements. The carrer and the stego are objectves Three knds of characterstcs are consdered n the classfer [14,15]. (1) Tme doman measurements nclude the SNR, segment SNR (SNRseg), Czenakowsk dstance (CAD), and Mnkowsky measurement (MM). (2) Frequency doman measurements nclude the logarthm lkelhood rato, Itakura-Sato dstance, COSH dstance, nverse spectrum dstance, multtude spectrum dstorton, and phase spectrum dstorton. (3) Perceptual doman measurements nclude the BARK dstorton. Lnearty regresson, classfcaton tree, vector support, and projecton were selected as the classfcaton algorthms n the classfer. Generally speakng, the two classfer results are detecton and false detecton. The testng sets nclude 200 groups of AAC fles compressed from khz to 48 khz, 16 bts, mono and stereo PCM btstreams. The test results are shown n Table 1. The data n Table 1 shows that the measurement probablty of the AAC data hdng algorthm s almost 50% whle the possbltes for other hdng algorthms, LSB, ECHO, Hde4PGP, and Steghde, are much hgher. Thus, the AAC data hdng algorthm has comparatvely hgher securty.

7 XU Shuzheng ( ) et al Performance Analyss of Data Hdng n MPEG-4 AAC Audo 61 Table 1 Performance comparson of dfferent data hdng algorthms Algorthm Probablty of detecton/probablty of false detecton Lnearty regresson Vector support Classfcaton tree Projecton LSB 0.72/ / / /0.44 ECHO 0.78/ / / /0.22 Hde4PGP 0.75/ / / /0.32 Steghde 0.67/ / / /0.39 AAC(BPF * 8) 0.52/ / / /0.44 * BPF: Byte per frame. 4 Concludng Remarks Ths paper frst gves an overvew of data hdng compressed audo sgnals and the state-of-the-art technologes for source compresson especally AAC formats. A data hdng scheme for MPEG4 AAC s presented based on the human audtory system. The data hdng occurs n the quantzaton process of the MDCT procedure of the AAC codng, and the hdden data capacty can be adaptve to the orgnal data rate by actve modfcaton of scale factors wthout degradaton of audo qualty. The mplementaton s very effcent wth good robustness acheved by enablng optmum coordnaton of the AAC quantzaton and data hdng processes. Tests confrm the potental of ths new technology and show t to be an attractve soluton for property rghts protecton and covert communcatons. References [1] Pettcolas F A P, Anderson R J, Kuhn M G. Informaton hdng A survey. In: Proceedngs of the IEEE Specal Issue on Protecton of Multmeda Content, 1999, 87(7): eee99-nfohdng.pdf. [2] Cheng S, Yu H, Xong Z. Enhanced spread spectrum watermarkng of AAC audo. In: Proceedng ICASSP 02. Orlando, FL, USA, 2002: [3] Neubauer C, Herre J. Audo watermarkng of MPEG-2 AAC btstreams. In: 108th AES Conventon, Audo Engneerng Socety Preprnt Pars, France, 2000: [4] Koenen R. Overvew of the MPEG-4 standard. Document, M-arz ISO/IEC JTC1/SC29/WG11 N, [5] Gruhl D, Lu A, Bender W. Echo hdng. Lecture Notes n Computer Scence, 1996, 1174: [6] Xu Chansheng, Wu Jankang, Sun Qubn, et al. Applcatons of dgtal watermarkng technology n audo sgnals. Journal of Audo Engneerng Socety, 1999, 47(10): [7] Garca R A. Dgtal watermarkng of audo sgnals usng a psychoacoustc audtory model and spread spectrum theory. In: 107th AES Conventon. New York, USA, 1999: [8] Dgtal Rado Mondale (DRM) System Specfcaton (ETSI) , V2.1.1, [9] Stoll G, Kozamernk F. A method for subjectve lstenng tests of ntermedate audo qualty. ITU Workng Party 10-11Q, Sept [10] Frdrch J, Goljan M. Practcal steganalyss of dgtal mages State of the art. In: Proceedngs of SPIE: Securty and Watermarkng of Multmeda Contents IV. 2002: [11] Frdrch J, Goljan M, Du R. Relable detecton of LSB steganography n color and grayscale mages. In: Proceedngs of the ACM Workshop Multmeda Securty. Ottawa, Canada, 2001: [12] Dumtrescu S, Wu Xaoln, Wang Zhe. Detecton of LSB steganography va sample par analyss. IEEE Transactons on Sgnal Processng, 2003, 51(7): [13] Andrew D K. Improved detecton of LSB steganography n grayscale mages. Lecture Notes n Computer Scence, 2004, 3200: [14] Ozer H, Avcbas I, Sankur B, et al. Steganalyss of audo based on audo qualty metrcs. In: Securty and Watermarkng of Multmeda Contents. Santa Clara, USA, 2003: [15] Yang Bo. Extensve bark spectral dstorton measurement for objectve speech qualty assessment. Journal of Unversty of Electronc Scence and Technology of Chna, 2006, 35(3):