WO2006077546A2

WO2006077546A2 - Registration phase

Info

Publication number: WO2006077546A2
Application number: PCT/IB2006/050200
Authority: WO
Inventors: Wytse H. Van Der Velde; Milan Petkovic; Claudine V. Conrado; Minne Van Der Veen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-01-24
Filing date: 2006-01-19
Publication date: 2006-07-27
Also published as: TW200704106A; JP2008529340A; WO2006077546A3; CN101107609A

Abstract

The present invention relates to a method, a device and a system for preventing unauthorized introduction of content items in a network containing compliant devices. A basic idea of the present invention is to provide a CA (206) with a fingerprint of a content item to be introduced in a network at which the CA is arranged. Further, the CA is provided with an identifier of a content introducer (201), which introduces the particular content item in the network. The CA compares the fingerprint to a predetermined set of fingerprints, and content item introduction is allowed if the content itemfingerprint cannot be found among the fingerprints comprised in the set. On introduction of the content item, the CA generates a watermark identifier, a watermarking key and a signed content ID certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer. Then, the watermark identifier is inserted in the content item by means of the watermarking key.

Description

Registration phase

The present invention relates to a method, a device and a system for preventing unauthorized introduction of content items in a network containing compliant devices.

In prior art DRM systems, content rights are associated with content items, such as audio files, movies, electronic books etc. Content rights typically contains rules (e.g. play, copy, distribute etc.) and necessary cryptographic keys for encrypting/decrypting the content item(s) with which they are associated. Content rights should only be transferred to devices that are compliant and operated by users that have appropriate user rights, i.e. rights specifying who can use the content rights. Note that a content right and a user right may be merged in one single license, as is known from Open Mobile Alliance (OMA) DRM. Compliant devices comply with a given standard and adhere to certain operation rules. They also communicate by means of a certain protocol such that they answer questions and requests, which are posed to them, in the expected way. Compliant devices are considered to be trusted, which e.g. means that they will not illegally output content on a digital interface and that ownership of a device is not important. Device compliancy management, i.e. compliant device identification, renewability of devices, and revocation of devices, can be effected by using known techniques. In general, content providers do not want to authorize users to create their own content rights, due to the risk of uncontrollable distribution of commercial content items. Consequently, the content provider digitally signs the content rights before they are distributed. Further, it must be enforced that the compliant devices check the signatures of the content rights and refuse content rights which are not properly signed by the content provider. Typically, the devices are comprised in a network or domain.

The above described approach is suitable for DRM systems in which only content provider(s) distribute content rights. However, if users wish to introduce personal content items, such as e.g. photos or home videos, they have to involve the content provider to create content rights for the personal content items. This is undesired, since the content provider should not be able to control personal content. In DRM systems in which commercial content items as well as personal content items are distributed, a compliant device may be authorized to create a content right for a specific personal content item. This content right may be signed by the compliant device, and if it is not signed, any compliant device shall reject the content right. This has the effect that personal content only can enter the network of devices via a compliant device. In environments with stricter security requirements, the content rights may be signed by a trusted third party authority, i.e. a party which is trusted by concerned communicating parties.

A problem to be solved in prior art DRM systems, in which commercial content items as well as personal content items are introduced, is that they are susceptible to attacks involving substitution of content item identifiers. A content item identifier uniquely identifies a corresponding content item in the system.

In DRM systems in which commercial content items as well as personal content items are distributed, any user is authorized to create a content right for a specific personal content item, which content right may be signed by a compliant device as mentioned above or by the user himself, and hence the user effectively becomes a content provider in his own right. Any user may also acquire commercial content items from a content provider and introduce them in the system. A malicious user may substitute a specific personal content item for a commercial content item following the creation of the content right associated with the specific personal content item. This will involve hacking of the compliant device to obtain a key to decrypt the commercial content item, such that the commercial content item comes in the clear. The malicious user then has to re-encrypt the commercial content, which has been obtained in an unauthorized manner, with a content key that is present in the content right that is associated with the specific personal content. Thereafter, the re-encrypted commercial content item is associated with the content identifier of the specific personal content item. The malicious user may then use this commercial content item with the same rights as his own personal content item. As a highly undesired consequence, a great number of commercial content items may be introduced and distributed in the network, if it is encrypted with the leaked content key. Hence, to avoid this attack, a secure link between a content item and a corresponding content item identifier is required. This has been solved by employing fingerprints of content. These fingerprints are used to uniquely identify the content to which they refer. A fingerprint of a content item is a representation of the information signal in question, which does not change when the content item is modified slightly. Such fingerprints are sometimes also known as "(robust) hashes". The term robust hashes refers to a hash function which, to a certain extent, is robust with respect to data processing and signal degradation, e.g. due to compression/decompression, coding, AD/DA conversion, etc. Robust hashes are sometimes also referred to as robust summaries, robust signatures, or perceptual hashes.

A known method of generating fingerprints is described in detail in WO 02/065782, which belongs to the applicant of the present patent application. However, a skilled person realizes that there exist many other methods of generating fingerprints. The compliant device adds fingerprint information to the content right before signing it. When a content right is used, the compliant device must check whether the fingerprint information that is included in the content right also can be found in the actual content item. If the fingerprint information cannot be found in the actual content item, the content right must be rejected. Moreover, digital watermarks associated with authorized content providers are employed to enhance security. A watermark is applied by an authorized content provider to a particular content item, and a compliant device should check that there is no watermark present in the content item for which a user wishes to create a content right. Watermarked content is originally coming from an "official" (i.e. authorized) and commercial content provider and therefore users are not allowed to create their own content rights for such content. In case the content item is watermarked, a compliant device will only access the content if it has the appropriate content rights signed by the official content provider (of which the public key is known). If no watermark is detected, the content is classified as 'personal content' and the accompanying content rights may be signed by any compliant device.

However, a problem that remains in the approach of employing fingerprints is that it does not prevent a user from unauthorized introduction and distribution of commercial content in the network. As can be seen from the above, in DRM systems in which commercial content items as well as personal content items are introduced and distributed, any user can create content rights for any content item.

Further, the use of watermarks for identifying commercial content items makes DRM systems vulnerable to watermark attacks. Since digital watermarks must be detected by the compliant devices to identify commercial content items, the security of the system relies on the secrecy of a watermark detection key. If this key is leaked, the security is breached. Moreover, by using watermarking, older content items to which watermarks are not applied are not recognized by the system as commercial content, and can still be illegally distributed. Hence, there are problems in terms of backwards compatibility. Also, in prior art systems employing watermarking for identification of commercial content, personal content items are not watermarked and are therefore untraceable when obtained in clear text and outside the protected area of the DRM system.

An object of the present invention is to provide a solution for preventing unauthorized distribution of commercial content that addresses the above given problems, in particular problems concerning security and backwards compatibility. This object is attained by a method of preventing unauthorized distribution introduction of content items in a network containing compliant devices in accordance with claim 1, a device for preventing unauthorized introduction of content items in a network containing compliant devices in accordance with claim 9 and a system for preventing unauthorized introduction of content items in a network containing compliant devices in accordance with claim 13.

According to a first aspect of the present invention, there is provided a method comprising the steps of providing a fingerprint of a content item to be introduced in the network and an identifier of a content introducer introducing said content item in the network, comparing said fingerprint to a predetermined set of fingerprints, wherein introduction of the content item is allowed if said fingerprint cannot be found in said set of fingerprints, and generating a watermark identifier, a watermarking key and a signed content identifier certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer. Further, the method comprises the step of inserting the watermark identifier in the content item by means of the watermarking key.

According to a second aspect of the present invention, there is provided a device comprising means for receiving a fingerprint of a content item to be introduced in the network and an identifier of a content introducer introducing said content item in the network, means for comparing said fingerprint to a predetermined set of fingerprints, wherein introduction of the content item is allowed if said fingerprint cannot be found in said set of fingerprints; and means for generating a watermark identifier, a watermarking key and a signed content identifier certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer. According to a third aspect of the present invention, there is provided a system for preventing unauthorized introduction of content items in a network containing compliant devices, which system comprises at least one compliant device and a certificate authority. The compliant device is arranged with means for determining a fingerprint of a content item to be introduced in the network and an identifier of a content introducer introducing said content item in the network, and means for sending the fingerprint and the identifier to the certificate authority. The certificate authority is arranged with means for comparing said fingerprint to a predetermined set of fingerprints, wherein introduction of the content item is allowed if said fingerprint cannot be found in said set of fingerprints, means for generating a watermark identifier, a watermarking key and a signed content identifier certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer and further with means for distributing the watermark identifier, the watermarking key and the signed content identifier certificate to the compliant device. Moreover, the compliant device further comprises means for inserting the watermark identifier in the content item by means of the watermarking key.

A basic idea of the present invention is to provide an authorized certificate authority (CA) with a fingerprint of a content item to be introduced in a network in or on behalf of which the CA is arranged. Further, the CA is provided with an identifier of a content introducer, which introduces the particular content item in the network. The content introducer may comprise a user, i.e. an individual, but it may also comprise a content provider in more traditional sense, e.g. Warner Brothers. The CA compares the fingerprint to a predetermined set of fingerprints, and introduction of the content item in the network is allowed if the fingerprint of the content itemcannot be found among the fingerprints comprised in the set. On condition that the content item may be introduced in the network, the CA generates a watermark identifier, a watermarking key and a signed content identifier (ID) certificate comprising at least said fingerprint and a unique content identifier for the content item, and the identifier of the content introducer. Then, the watermark identifier is inserted in the content item by means of the watermarking key. In prior art DRM systems, in which commercial content items as well as personal content items are introduced and distributed, any user is authorized to create a content right for a specific personal content item and hence effectively becomes a content provider in his own right. Since compliant devices do not have access to information regarding ownership of a content item, any user can create content rights for any content item. According to the present invention, the signed content identifier (ID) certificate comprising the fingerprint and the unique content identifier for the particular content item with which it is associated is introduced in the network of compliant devices. Note that the CA may be a trusted third party, or alternatively a trusted compliant device to which the authority to sign certificates has been distributed by the trusted third party. Further, it should be realized that even though the fingerprint of the content item as well as the unique content identifier for the content item both are unique, the fingerprint is actually calculated from the content item, whereas the unique content identifier typically is selected by the CA and assigned to the content item. The unique content identifier is linked to the fingerprint by the content ID certificate, which makes it a valid content identifier. The unique content identifier may e.g. be a numeral, or a string of characters representing the title and/or the artist associated with the particular content item.

The signing of the content ID certificate is effected in order to prevent malicious users from tampering with it. Whenever a user wants to use a content right to access a corresponding content item, the compliant device on which the content item is to be rendered verifies correctness of the signature of the content ID certificate and compares the fingerprint of the content item with the fingerprint that is comprised in the content ID certificate. In the prior art, the content right can be used to access the content item if there is a match. As previously mentioned, content ID substitution attacks may be prevented by creating a secure link between a content item and a unique content item identifier. However, this does not hinder unauthorized introduction and distribution of content items in the network. If a malicious user has obtained cryptographically protected, i.e. encrypted, commercial content via the DRM system, he may hack the compliant device which handles the content, in order to procure a secret decryption key to create a clear text copy of the commercial content. Hence, the malicious user can create a new content right for the commercial content. To overcome this problem, the present invention links a user (i.e. a content provider) and a content item.

This is accomplished by including, in the content ID certificate, an identifier, e.g. a public key, of the user/content provider who introduced the content item in the network, although other identifiers are possible. The user/content provider who introduced the content item in the network is occasionally referred to herein as a "content introducer". When a user is to create a content right for a particular content item, the compliant device which is employed will check that the user's identifier is present in the content ID certificate signed be the CA. If the user's identifier is present in the content ID certificate, the user is deemed authorized to create content rights for the particular content item. This is a result of the fact that if the user's identifier is included in the signed certificate, it must have been this particular user that introduced the content item in the network, which introduction has been authorized by the CA. On the contrary, if the content ID certificate does not comprise the user's identifier, the user is not authorized to create content rights for the particular content item. Hence, unauthorized introduction and distribution of content in the network is prevented.

Further, in the prior art DRM systems previously described, every commercial content item is watermarked. Any compliant device will verify that personal content which is to be introduced is not watermarked. If a user makes an unauthorized attempt to introduce watermarked content, the compliant device will not allow it to be introduced. However, watermarking of commercial content causes problems concerning security and backwards compatibility. According to the present invention, each personal content item introduced in the network must be registered in a registration phase. During the registration phase, which has been described hereinabove, a fingerprint for the content item to be introduced is provided to the CA, in order to identify the content item as a possible commercial content item. Hence, in the present invention, watermarking is not employed for identification.

The watermark identifier employed in the present invention enables tracing of the content introducer, i.e. the user/content provider that originally introduced the content item, in case a personal content item is misused. Further, in case a malicious user tampers with the fingerprint of a content item and introduces a commercial content item as a personal content item, the watermark facilitates identification of the unauthorized distributor (i.e. the malicious user).

In an embodiment of the present invention, the content introducer communicates directly with the CA, wherein the step of providing the fingerprint of a content item to be introduced in the network and the identifier of the content introducer includes determining, from the content item, the fingerprint of the content item, as well as the identifier of said content introducer, at the CA.

In another embodiment of the present invention, the content introducer communicates with the CA via his/her compliant device, wherein the step of providing the fingerprint of a content item to be introduced in the network and the identifier of the content introducer is effectuated in two steps. First, the compliant device via which the content introducer introduces said content item in the network determines the fingerprint from the content item, as well as the identifier of the content introducer. The identifier is typically provided to the compliant device by means of inserting a smart card into the device, which smart card contains the content introducer's identifier, e.g. a public key. Alternatively, the compliant device is arranged with a key pad, via which the content introducers enters his/her identifier in the form of a personal code or string of characters. Then, the fingerprint of the content item and the identifier of the content introducer are sent to the CA for further processing.

In the case where the content introducer acts towards the CA via a compliant device, it is necessary that the CA distributes the watermark identifier, the watermarking key and the signed content identifier certificate to the compliant device via which the content introducer introduces said content item, after creation of these data, such that the compliant device may insert the watermark identifier in the content item.

According to yet another embodiment of the present invention, the CA stores a data post comprising the watermark identifier, the identifier of the content introducer and the fingerprint of the content item. It is also possible that the unique content identifier is included in the data post, if there is a desire to keep track of the content identifier as well.

As previously mentioned, the watermark identifier enables tracing of the content introducer, i.e. the user/content provider which originally introduced the content item, in case a personal content item is misused.

From the point of view of the CA, the watermark identifier is a pointer to a data post for each introduced content item in a database. The watermark identifier may be retrieved from the database, which is stored in an appropriate storage medium at the CA, by using the fingerprint of the content item. Subsequently, the content introducer may be found in the database. Further, as described hereinabove, in case a malicious user tampers with the fingerprint of a content item (or if the fingerprint cannot be found in the database, which means that the CA never has authorized the introduction of the content item) and introduces a commercial content item as a personal content item, the watermark facilitates identification of the unauthorized distributor (i.e. the malicious user). Moreover, the CA can use the stored fingerprint of the personal content item to protect that personal content item from being illegally reintroduced in the network. Due to restrictions in the amount of data that the CA can process, the CA may allow fingerprint identification for commercial content items only, allowing an option for identification of personal content items (and therefore extra protection for the personal content items). Possibly, users may have to pay an additional security fee for that feature. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

A detailed description of preferred embodiments of the present invention will be given in the following with reference made to the accompanying drawings, in which: Fig. 1 shows an authorization hierarchy in which the present invention is applied;

Fig. 2 shows a registration procedure used for content items to be introduced in the network, in accordance with an embodiment of the present invention; and

Fig. 3 shows an authentication procedure which is performed when a user wishes to access a content item, in accordance with an embodiment of the present invention.

Fig. 1 shows an authorization hierarchy 100 implementing the present invention. Continuous lines indicate authorization steps, which involve the use of public key certificates. These certificates are well known in the art and are hence not shown in Fig. 1. Dotted lines indicate issuing of certificates and/or rights.

A System Authority (SA) 101 is at the top of the hierarchy. All compliant devices has access to the public key of the SA. Typically, the SA public key is built-in into the hardware of each compliant device 102. With this public key, a compliant device can verify any certificate that has been issued by the SA 101. At the next level in the hierarchy, a Certificate Authority (CA) 103, a Device Authority (DA) 104 and a User ID Authority

(UIDA) 105 are arranged. The CA 103 authorizes content providers 109 within the system. For example, EMI and Disney may constitute content providers within the network, but as previously mentioned, a compliant device or a user may also represent a content provider. In fact, in DRM systems in which commercial content items as well as personal content items are distributed, any user is authorized to create (via a compliant device) a content right for a specific personal content item and hence effectively becomes a content provider in his own right. Consequently, in a DRM system in which the present invention is applied, a large number of content providers exist, since the term "content provider" in this context includes both individual users and traditional content providers such as record and motion-picture companies and content distributors.

The CA 103 issues content ID certificates 106 and provides these to the content providers 109. The CA 103 may be a trusted third party or may alternatively be a compliant device. This is primarily a question of flexibility; if a compliant device is authorized to act as CA, it brings flexibility to the system. On the contrary, a third party provider may not want to "distribute" the right to issue content ID certificates to a compliant device for security reasons. The content ID certificate 106 has been described in detail hereinabove and comprises: (a) the unique content ID and

(b) the fingerprint for a content item introduced in the network, as well as

(c) the identifier, e.g. the public key, of a user having introduced the content item in the network and

(d) a signature of the CA. Note that it is possible that, in case the CA is a trusted third party, the content

ID certificate is created at a content provider in the form of a compliant device, but signed at the CA.

A content provider 109 within the network is authorized to issue content rights 111 for a content item, if the content provider has been provided with a valid content ID certificate 106. Each content right contains the content ID and content key(s) that enable access to cryptographically protected content items with which the content right is associated (which association is made effective by means of the content ID in the content right, since it is compared to the content ID attached to the encrypted content item). The content right 111 also specifies a valid User Right Authority (URA) 112 for a particular content item, in that the content right 111 contains the public key of the URA 112. Hence, the content provider 109 may delegate issuance of user rights 113 to another party, namely the URA 112. This makes the system flexible, because it can support different usage models, including content distributed by a content provider, personal content (when a user/compliant device acts as content provider) and content imported from another DRM system. The content provider 109 who issues the content right 111 also signs it. In practice, the content provider itself is authorized to be URA, and hence issues the content rights 111 and the user rights 113. In fact, the content right and the user right for a particular content item may be combined into one single right. The URA 112 issues a user right 113 for a certain content item. A user right indicates whether a user is allowed to use a content right to access a content item. The user right comprises a content ID, which is the link between the user right, the content right and the content item. As described hereinabove, these three components all comprise a content ID. The user right further comprises a rights expression that indicates how a user, which user is designated by means of a user ID in the form of a public key included in the user right, may use the content item. Finally, the user right is signed by the URA.

In terms of security aspects involved in handling different types of rights, there is a distinction between user rights 113 and content rights 111. User rights may be freely distributed, because they do not contain any secrets, and the signature prevents modifications. Content rights, on the other hand, contains cryptographic keys for accessing content items. Hence, content rights may only be transferred to compliant devices. Further, the transfer of content rights between devices requires secure communication means, which may be based on secure authenticated channels. Consequently, the content right 111 requires both confidentiality and integrity, whereas the user right 113 requires only integrity.

User and device management involves personalization and certification of users and devices, which are then introduced into the system and declared compliant (to certain required properties, as has previously been described). The Device Authority (DA) 104 is a trusted party that authorizes the Device ID Authorities (DIDA) 110 for several device manufacturers. Each device manufacturer (e.g. Philips, Sony) has its own DIDA 110 that gives devices a unique identity and an associated public key by means of a signed device ID certificate 107, hence indicating compliance.

The User ID Authority (UIDA) 105 is responsible for issuing user ID devices (not shown in Fig. 1). This is typically performed during a manufacturing phase. The UIDA 105 associates a user ID device, which device typically comprises e.g. a tamper resistant smartcard or a SIM card, with a certain person by issuing a signed user ID certificate 108 containing the name, or any other identifier, of the user together with the public key of the user ID device. The private key that corresponds to this public key is considered to be the user's private key. However, the user is not given personal access to this private key. This prevents a user from distributing the private key to someone who thus could impersonate him. Therefore, the user's private key is securely stored on the user ID device, which is tamper resistant. The user ID device serves as a token, proving the user's presence. The user ID device should be easy to handle, robust, provide secure computing and hard to clone. Each authority illustrated in Fig. 1 typically comprise one or more microprocessors or some other device with computing capabilities, e.g. an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a complex programmable logic device (CPLD), etc., in order the create the various certificates and rights. In the creation of the certificates and rights, as well as in intercommunication between the different authorities, the microprocessors execute appropriate software that is downloaded to the respective authorities and stored in a suitable storage area, such as a RAM, a Flash memory or a hard disk. For intercommunication to be possible, the authorities are arranged with interlaces that enables the communication. Before a certificate can be used, it has to be validated. Certificate validation implies that at least the integrity (using the signature) and the authenticity (using the chain of certificates that links a certificate to the certificate of the authority, all the way up to the SA) of a certificate is checked.

With reference made to Fig. 2, which shows an embodiment of the present invention, when a new content item is to be registered in the network, a content introducer 201, in this case a user, presents (step 202) the content item to a compliant device 203. The compliant device determines the fingerprint of the content item. Moreover, by means of e.g. a smart card 204, the content introducer provides the compliant device with an identifier. The compliant device 203 forwards (step 205) the fingerprint of the content item and the identifier of the content introducer to a trusted certificate authority (CA) 206. The unique content identifier is typically selected by the CA and comprises e.g. a string of characters defining artist and title, or a reference number. The compliant device 203 and the CA 206 each comprises a microprocessor 217, 218 in order to perform cryptographic operations and other computing operations. The microprocessors 217, 218 execute appropriate software that is downloaded to the compliant device 203 and the CA 206 and stored in a corresponding RAM 219, 220. The CA is equipped with a database stored in a storage medium 207, such as a RAM or a hard disk. The CA searches (step 208) for the fingerprint of the content item to be introduced in the database of fingerprints of existing commercial content or registered personal content. Introduction of the content item in the network is allowed if the fingerprint of the content itemcannot be found among any of the fingerprintsstored in the storage medium 207.

If the content item may be introduced in the network, the CA 206 generates a watermark identifier, a watermarking key and a signed content ID certificate comprising the fingerprint and the unique content identifier of the content item, and the identifier of the content introducer. These data are sent (step 209) to the compliant device 203. The compliant device then inserts the watermark identifier in the content item by means of the watermarking key. Alternatively, the watermarking is performed by the CA 206. After watermarking the content item, the compliant device 203 can create (step 211) other appropriate DRM certificates, e.g. a content right 212 and a user right 213, and the content item will be ready for sharing. The compliant device also uses the received content ID certificate 214 and a user ID certificate 215 (as described in connection to Fig. 1). The CA stores (step 210) a data post comprising the watermark identifier, the identifier of the content introducer and the fingerprint (and possibly the unique content identifier) of the content item in a storage medium 216. Note that the storage medias 207, 216 may be merged into one single storage medium, and may further be incorporated in the CA 206.

Referring to Fig. 3, when a user 301, in the following referred to as Alice, wishes to access a content item, she will need the following: (a) a content ID certificate, (b) a content right,

(c) a user right, and

(d) a user ID certificate.

It is assumed that device compliancy has already been checked, which is why the above list does not comprise a device ID certificate. The content item has been loaded into a compliant device 302 in encrypted form. The compliant device may e.g. be a CD player, and the content item to be rendered on the device may be an audio CD. The compliant device 302 comprises a microprocessor 313 in order the create the various certificates and rights and to perform cryptographic operations and other computing operations described in the following. The microprocessor 313 executes appropriate software that is downloaded to the compliant device and stored in a RAM 314.

The compliant device 302 verifies (step 303) that the user ID certificate 304 is valid by checking the signature using the built-in public key in the compliant device. Alice 301 will also have to authenticate herself by proving she knows the secret key corresponding to the public key comprised in the user ID certificate. As previously mentioned, the user is not given personal access to this private key in order to prevent the user from distributing the private key, and thus prevent impersonation. Therefore, the user's private key is securely stored on a user ID device 305, e.g. a tamper resistant smartcard, which is inserted (step 306) into, and read by, the compliant device 302. Further, the compliant device verifies (step 307) the signature of the user right 308, to ensure that the user right is valid. To do this, the compliant device checks the User Right Authority (URA) field in the content right 309 and verifies that the specified URA signed the user right. The compliant device 302 verifies that Alice 301 can use the user right 308. This is done by comparing the user ID, i.e. a user public key, in the user right with the user ID on Alice's user ID certificate 304. The compliant device verifies (step 310) that the content provider was allowed to sign the content right. Thus, the device checks the signature of the content right 309 using the public key comprised in the content ID certificate 311. To do this, the compliant device must, by using its built-in public key, first verify (step 312) the content ID certificate by checking the signature of the content ID certificate provided by the CA (see Fig. 1). As described in the above, the content right 309 is created and signed by the actor who introduced the corresponding content item in the network. Hence, the public key comprised in the content ID certificate 311 is the public key of the user (i.e. content provider) having introduced the content item in the network, and this public key corresponds to the private key that was employed to sign the content right 309. Finally, the compliant device 302 will have to verify if the content right can be used to access the encrypted content. To this end, the device computes a fingerprint of the content item and compares it with the fingerprint in the content ID certificate 311. If there is a match, Alice 301 is allowed to access the content item on the compliant device 302. If any of the above steps fail, Alice will not be given access to the content. According to the present invention, a content ID certificate is introduced in the network of compliant devices. The signing of the certificate by the authorized certificate authority (CA) prevents malicious users from tampering with the content ID certificate. The fingerprint of the content item is included in the content ID certificate to hamper content ID substitution attacks. The problem related to unauthorized introduction and distribution of content items in the network by means of (unauthorized) creation of content rights is overcome by including, in the content ID certificate, the public key of the content introducer. When a user (or a third party content provider) is about to create a content right for a particular content item, the compliant device which is employed will check that the user's public key is present in the content ID certificate signed be the CA, as described hereinabove. If the user's public key is present in the content ID certificate, the user is deemed authorized to create content rights for the particular content item. Hence, unauthorized introduction and distribution of content in the network is prevented. Further, the watermark identifier introduced in the registration phase described above in accordance with the present invention enables tracing of the content introducer, i.e. the user/content provider which originally introduced the content item, in case a personal content item is misused. Further, in case a malicious user tampers with the fingerprint of a content item and introduces a commercial content item as a personal content item, the watermark facilitates identification of the malicious and unauthorized user. Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. For example, the content ID certificate could also comprise the public key of a compliant device via which a content item is introduced. This public key may be used to create content rights in accordance with format of licenses used in OMA DRM. The content ID certificate could additionally or alternatively comprise information concerning type of certificate. This may be specified in a rights field, e.g. right = ownership. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.

Claims

CLAIMS:

1. A method of preventing unauthorized introduction of content items in a network containing compliant devices (203), said method comprising the steps of: providing (202) a fingerprint of a content item to be introduced in the network and an identifier of a content introducer (201) introducing said content item in the network; - comparing (207) said fingerprint to a predetermined set of fingerprints, wherein introduction of the content item is allowed if said fingerprint cannot be found in said set of fingerprints; generating (209) a watermark identifier, a watermarking key and a signed content identifier certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer; and inserting (211) the watermark identifier in the content item by means of the watermarking key.

2. The method according to claim 1, wherein the step of providing (202) a fingerprint comprises the step of determining the fingerprint of the content item to be introduced in the network and the identifier of said content introducer (201).

3. The method according to claim 1, wherein the step of providing (202) a fingerprint comprises the step of receiving (205) the fingerprint of the content item to be introduced in the network and the identifier of said content introducer (201).

4. The method according to claim 3, wherein the received fingerprint of the content item and the identifier of said content introducer (201) are determined at a compliant device (203) via which the content introducer introduces said content item in the network.

5. The method according to claim 3, further comprising the step of distributing (209) the watermark identifier, the watermarking key and the signed content identifier certificate to the compliant device (203) via which the content introducer (201) introduces said content item.

6. The method according to claim 1, further comprising the step of storing (201) a data post comprising the watermark identifier, the identifier of said content introducer (201) and the fingerprint of the content item.

7. The method according to claim 6, wherein the data post further comprises a unique content identifier for the content item.

8. The method according to claim 1, wherein the identifier of the content introducer (201) comprises a public key of said content introducer.

9. A device (206) for preventing unauthorized introduction of content items in a network containing compliant devices (203), said device comprising: means (218) for receiving a fingerprint of a content item to be introduced in the network and an identifier of a content introducer (201) introducing said content item in the network; means (218) for comparing said fingerprint to a predetermined set of fingerprints, wherein introduction of the content item is allowed if said fingerprint cannot be found in said set of fingerprints; and means (218) for generating a watermark identifier, a watermarking key and a signed content identifier certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer.

10. The device (206) according to claim 9, further comprising means (218) for inserting the watermark identifier in the content item by means of the watermarking key.

11. The device (206) according to claim 9, further comprising means (218) for receiving the identifier of the content introducer (201) by means of reading a smart card (204) inserted into the device, which smartcard contains the identifier of the content introducer.

12. The device (206) according to claim 9, further comprising means (218) for selecting a unique content identifier for the content item to be introduced in the network.

13. A system for preventing unauthorized introduction of content items in a network containing compliant devices, said system comprising: at least one compliant device (203); and a certificate authority (206), wherein the compliant device (203) is arranged with

- means (217) for determining a fingerprint of a content item to be introduced in the network and an identifier of a content introducer (201) introducing said content item in the network; and

- means (217) for sending the fingerprint and the identifier to the certificate authority; and the certificate authority (206) is arranged with means (218) for comparing said fingerprint to a predetermined set of fingerprints, wherein introduction of the content item is allowed if said fingerprint cannot be found in said set of fingerprints; means (218) for generating a watermark identifier, a watermarking key and a signed content identifier certificate comprising at least said fingerprint and a unique content identifier for the content item and the identifier of the content introducer; and means (218) for distributing the watermark identifier, the watermarking key and the signed content identifier certificate to the compliant device; the compliant device further comprising: - means (217) for inserting the watermark identifier in the content item by means of the watermarking key.

14. A computer program product comprising computer-executable components for causing a device to perform the steps recited in any one of claims 1-8 when the computer- executable components are run on a processing unit included in the device.