WO2007093946A1

WO2007093946A1 - Improved method of content protection

Info

Publication number: WO2007093946A1
Application number: PCT/IB2007/050444
Authority: WO
Inventors: Johan C. Talstra; Antonius A. M. Staring
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-02-14
Filing date: 2007-02-12
Publication date: 2007-08-23

Abstract

A method of enabling authentication of content, comprising computing a number of respective message authentication codes for the content, each respective message authentication code being computed using a respective group key associated with a group of devices suitable for accessing the content. Preferably for each of a plurality of sections of the content respective message authentication codes are computed using the respective group keys. Also a device suitable for accessing content and comprising means for authenticating the content, in which the means for authenticating are configured for computing a message authentication code for the content and for comparing the computed message authentication code against at least one of a plurality of recorded message authentication codes that are provided with the content. Also a system comprising a plurality of such devices.

Description

Improved method of content protection

Unauthorized content distribution is a grave concern for content owners. Such distribution may occur in the form of unlicensed replication of content on read-only carriers such as DVD discs, typically in large numbers, which is then sold to the public. Other forms may be e.g. distribution over a peer-to-peer network or from an unauthorized Web site. The source material e.g. might be a movie recorded on a digital camcorder in the back of a movie theater, or a stolen/leaked pre-production screener. To combat this form of piracy, copy protection systems like the Advanced Access Content System (AACS) introduce the process of Content Certificate verification.

Such process typically operates as follows. Legitimately produced content should be accompanied by a digital certificate covering that content. The certificate is digitally signed by the licensing authority of the copy protection system that applies to the content or distribution channel. When the content is played back in a compliant player (i.e. one that complies with the copy protection system in question), this player checks for the presence of a valid content certificate. The content must also be associated with that certificate. A carrier produced by an unauthorized third party or a Web site offering unauthorized downloads would not play in a compliant player, because the third party cannot create a valid content certificate.

Generally there are two steps in the certificate verification process that are computationally intensive. First, the creation and verification of a digital signature involves the computation of a cryptographic hash over the content. Calculation of this hash requires an enormous amount of data processing, as the content consists of multiple gigabytes of information.

Second, verifying the digital signature requires public key cryptography. Typically this consists of a point-multiplication on an elliptic curve cryptosystem (ECC) or a modular exponentiation using e.g. RSA. On today's (embedded) processors this can take many hundreds-of-thousands of 32x32-bit multiplications.

In systems like AACS the problem of hash calculation is reduced with an indirection as follows. The content is divided into many sections and the hashes of these sections are put in a separate file on a carrier. The licensor only signs this file and not the content as a whole. A player selects a limited number of sections of the content and compares the hash of those sections with values stored in the file. Different players may select different sections. This means a carrier with unauthorized content still has to carry hashes for all sections. As the third party does not have the necessary private key, he will be unable to create the file with signed hashes. Still, the burden of doing public key operations remains with the players.

To alleviate this burden, one may replace the public key signature with a Message Authentication Code (MAC) scheme based on symmetric cryptography. The problem with such a MAC scheme is that the key used by the player to verify the MAC is the same key as is used to generate this MAC. Therefore a professional pirate can circumvent such scheme by reverse engineering a single player implementation (e.g. a PC software player), to obtain the key(s) necessary to generate valid MACs independent from the licensor.

It is an object of the present invention to improve on the above.

This object is achieved according to the invention in a method comprising computing a number of respective message authentication codes for the content, each respective message authentication code being computed using a respective group key associated with a group of devices suitable for accessing the content. Preferably respective message authentication codes are computed for each of a plurality of sections of the content using the respective group keys.

The invention makes use of the market logistics which apply equally to content owner and professional pirate. Both parties need a kind of sustainable business required to recoup their investment. Such business can only be realized if both their carriers play with a very high probability (>90%) in any given player. If a carrier has a lower probability of working, potential customers will regard it as unreliable and not purchase it.

According to the invention, to create a disc that is playable with very high probability, a number of respective message authentication codes needs to be provided on the carrier, each usable only with one particular group key. Although the professional pirate can still reverse engineer an individual player and obtain the group MAC key in such player, this information only allows him to create a carrier which plays in the group of which this player is a member. By choosing the number of groups sufficiently large, each group is only a small fraction of the total market. This means that with a large probability, the pirated discs are unlikely to play in a randomly chosen player. To achieve the desired high probability of working, the pirate has to obtain the group MAC keys for a large number of groups, which means reverse engineering a very large number of players.

The invention further provides a device suitable for accessing content and comprising means for authenticating the content, configured for computing a message authentication code for the content and for comparing the computed message authentication code against at least one of a plurality of recorded message authentication codes that are provided with the content.

Advantageous embodiments are set out in the dependent claims.

The invention will now be discussed in more detail with reference to the figures, in which:

Fig. 1 schematically illustrates an arrangement for content mastering, distribution and rendering; and

Fig. 2 schematically illustrates a scheme for managing revocation of group MAC verification keys.

Throughout the figures, same reference numerals indicate similar or corresponding features. Some of the features indicated in the drawings are typically implemented in software, and as such represent software entities, such as software modules or objects.

Fig. 1 schematically illustrates an arrangement for content mastering, distribution and rendering. A content authoring facility 100 creates content such as music, songs, movies, animations, speeches, videoclips for music, TV programs, pictures, games, ringtones, spoken books or interactive services. To make the content available, it is sent to mastering facility 110 where an arrangement 111 for mastering and replication on carriers 150 such as Digital Versatile Discs is available. The process of mastering and replication involves the application of a copy protection scheme. To this end, a sub-arrangement 112 is provided where the content is encrypted and a hash of the content is computed. This hash is transmitted to a licensing authority 120. The licensing authority 120 uses MAC module 121 to apply a Message Authentication Code algorithm to the received hash using group keys available in storage means 122 and sends the result back to the arrangement 111.

Not all MAC algorithms are based on a hash of the content as input. For example, one may employ a symmetric key block cipher in Cipher Block Chaining mode, e.g. the CMAC algorithm as specified in NIST Special Publication 800-38B, "Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication", available on the Internet at http://csrc.nist.gov/publications/nistpubs/800-38B/SP_800-38B.pdf When employing a MAC algorithm that does not involve computation of a hash, the sub-arrangement 112 instead provides the encrypted content itself to the licensing authority 120. The MAC module 121 in that embodiment applies the MAC algorithm to the received content using group keys available in storage means 122 and sends the result back to the arrangement 111. Alternatively to providing the content as a whole, one or more sections of the content may be provided to the licensing authority 120.

In accordance with the present invention, the players are grouped into a large number of groups. Preferably over 1,000 groups are created. Each group is assigned a group MAC verification key. The licensing authority 120 computes a respective MAC for the encrypted content for each group, each time using that group's MAC verification key. The MACs are recorded on the carrier as MACs 153-1, 153-2, 153-3. The MACs as recorded on the carrier are preferably provided with an identification of the applicable group.

The licensing authority 120 may further comprise key generating means 123 to e.g. provide cryptographic keys to the mastering facility 110 for encrypting the content.

During the mastering and replication process the carrier 150 may be provided with such items as a ROM mark 151, a Media Key Block 152, the set of MACs 153-1, 153-2, 153-3 and of course the encrypted content 154.

The licensing authority 120 may be realized as a computer system that is inside the mastering facility connected to the mastering and/or replication equipment. Another option is to have the licensing authority 120 at a separate location, allowing a mastering facility to contact it over a network such as the Internet.

Subsequently the carrier 150 is marketed and sold or otherwise disposed of to a consumer, who at some point will insert the carrier 150 in his player 160 to access the content. The player 160 could be e.g. a radio receiver, a tuner/decoder, a CD or DVD player, a television, a VCR, a digital recorder, a mobile phone, a tape deck, a personal computer, a personal digital assistant, a portable display unit, a car entertainment system, and so on.

In accordance with the present invention, the player 160 is provided with a MAC verification module 161. The player 160 is provided with an identification of its group, for example a group number programmed into the device 160 in the factory. The player 160 is also provided in storage module 162 with the group MAC verification key for the group of which it is a member. Using this identification the player 160 accesses the MAC for this group from the carrier 150. For example if the player 160 is a member of group 1, it will access MAC 153-1; if it is a member of group 2, it will access MAC 153-2, and so on. The MAC verification module 161 computes a MAC for the content using the group MAC verification key, for example by first computing a cryptographic hash for the content and then computing a MAC using the hash and the group MAC verification key as input. This MAC is compared against the MAC for the group as recorded on the carrier 150. If the MACs are identical, a positive signal is sent to a decision module 163, otherwise a negative signal is sent.

Alternatively the player 160 may compare the computed MAC against each MAC recorded on the carrier 150. This avoids the need to provide the player 160 with an identification of its group, but requires slightly more processing power as a potentially large number of comparisons may need to be performed. Alternatively to computing a MAC over the content as a whole, the MAC module 121 may compute respective MACs over one or more sections of the encrypted content as supplied by the sub-arrangement 112. The items 153-1, 153-2, 153-3 as recorded on the carrier 150 then comprise for each group one or more MACs, one for each section. The MACs as recorded on the carrier are preferably provided with an identification of the applicable group and section.

This effectively creates a matrix of MACs as shown in the following table:

As can be seen in the table, this embodiment may require the storing of m times n group MACs, if there are n device groups and m content sections. For large content and/or large group sizes, it will usually be advantageous to store only a subset of the group MACs. The MAC verification module 161 then is configured to select only a subset of all sections and to compute respective MACs for only the selected sections. Preferably such selection is done randomly or pseudo-randomly. This has the advantage that not all sections need to be verified, which saves time. By making the selection (pseudo-) randomly, it is achieved that a third party wishing to pass off unauthorized content as authorized still has to create MACs for all sections. Otherwise the content might still be detected as being unauthorized. In an embodiment the sub-arrangement 112 may be configured to only supply a subset of all sections of the encrypted content to the licensing authority 120. This saves transmission capacity and time for creating the MACs. When this option is used, the MAC verification module 161 needs to compare only the MACs computed for the sections of the subset against the MACs 153-1, 153-2, 153-3. Preferably then the carrier 160 is provided with an indication of which sections were comprised in the subset, so that the MAC verification module 161 can access those sections and compute the MACs.

In an embodiment the sections are chosen differently for each group. This achieves that a device in a group only needs to verify one group MAC, and at the same time that a valid group MAC needs to be present for every section for the content to be accepted as authentic. For example, a selection could be made of MACn, MAC22, MAC33, ..., MACm_n.

In certain situations it may even be sufficient to have the MAC module 121 compute the respective MACs for a single section of the encrypted content. The MAC verification module 161 then only needs to compute a MAC for that single section.

The decision module 163 allows access to the content if the verification performed by the MAC verification module 161 is positive. The content is then decrypted by decryption module 164 and can be rendered using rendering module 165. The exact way in which a content item is rendered depends on the type of device and the type of content. For instance, in a radio receiver, rendering comprises generating audio signals and feeding them to loudspeakers. For video content, rendering generally comprises generating audio and video signals and feeding those to a display screen and loudspeakers. For other types of content a similar appropriate action must be taken.

Alternatively to rendering, the content may also be copied onto a different carrier such as a hard disk. The player 160 may be connected to a network and may be configured to provide the content to a different device which performs the actual rendering. The device 160 may be provided with a content protection system (CPS) 166 that uses one or more device keys available in storage module 167. Note that storage module 167 and storage module 162 may be implemented using a single storage medium.

Alternatively to the distribution of the content on carrier 160, the content may also be made available through different channels. For example, the content may be offered for download on a Web site or through a pay-per-view television scheme.

The above system is secure for as long as only a relatively small number of group MAC verification keys become compromised. Its security will deteriorate over time as more devices are hacked and a sufficient number of group MAC verification keys become available to unauthorized third parties. It is then possible for these third parties to create unauthorized carriers (or to offer unauthorized content through other channels) with a large number of validly created group MACs. Most compliant players would accept such content as valid.

Fig. 2 schematically illustrates a scheme for managing revocation of group MAC verification keys. This scheme solves the problem noted in the previous paragraph. The nodes of the tree 200 have been labeled in the canonical way using numbers with binary digits. At the highest level, there is the root node 201. Below there are two nodes 211, 212 labeled "0" and "1". The next level comprises nodes 221, 222, 223, 224 also with respective labels. Below that there are nodes 231-238, and at the bottom the leaf nodes 240 are provided.

Devices correspond to leaf nodes. Device 0 (indicated in Fig. 2 with decimal number "0") corresponds to the leaf node with label "0000"; device 1 corresponds to the leaf node with label "0001", and so on. In practice there would of course be many more leaf nodes, as there may well be hundreds of thousands of devices in the system. Assigned to every node of the binary tree are randomly chosen MAC verification keys. A key assigned to a node with number n will be referred to as K_n. For example the key of node 212 is Ko, a key assigned to node 222 is Koi, the key for node 234 is Koii and a key for the last shown leaf node is K₁₁₁₁. The root node may or may not have been assigned a key. All keys on the path from the root node 201 to a particular leaf node are available to the device corresponding to that particular leaf node. For example, device 7 holds keys K₀, K_Oi, K_Oπ and K_Oπi and device 15 has keys K₁, K₁₁, K₁₁₁ and K₁₁₁₁. In some systems these keys are often referred to as device keys, although it would be better to call them group keys, because such a key is shared by the group of devices contained in the sub-tree rooted at that particular node.

This assignment of group keys makes it possible to selectively address subsets of all devices. Suppose that the group MAC verification keys stored in devices 0, 7, 8 and 9 have been reverse engineered and used by professional pirates. This means that future legitimate content should no longer be accompanied by a MAC that can be verified using any of these group MAC verification keys.

To achieve this, the MAC module 121 now uses the keys Koooi, K_Ooi, K₀ io, Kono, Kioi and Kn to create the plurality of message authentication codes that is to accompany the content. As can be seen in Fig. 2, these are the keys available only to the devices in the shaded areas of the tree. This new choice of keys avoids the keys available to the devices 0, 7, 8 and 9.

The MAC verification module 161 now uses at least one of the group keys available from storage module 162 to compute a MAC for the content. Next, the computed MAC is compared against at least one of the group MACs on the carrier 150.

Preferably, the group MACs on the carrier 150 may are accompanied by respective identifications of the group MAC verification keys that are to be used for verification of these group MACs. This enables the MAC verification module 161 to easily select the stored group MAC for which it has a group MAC verification key available. Alternatively the MAC verification module 161 compares the computed MAC against each of the group MACs to determine if one of them matches the computed MAC.

In copy protection systems such as Copy Protection for Recordable Media and Copy Protection for Pre-recorded Media, Video Content Protection System and AACS a comparable scheme is used to assign decryption keys. The decryption keys are used to decrypt content keys with which in turn the content can be decrypted. However in these systems only confidentiality of the content key is achieved. In the present invention, the scheme is used to make it possible to verify origin authentication and integrity of the content itself. Moreover in those systems the keys are not used to compute a MAC over the content itself. In a preferred embodiment the group MAC verification keys initially used are not the root key or keys Kg or Ki. Rather, to make it very hard for the professional hacker, the initial number of group MAC keys is already chosen to be very large, preferably 1 ,000 groups or more. This means that none of the group keys of the top ten layers of the tree should be used. In an advantageous embodiment, these keys from the top ten layers do not even exist.

In international patent application WO2005/008385, technologies are disclosed to transfer responsibility and control over security from player makers to content authors by enabling integration of security logic and content. An exemplary optical disc carries an encrypted digital video title combined with data processing operations that implement the title's security policies and decryption processes. Player devices include a processing environment (e.g., a real-time virtual machine), which plays content by interpreting its processing operations. Players also provide procedure calls to enable content code to load data from media, perform network communications, determine playback environment configurations, access secure nonvolatile storage, submit data to codecs for output, and/or perform cryptographic operations. Content can insert forensic watermarks in decoded output for tracing pirate copies. If pirates compromise a player or title, future content can be mastered with security features that, for example, block the attack, revoke pirated media, or use native code to correct player vulnerabilities.

US patent application US20040190868A1 discloses a recording apparatus that comprises a receiving unit operable to receive content, a control unit operable to determine a recording method of the content on a recording media, and a R/W unit operable to write in and read out on the recording media. The control unit includes a recording media identification unit operable to identify a type of the recording media via the R/W unit, a source identification unit operable to judge a type of a source about whether or not the received content is a content subject to a content protection, a recording method selection unit operable to select a recording method of the content on the recording media, and a recording method conversion unit. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method of enabling authentication of content, comprising computing a number of respective message authentication codes for the content, each respective message authentication code being computed using a respective group key associated with a group of devices suitable for accessing the content.

2. The method of claim 1 , comprising computing for each of a plurality of sections of the content respective message authentication codes using the respective group keys.

3. The method of claim 2, in which the plurality of sections represents a subset of all sections into which the content is divided.

4. The method of claim 1, comprising receiving a cryptographic hash for the content and computing the respective message authentication codes using the received cryptographic hash.

5. The method of claim 2, comprising receiving respective hashes for the respective sections of the plurality of sections and computing the respective message authentication codes for each section using the received cryptographic hash for the section in question.

6. A device suitable for accessing content and comprising means for authenticating the content, in which the means for authenticating are configured for computing a message authentication code for the content and for comparing the computed message authentication code against at least one of a plurality of recorded message authentication codes that are provided with the content.

7. The device of claim 6, in which the means for authenticating are configured for determining a group to which the device belongs and for comparing the computed message authentication code against the one recorded message authentication code that is applicable to said group.

8. The device of claim 7, comprising storage means for storing a plurality of group keys, the means for authenticating being configured for using at least one of the stored group keys to compute the message authentication code.

9. A system comprising a plurality of devices as claimed in claim 6, which devices have been grouped into respective groups, and in which the devices of a single group share at least one common group key for computing a message authentication code for content to be authenticated.

10. The system of claim 9, in which the devices have been assigned a plurality of group keys, each group key of said plurality being shared with a subset of all devices, at least one of the subsets being a subset of another subset.