WO2008032002A1

WO2008032002A1 - Method of securing access to a content recorded on a storage means

Info

Publication number: WO2008032002A1
Application number: PCT/FR2007/051944
Authority: WO
Inventors: Jean-Claude Pailles; Philippe Raipin Parvedy
Original assignee: France Telecom
Priority date: 2006-09-15
Filing date: 2007-09-14
Publication date: 2008-03-20

Abstract

The invention relates to a method of secured access to a digital content (33) recorded in encrypted form on an associated storage means (3) of an electronic module (1) interrogatable remotely and able to provide a reading apparatus (5) with at least one decryption key (Kj) intended for decrypting the content of the storage means (3). The reading apparatus (5) generates (E7) a proof of non-revocation whose validity is verified (E9) by the electronic module (1). If the proof of non-revocation is valid, the electronic module (1) provides the decryption key to the reading apparatus (5).

Description

Title of the invention

A method of securing access to content stored on a storage medium.

Field of the invention

The present invention is in the field of the protection of contents recorded on storage means constituted, for example, by optical storage media. It concerns Digital Rights Management (DRM) technologies that are currently used to control access to digital data recorded on optical discs, such as compact discs (W-CDs, RW-CDs), digital versatile discs (DVD, HD-DVD) or Blu-Ray ™ discs.

The data recorded on these storage media are generally of commercial value and therefore are most often subject to copyright. Anyone authorized to distribute such data via such media, therefore wishes to maintain control over the reproduction and use of such data, once the media sold to a third party. Protecting optical discs of the CD / DVD type against copying is a goal sought in many cases of use, but remains difficult to achieve. Moreover, the protections sought are not necessarily limited to preventing copying.

For this purpose, copy protection standards, such as the Advanced Access Content System (AACS) or Content Protection for Recordable Media (CPRM) standards, have been put in place. In general, these protection standards consist of recording digital content on the disk in encrypted form and allowing it to be read in clear only by means of a reading device having a secret key, from which it can calculate the keys of decryption of the contents of the disc. However, bit-by-bit copying of discs is not prevented by systems implementing the AACS standard.

According to another approach, the decryption keys of the digital content recorded on the disk can be stored in an RFID tag ("Radio Frequency Identification") fixed on the disk, the reading device accessing these keys, only after a procedure of reciprocal authentication.

If the latter solution makes it possible to subordinate the provision of the decryption keys following an identification / mutual authentication step previously carried out between the reading device and the RFID tag, it does not make it possible to take into account the revocation management of some reading devices.

Indeed, some content providers or Authority wish to revoke a set of playback devices compromising these contents, so that the set of compromising devices can no longer read the content of new disks marketed.

Object and brief description of the invention

The present invention aims to overcome the aforementioned drawbacks and to provide a reliable and economical solution for the protection of data storage media using optical disks.

These goals are achieved by a method of securely accessing a digital content recorded in encrypted form on a data storage means, this storage means being associated with a remote interrogable electronic module, the electronic module being able to provide the least a decryption key to decrypt the content, during a decryption step. The method according to the invention comprises the following steps: receiving a proof of non-revocation of the reading apparatus generated according to a unique identifier of the reading apparatus and data of the data storage means;

- verification of the validity of the proof of non-revocation by the electronic module; and

- If the proof of non-revocation is valid, provision of said at least one decryption key by the electronic module to the reading device, so as to perform the content decryption step.

Thus, the access control level of the encrypted digital data on a storage means is improved insofar as the possible revocation of the reading devices of the storage means is taken into account.

For this purpose, the keys for decrypting the contents of the storage means are provided by the electronic module to the reading device, only after verification that the reading device has not been revoked by an Authority.

In this way, if a reading apparatus is revoked, the electronic module does not provide the keys for decrypting the contents of the storage means to this reading apparatus. The data for generating the proof of non-revocation is stored on the data storage means and not on the electronic module. Thus, the size of the memory of the electronic module may be small.

According to another characteristic of the invention, the method according to the invention further comprises a step of authentication of the reading device by the electronic module, so that said at least one decryption key is provided by the electronic module. to the reading device, in case of positive authentication of the reading device. Thus, the electronic module provides the keys for decrypting the contents of the disc to an authentic reading device and not revoked.

According to a first embodiment of the invention, the generation of the proof of non-revocation comprises the formation, according to a unique identifier of the reading apparatus, of a set of p indices taken from among indexes indexing elements of a block of data of said data storage means, each of the MKBi elements of this block being previously obtained by an Authority by applying a public cryptographic function E and a key C specific to the content of the data storage means, so that: MKBi = E (C, ï) where the index i is a natural integer included in 1 and n, the proof of non-revocation being constituted by a set of constituent elements of this proof denoted Pi such that Pi = MKBi for any index i belonging to the set (G) of p indices.

When a reading device is revoked, it is sufficient that the Authority does not inform the MKBi elements of the data block (MKB) of the storage means corresponding to this device, thus preventing the latter from calculating any proof of non-revocation. It will be noted that the specific key C is previously chosen by the Authority for a given content and entered in the electronic modules of the storage means having this content. In addition, each MKBi element is previously calculated by the Authority according to this specific key. The proof of non-revocation is calculated, based on the unique identifier of the reading device, so that the value of this proof is specific to this reading device.

Thus, it is possible to foil a particular type of attack which would be to obtain the value of the proof of non-revocation of an unrevoked reading device and to broadcast it to terminals previously revoked, so that they can decipher the contents of the disc.

According to a characteristic of the first embodiment of the invention, the step of verifying the validity of the proof of non-revocation by the electronic module comprises:

a first verification sub-step, during which the electronic module verifies that each index of the constituent elements of the proof of non-revocation received is associated with the identifier of the reading device; and a second verification sub-step, in which the electronic module verifies that the components of the proof of non-revocation received correspond to the elements E (Qi), these elements being calculated by the electronic module from the indices i of the received elements, the cryptographic function (E) and the key (C) specific to the content of the storage means.

In this way, the electronic module transmits the decryption keys to the reading device, if at least one of the constituent elements (Pi) of the proof of non-revocation received is equal to the corresponding value E (Qi) calculated by the module electronic. According to a second embodiment of the invention, in a data block of the storage means are recorded non-revocation proof values each corresponding to the root node of a subtree whose ends each correspond to a reading device not revoked, each subtree corresponding to a part of a binary tree. Still according to this second embodiment of the invention, the generation of the proof of non-revocation of a reading apparatus comprises calculating the value of the proof of non-revocation recursively, starting from the value of the proof. non-revocation assigned to the root node of the subtree containing the reading apparatus, and according to the unique identifier of the reading apparatus. This second embodiment makes it possible to obtain, in a simple and compact manner, a proof of non-revocation value of the reading apparatus which is specific to each reading apparatus and therefore has the same advantages as those of the first embodiment. described above.

Moreover, by structuring the data of the MKB of the storage means according to a binary tree, it is possible to compress the information to be stored in the MKB in large proportions, so that a reading apparatus is able to calculate the proof of no revocation specific to it.

According to a characteristic of the second embodiment, the step of verifying the validity of the proof of non-revocation comprises the following substeps:

calculating the value of the non-revocation proof recursively, from a value of the reference non-revocation proof attributed to the root of the binary tree and according to the unique identifier; and

- comparison of the value of the proof of non-revocation calculated by the electronic module to the value of the proof of non-revocation provided by the reading device. It should be noted that the reference non-revocation proof value is chosen by the Authority for a given content, and entered in the electronic modules associated with the storage means having this same content.

Thus, if the value of the proof of non-revocation calculated by the electronic module is equal to the value of the proof of non-revocation provided by the reading device, then the electronic module transmits the decryption keys of the content to the device reading.

According to a third embodiment of the invention, the data of the data storage means comprise a set of records in which are stored a first and a second identifiers and a seal calculated according to said identifiers and a coding key. In this embodiment, the generation of the proof of non-revocation comprises a step of searching in said set, of a record for which the identifier of the reading apparatus is greater than the first identifier and less than the second identifier, and the proof of non-revocation contains the record sought, that is, the record corresponding to the result of the search.

Thus, if the playback device is revoked, no record will match the search and the playback device will not be able to generate proof of non-revocation.

According to a characteristic of this third embodiment, the step of verifying the validity of the proof of non-revocation by the electronic module comprises:

a first verification sub-step, during which the electronic module verifies that the received identifier has a value between the first identifier and the second identifier,

a second verification sub-step, during which the electronic module verifies the seal received using the first and second identifiers and a key associated with the coding key used to calculate the seal.

According to one characteristic of the invention, the method according to the invention further comprises the following steps:

authentication of the electronic module by the reading device; and

interrogating, by the reading device, a local list of unauthorized electronic modules to check whether the electronic module is authorized, the local list being updated by the reading device from a registered list; on the storage medium.

A mechanism for protecting against the illegal copy of an electronic module associated with a storage means is thus implemented. In this way, the use of storage means is prevented duplicated, whose associated electronic module has been fraudulently cloned, for example, by reproduction of the unique identifier and secret data contained in a source electronic module. For this, the storage means is used as means for updating, in the reading device, a black list of electronic modules that have been illegally reproduced.

Thus, after having authenticated an electronic module associated with a storage means, it is possible to prevent the use of this storage means, if the identifier of the electronic module is contained in a black list of unauthorized electronic modules.

According to another characteristic of the invention, the step of decrypting the digital content of the storage means is performed according to usage rules recorded on the storage means and as a function of usage data stored on the electronic module. The usage rules are applied in combination with usage data that are read and updated on the electronic module by the reading device. The rules of use may be defined by content providers who wish to limit the use of a means of storage in a computer or in a limited number of computers (eg software), limit its reading to a predefined date of use or limit its copy and / or reading to a predetermined number of times.

According to another characteristic of the invention, the digital content of the storage module comprises a set of variable sequences, each being formed by the juxtaposition of two versions of the same content that has been encrypted by separate encryption keys. In addition, the electronic module provides the reading device, for any variable sequence a single decryption key for decrypting only one or the other of the two versions, the decryption key being selected according to a code digital marking specific to the reading device and supplied to the electronic module by the latter.

Thus, a traceability of reading devices can be effected effectively, in particular because the decryption keys are provided by the electronic module, this module can be more easily secured than the reading device and therefore more difficult to attack.

The invention also relates to an electronic module adapted to be associated with a data storage means, said storage means comprising digital content data in encrypted form and intended to be read by a reading device, the electronic module comprising means for storing protection data necessary to provide at least one key for decrypting the content. The electronic module according to the invention further comprises: means for receiving a proof of non-revocation of the reading device generated as a function of a unique identifier of the reading device and data of the storage means of data ;

- verification means to verify the validity of the proof of non-revocation; and processing means for supplying said at least one decryption key to the reading device, in the event of validity of the proof of non-revocation.

The invention also relates to the use of a storage means comprising a digital content in encrypted form intended to be read by a reading apparatus and a non-revocation proof value data block, the data is used to generate a proof of non-revocation to be transmitted to an electronic module capable of providing at least one decryption key of the digital content in the event of validity of the proof of non-revocation. The invention also relates to a revocation device able to revoke a reading device intended to read an encrypted digital content of a storage means associated with an electronic module according to the invention. The revocation device being connected to the reading apparatus comprises:

means for generating a proof of non-revocation of the reading apparatus according to a unique identifier of the reading apparatus and data of the data storage means; and

means for sending this proof of non-revocation to the electronic module.

The invention also relates to a reading apparatus comprising decryption means for reading an encrypted digital content of a storage means associated with an electronic module according to the invention.

The reading apparatus according to the invention comprises means for receiving decryption keys provided by the electronic module.

According to a characteristic of the invention, the reading apparatus comprises a module for controlling the decryption of the content of the storage module capable of supplying the said at least one decryption key to the decryption module in the case where the electronic module is authorized, and / or according to usage rules and usage data.

This control module, decision-making organ of the reading apparatus, must be modified with respect to the prior art to allow the implementation of the invention (taking into account non-revocation parameters), to decide whether or not no decryption keys must be provided to the decryption module.

According to one embodiment of the invention, the revocation device is integrated in the reading apparatus.

The invention also relates to a computer program intended to be executed by a microprocessor implemented by an electronic module according to the invention to allow secure access to a content digital recording recorded in encrypted form on a storage means, the digital content being intended to be Iu by an optical reading device, the electronic module being associated with the storage means and being able to provide the reading device with at least one key decryption of digital content. The program according to the invention comprises instructions for:

- check the validity of a proof of non-revocation generated according to a unique identifier of the reading device and data of the data storage means; and - providing said at least one decryption key to the reading device, in the case where the proof of non-revocation (P) is valid.

According to another characteristic of the invention, the computer program according to the invention further comprises instructions for authenticating the reading apparatus. In a complementary manner, the invention also aims at a computer program intended to be executed by a processor of a revocation device to allow secure access to a digital content recorded in encrypted form on a storage medium with which a data storage device is associated. electronic module according to the invention capable of supplying the reading device with at least one key for decrypting the content. The computer program according to the invention comprises instructions for:

generating a proof of non-revocation of the reading apparatus according to a unique identifier of the reading apparatus and data of the data storage means; - send proof of non-revocation to the electronic module.

According to another characteristic of the invention, the computer program to be executed by the reading apparatus further comprises instructions to authenticate with the electronic module.

According to another characteristic of the invention, the computer program intended to be executed by the reading apparatus comprises in in addition to instructions for authenticating the electronic module.

It should be noted that the above-mentioned programs can use any programming language and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in n any other desirable form.

Brèwe description of the drawings

Other characteristics and advantages of the invention will emerge from the following description of particular embodiments of the invention, given by way of non-limiting examples, with reference to the appended drawings, in which:

- Figure 1 schematically illustrates a system comprising an optical pickup apparatus, an optical disk and an RFID tag according to the invention;

FIG. 2 describes in flowchart form an exemplary implementation of the method for securely accessing the encrypted content of a disk according to the invention;

FIG. 3 illustrates a bit-tree structure of a data block MKB of the optical disk, according to a second embodiment; and

FIG. 4 schematically illustrates the provision of decryption keys by the RFID tag to the reading device, in the case of using a digital marking technique.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention applies, in general, to any medium or digital storage means. In particular, it may be an optical disk on which digital data is recorded. (eg software, music, videos, etc.) whose use is to be controlled (eg copy, number of readings, type / number of device (s) used to read the data). By way of non-limiting examples of this type of support, mention may be made in particular of CD, DVD, DVD-HD or Blu-Ray ™ type disks. The invention also applies to storage means constituted by non-volatile memories of EEPROM type ("Electrically Erasable Programmable Read OnYy Memory"), Flash-EPROM, or the like, generally "sold under the name of USB stick (" Universal Serial Bus ") or Flash Multimedia Memory Card (MMC) It should be noted that the use of these digital storage media means both the reading or copying of their content (eg music or video) by a user. any program of reading or copying, as loading software read on the disk.

In addition, the invention applies to any type of reading device capable of reading or recording data on the means or storage media mentioned above. For example, ^'in the case of the optical disk, the device can be a PC or a player / CD / DVD home. In the case of storage means constituted by a memory card, the reading device may be, for example, a PC computer or a mobile terminal (mobile phone, PDA, etc.) equipped with at least one card reader memory (for example, MMC cards).

FIG. 1 schematically illustrates a system consisting of an electronic module (1), data storage means (3) and a reading device (5) capable of reading the storage means (3). In our exemplary embodiment, the electronic module (1) is an RFID tag (1) and the data storage means (3) is an optical disk (3).

In practice, the RFID tag (1) is fixed on one of the faces of the optical disk (3) which is itself intended to be read by the reading device (5). The reading apparatus (5) also comprises means for reading RFID tags. For this, the reading device (5) is equipped with a coil for sending / receiving radio frequency signals to / from the RFID tag (1). The signals between the reel of the reader (5) and the RFID tag (1) are transmitted in both directions by modulation of a magnetic field.

By way of non-limiting example, in our exemplary embodiment, the reading device (5) is an optical disk drive (CD, DVD). Of course, other reading devices as mentioned above are conceivable, such as, for example, a mobile phone capable of reading MMC cards and equipped with means for reading RFID tags.

According to an alternative embodiment of the invention, not shown in the figures, the RFID tag reading means are dissociated from the reading apparatus (5), while being connected thereto. This makes it possible to implement the invention in disk reading devices that are not yet equipped with means for reading RFID tags.

Similarly, the RFID tag associated with the disk does not need to be attached to the disk and can be read independently. Preferably, the interactions on the one hand between the reading device and the optical content of the disk and on the other hand between the reading device and the label are simultaneous, so as not to impede the user with a problem. waiting time too long.

In our exemplary embodiment, the RFID tag (1) is constituted by a remote searchable electronic device (1) comprising an electronic chip connected to a coil forming an antenna.

The electronic chip comprises memory means

(16) for storing protection data necessary for the use of encrypted digital content data stored on the disk (3). These protection data comprise for example the decryption keys of the digital content of the disk (3).

In our exemplary embodiment, the optical disk (3) of FIG. 1 comprises: a data block (31) commonly called MKB ("Media Key Block");

an encrypted digital content (33) comprising a series of encrypted sequences which constitute the "payload" of the disk (3) and occupy the major part of its storage space; a memory zone (35) that can not be modified and stores usage rules (LJR) related to the use of the disk (3); and

- a blacklist (37) listing identifiers of RFID tags (1) not authorized by an Authority.

In known manner, the encrypted digital content (33) of the optical disk (3) comprises a plurality of sequences or titles (Cj) denoted {Cj} i <j <N (j being a natural number varying from 1 to N, where N is the maximum number of sequences). In our example, these sequences are encrypted according to a plurality of encryption keys (Kj) with l≤j≤N, noted {Kj} i≤j < _N . In our example, the RFID tag (1) of FIG. 1 comprises:

an authentication module (10) able to authenticate the reading device (5);

reception means (not shown) for receiving a proof of non-revocation of the reading apparatus (5), these means being constituted, in this example, by the aforementioned coil serving as an antenna;

- a verification module (12) of a proof of non-revocation (P) of the reading device (5);

a processing module (14) capable of supplying the reading apparatus (5) with keys for decrypting the content (33) of the disc (3); a memory area (16) comprising protection information necessary for the provision of the decryption keys;

- an authentication module (18) of the RFID tag (1), allowing it (1) to authenticate with the reading device (5); - A storage module (19) for storing and providing usage data related to the use of the disk (3).

The decryption keys (Kj) are stored in the memory area (16) of the RFID tag (1) which is adapted to send them to the reading device (5), under certain conditions, for example related to an authentication of the device (5) and / or the RFID tag (1) and rules of use of the disk (3).

The reading apparatus (5) of Fig. 1 comprises:

an authenticity module (51) enabling the reading device (5) to authenticate itself to the RFID tag (1); a storage area (52) comprising data necessary for authentication of the reading device (5) with the RFID tag

CD;

- a generation module (53) of a proof of non-revocation, able to send this proof to the RFID tag (1); sending means (not shown) for sending its proof of non-revocation, these means being constituted, in this example, by a aforementioned coil serving as an antenna;

a decryption module (54) for the content (33) of the disk (3) from the decryption keys (Kj) provided by the RFID tag (1); a decryption control module (55) for initiating the decryption performed by the decryption module (54); said control module (55) comprising a module (59) for obtaining the decryption dice;

a verification module (56) to check whether the RFID tag (1) is authorized, this verification module (56) comprising a local blacklist; (560) updatable from information in the blacklist (37) of the disk (3);

an authentication module (57) for authenticating the RFID tag (1);

an updating module (58) for usage data stored in the storage module (19) of the RFID tag (1).

The functions described below performed by the aforementioned modules of the RFID tag (1) are executed by processing means (microprocessor) constituted by the electronic chip of the RFID tag (1). In known manner, each function (eg authentication, verification of the proof of non-revocation) is stored in the label in the form of a computer program (written for example in C or assembler language) which will be executed by the processing means of the electronic chip.

Similarly, the functions described below performed by the aforementioned modules of the reading apparatus are performed by a computing unit of the reading apparatus such as a microprocessor.

An exemplary embodiment of the method for secure access to the encrypted content (33) of the disk (3) according to the invention will now be described with reference to FIG. 2 and FIG. 1 already described. In FIG. 2, the process steps shown on the left are made by the RFID tag (1), while those shown on the right are made by the reading device (5).

In a first authentication step (El), the RFID tag (1) authenticates the reading device (5). According to a first embodiment of the first authentication step (E1), the authentication module (51) of the reading device (5) sends the authentication module (10) the RFID tag (1). ) a unique identifier (d) of the reading device (5) accompanied by a certificate (Cd) specific to the reading device (5), during a first sub-step (EiO). A public key (Pd) included in the certificate is transmitted during this same step (ElO) to the RFID tag (1).

During a second substep (E12), the RFID tag (1) verifies the validity of the certificate (Cd) previously provided by the reading device (5) using standard verification mechanisms. For example, the authentication module (10) can perform the following operation; Cd ^e x

/ VOd (Ha), where e is a modular exponent whose value depends on the protection system; and Na represents a public key provided by the Authority, this public key being known from the RFID tag (1) and the reading device (5).

It should be noted that the value of the modular exponent e is preferably chosen to be equal to 3 or 2 in order to minimize the calculation load of the RFID tag (1). By verifying the validity of the certificate (Cd), the authentication module (10) verifies the validity of the public key (Pd), then stores it in the case of a positive verification.

During a third substep (E14), the RFID tag (1) sends the result of this verification to the reading device (5). If this check is positive, the public key (Pd) obtained from the certificate

(Cd) above corresponds to the public key (Pd) of the reading device (5).

Note that this public key (Pd) can be used to send the decryption keys (Kj) in encrypted form to the reading device (5).

According to a second variant (not shown) of realization of the first authentication step (El), the RFID tag (1) wishing to obtain a proof of identity of the reading device (5) created, in a known manner, a "challenge", by generating a random number that it (1) sends to the latter (5), during a first substep (ElO).

During a second substep (E12), the reading device (5) generates a signature (Sd) from the "challenge" previously received, using a signature function; previously recorded on the reading device (5), then sends to the RFID tag (1), in a third substep (E'14), the aforementioned signature (Sd) accompanied by the unique identifier of the reading apparatus (5) and a certificate (Cd) specific to the reading apparatus (5).

From this information, the RFID tag (1) proceeds, in a fourth substep (E'16), to verify the certificate (Cd) and the signature (Sd) provided by the device. reading (5). If this check is positive, then the reader (5) is correctly authenticated with the RFID tag (1).

It will be noted that the first variant generates fewer calculations in the RFID tag than the second variant.

During a test step (E3), if the reading device (5) is not authentic, then the process ends (step E50). On the contrary, if the reading apparatus (5) is authentic, that is to say correctly authenticated, then the RFID tag (1) receives and stores (step E5) a proof of non-revocation (P) provided by the reading apparatus (5). This proof of non-revocation (P) was previously calculated by the reading apparatus (5), from information contained in the data block (31) of the disk (3), during a generation step ( E7).

During a verification step (E9), the verification module (12) of the RFID tag (1) checks the validity of the non-revocation proof (P) provided by the reading device (5). For this, the verification module (12) calculates a non-revocation proof value (P ') which it then compares with the value of the proof of non-revocation (P) previously stored in the previous step (E5). ,

If these two values differ (test step (EI1)), then the process ends (step E50). If not, the proof of non-revocation (P) provided by the reading device (5) is valid, and the step of verification (E9) of the proof of non-revocation carried out by the RFID tag (1) is said to be positive.

In this case, the processing module (14) performs the operations necessary to provide, during a step (E13), the decryption keys (Kj) to the reading device (5) which then stores them in the module control (55) decryption. For this, the processing module (14) obtains these decryption keys (K i) from the memory area (16) of the RFID tag (1). ^•

For greater security, the decryption keys (Kj) are sent, during a sending step (E13), to the reading device (5) by the RFID tag (1). Preferably, this sending is done in a secure manner, to avoid obtaining clear keys at the interface between the RFID tag (1) and the reading device (5). For this, the processing module (14) of the RFID tag (1) encrypts the decryption keys (Kj) with the public key (Pd) of the reading device obtained safely during the step (E12 ).

To limit the calculations performed by the RFID tag (1), the Rabin scheme can be used. In this way, the RFID tag (1) only has to perform a modular exponentiation with a modular exponent equal to two. Thus, an RFID tag with reduced computation capability can be used.

For more information on the Rabin scheme, one skilled in the art can refer to the following document: MO Rabin, "Digitalized Signatures and Public-Key Functions as Intractable as Factorization", Technical Report, MIT / LCS / TR-212, MIT, 1979.

The use of the Rabin scheme has the advantage of requiring a reduced computing capacity, thus making it possible to limit the need for calculation capacity of the RFID tag (1), and consequently to reduce its complexity and its cost of implementation. implemented. According to the first embodiment variant of the first authentication step (E1), the processing module (14) encrypts the decryption fields (Kj) with the public key (Pd), thus making it possible to ensure that only the apparatus having provided this public key (Pd) can obtain the decryption keys (Kj) from this public key (Pd).

Advantageously, the content decryption keys (Kj) are sent by the RFID tag (1) in encrypted form in order to increase the security level associated with obtaining these keys.

A particular example of implementation of this step of providing (E13) key decryption content in encrypted form is now described in detail.

During a first substep, the RFID tag (1) receives from the reading device (5) a request to obtain a particular decryption key (Kj). In response to this request, the RFID tag (1) calculates the decryption key (Kj) required.

During a second encryption sub-step, the RFID tag (1) encrypts the decryption key (Kj) from a known encryption function (F) and a so-called encryption key (k). session key and chosen randomly or pseudo-randomly. For example, the encryption function (F) is the mathematical operator "or exclusive" (XOR: exclusive OR), noted Θ. In this way, the RFID tag (1) generates an encrypted key (U) obtained by the following relation: U = Kj Θ k.

Then, the RFID tag (1) encrypts the encryption key (k) with the public key (Pd) previously obtained in step (ElO) ₇ and then sends the result z = Pd (k) accompanied by the encrypted key (U) to the reading apparatus (5).

Advantageously, the same session key (k) is used to encrypt any required decryption key (Kj) with the public key (Pd) of the reading apparatus (5), thus avoiding the RFID tag (i ) to perform this encryption operation with each new request for a particular decryption key by the reading device (5).

In a substep of calculation, the reading apparatus (5) obtains the encryption key k using the secret reader key (Sd), performing the following operation: Sd (z): (= Sd (Pd (k)) = k). During a second substep of calculation, the reading apparatus obtains the decryption key (Kj) by applying the known encryption function (F), according to the following operation: k ® U (= Kj).

Advantageously, the release of the decryption keys (step E29) from the decryption module (54) by the control module (55) only occurs under certain additional conditions. These conditions take into account additional protection settings to increase the level of data protection on the disk.

A first of these conditions is to verify that the RFID tag (1) associated with the disk (5) is an authentic and authorized tag. For this purpose, during a second authentication step (E15), the authentication module (57) of the reading device (5) interacts with the authentication module (18) of the RFID tag (1). ) to perform the authentication of the RFID tag (1) with the reading device (5). During a first substep (E150), the reading device (5) wishing to obtain a proof of identity of the RFID tag (1) creates a "challenge" that it (5) sends to the latter (1).

During a second substep (E152), the RFID tag (1) generates a signature (Se) from the received "challenge", using a signature function previously recorded on the RFID tag (1). , then sends to reading apparatus (5), in a third substep (El 54), the aforementioned signature (Se) accompanied by a unique identifier of the RFID tag (1) and a certificate ( This) specific to the RFID tag (1), From this information, the reading apparatus (5) proceeds _; in a fourth substep (E156), checking the certificate (Ce) and the signature (Se) provided by the RFID tag (1).

If this verification is positive (test step (El?)), Then the RFID tag (1) is correctly authenticated with the reading device (5). Otherwise, the process ends (end step (E60)).

Preferably, this second authentication step (E15) is based on the Fiat-Shamir protocol. For more information on this protocol, one skilled in the art can refer to the following document: U. Feige, A. Fiat and A. Shamir, "Zero Knowledge

Proofs of Identity, "Journal of Cryptology, Volume 1, pages 77-94, 1988.

The use of such a protocol has the advantage of providing a high level of security, while limiting the need for computing capacity of the RFID tag (1), and therefore reduces its complexity and cost of ownership. Implementation.

During the test step (E17), the authentication module (57) verifies that the RFID tag (1) is authentic, that is to say that it has been correctly authenticated during the second step authentication (E15). If this is the case, the reading device (5) checks whether the label

RFID (1) is allowed. For this, the verification module (56) consults, (step (E19)) the blacklist (37) stored on the disk (3), updates the local blacklist (560) stored in the verification module (56). ) of the reading device (5) from the blacklist information (37) of the disk (3) and then verifying that the identifier of the authenticated RFID tag (1) does not belong to the local blacklist (560) update (test step (E21)). If the RFID tag (1) does not belong to the local blacklist (560), the tag (1) is allowed. If not, the label is unauthorized, which causes the process to stop (end step (E60)). Thus, any optical disc using RFID tags illegally cloπées and listed in the blacklist (37) of labels

RFID to ban, can not be used, because the decryption keys of its contents will not be transmitted to the decryption module (54).

A second condition that can be tested by the reading apparatus (5) is to verify that the use of the disc (3) is allowed by applying certain predefined usage rules.

For this, the decryption control module (55) reads, during a reading step (E23), rules of use (RU) stored in the non-modifiable memory area (35) of the disk (3) and reads, during a reading step (E25), usage data (DU) stored in the storage module (19) of the RFID tag (1). From these rules (RU) and usage data (DU), the decryption control module (55) determines, during a test step (E27), whether the use of the disk (3) is allowed. . Otherwise, the process ends at the end step (E60).

For example, the usage rules specify a maximum number of allowed disk read / write usage, while the usage data indicates the number of times a disk has already been used (read or copied).

It will be noted that the usage data (DU) is updated by the updating module (58) of the reading device (5) after each use of the disk (3). As an example, these usage data contain a counter of the number of uses of the disk made in order to limit its use to a maximum number of predetermined uses. However, it should be ensured that these usage data can not be tampered with fraudulently, for example during an operation that would consist of resetting the data to an earlier value.

For this purpose, the RFID tag (i) has a unidirectional counter (n) which can only be incremented by an entity outside the box, proving that the usage data has not been reset to an earlier value. For this, the reading device uses a conventional method of signature in which it signs the usage data using a signature containing the value of the counter (n). Before each use of the disk (3), the value of the counter (n) stored in the storage module (19) of the tag (1) is read by the decryption control module (55). The control module (55) verifies the signature of the usage data which depends on the value of the counter (n). After each successful use of the disk (3), the usage data and the signature are updated: the decryption control module (55) causes the counter (n) to increment and updates the signature, so the storage module (19) of the RFID tag (1) changes the value of the counter (n) to a new value equal to n + 1.

Thus, for optimal security, the decryption keys (Kj) for decrypting the contents of the disk will be provided to the decryption module (54) of the device! (5) only if the following conditions are fulfilled:

the reading device (5) is authentic and not revoked (conditioning the provision of the decryption keys to the reading device (5) by the RFID tag (I));

the RFID tag (1) is authentic and authorized; and

- The rules of use (RU) stored on the disk (3) and the usage data recorded and updated on the RFID tag (1) allow the use of the disk (3). Of course, for a faster execution of the secure access method according to the invention, the test of each of the aforementioned conditions can be performed in parallel.

In the case where the use of the disk is authorized (test step E27), the control module (55) sends the decryption keys (Kj) during a sending step (E29). In the case where these decryption keys (Kj) have themselves been previously encrypted by the processing module (14) of the RFID tag (1) with the public key (Pd), the control module (55) performs a decryption operation (with the secret key (Sd) ) corresponding to the public key (Pd) to obtain in clear the decryption keys (Kj) of the contents of the disk (3).

On receipt of the keys (Kj), the decryption module (54) executes the decryption of the encrypted digital content (33) of the disk (3), during a decryption step (E31), so as to obtain in clear the content decrypted digital (37) disk (3). Three embodiments of the generation step (E7) of the proof of non-revocation (P) by the reading apparatus (5) and the corresponding verification step (E5) of this proof (P) by the RFID tag (1) will now be described.

The step of generating a proof of non-revocation (P) according to a first embodiment will now be described in detail.

In this first embodiment, the data block (31) of the disk (3) is constituted by the MKB ("Media Key Block") which is established by an Authority as follows.

The MKB is formed of an array comprising a set of n elements whose values are denoted MKBi, each of these values being the result of an encryption operation by a conventional function and a specific key according to the following operation: MKBi = E (Qi), where E is a conventional encryption function, such as DES ("Data Encryption Standard") or AES ("Advanced Encryption Standard"); This is a specific key of a disk with a given content; and i is an index (natural integer) ranging from 1 to n (typically, n = 10 ⁶ ).

During a first substep, the reading device (5) associates with its unique identifier (d), a set (G) of p indices selected from among all the n indices used to index the set of n MKBi elements. It should be noted that the choice of p among n gives a large number of possible combinations, ie: C _p "≈ n ^p / p!

10 ^24/24 !). In this way, it is possible to manage a large number of reading devices from a given MKB (31) which can typically support up to ⁹ read devices.

To achieve this association, the reading apparatus (5) uses a public function W, which associates with the unique identifier (d) the set (G) of p indices: d \ → W (d) = {i _[ , i ₂ , ..., i _p \ ≡ G. For example, p = 4 with ii = 3, 12 = 12, Ï ₃ = 15, i ₄ = 6. In this example, the public W function is a hash function, such as functions belonging to the SHA (Secure Hash Algorithm) family.

In a second sub-step, for each index of the set (G) of the indices

selected, the reading device

(5) reads from the MKB (31) of the disk (3), the value of the MKBi element corresponding to this index and assigns this value to a non-revocation proof value denoted Pi, so as to form a set of constituent elements of the proof of non-revocation (P) such that {Pi = MKBi}, where i is one of the p indices of the set (G) corresponding to the identifier d.

The reading apparatus (5) then sends each value of the constituent elements (Pi) of the proof of non-revocation (P) to the label

RFID (1) which is then responsible for verifying the compliance of each of these values during a verification step which will now be described in detail.

To perform this verification, the RFID tag (1) must know the public hashing function (W), the public classic encryption function (E) and the specific die (C) of a given content disc mentioned above. . This information is previously recorded on the RFID tag (1). In a first sub-step, the verification module (12) of the RFID tag (1) verifies, from the function of hachagβ (W) that each index of the constituent elements (Pi) of the proof of no revocation (P) received is associated with the unique identifier (d) of the reading apparatus (5). For example, if p = 4 then W (d) = {ii _/ h, h, U} where W is a hash function providing 80 bits, i.e. 4 times 20 bits, which gives 4 indices in an MKB block comprising 1 million entries.

In a second sub-step, the RFID tag (1) calculates the elements E (Qi), from the specific key C and the aforementioned indices and verifies that the constituent elements (Pi) of the proof of non-revocation received correspond to the calculated elements E (Qi). In other words, for each index i of the set (P) of p indices, the RFID tag (1) satisfies the following equality: Pi = E (Qi).

In the case where none of the verifications is positive, that is to say if none of the above equalities is valid, then the RFID tag (1) stops the process and no longer provides the decryption keys. (Kj) content. In this case, the reading device is revoked. In fact, for any reading device revoked, the authority records, in p boxes of the MKB of the disk corresponding to this device, non-significant MKBi values, so that this device can not manufacture a proof of non-revocation as described above. during the generation step (E7).

A second embodiment of the steps of generating a proof of non-revocation (P) by the reading device (5) and verification of this proof of non-revocation (P) by the RFID tag (1) will now to be described.

In this second embodiment, a structure according to a binary tree is used to represent all reading devices to be managed by an Authority. A binary tree structure will now be described with reference to FIG. A binary tree (60), in the sense of the graph theory, is formed of a main root (0), a plurality of nodes (6, 8, 5) interconnected by branches. Some of these nodes are leaves (5,

R1-R9) located at the ends of the branches of the shaft (60). Each leaf of the tree corresponds to a reading device.

For example, the binary tree (60) shown in Fig. 3 has seven levels and therefore has ²⁶ = 64 sheets, thereby managing a total of 64 reads. In practice, this type of tree typically contains 33 levels, thus making it possible to control at most 2 ³² ~ 4 x IQ ⁹ reading devices.

On the binary tree (60) of FIG. 3, triangle shaped gray areas (A1, A2, A3) correspond to subtrees containing revoked reading devices. In this example, of the 64 reading devices represented, a total of 9 of them are revoked (R1 to R9). By definition, a subtree is said to be "maximum subtree" if it does not contain any replayed readers and if the root father of that subtree has revoked playback devices.

In the example of Figure 3, the subtree (2) is a maximum subtree (2) because it contains only non-revoked reading devices and the father (6) of the root (8) of this subtree (2) has revoked devices (R4, R5, R6, R7, R8, R9). In this example, the father (6) has a total of 6 revoked devices (R4, R5, R6, R7, R8, R9) contained in the other subtree (4) which depends on this same father (6). Only the non-revocation proof values corresponding to the root of each maximum sub-tree, in association with its position in the binary tree (60), are stored in the MKB (31) of the disk (3).

In this way, it is possible to significantly reduce the amount of information to be stored in the MKB (31) of the disk (3) necessary to enable the reading apparatus (5) to to compile a specific proof of non-revocation (P). This reduction is due to the fact that normally a small fraction of the devices will be revoked. A reference non-revocation proof (PO) value is assigned to the root (0) of the binary tree (60). The generation step (E7) of a proof of non-revocation (P) by the reading apparatus (5) according to this second embodiment will now be described in detail.

The generation module (53) is adapted to calculate the value of the proof of non-revocation (P) of any node, from the value of the proof of non-revocation of the parent node on which this node depends. For that, it knows two functions allowing it to traverse the tree (60): a left function OW ₀ and a right function OWi, defined by the following relations;

P _{0 /} m ₊ l = OW ₀ (Pm) P _l , m _{+ l} = QWi (P _m ) where Po, m ₊ i _/ Pi, _{m +} i represent the value of the proof of non-revocation of a child node located left of his father, respectively to the right of his father. In other words, the value of the proof of non-revocation of the son to the left (respectively to the right) of his father is obtained by applying the left function OW ₀ (respectively OWi) on the value of the proof of non revocation of his father ( P _m ). These two functions are public and similar to hashing functions of the "One-Way functions" type.

Knowing these two functions (OW ₀ , OWi), the reading device (5) calculates its proof of non-revocation (P) recursively, starting from the value of the proof of non-revocation (P8) of the root ( 8) of the maximum subgroup (2) to which it belongs, and using the value of its unique identifier (d).

By way of example, the unique identifier (d) of the reading device (5) consists of a sequence of bits taken from the set {0, 1} indicating the path to reach the reading apparatus (5), where the element "0" indicates a bifurcation to the left and where the element "1" indicates a bifurcation to the right, descending into the tree from the root, i.e., when passing from a parent node (m) to a child node (m + 1).

In the example of FIG. 3, the sequence [110111] indicates the path in the binary tree, starting from the root (0) of the binary tree (60), so as to reach the reading apparatus ( 5) whose identifier (d) is equal to this binary sequence [110111]. In practice, a sequence of 1000 bits must be used to resist certain attacks not described here.

During a first substep, the reading device (5) consults the MKB (31) of the disk (3). If the reading apparatus (5) in question is not revoked, then it obtains in the MKB (31) the proof value of non-revocation (P8) of the root (8) of the maximum subtree (2). to which he

(5) belongs.

During a second substep, by successive application of the right (OWi) and / or left (OW ₀ ) functions, the reading device (5) calculates the value of its proof of non-revocation (P), starting from of the non-revocation value (P8) of the root (8) of this maximum subtree (2) and following the path described in its identifier (d), from the position of the root (8) of the sub maximum tree (2).

The reading device (5) then sends this non-revocation proof value (P) to the RFID tag (1) which then takes care of its verification.

For this purpose, the RFID tag (1) must know the aforementioned functions (OWi) and left (OW ₀ ), the unique identifier (d) of the reading device (5) and the value of the proof of non-revocation of reference (Po) associated with the root (0) of the binary tree (60). From these information, the RFID tag (1) recursively calculates the value of the proof of non-revocation (P ') of the reading device (5).

Once this calculated value (P '), the RFID tag (1) Ia compares to the value (P) provided by the reading device (5). If these two values are equal, the reading device (5) is not revoked. In this case, the RFID tag (1) provides the decryption keys (Kj) of the content to the reading device (5).

A third embodiment of the steps of generating a proof of non-revocation (P) by the reading device (5) and checking this proof of non-revocation (P) by the RFID tag (1) will now to be described.

In this third embodiment, the data block (31) of the disk (3) is constituted by the MKB which is established in the following manner. The MKB consists of a table with a set of m records.

Each record relates to a set of unrevoked reading devices for which the identifiers are consecutive. Groups of consecutive identifiers corresponding to non-revoked reading devices are thus formed. All of the groups constitute all the identifiers of the unrevoked devices. For each group, we record the lowest value identifier dl, the highest value identifier d2 and a seal S. S is a value calculated by a

Authority and is the result of an encoding operation by a conventional function F as a function of dl, d2 and a coding key k shared by the Authority and the RFID tag (1). More precisely, S = F _k (d1, d2).

For example, F is a digest signed with the key k and k is a secret key held by the Authority and the RFID tag (1).

Alternatively, F is a function DES in MAC mode (for "Message Authentication Code") and k is a secret key or F is a RSA type function and k is the secret key of a couple (secret key, public key) known to the Authority.

During a first generation sub-step, the reading device (5) reads the array MKB until it finds a record containing on the one hand a value of dl smaller than the value d of its identifier and, on the other hand, a value of d2 greater than d (dl <d <d2).

In the case where no record matches, this means that the reading apparatus (5) is revoked and the process stops.

In the opposite case where a record is found, the reading apparatus (5) transmits this record, also called proof of non-revocation, as well as the value d to the RFID tag (1) in a second generation sub-step. .

If the value d has already been transmitted to the tag, especially during the authentication step El, only the content of the record can be transmitted.

During a first verification step, the RFID tag (1) receives this record and verifies that the identifier d of the reading device has a value between the received values d1 and d2.

If the value of d is not between these values, the RFID tag stops the process and does not provide the decryption keys (Kj) of the content.

Otherwise, the electronic tag, during a second verification step, checks the seal S received with a key inserted into the RFID tag (1) during its manufacture. In the case where the function F used for the calculation of the seal S is a secret key function, the key inserted in the electronic tag is the coding key k used by the Authority. If the function F is a public key function, the key inserted in the electronic tag is the public key associated with the secret key used by the Authority. If the check is positive, it means that the reading device is not revoked. In case of successful attack of a reading device and obtaining a content in clear, techniques of "Digital Watermarking" of hiding in the contents of a disk information called fingerprints or fingerprints allow an Authority to recognize, in view of this content, which reading device is attacked and therefore to revoke it.

FIG. 4 illustrates, by way of example, the provision of decryption keys by the RFID tag (1) to the reading device (5) for decrypting the contents of a disk (3) which includes fingerprints. digital markings.

In known manner, the content of a prerecorded disc is composed of a set of sequences (Ci,... ₁₃ ,..., Ci), some of which are recorded in a redundant manner. Each redundant sequence corresponds to a different version of an existing sequence. In the example of FIG. 4, a first redundant sequence (C'5) juxtaposed with a fifth sequence (C5) has been encrypted using a variant (K'5) of the encryption key (K5) used to encrypt the content of the fifth sequence (C5). Thus, the sequence formed by the juxtaposition (C5, C'5) of the fifth sequence (C5) and the first redundant sequence (C5 ') is called "variant sequence" (Vl).

When reading a variant sequence, the RFID tag (1) provides the reading device (5) only one of the two decryption keys (K'5 or K5) respectively corresponding to one two constituent sequences of the variant sequence (C5 or C'5). The symbol # in FIG. 4 represents the absence of a decryption key supply for the corresponding sequence. Thus, any decryption key of a variable sequence is called variant key (Krj) since it can take one of two values (Kj or K'j). The choice of the value of a variant key (Krj) is determined according to a code (FP _d ) said code "fingerprint" or digital marking code. This code (FP _d ) stores the identifier of each variable sequence of an optical disk in association with information specifying the value of the key to be chosen to decrypt each variant sequence. For example, this information is a binary value taken from the set {0, 1}, where the value "0" indicates the choice of the variant key Kj relative to the conventional sequence Cj, while the value "1" indicates the choice of the key K'j relating to the redundant sequence Cj. In the example of FIG. 4, the code (FP _d ) associated with the reading apparatus is {1001}. During the authentication step (El) of the reading device (5) by the RFID tag (1) described above with reference to FIG. 2, the reading device (5) sends the code (FP _d ) mentioned above on the RFID tag (1) that it stores after reception. This code (FP _d ) is part of the certificate (Cd) provided by the reading device (5) and previously verified by the RFID tag (1) during the step (E12) described above, so that the code (FP _d ) can not be modified by a reading device that has been compromised.

According to one particularity of the invention, any variant key (Krj) of a variant sequence (Cj, Cj) is calculated according to the following operation:

Krj = G (Kfp, B, j) where

G is a cryptographic function (of type DES or AES); B is the bit associated with the position j of the code (FP _d ) mentioned above; Kfp is a secret key of the RFID tag (1) common to a set of RFID tags (1) associated with a disk of the same content.

The result of this operation provides either the decryption key Kj relative to the content of the sequence Cj, or the decryption key K'j relative to the content of the redundant sequence Cj, as a function of the value of the bit B. For example, the decryption key Kj (respectively K'j) relative to the content of the sequence Cj (respectively of the sequence redundant Cj) is provided if B = O (respectively if B = I). Advantageously, this operation is carried out within the RFID tag (1), thus rendering ineffective the attack of a reading device that would generate any fingerprint code (FP _d ). Indeed, only the authority is able to constitute a set of data (d, Cd, Pd, FP _d ) which will be considered coherent by the label, after all the controls described above.

A reading device that has illegally copied a disc whose exact content of the sequences is known to the Authority may be identified by this Authority, from the fingerprint code (FP _d ) that it has transmitted in the copied disc, through its particular choice of keys at the level of each variant sequence.

Claims

A method for securely accessing a digital content (33) recorded in encrypted form on a data storage means (3), said storage means (3) being associated with a remote interrogable electronic module (1), said electronic module (1) being able to provide at least one decryption key (Kj) for decrypting said content (33) during a decryption step (€ 31), said method being characterized in that it comprises the following steps : - receiving a proof of non-revocation (P) of the reading apparatus (5) generated according to a unique identifier (d) of the reading and data device (31) of said storage means ( 3) data; - verification (E9) of the validity of the proof of non-revocation (P) by the electronic module (1); and - if the proof of non-revocation is valid, supplying said at least one decryption key (Kj) by the electronic module (1) to the reading device (5), so as to carry out the step of decrypting (E31) said content (33).

2. Method according to claim 1, characterized in that it further comprises an authentication step (El) of the reading device (5) by the electronic module (1), so that said at least one key decryption (Kj) is provided by the electronic module (1) to the reading device (5), in case of positive authentication of the reading device (5).

3. Method according to any one of claims 1 and 2, characterized in that the generation (E7) of the proof of non-revocation (P) comprises

The training, according to a unique identifier (d) of the reading apparatus (5), of a set (G) of p indices taken from among indexes indexing elements (MKBi) a data block (31) of said data storage means (3), each of said elements (MKBi) of said block (31) being previously obtained by application of a public cryptographic function (E) and a key (C) specific to the content (33) of the data storage means (3), so that: MKBi = E (Q i) where 1 <i <n, the proof of non-revocation (P) being constituted by a set of constituent elements of this proof denoted Pi such that Pi = MKBi, for any index i belonging to the set (G) of p indices.

4. Method according to claim 3, characterized in that the verification step (E9) of the validity of said proof of non-revocation (P) by the electronic module (1) comprises:

a first verification sub-step, during which the electronic module (1) verifies that each index of the elements (Pi) constituting the proof of non-revocation (P) received is associated with the identifier (d) of the reading apparatus (5); and

a second verification sub-step, during which the electronic module (1) verifies that the constituent elements (Pi) of the proof of non-revocation received correspond to the elements E (Qi), these elements being calculated by the electronic module ( 1) from the indices i of the elements (Pi) received, the cryptographic function (E) and the key (C) specific to the content (33) of the storage means (3).

5. Method according to any one of claims 1 and 2, characterized in that in a data block (31) of the storage means (3) are recorded non-revocation proof values each corresponding to the root node (8) a sub-tree (4) whose ends each correspond to a non-revoked reading apparatus (5), each sub-tree (4) corresponding to a part of a binary tree (60); and in that the generation comprises calculating the value of said non-revocation evidence (P) recursively, from the value of the non-revocation evidence (P8) assigned to the root node (8) of the subtree (2) containing said reading apparatus (5), and according to the unique identifier (d) of the reading apparatus (5).

6. Method according to claim 5, characterized in that the verification step (E9) of the validity of the proof of non-revocation (P) comprises the following sub-steps: - calculation of the value of said proof of non-revocation (P) recursively, from a reference non-revocation proof (PO) value assigned to the root (0) of said binary tree (60) and as a function of the unique identifier (d);

- comparing the value of the proof of non-revocation calculated by the electronic module (1) with the value of the proof of non-revocation provided by the reading device (5).

7. Method according to any one of claims 1 and 2, characterized in that the data (31) of said data storage means (3) comprise a set of records in which are stored a first (d1) and a second ( d2) identifiers and a seal (S) calculated according to said identifiers and an encryption key (k) and in that the generation of the proof of non-revocation comprises a search step in said set of a record for which the identifier (d) of the reading apparatus (5) is greater than the first identifier (d1) and less than the second identifier (d2), and in that the proof of non-revocation contains the desired record.

8. Method according to claim I ₁ characterized in that the verification step (E9) of the validity of the proof of non-revocation (P) by the electronic module (1) comprises:

a first verification sub-step, during which the electronic module (1) verifies that the received identifier (d) has a value between the first identifier (d1) and the second identifier (d2), -a second sub-step; verification, in which the electronic module (1) verifies the received seal using the first and second identifiers and a key associated with said encryption key

9. Method according to any one of claims 1 to 8, characterized in that it further comprises the following steps:

- authentication (E13) of the electronic module (1) by the reading device (5); and interrogating (E19) a local list (560) listing unauthorized electronic modules (1) to check whether said electronic module (1) is authorized, said local list (560) being updated from a list (37) recorded on the storage means (3),.

10. Method according to any one of claims 1 to 9, characterized in that the step of decryption (E31) of the digital content (33) of the storage means (3) is performed according to usage rules (RU) stored on the storage means (3) and according to usage data (DU) stored on the electronic module (I).

11. Method according to any one of claims 1 to 10, characterized in that the digital content (33) of the storage module (3) comprises a set of variable sequences (Vl, V2, V3, Vi.), Each ( Vi) formed by the juxtaposition of two versions (Ci, Ci) of the same content that has been encrypted by separate encryption keys (Ki, KI), and in that the electronic module (1) provides to the reading apparatus (5), for any variable sequence (Vi), a single decryption key (K'i or Ki) for decrypting only one or the other of the two versions, said decryption key being selected according to a digital marking code (FP _d ) specific to the reading device (5) and supplied to the electronic module (1) by the latter (5).

12. An electronic module (1) adapted to be associated with a data storage means (3), said storage means (3) comprising digital content data (33) in encrypted form and intended to be read by a data storage device. reading (5), the electronic module (1) comprising storage means (16) for storing protection data necessary to provide at least one decryption key (Kj) of said content (33), said electronic module (1) being characterized in that it further comprises:

means for receiving a revocation proof (P) of the reading apparatus (5) generated as a function of a unique identifier (d) of the reading and data device (31) of said reading means; storage (3) of data; - verification means (12) for verifying the validity of the proof of non-revocation (P); and

processing means (14) for supplying said at least one decryption key (Kj) to the reading device (5), in case of validity of the proof of non-revocation (P).

13. Use of a data storage means (3) comprising a digital content (33) in encrypted form for reading by a reading apparatus (5) and a non-revocation proof value data block, characterized in that said data is used to calculate a proof of non-revocation to be transmitted to a module electronic device (1) capable of providing at least one decryption key of the digital content (33) if the proof of non-revocation is valid.

14. A revocation device capable of revoking a reading device (5) for reading an encrypted digital content (33) from a storage medium

(3) associated with an electronic module (i) according to claim 12, said device being connected to the reading apparatus (5) and being characterized in that it comprises:

means for generating (53) a proof of non-revocation (P) of the reading device (5) as a function of a unique identifier (d) of the reading and data device (31) of said means for storing (3) data; and

means for sending said proof of non-revocation (P) to the electronic module (1).

15. Reading apparatus (5) comprising means for decrypting (54) an encrypted digital content (33) and stored on a storage means (3) associated with an electronic module (1) according to claim 12, characterized in that it comprises reception means for receiving the decryption keys provided by the electronic module (1), in case of validity of the proof of non-revocation.

16. The reading apparatus (5) according to claim 15, characterized in that it comprises a control module (55) for decrypting the content (33) of the storage module (3) capable of supplying said at least one key of decryption (Kj) to the decryption module (54) in the case where the electronic module (1) is authorized, and / or according to usage rules (RU) and usage data (DU).

A computer program for execution on an electronic module (1) according to claim 12 for enabling secure access to digital content (33) recorded in encrypted form on storage means (3) and for Iu being read by a reading device (5), said electronic module (1) being associated with the storage means (3) and being able to supply the reading device with at least one decryption key (Kj) of said content (33). ), said program being characterized in that it comprises instructions for:

- check the validity of a proof of non-revocation (P) generated according to a unique identifier (d) of the reading and data device

(31) said data storage means (3); and

- Provide said at least one decryption key (Kj) to the reading device (5), in the case where the proof of non-revocation (P) is valid.

A computer program to be executed by a revocation device according to claim 14, to allow secure access to a digital content (33) recorded in encrypted form on a storage means (3) to which is associated with a electronic module (1), said electronic module (1) being able to supply at least one decryption key (Kj) of said content (33) to the reading device (5), said program being characterized in that it comprises instructions for:

generating a proof of non-revocation (P) based on a unique identifier (d) of the reading and data device (31) of said data storage means (3); - send the said proof of non-revocation to the electronic module (1);