Method and apparatus for efficiently encrypting/decrypting digital content according to broadcast encryption scheme

A method of and apparatus for encrypting and/or decrypting content according to broadcast encryption scheme. The decryption method includes: determining whether or not a revoked device among devices that have licenses for predetermined content is present; and according to the determination result, selectively decrypting a content key encrypted by using a key to prevent the revoked device from decrypting the predetermined content. By doing so, an unnecessary encryption process and decryption process that occur when there is no revoked device are avoided.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims the benefit of priority from Korean Patent Application No. 10-2005-0038493, filed on May 9, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention broadly relates to digital content protection and, more particularly, to an apparatus for and a method of efficiently encrypting and/or decrypting digital content according to a broadcast encryption scheme.

2. Description of the Related Art

Recently, the transmission of digital contents using a variety of communication media including the Internet, ground wave, cable, and satellites and sales and rental of digital contents using high capacity recording media have rapidly increased. Accordingly, digital rights management (DRM) that is a solution to protect copyright of digital contents has been emerging as an important issue. Among technologies related to the DRM, research on a broadcast encryption scheme, (by which widely distributed digital contents are protected by encrypting the digital contents broadcast by using recording media, such as a CD, a DVD and the Internet), has been actively conducted.

Referring toFIG. 1, the conventional broadcast encryption scheme uses a 2-stepped encryption process, in which digital content is encrypted by using a content key and the content key used for the encryption is encrypted by using a revocation key. Generally, digital content encrypted by using a content key CK is expressed as E(CK, Content), and the content key encrypted by using a revocation key {Ki} is expressed as {E(Ki, CK)}. Here, the curly brackets { } indicate that the revocation key {Ki} is a key set formed with a plurality of Ki's.

Also, in order to decrypt the content encrypted according to the conventional broadcast encryption scheme, a 2-stepped decryption process should be performed in which the encrypted content key is decrypted by using a revocation key and the encrypted content is decrypted by using the obtained content key.

The revocation key described above is allocated to each of devices to which the broadcast encryption scheme is applied, and among these devices, a device that cannot be protected any more by the broadcast encryption scheme due to exposure of the revocation key or to other reasons is revoked. Thus, revoked device cannot decrypt a digital content complying with the broadcast encryption scheme by using a revocation key that the revoked device has.

FIG. 2illustrates an example of a tree used in the conventional broadcast encryption scheme.

Referring toFIG. 2, one parent node has two child nodes in the tree shown inFIG. 2and the tree is a binary 4-level tree formed with 4 levels. Also, one key is allocated to each of the nodes of the tree.

The top node among the nodes of the tree is referred to as a root node, and a node at the bottom is referred to as a leaf node. According to the conventional broadcast encryption scheme, each of the devices corresponds to one of the leaf nodes and keys {Ki} of nodes, positioned on a path from a respective leaf node of the respective device to the root node, are allocated to the device.

For example, in case of key set allocation of a device1, keys of nodes positioned on the path from the leaf node corresponding to the device1to the root node are K1, K2, K4, and K8and these keys are allocated to the device1.

If all devices are not revoked, a content server performing broadcast encryption encrypts a content key by using only the key K1of the root node, and a device performing decryption corresponding to the broadcast encryption decrypts the encrypted content key by using only the key K1of the root node. Also, if only devices1and2are revoked among the devices, the content server performing broadcast encryption encrypts a content key by using revocation keys K3and K5, and a device performing decryption corresponding to the broadcast encryption decrypts the encrypted key by using the revocation keys K3and K5.

However, according to the conventional broadcast encryption scheme, the content server performing broadcast encryption should encrypt a content key by always using a revocation key, and the device performing decryption corresponding to the broadcast encryption should decrypt the encrypted content by always using the revocation key. In other words, a revocation key is used whether a revoked device is present or not. Accordingly, this increases the computational load on the content server and the device to which the conventional broadcast encryption scheme is applied.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method of protecting digital content by selectively encrypting and/or decrypting a content key according to whether there is a revoked device among devices to which a broadcast encryption scheme is applied.

The present invention also provides a computer readable recording medium having embodied thereon a computer program for executing the method described above.

Illustrative, non-limiting embodiments of the present invention may overcome the above disadvantages and other disadvantages not described above. The present invention is not necessarily required to overcome any of the disadvantages described above, and the illustrative, non-limiting embodiments of the present invention may not overcome any of the problems described above. The appended claims should be consulted to ascertain the true scope of the invention.

According to an aspect of the present invention, there is provided a decryption method including: determining whether or not there is a revoked device among devices having licenses for predetermined content; and according to the determination result, selectively decrypting a content key encrypted by using a key to prevent the revoked device from decrypting the predetermined content, wherein the content key is the key used to encrypt the predetermined content.

According to another aspect of the present invention, there is provided a decryption apparatus including: a determination unit determining whether or not there is a revoked device among devices having licenses for predetermined content; and a decryption unit selectively decrypting a content key encrypted by using a key to prevent the revoked device from decrypting the predetermined content, according to the determination result of the determination unit, wherein the content key is the key used to encrypted the predetermined content.

According to yet another aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for executing the decryption method described above.

According to yet another aspect of the present invention, there is provided an encryption method including: determining whether or not there is a revoked device among devices having licenses for predetermined content; and according to the determination result, selectively encrypting a content key used to encrypt the predetermined content by using a key to prevent the revoked device from decrypting the predetermined content.

According to a further aspect of the present invention, there is provided an encryption apparatus including: a determination unit determining whether or not there is a revoked device among devices having licenses for predetermined content; and an encryption unit selectively encrypting a content key used to encrypt the predetermined content by using a key to prevent the revoked device from decrypting the predetermined content, according to the determination result of the determination unit.

According to an additional aspect of the present invention, there is provided a computer readable recording medium having embodied thereon a computer program for executing the encryption method described above.

According to an additional aspect of the present invention, there is provided a computer readable recording medium having a data structure recorded thereon, the data structure including: a first field recording encrypted content; and a second field recording license information on the content, wherein the second field includes a first subfield that records a value indicating whether or not there is a revoked device among devices having licenses for the content.

DETAILED DESCRIPTION OF THE EXEMPLARY NON-LIMITING EMBODIMENTS OF THE PRESENT INVENTION

The present invention will now be described more fully by describing exemplary, non-limiting embodiments with reference to the accompanying drawings. In the drawings same reference characters denote analogous elements.

Referring toFIG. 3, a home network according to an exemplary embodiment of the present invention is formed with a content server10and six devices11through16.

The content server10encrypts a content key by using a revocation key {Ki} only when there is a revoked device among the devices11through16, and if there is no revoked device among the devices11through16, the process of encrypting a content key by using the revocation key {Ki} is omitted.

Accordingly, only when there is a revoked device among the devices11through16, the devices11through16decrypt the content key {E(Ki, CK)} encrypted by using the revocation key {Ki}, and if there is no revoked device among the devices11through16(as depicted inFIG. 3), the devices11through16omit the process of decrypting the content key {E(Ki, CK)} encrypted by using the revocation key {Ki}.

Thus, by encrypting a content key using the revocation key {Ki} only when there is a revoked device and decrypting the encrypted content key, unnecessary encryption process and decryption process that occurred when no revoked device is present can be eliminated.

FIG. 4is a diagram of the structure of a content encryption apparatus according to an exemplary embodiment of the present invention.

Referring toFIG. 4, the content encryption apparatus according to the exemplary embodiment of the present invention is mounted on the content server10shown inFIG. 3, and is formed with a reception unit101, a revocation determination unit102, a first encryption unit103, a second encryption unit104, a third encryption unit105, a packet generation unit106, and a transmission unit107.

The reception unit101receives a content and license information with respect to the content. The reception unit101may receive this content and the license information related to the content provided by the content producer through a network such as the Internet, or may receive them through a recording medium such as a DVD.

By referring to the license information received in the reception unit101, the revocation determination unit102determines whether or not there is a revoked device among the devices11through16having licenses on the content received in the reception unit101, and according to the determination result, the revocation determination unit102connects the first encryption unit103to the second encryption unit104or to the third encryption unit105.

More specifically, if the revocation key {Ki} in the license information received in the reception unit101is the root key K1, the revocation determination unit102determines that there is no revoked device among the devices11through16having licenses for the content received in the reception unit101, and if the revocation key {Ki} in the license information received in the reception unit101is not the root key K1, the revocation determination unit102determines that there is a revoked device among the devices11through16having licenses for the content received in the reception unit101.

The first encryption unit103encrypts the content received in the reception unit101with a content key corresponding to this content. As the result of the encryption in the first encryption unit103, the encrypted content E(CK, Content) is generated. When the content server10is manufactured, the content key is stored in a place where the key can be protected safely from an intrusion from the outside.

According to the determination result of the revocation determination unit102, the second encryption unit104selectively encrypts the content key used for the encryption in the first encryption unit103by using a revocation key {Ki} to prevent a revoked device from decrypting the content received in the reception unit101. As the result of the encryption in the second encryption unit104, an encrypted content key {E(Ki, CK)} is generated.

More specifically, if it is determined by the revocation determination unit102that there is a revoked device, that is, if the revocation key {Ki} in the license information received in the reception unit101is not the root key K1, the second encryption unit104is connected to the first encryption by the revocation determination unit102and as the result, the content key used for the encryption in the first encryption unit102is encrypted by using a revocation key {Ki}. Also, if it is determined by the revocation determination unit102that there is no revoked device, that is, if the revocation key {Ki} in the license information received in the reception unit101is the root key K1, the second encryption unit104is not connected to the first encryption by the revocation determination unit102, and as the result, the content key is not encrypted by using the revocation key K1.

According to the determination result of the revocation determination unit102, the third encryption unit105generates an encrypted content key E(Kd, CK) by encrypting the content key used for the encryption in the first encryption unit103, or generates a double-encrypted content key E[Kd, {E(Ki, CK)}] by encrypting the content key encrypted by the second encryption unit104.

More specifically, if it is determined by the revocation determination unit102that there is a revoked device, the third encryption unit105generates the double-encrypted content key E[Kd, {E(Ki, CK)}] by encrypting the content key encrypted by the second encryption unit104. Also, if it is determined by the revocation determination unit102that there is no revoked device, the third encryption unit105is connected to the first encryption unit103by the revocation determination unit102and generates the encrypted content key E(Kd, CK) by encrypting the content key used for the encryption in the first encryption unit103.

According to an encryption algorithm between the content server10and the devices11through16, the third encryption unit105encrypts the content key used for the encryption in the first encryption unit103, or encrypts the content key encrypted by the second encryption unit104. For example, if the encryption algorithm between the content server10and the devices11through16is a secret key encryption scheme, the content server10and each of the devices11through16share one secret key and by using this secret key, the third encryption unit105encrypts the content key used for the encryption in the first encryption unit103, or encrypts the content key encrypted in the second encryption unit104.

Also, if the encryption algorithm between the content server10and the devices11through16is a public key encryption scheme, the content server10has respective public keys of the devices11through16and each of the devices11through16has a private key paired with one public key. By using these public keys, the third encryption unit105encrypts the content used for the encryption in the first encryption unit103, or encrypts the content key encrypted in the second encryption unit104.

These secret and public keys can be used with an identical value in a predetermined domain, such as for instance a home network shown inFIG. 3, or can be used with different values for devices11through16disposed in the home network shown inFIG. 3. In the former case, content can be freely distributed in a predetermined domain, such as a home network, and the secret key and the public key are also referred to as a domain key. Meanwhile, in the latter case, content can be distributed to only devices having secret keys among devices disposed in a home network and the secret key and the public key are also referred to as a device key.

The packet generation unit106generates a packet including license information received in the reception unit101, the content encrypted by the first encryption unit103, and the content key encrypted by the third encryption unit105.

FIG. 5is a diagram showing the format of a packet according to an exemplary embodiment of the present invention.

Referring toFIG. 5, the packet according to the exemplary embodiment of the present invention is formed with a license information field51, an encrypted content key field52, and an encrypted content field53.

License information that is based on the license information received in the reception unit101is recorded in the license information field51. The content key encrypted by the third encryption unit105is recorded in the encrypted content key field52. The content key encrypted by the first encryption unit103is recorded in the encrypted content field53.

Referring again toFIG. 5, the license information field51is formed with a revocation presence subfield511, a usage information subfield512, a device information subfield513, a key information subfield514, and a content information subfield515. Those skilled in the art of the exemplary embodiment will understand that the license information field51should include the revocation presence subfield511, and other fields can be included selectively.

A value indicating the determination result of the revocation determination unit102, that is, a value indicating whether or not there is a revoked device, is recorded in the revocation presence subfield511. More specifically, if it is determined by the revocation determination unit102that there is a revoked device, True (binary 1), a value indicating that there is a revoked device, is recorded in the revocation presence subfield511. Also, if it is determined by the revocation determination unit102that there is no revoked device, False (binary 0), a value indicating that there is no revoked device, is recorded in the revocation presence subfield511. That is, the revocation presence subfield511may be a flag indicating whether or not there is a revoked device.

Information on the use of a content, such as copy control information (CCI), is recorded in the usage information subfield512. The CCI is information indicating whether or not the content can be copied, the frequency of permitted copy, and so on.

The identification (ID) of a device having a license for a content to be included in a packet to be generated in the packet generation unit106is recorded in the device information subfield513.

Key information on an encryption algorithm between the content server10and the devices11through16is recorded in the key information subfield514. For example, if the encryption algorithm between the content server10and the devices11through16is a public key encryption method, a public key is recorded in the key information subfield514.

Information on a digital content to be included in the packet to be generated in the packet generation unit106, for example, the content editor, the content owner, and the contents of the digital content, is recorded in the content information subfield515.

The transmission unit107transmits the packet generated in the packet generation unit106to the devices11through16through the home network.

FIG. 6is a diagram of the structure of a content decryption apparatus according to an exemplary embodiment of the present invention.

Referring toFIG. 6, the content decryption apparatus according to the exemplary embodiment is mounted on one of the devices11through16, shown inFIG. 3. By way of an example, the content decryption apparatus may be mounted onto a device11depicted inFIG. 6and is formed with a reception unit111, a packet interpretation unit112, a revocation determination unit113, a first decryption unit114, a second decryption unit115, a third decryption unit116, and a content output unit117.

With reference toFIG. 6, a case where the content decryption apparatus according to the exemplary embodiment is mounted on the device11among the devices11through16shown inFIG. 3will be explained. The following explanation analogously applies to the devices other than the device11when the content decryption apparatus according to the exemplary embodiment of the present invention is mounted.

The reception unit111receives a packet from the content server10through a home network.

The packet interpretation unit112interprets the packet received in the reception unit111, and as the result, detects that the received packet is formed with a license information field51, an encrypted content key field52, and an encrypted content field53, and detects, in particular, that the license information field51is formed with a revocation presence subfield511, a usage information subfield512, a device information subfield513, a key information subfield514, and a content information subfield515.

By referring to the value of the revocation presence subfield511detected according to the interpretation result of the packet interpretation unit112, the revocation determination unit113determines whether or not there is a revoked device among the devices11through16having licenses to obtain content included in an encrypted form in the packet received in the reception unit111, and according to the determination result, the packet interpretation unit112connects the first decryption unit114to the second decryption unit115or to the third decryption unit116.

More specifically, if the value of the revocation presence subfield511detected according to the interpretation result of the packet interpretation unit112indicates that there is a revoked device, that is, the value is True (binary 1), the revocation determination unit113determines that there is a revoked device among the devices11through16having licenses for content included in an encrypted form in the packet received in the reception unit111. Also, if the value of the revocation presence subfield511detected according to the interpretation result of the packet interpretation unit112indicates that there is no revoked device, that is, the value is False (binary 0), the revocation determination unit113determines that there is no revoked device among the devices11through16having licenses for content included in an encrypted form in the packet received in the reception unit111.

The first decryption unit114decrypts an encrypted content key E[Kd, {{E(Ki, CK)}}] or E(Kd, CK) recorded in the encrypted content key field52detected according to the interpretation result of the packet interpretation unit112. As the result of the decryption in the first decryption unit114, the encrypted content key {{E(Ki, CK)}} or the content key CK is generated.

If there is a revoked device, there will be a double-encrypted content key [Kd, {E(Ki, CK)}] recorded in the encrypted content key field52, and if there is no revoked device, there will be an encrypted content key E(Kd, CK) recorded in the encrypted content key field52. However, regardless of whether [Kd, {E(Ki, CK)}] or E(Kd, CK) is recorded in the encrypted content key field52, the first decryption unit114only needs to decrypt data recorded in the encrypted content key field52mechanically by using a predetermined key Kd.

The first decryption unit114decrypts the encrypted content key E[Kd, {E(Ki, CK)}] or E(Kd, CK) according to the encryption algorithm between the content server10and the devices11through16.

For example, if the encryption algorithm between the content server10and the devices11through16is a secret key encryption scheme, the content server10and each of the devices11through16share one secret key and by using this secret key the first decryption unit114decrypts the encrypted content key E[Kd, {E(Ki, CK)}] or E(Kd, CK) by using this secret key.

Also, if the encryption algorithm between the content server10and the devices11through16is a public key encryption scheme, the content server10has respective public keys of the devices11through16and each of the devices11through16has a private key paired with one public key. The first decryption unit114decrypts the encrypted content key E[Kd, {E(Ki, CK)}] or E(Kd, CK) by using this private key.

By using a revocation key {Ki} to prevent a revoked device from decrypting a content included in an encrypted form in the packet received in the reception unit111, the second decryption unit115selectively decrypts the encrypted content key {E(Ki, CK)} according to the determination result of the revocation determination unit113. As the result of the decryption in the second decryption unit115, a content key CK is generated.

More specifically, if it is determined by the revocation determination unit113that there is a revoked device, that is, if the value of the revocation presence subfield511is True (binary 1), the second decryption unit115is connected to the first decryption unit114by the revocation determination unit113, and as the result, the second decryption unit115decrypts the encrypted content key {E(Ki, CK)}, that is, the decryption result of the first decryption unit114, by using a revocation key {Ki}.

Also, if it is determined by the revocation determination unit113that there is no revoked device, that is, if the value of the revocation presence subfield511is False (binary 0), the second decryption unit115is not connected to the first decryption unit114by the revocation determination unit113, and as the result, the second decryption unit115does not decrypt the encrypted content key {E(Ki, CK)} by using a revocation key {Ki}. In this case, the content key CK is already generated by the decryption in the first decryption unit114and the encrypted content key {E(Ki, CK)} does not exist.

The third decryption unit116decrypts the encrypted content E(CK, Content) recorded in the encrypted content field53detected according to the interpretation result in the packet interpretation unit112, by using the content key CK decrypted by the first decryption unit114or the content key CK decrypted by the second decryption unit115according to the determination result of the revocation determination unit113. As the result of the decryption in the third decryption unit116, the content that the content server10desires to provide to the device11is generated.

More specifically, if it is determined by the revocation determination unit113that there is a revoked device, the third decryption unit116decrypts the encrypted content E(CK, Content) recorded in the encrypted content field53detected according to the interpretation result in the packet interpretation unit112, by using the content key CK decrypted by the second decryption unit115. Also, if it is determined by the revocation determination unit113that there is no revoked device, the third decryption unit116is connected to the first decryption unit114by the revocation determination unit113and decrypts the encrypted content E(CK, Content) recorded in the encrypted content field53detected according to the interpretation result in the packet interpretation unit112, by using the content key CK decrypted by the first decryption unit114.

The content output unit117outputs the content decrypted by the third decryption unit116to a user. The content output unit117processes the content in a method corresponding to the characteristic of the content and outputs the content to the user. For example, if the content is in a compressed form, the content output unit117decompresses the content and outputs the result to the user. However, this is provided by way of a non-limiting example and those skilled in the art of the present invention will understand that the content output unit117can process a variety of jobs in order to output the content decrypted by the third decryption unit116to the user.

FIG. 7is a flowchart of a content encryption method according to an exemplary embodiment of the present invention.

Referring toFIG. 7, the content encryption method according to the exemplary embodiment includes the following operations that may be processed successively in the content encryption apparatus of the content server10shown inFIG. 4. Accordingly, though omitted in the following explanation, the descriptions presented above in relation to the content encryption apparatus of the content server10shown inFIG. 4may also apply to the content encryption method according to the exemplary embodiment.

In operation71, the content server10receives content and license information on this content.

In operation72, by referring to the license information received in the operation71, the content server10determines whether or not there is a revoked device among the devices11through16, that have a license for the content received in the operation71.

In operation73, the content server10encrypts the content received in the operation71with a content key corresponding to this content. As the result of the encryption in the operation73, an encrypted content E(CK, Content) is generated.

In operation74, if it is determined in the operation72that there is a revoked device, the content server10encrypts the content key used for the encryption in the operation73, by using a revocation key {Ki} to prevent the revoked device from decrypting the content received in the operation71. As the result of the encryption in the operation74, an encrypted content key {E(Ki, CK)} is generated.

In operation75, if it is determined in the operation72that there is no revoked device, the content server10generates a double-encrypted content key E[Kd, {E(Ki, CK)}], by encrypting the content key encrypted in the operation74with a key Kd according to an encryption algorithm between the content server10and the devices11through16. Also, in operation75, if it is determined in the operation72that there is no revoked device, the content server10generates an encrypted content key E(Kd, CK), by encrypting the content key used for the encryption in the operation73with a key Kd according to an encryption algorithm between the content server10and the devices11through16.

In operation76, the content server10generates a packet including the license information received in the operation71, the content encrypted in the operation73, and the content key encrypted in the operation75.

In operation77, the content server10transmits the packet generated in the operation76to the devices11through16through a home network.

FIG. 8is a flowchart of a content decryption method according to an exemplary embodiment of the present invention.

Referring toFIG. 8, the content decryption method according to the exemplary embodiment includes the following operations that are processed in a time series successively in the content decryption apparatus such as for instance the content encryption apparatus of the device11shown inFIG. 6. Accordingly, though omitted in the following explanation, the description presented above in relation to the content decryption apparatus of the device11shown inFIG. 6may also apply to the content encryption method according to the exemplary embodiment of the present invention.

In operation81, the device11receives a packet from the content server10shown inFIG. 3, for example, through a home network

In operation82, the device11interprets the packet received in the operation81and as the result, detects that the received packet is formed with a license information field51, an encrypted content key field52, and an encrypted content field53, and detects in particular, that the license information field51is formed with a revocation presence subfield511, a usage information subfield512, a device information subfield513, a key information subfield514, and a content information subfield515.

In operation83, the device11decrypts the encrypted content key E[Kd, {E(Ki, CK)}] or E(Kd, CK) recorded in the encrypted content key field52detected according to the interpretation result of the operation82, by using a key Kd according to an encryption algorithm between the content server10and the devices11through16. As the result of the decryption in the operation83, an encrypted content key E[Kd, {E(Ki, CK)}] or E(Kd, CK) is generated. As described above, E[Kd, {E(Ki, CK)}] is a double-encrypted content key encrypted by using a key Kd according to the encryption algorithm between the content server10and the devices11through16, and a revocation key {Ki} to prevent decryption of the content included in an encrypted form in the packet received in the reception unit11. Meanwhile, E(Kd, CK) is a content key encrypted by using only the key Kd according to the encryption algorithm between the content server10and the devices11through16.

In operation84, by referring to the value of the revocation presence subfield detected according to the interpretation result of the operation82, the device11determines whether or not there is a revoked device among the devices11through16that have the licenses for the content included in an encrypted form in the packet received in the operation81.

In operation85, if it is determined in the operation84that there is a revoked device, the device11decrypts the encrypted content key {E(Ki, CK)} that is the result of the decryption in the operation83, by using a revocation key {Kl} to prevent decryption of the content included in an encrypted form in the packet received in the operation81. As the result of the decryption in the operation85, a content key CK is generated.

In operation86, if it is determined in the operation84that there is a revoked device, the device11decrypts the encrypted content E(CK, Content) recorded in the encrypted content field53detected according to the interpretation result of the operation82, by using the content key CK decrypted in the operation85.

Also, in operation86, if it is determined in the operation84that there is no revoked device, the device11decrypts the encrypted content E(CK, Content) recorded in the encrypted content field53detected according to the interpretation result of the packet interpretation unit112, by using the content key CK decrypted in the operation83. As the result of the decryption in the operation86, the content that the content server10desires to provide to the device11is generated.

In operation87, the device11outputs the content decrypted in the operation86to the user.

According to the exemplary embodiments of the present invention, only when there is a revoked device among devices to which the broadcast encryption scheme is applied, a content key is encrypted by using a revocation key, and the encrypted content key is decrypted. By doing so, an unnecessary encryption process and decryption process that occurs when there is no revoked device are not performed any more. As a result, contents can be more efficiently protected.

The exemplary embodiments of the present invention can be written as computer programs and can be implemented in general-use digital computers that execute the programs using a computer readable recording medium. Also, the data structure used in the exemplary embodiments of the present invention described above can be recorded on a computer readable recording medium through a variety of ways.

Examples of the computer readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), and optical recording media (e.g., CD-ROMs, or DVDs).