The invention relates to a method and to a system for protecting by watermarking against non-authorised use, e.g. non-authorised recording or copying, original audio or video data which are to be presented, e.g. in a digital cinema.
Audio Watermarking is one security technology for content protection. The watermark (WM) is a signature embedded within the data of the original audio signal and, in addition to being inaudible to the human ear, should also be statistically undetectable, and should be resistant to any attempts of removing it. For example in a Digital cinema application, the requirements of a watermarking system can be specified on the basis of a set of properties: in addition to the general requirements of quality of the watermarked copies and robustness of the embedded watermarks, such cinema application includes additional security constraints which are to be complied with.
‘Digital Cinema Initiatives’ (DCI) is an entity founded by seven motion picture studios to establish specifications for an industry-wide standard for Digital cinema. A DCI Specification Version 1.2 was published on http://www.dcimovies.com. The workflow for movie production includes mastering, distribution and playback in the theatres, as depicted in
In the Digital cinema scenario different components are specified, e.g.:
The Digital cinema distribution master DCDM shall be based on a hierarchical image structure that supports 2K and 4K master. The Digital cinema system is built on a file-based design, i.e. the complete content is made up of data stored in files which are organised around the image frames. The included DCDM audio data has a bit depth of 24 bit/sample and a sample rate of 48 or 96 kHz. The file format is PCM WAVE and the data are not compressed.
In the Digital cinema system, the security system as depicted in
Watermarking is implemented in embedder 22 within the media block MEDBL of the server located in the theatre.
The most important requirement regarding the watermarking of audio data is that the watermarking is not audible in ABX listening tests. The payload to be embedded includes an identification of the theatre by using 19 bit word length, corresponding to 219=524.288 possible locations. In addition a time stamp is embedded which changes every 15 minutes and is repeated every year, requiring 16 bits for its binary representation:
4 time stamps/h*24 h/d*366 d/y=35136 time stamps/h
Thus, the total length l(w) of watermark w information bits is l(w)=19+16 bits=35 bits. The data-rate is set to
l(w)bit/5 min=7 bit/min
Since the embedding of the watermark is to be performed just before playback in the theatre it is to be performed in real-time or even faster. The minimal time segment required for detection should not exceed 30 minutes.
In the following, the original or carrier object is denoted by co. In the notation used, co[i], i=1, . . . , lc
The intention of the publication O. Billet et al., “Efficient Traitor Tracing from Collusion Secure Codes”, 10 Aug. 2008, Information Theoretic Security, Lecture Notes in Computer Science, Springer Berlin Heidelberg, pp. 171-182, XP019102288, is to enable the tracing of traitors even if several traitors collude to produce a pirate decoder from the shared secrets (keys) which are illicitly extracted from the decryption box (decoder). Therefore it is a tracing of traitors by cryptographic means not using a watermarking technique. In case a pirate decoder can decrypt the content, no further mechanisms are available for tracing the illicit user on the basis of the content. In turn, for tracing unauthorised users the tracer needs access to the pirate decoder. In case of watermarking, the tracer only needs access to the pirate content with the ID of the user (or the DCI watermark) embedded in the content in contrast to a set of keys integrated in the decoder.
In contrast, the invention solves the problem that an adversary can potentially access the original content if embedding is performed at receiver site as it is the case in the digital cinema scenario. It can be combined with the method described above to increase the security level by providing additional security against collusion attacks due to the leaking of the security keys out of the decryption box (in the digital cinema case the DCI server).
Due to the embedding of the watermark just before the presentation of the movie as shown in
These kinds of problems will occur in all applications where embedding is done at the receiver site and the watermarking algorithm is implemented in hardware.
A problem to be solved by the invention is to increase the overall security of such protection processing or system. This problem is solved by the method disclosed in claim 1. A system that utilises this method is disclosed in claims 2 and 3.
The invention shifts the burden of embedding the watermarks from the receiver site (theatre) to the sender site (postproduction, studio) by using pre-watermarked and encrypted copies:
Several frame-wise pre-watermarked and encrypted copies of an audio PCM file are delivered. Frame-wise or block-wise watermarking and encryption is carried out at sender site (post-production or studio), and the frame/block-wise encrypted files are packed and delivered to the receiver site (theatre). The different versions of the copies or tracks represent encrypted candidate watermarking with different symbols of the watermark. For example, the first copy of the audio or video signal is watermarked at sender site with value ‘0’ watermark symbols for each block or frame, and the second copy is watermarked at sender site with value ‘1’ watermark symbols for each block or frame. A further copy can contain sync watermark symbols.
In addition to the files delivered, a key sequence is transmitted to each receiver consisting of the keys for decrypting individual frames or blocks identified by the frame/block number and the symbol embedded. At receiver site the decryption can be performed on the proper frames only, according to the transmitted key sequence. The pre-watermarked and decrypted frames are used to create a single watermarked copy that implicitly carries the correct watermark information bits, e.g. ID and time stamp in a DCI application. For creating that single watermarked copy, based on the bits or values of the desired watermark information bit word, the corresponding frames or blocks from the received or stored copies are taken and are assembled in a successive manner.
Due to pre-watermarking of the content at sender site, the embedding of the watermark has the following advantages:
Nevertheless, the invention described so far has still two disadvantages:
Therefore the encryption can be carried out block-wise as described below.
In principle, the inventive method is suited for protecting by watermarking against non-authorised use, e.g. non-authorised recording or copying, an original audio or video data signal that is to be presented, said method including the steps:
In principle, the inventive sender system is suited for protection by watermarking against non-authorised use, e.g. non-authorised recording or copying, of an original audio or video data signal that can be presented at a receiver site, said sender system including:
In principle, the inventive receiver system is suited for protection by watermarking against non-authorised use, e.g. non-authorised recording or copying, of an original audio or video data signal that is to be presented, said receiver system receiving or storing at least two differently pre-water-marked versions for successive blocks or frames of said original audio or video data, wherein said versions were derived by applying a repeated watermark symbol value to a version and different watermark symbol values to the different versions, and by encrypting said versions using corresponding encryption keys, wherein in addition each block or frame of said differently pre-watermarked versions is encrypted individually using a corresponding key sequence, said receiver system also receiving or storing said encryption keys and said key sequence, and said receiver system including:
Advantageous additional embodiments of the invention are disclosed in the respective dependent claims.
Exemplary embodiments of the invention are described with reference to the accompanying drawings, which show in:
Pre-Watermarking and Encryption
According to the invention, several block-wise pre-watermarked tracks for one audio PCM file are delivered. All pre-watermarked tracks are encrypted and stored in a file. At receiver site the pre-watermarked tracks are decrypted and, depending on the watermark information word (e.g. ID of the theatre and the time-stamp), the watermarked audio stream is constructed for the presentation.
The inventive processing is portable to other watermarking systems if it conforms to the following criteria:
Besides the above-mentioned characteristics, in the special case of Adaptive Spread Phase Modulation (ASPM) processing the following can be assumed:
Before the audio tracks are delivered, the original file data corg is watermarked times with special watermarks wi, i=1, . . . ,
, resulting in
watermarked copies.
={0, 1, S1, S2, S3},
=5). For example, in file version c0 the bit pattern ‘000 . . . 00’ is used to generate watermark symbol w0 for the time periods t4 to tn, and in file version c1 the bit pattern ‘111 . . . 11’ is used to generate watermark symbol w1. Such data preparation is referred to as pre-watermarking. The embedding of these watermarks is performed by using a unique key K for generating the bit pattern representing the different bits or sync words ‘0’, ‘1’, ‘S1’, ‘S2’, ‘S3’. In the special case of ASPM processing being used, three different sequential synchronisation sequences (‘S1’, ‘S2’, ‘S3’) have to be inserted at the beginning of the watermarks w0/w1 as shown in
=2 if all watermarked blocks are counted, as shown in
In order to increase the security, the watermark keys can be different for the individual audio files. However, this results in an identification problem during watermark detection. But this problem can be solved by incorporating fingerprinting techniques.
The watermarked copies of the track are encrypted e.g. as a whole by Ci=EK
Before presenting an audio track at receiver site the pre-watermarked copies are decrypted cw
The content of the watermark information word ‘w’ can be determined individually by the present receiver.
Advantageously, the construction speed of the watermarked track depends on the I/O performance only and not on the kind of watermarking processing being used. The watermarking performed in the data preparation step is the same for all receivers (cinemas) and therefore has to be performed only once for an audio track.
In the embodiment described before, it could be possible to construct watermarked tracks carrying the wrong watermark (i.e. cinema ID). The following embodiment modifies the pre-watermarking and construction processing.
The watermarking performed to create the pre-watermarked copies is identical to the previous embodiment. However, the watermarked copies are encrypted block-wise.
Each block Cji=EK
For Digital cinema application, the watermark w={wID,wtime} consists of the ID of the cinema and the time stamp. The key sequence can be partitioned into a fixed part consisting of the synchronisation blocks KS={KS11, KS22, . . . , KSnn
K
w
={K
S
,K
ID
,K
time
}, n=n
S
+l(wID)+l(wtime)=38 (1)
consists of 35 bits for the DCI watermark and 3 synchronisation bits.
Different time stamps can be implemented by varying the last sub-sequences Ktime. This offers the possibility to limit the time stamps to a certain period by transmitting only the corresponding key sequences.
By using the key sequence Kw
In addition to the previous embodiment, this embodiment has the advantage that the correct watermark (and thereby the ID of the cinema) is implicitly embedded because only the proper blocks can be decrypted by the assigned key sequence. But there are remaining disadvantages:
A reduction of bandwidth can be achieved by taking into account the static and the dynamic part of the watermark. This can be performed by sending the pre-watermarked and block-wise encrypted part of the stream directly with the fixed watermark portion carrying synchronisation bits and the ID of the watermark. Only for the variable part the two versions of the pre-watermarked and encrypted blocks are sent to the receiver (theatre). The time stamp range can be controlled at sender site.
The watermark embedding is the same as presented in the previous embodiment, whereby selecting the blocks for decryption has to be done only for the variable part wtime of the watermark. This is depicted in
A further reduction of the necessary bandwidth can be achieved by sending for the variable portion wtime of the watermark a track marked with one watermark symbol and the difference of the watermark signals for different watermark symbols. As depicted in
Because the watermark difference signals have a lower dynamic range than the original watermark signal they can be compressed effectively.
For embedding the watermark by construction, in addition to the selection of the blocks from the variable watermark portion wtime as presented in the previous embodiment a block-wise addition of the difference signals according to equation (3) is performed if a symbol other than that of the pre-watermarked copy is to be embedded.
As mentioned earlier, the invention is applicable to Digital cinema applications. It is further applicable to all applications where a watermark embedding step has to be performed at receiver site in an insecure device.
Number | Date | Country | Kind |
---|---|---|---|
10305857.4 | Aug 2010 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP11/62807 | 7/26/2011 | WO | 00 | 1/31/2013 |