WO2010113040A2

WO2010113040A2 - Watermarking method and system

Info

Publication number: WO2010113040A2
Application number: PCT/IB2010/001119
Authority: WO
Inventors: Shiguo Lian; Yuan Dong
Original assignee: France Telecom
Priority date: 2009-03-31
Filing date: 2010-03-15
Publication date: 2010-10-07
Also published as: EP2414979A2; WO2010113040A3

Abstract

A method for collaboratively watermarking a media content available on a media server and produced by users in a first list of users, said media content comprising a first watermark embedded therein identifying the users of said first list of users, said method comprising, for an additional user that does not belong to the first list of users and modifies the media content, the acts of obtaining the modified media content and embedding by the media server into the modified media content a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user.

Description

WATERMARKING METHOD AND SYSTEM

Field of the Invention

The present invention relates in general to digital right management and more specifically to media watermarking.

Background of the Invention

Watermarking is a suitable solution for digital right management in media distribution. Indeed, a watermark corresponding to a user may be embedded into a media content so as to further identify said user. In other words, a watermark corresponds to a unique user code, such as user ID (identification) that allows in particular identifying the user associated with a given copy of the media content. Watermarking is widely used for copyright protection as it allows embedding ownership information into a media content imperceptibly and is further used to identify the ownership of said media content. For example, document US6442283 "Multimedia data embedding" (Tewfik et al, August, 2002) proposes a method to embed a watermark into a media content. Document US6539475 "Method and system for protecting digital data from unauthorized copying" (Cox et al., March 2003) proposes a method to embed identifier information into a media content. Document US7044395 "Embedding object identifiers in media objects compatible with object identification standards" (Davis, Bruce L., Rhoads, Geoffrey B.) proposes a method using watermarking to resist unauthorized copying. However, few existing solutions consider the content distribution in networks, especially in social networks that is becoming more and more popular. Document US6647128 "Method for monitoring internet dissemination of image, video, and/or audio files" (Rhoads, Geoffrey B., November 2003) proposes a method to detect whether a copyrighted multimedia content is distributed over the Internet, including social networks, by detecting the watermark contained in the media content and deriving whether the media content is copyrighted or not. This method aims at preventing the illegal distribution of multimedia content.

However, social network is now not only a channel for free content sharing that does not consider copyright issues, it is also becoming a platform for personal advertisement or shared media content generation. For example, several users may work together to draw a picture, make a song or edit a film. Thus, copyright protection may be required. A user who uploads a media content may want to signify to the other users that the media content has been produced by himself. Similarly, users who generate a media content together expect to share the content's copyright. Document US2002/0181732 disclosed a method for collaboratively watermarking a media content that includes portions produced by respective users in a group of users. However, this method involves providing partial watermarks that are distributed by a central server to each user. Each watermark is embedded by each user in a portion of media content respectively produced said each user. Then all the portions are combined to derive the whole watermarked media content. A drawback of this solution is that each user has to embed his partial watermark in his portion of media content and send said watermarked portion to the central server.

Today there is no solution to efficiently collaboratively watermarking a media content generated by a plurality of users.

Today there is a need for a watermarking solution that can be easily implemented on the existing communication infrastructures.

Summary of Invention It is an object of the present system to overcome disadvantages and/or make improvement over the prior art.

To that extend, the invention proposes a method for collaboratively watermarking a media content available on a media server and produced by users in a first list of users, said media content comprising a first watermark embedded therein identifying the users of said first list of users, said method comprising, for an additional user that does not belong to the first list of users and modifies the media content, the acts of: - obtaining the modified media content,

- embedding by the media server into the modified media content a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user. The present invention allows protecting the copyright of a media content shared by a plurality of users on an entity (e.g. a server) of a social network. Said media content is first embedded with a watermark corresponding to the user who first creates/initiates and/or uploads it into the social network. A watermark corresponding to the users who have modified the media content is further embedded each time a user edit and modify the media content in the social network. This allows protecting, for the shared media content, the copyright for all the users who have contributed to the media content. The users who have been cooperatively involved in the media content edition may then share the copyright of the media content. Indeed, in the partial watermarking of the prior art, the users had to watermark themselves the media content each with a partial watermark then the media content had to be combined. With the present invention, users do not need to perform any action each time they modify the content. The watermarking may be performed for example by a social network edition software or a specific application in the social network: for example, copyright related operations may be performed by an on-line application provided by a social network manager, while the users' experience is not affected. Moreover, there is no media content combination and the quality of the media content is preserved.

Another advantage of the method according to the invention is that all the media content in the social network may be watermarked.

Furthermore, the watermark in a given media content may reveal for example who uploaded or edited the content.

The invention also proposes a media server according to claim 6.

The invention also proposes a system according to claim 10.

The invention also proposes a readable computer program according to claim 11.

Brief Description of the Drawings

Embodiments of the present invention will now be described solely by way of example and only with reference to the accompanying drawings, where like parts are provided with corresponding reference numerals, and in which:

Figure 1 schematically illustrates the context according to an exemplary embodiment of the present invention;

Figure 2 schematically illustrates the method according to an exemplary embodiment of the present invention;

Figure 3 schematically illustrates a flowchart according to an exemplary embodiment of the present invention;

Figure 4 schematically illustrates a watermark generation according to an exemplary embodiment of the present invention; Figure 5 schematically illustrates a watermark database according to an exemplary embodiment of the present invention; Figure 6 schematically illustrates a watermark sequence generator according to an exemplary embodiment of the present invention;

Figure 7 schematically illustrates a watermark embedding during uploading or creation of a media content according to an exemplary embodiment of the present invention;

Figure 8 schematically illustrates a watermark embedding during media content modification according to an exemplary embodiment of the present invention; Figure 9 schematically illustrates a watermark identification according to an exemplary embodiment of the present invention;

Figure 10 schematically illustrates a watermark generation according to an exemplary embodiment of the present invention.

Description of Preferred Embodiments

The following are descriptions of exemplary embodiments that when taken in conjunction with the drawings will demonstrate the above noted features and advantages, and introduce further ones. In the following description, for purposes of explanation rather than limitation, specific details are set forth such as architecture, interfaces, techniques, devices, etc., for illustration. However, it will be apparent to those of ordinary skill in the art that other embodiments that depart from these details would still be understood to be within the scope of the appended claims.

Moreover, for the purpose of clarity, detailed descriptions of well-known devices, systems, and methods are omitted so as not to obscure the description of the present system. Furthermore, routers, servers, nodes, gateways or other entities in a communication network are not detailed as their implementation is beyond the scope of the present system and method. Unless specified otherwise, the exemplary embodiment will be described hereafter in its application to a media server of a communication network. This is in no way limiting as the present teachings may be applied to other systems for collaboratively watermarking a media content using the method described in the present invention.

In addition, it should be expressly understood that the drawings are included for illustrative purposes and do not represent the scope of the present system.

Figures 1 describes an illustrative embodiment of the system according to the invention. A media content 100 may be edited by a plurality of users user 1, user 2... user n. Users may belong to a list of users that are allowed to access said media content 100. For example, the media content 100 is shared by the plurality of users in a communication network. In such a configuration, the media content may either be stored for instance on a media server (e.g. managed by a social network application such as Youtube ™) or on the computer of a user. A user 1 firstly creates or initiates the media content 100 either on his computer or directly on a media server. The media content is then embedded with a first watermark Wl (or watermarked) that corresponds to said user 1. The watermarking operation may either be performed when user 1 uploads the media content 100 from his computer to the media server or also when the media content 100 is created directly on the media server. Consequently, when another user, user 2, accesses (or edits) and modifies the media content 100, a check is performed to detect an embedded watermark. Wl is detected and a new watermark corresponding to users 1 and 2 is further embedded. Each time a new user modifies the media content 100, a new watermark corresponding to all the users who have modified the media content 100 is further embedded into said media content 100.

In an illustrative embodiment according to the invention, a social network (e.g., Youtube ™, Tudou ™, etc..) provides the media server to share media content and facilities to create, produce, edit, modify said media content. In those services, two kinds of functionalities are highlighted: share the content (users may upload their media content and share it with other users) and share the producing facilities or abilities, wherein users may work together on a given media content. A user may perform operations such as user registration/login, media content uploading, shared media content edition and media content consuming. These operations are supported by the social network manager (in software forms). User registration/login records the user profile or access histories. Content uploading means to upload the content to the social network. Shared content edition means to edit the content in the social network. Generally, there are two types of edition, i.e., online edition or offline edition. The former one needs the online edition software supported by the social network manager, while the latter one may be performed using a general edition software. Content consuming means to play the content in the social network.

Figure 11 describes an illustrative embodiment of the system according to the invention.

For purposes of simplifying a description of the present system, the terms "operatively coupled", "coupled", and formatives thereof as utilized herein refer to a connection between devices and/or portions thereof that enables operation in accordance with the present system. For example, an operative coupling may include one or more of a wired connection and/or a wireless connection between two or more devices that enables a one and/or two-way communication path between the devices and/or portions thereof. For example, an operative coupling may include a wired and/or wireless coupling to enable communication between a social network server and one or more user devices. A further operative coupling, in accordance with the present system may include one or more couplings between two or more user devices, such as via a telecommunication network in accordance with an embodiment of the present system.

A media server 1100 of a communication network 1110, for instance a social network, may be accessed by a plurality of users i, j, k etc... A social network manager 1120 (typically a software application) is installed on or operatively coupled to said media server 1100 to manage users and media contents of the social network 1105. Media contents are stored in a Media content database 1150 or uploaded by users. A watermark unit 1130 is used to watermark media content according to the method of the present invention. A watermark database 1140 allows storing watermarks corresponding to users that create, upload, access or modify a media content. One may note that illustrating the entities 1100 to 1150 as separate parts is in no way limiting as these parts could be hosted by the same node or server, for instance media server 1100, by a same telecommunication device, or being operatively linked to each other. One may note that illustrating the media server 1100 as comprising the social network manager 1120 comprising itself three parts is in no way limiting as all these parts or entities could be hosted by the same node or server, or being operatively linked to each other. The multi-parts presentation helps illustrating the different tasks performed by the media server 1100 and the social network manager 1120 in the present system. Figure 12 describes an illustrative embodiment of the method according to the invention.

The method according to the invention allows collaboratively watermarking a media content available on a media server and produced by users in a first list of users, said media content comprising a first watermark embedded therein identifying the users of said first list of users, said method comprising, for an additional user that does not belong to the first list of users and modifies the media content, the acts of: - obtaining the modified media content (act 1210),

- embedding by the media server into the modified media content a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user (act 1220).

Figure 2 describes an illustrative embodiment of the method according to the invention in reference to Figure 11.

Here the first list of users is limited to user i, user j is an additional user distinct from user i and, when user j obtains the media content, it comprises a watermark embedded therein that corresponds to the first list of users, namely to user i.

A media content 100 may be first created or initiated by a user i. The creation may be performed for instance:

- on the user's computer then uploaded through the communication network 1110 onto the social network manager

1120 of the media server 1100 or,

- remotely through the communication network 1110 directly on the social network manager of the media server 1100 wherein an specific application allows such a media content creation. When the media content creation is completed on the social network manager or when it is uploaded onto the social manager, a watermark Wi corresponding to user i is embedded into said media content. Said watermark Wi may be created by the watermark unit 1130 before the media content creation or upload is completed and then stored in the watermark database 1140 or may be created at the time of embedding by the watermark unit 1130 and also stored in the watermark database 1140.

The media content may be optionally stored in the media content database 1150.

When another user j wishes to access and modified the media content (providing he is authorized to do so) in an act 210, the media content database 1150 is accessed (for example, by the social network manager 1120) and the media content comprising watermark Wi corresponding to user i is retrieved. A watermark Wj corresponding to user j may be created or may have been previously created and stored in watermark database 1140. User j may then modify the media content (for instance using a specific edition application provided by the social network manager 1120).

In an additional embodiment of the present invention, user j may download the media content before modifying it then upload it when modifications are done onto the social network manager 1120 for further watermarking and storage according to the present invention.

When modifications or upload are completed, the media content is embedded with a watermark Wi+j corresponding to users i and j so as to further identify the users who have accessed and modified the media content.

Before embedding watermark Wi+j, watermark i may be removed or left into the media content.

Watermark Wi+j allows identifying users i and j using for instance a table stored on the watermark database 1140 and linking users with their corresponding watermark (described here under in reference to Figure 5). Watermark Wl+j may be reversible, i.e. created from watermark Wi and watermark Wj allowing identifying users i and j or may be created independently from watermarks Wi and Wj. In the latter case, a correspondence table allows further identifying users i and j from the watermark WiJ.

If user j, further create a new media content, it will be embedded with watermark Wj or a new watermark W'j (for instance if watermarks depend on time).

Users may belong to an authorized list that allows only said users to access and modified a given media content. There may be an optional registration step that allows registering said users to the authorized list and optionally create their corresponding watermarks by the watermark unit 1130 and store them in the watermark database 1140.

Figure 3 describes a flowchart according to an exemplary embodiment of the method according to the invention, wherein a user i 301 and a user j 303 share a media content via a social network manager 302. In a preliminary act of user registration 310:

- user i 301 proceeds to user registration 311 to access the social network manager 302,

- watermark initialisation 312 is performed: watermark i corresponding to user i 301 is initialized or created (then may be stored in a database),

- user j 303 proceeds to user registration 315 to access the social network manager 302,

- watermark initialisation 316 is performed: watermark j corresponding to user j 303 is initialized or created (then may be stored in a database). User registration for user i and user j may be performed independently from each other, or registration of one user may result for instance from an invitation from the other user as known in social networks. In an additional embodiment of the method according to the invention, a given watermark corresponding to a given user may be initialized, initiated or created independently from the user registration step.

User i may further upload a media content C 321 in act 320 of content uploading. Said media content C 321 is then embedded, on the social network manager 302, with the watermark i in act 325, the resulting media content being C. Then, user j, in an act 330 of shared content edition, accesses or edits and then modifies the shared content C in act 332 to generate a new media content which is further embedded (or updated) in act 334 with a watermark (i+j)

335 corresponding to users i and j.

Watermark (i+j) corresponds to both users i and j and indicates that both users have previously modified the media content. Watermark (i+j) may either be a combination of watermarks i and j or a brand new watermark, as long as it further allows identifying that users i and j (i.e. the set of users {i,j}) have modified (in other words, brought a contribution to) the media content so that they can share copyrights. Consequently, watermarks identifying each user and each set of users should be stored for further identification of users who have contributed to the generation of the media content and thus share copyrights.

In social networks, several users may work on one media content cooperatively. According to the present invention, the media content may be edited and modified by different users, wherein each of them shares the copyright of the media content. Furthermore, in an additional embodiment of the present method, if an additional user has previously modified a media content and modifies said media content again, a new watermark may be embedded or not. In a further embodiment of the present invention, the present method may also be applied to a media content which is only accessed or edited, but not necessarily modified.

In an additional embodiment of the present invention, in reference to Figure 3, the watermark i 325 may either be removed or left into the media content C when the watermark (i+j) 335 is embedded.

Figure 4 describes an illustrative embodiment of watermark generation according to the invention. In an optional act 400, a user i sends a registration request to the social network manager 302. In an optional act 410, the social network manager 302 checks the requested account availability. For example, whether the user's profile is filled out completely or whether the user name is repeated. In act 420, the social network manager 302 generates a watermark sequence, for instance W=W₀W_{1 ^}w_{11 1} (w =0 or 1 , j=O, l ,...,n-l , n>0), for user i. The watermark sequence may a n- length random sequence (composed of ¹O' or T). The sequence may be generated by a watermark sequence generator, an example of which is further described here under in reference to Figure 6.

In act 430, the social network manager 302 writes the watermark sequence into a Watermark Database 440. The Watermark Database 440 may be updated for example when a new user registers or a user modifies other users' content. Figure 5 describes an illustrative embodiment of watermark database 1 140 according to the invention.

The record or table of Watermark Database allows associating a watermark to the corresponding given user or given set of users and may comprise (but is not limited to) three fields:

- the watermark order 500,

- the watermark sequence 510,

- the corresponding user or set of users 520.

The watermark order 500 denotes the rank of the watermarks in the list of watermarks in the database. The watermark sequence 510 (e.g., W ) is the n-length sequence generated in act 420 in reference to Figure 4. The corresponding user or set of users 520 denotes the user(s) who cooperatively edited and modified (and may also comprise creation of) the content. For example, in reference to Figure 5, W₁ corresponds to the content uploaded by User 1 , and W₅ corresponds to the content edited by three users, namely User 0, User 1 and User 2. "User 0 & User 1" denotes the set of users composed of User 0 and User 1 (wherein both User 0 and User 1 have either created or modified the same content). More generally, the first watermark embedded into a media content produced by users in a first list of users allows, for example through the table of the watermark database 1140, identifying the users of said first list of users. Then, for an additional user that does not belong to the first list of users and modifies the media content, a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user is embedded by the media server into the modified media. Said second watermark may be stored and retrieved from the watermark database 1 140 prior to embedding or created for embedding then stored in the watermark database 1140 for further identification of the corresponding users of the second list of users.

Figure 6 describes an illustrative embodiment of watermark sequence generator according to the invention. This allows generating a watermark sequence 510 (e.g., Wi) according to act 420 in reference to Figures 4 and 5.

The illustrative embodiment described in Figure 6 is an example of watermark sequence generation, but does not limit the scope of the present invention as any suitable watermark sequence (or more generally watermark) generation scheme may be used.

In an act 610, a floating data K₀ ranging in [0, 1] for involved users i, j etc... 600 (i.e. users who have modified the media content) is computed. For example, if only User i creates/generates or uploads the media content, while no other

user is involved, then K₀ may be computed by: o ^₇- .

N is the total number of users to be supported by the system. Similarly, if two users are involved, i.e., User i and User j,

(i + j)moάN then K₀ may be computed by: ^Ao ^

Here, mod is the operation to get the residue. Similarly, if more than two users are involved, the same method may be used. For example, if there are three users, i.e., User i, User j and User k, then K₀ may be computed by:

(i + j + k)modN

^K° ⁼ N ^■

The current time 620 is used in act 630 to compute a floating data K₁, ranging in [0, 1]. For example, if the time is Month/Day/Hour/Minute/Second, then K₁ may be computed by: Month Day Hour Minute Second l^~ 12 ^T 24 60 60 ^■ wherein Month (0-11), Day (0-30), Hour (0-23), Minute (0-59) and Second (0-59).

Time is used for security. Firstly, it makes the watermark generation process depends on time when the content is operated and as time is random, the watermark is random and thus difficult to reconstruct or imitate. Secondly, it avoids the possibility of generating the same watermarks as the scheme benefits from time varying. In act 640, random sequence R=^₁T₁, ..., r_{n l} ( 0 < r, < 1 ) is generated using K₀ and K₁ according to: r_{o 1 n}

^Vι

"^""ⁿ

wherein f may be a polynomial function with f(x) ≤ 1 for x e

[0, 1] such as for example: /

^~ X) .

And f() means to iterate f() function for t (t>=50) times. In act 650, the watermark sequence W=wθwl...wn-1 (wj=0 or 1 , j=0, l,...,n-l, n>0) 660 may be computed according to:

Figure 7 describes an illustrative embodiment of watermark embedding during media content uploading according to the invention.

The social network manager of the media server embeds the user's watermark into the media content when the user uploads the content. Of course, the content should not already exist in the social network. If the media content is initiated or created on the media server and though not uploaded, the watermark sequence may also be embedded by the social network manager on the media server.

In act 700, a User i logs onto the media server of the social network. In act 720, the social network manager checks the content (C) to be uploaded 710 to see whether it already exists in the social network or whether it is legal. Act 720 may be performed either manually (person based) or automatically (computer based). If the media content exists or is illegal, the social network manager may refuse to put the content in the social network.

In act 740, the social network manager gets the watermark sequence corresponding to User i from the Watermark Database 730. This is done for instance by searching the database with keyword "i" in "Corresponding user share list". The watermark database stores watermarks that are embedded in each media content shared in the social network.

In act 750, the social network manager embeds the watermark sequence into the media content and thus produces a new content C. For watermark embedding, various existing watermark embedding methods may be used. After that, the watermarked media content is placed on the social network for sharing.

Figure 8 describes an illustrative embodiment of watermark embedding during media content modification according to the invention.

A User j logs onto the social network and edits the content C produced by User i, or uploaded by User i. In this example, User j uses the online software provided by social network manager to modify the content.

In act 820, the watermark Wd=wdθwdl ...wdn- 1 (wdj=0 or 1 , j=O, l ,...,n-l) embedded in content C 760 is detected. In act 830, the media content owners (e.g., the user who uploaded the media content, or the users form the User List who edit the content cooperatively) is/are identified using Watermark Database 730. Act 820 User List Identification may comprise several sub-steps as described in Figure 8 here under.

When the user(s) corresponding to watermark Wd have been identified, the watermark corresponding to users associated with Wd and user j is generated into the media content in act 850.

During content edition 860, said watermark is embedded into the media content in act 870 to produce a new content C" 880.

Figure 9 describes an illustrative embodiment of watermark identification according to the invention. Each watermark Wi stored in the Watermark Database 730 is read (act 910) and compared (act 920) with the detected watermark Wd 900, for i-th (i=0 to N-I , wherein N is the number of watermarks stored in the Watermark Database 730).

In act 920, the difference between Wd and Wi, Di=d(Wd, Wi)

may be computed as follows: ι ^{~ ~}Z-ι Φ υ

Here, |x| is the absolute value of x. And, T ( l>T>0) is the threshold to determine whether the detected watermark sequence exists in the Watermark Database or not. Generally, T=O.05 may be preferred. When Di is greater than T, then WD corresponds to Wi. The

User List 950 comprising users who have created and/or modified the media content is retrieved from Watermark Database in act 940.

Figure 10 describes an illustrative embodiment of watermark generation according to the invention. When the users corresponding to the embedded watermark of a content C have been identified through User List 950, a watermark corresponding to the users of the User List and to user j is searched in the Watermark Database 730 in act 1020 or generated in act 1040.

For example, if the detected User List is "Ud=User 0 & User 1", and the User j (j=2), then the new User List is "Un=User 0 & User 1 & User 2". If the detected User List is "Ud=User 0 & User 1", and the User j (j=l), then the new User List is "Un=User 0 & User 1 ".

Act 1020 allows finding whether the new User List exists in the Watermark Database or not. This is done by searching the new User List in the field of "Corresponding user share list".

Act 1040 allows generating a new watermark for the new User List. The watermark sequence generator is same to the one used in watermark initialization for user registration. The generated watermark is unique in order to avoid conflictions.

Act 1050 allows storing the generated new watermark sequence into Watermark Database. The watermark sequence corresponds to the new User List.

Act 1060 allows outputting the watermark sequence corresponding to the new User List.

Claims

1. A method for collaboratively watermarking a media content available on a media server and produced by users in a first list of users, said media content comprising a first watermark embedded therein identifying the users of said first list of users, said method comprising, for an additional user that does not belong to the first list of users and modifies the media content, the acts of:

- obtaining the modified media content, - embedding by the media server into the modified media content a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user.

2. A method according to claim 1, said method comprising a preliminary act of embedding into the media content an initial watermark that identifies a first user that has initiated the media content creation.

3. A method according to one of the previous claims, wherein the additional user is identified by a third watermark, the second watermark being a reversible function of the first and third watermarks.

4. A method according to any of the preceding claims, said method comprising, after the act of obtaining the modified media content, a preliminary act of withdrawing a previously embedded watermark.

5. A method according to any of the preceding claims, wherein the users from the second list of users are users authorized to modify the media content.

6. A media server for collaboratively watermarking a media content available on a media server and produced by users in a first list of users, said media content comprising a first watermark embedded therein identifying the users of said first list of users, said media server being operable, for an additional user that does not belong to the first list of users and modifies the media content, to:

- obtain the modified media content,

- embed into the modified media content a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user.

7. A media server according to claim 6, said media server being further operable to embed into the media content an initial watermark that corresponds to a first user that has initiated the media content creation.

8. A media server according to one of the previous claims 6 and

7, wherein the additional user is identified by a third watermark, the second watermark being a reversible function of the first and third watermarks.

9. A media server according to any of the preceding claims 6 to

8, said media server being further operable to withdraw a previously embedded watermark prior.

10. A system for collaboratively watermarking a media content available on a media server and produced by users in a first list of users, said media content comprising a first watermark embedded therein identifying the users of said first list of users, said system comprising: - a first user device from an additional user that does not belong to the first list of users, said first user device being operable to modify the media content:;

- a media server operable to: - obtain the modified media content,

- embed by the media server into the modified media content a second watermark identifying a second list of users, said second list of users comprising the first list of users and the additional user.

11. A computer program providing computer executable instructions stored on a computer readable medium, which when loaded on to a data processor causes the data processor to perform a method for collaboratively watermarking a media content according to claims 1 to 5.