KR101455827B1 - Method for managing group keys in multi-cast service - Google Patents

Abstract

The present invention relates to a method of managing a group key according to a logical hierarchical structure for a multicast service. A method of managing a group key in a key distribution center of the present invention includes receiving a participation request message including resource information of the user terminal from a user terminal; Determining a tree structure for group key provision based on the resource information; And distributing the group key to the user terminal device based on the determined tree structure, wherein, when forming the tree structure for distributing the group key, considering the amount of traffic and the amount of storage of the user terminal device as well as the communication traffic, It is possible to efficiently distribute the group key to the terminal device.

Multicast service, group key, binary tree, tree structure

Description

[0001] The present invention relates to a group key management method for a multicast service,

The present invention relates to a group key management method, and more particularly, to a method of managing a group key according to a logical hierarchical structure for a multicast service.

With the development of 4G network technology, it is expected that high-speed data service will be available at low cost, and demand for services such as IPTV, VoD, music streaming and personal streaming will increase. It is efficient to send such multimedia data at one time rather than sending it to each user. Thus, multicast services for services such as multimedia will be a major service in 4G networks. However, since the multicast service needs to be accessible only to authorized users, technologies for access control and techniques for efficiently providing such access control are being developed.

The present invention proposes a method of managing a group key according to a logical hierarchical structure for a multicast service.

A method for managing a group key in a key distribution center according to an aspect of the present invention includes: receiving a participation request message including resource information of a user terminal from a user terminal; Determining a tree structure for group key provision based on the resource information; And distributing the group key to the user terminal device based on the determined tree structure.

According to another aspect of the present invention, there is provided a method of managing a group key in a user terminal, the method comprising: transmitting a join request message including resource information of a user terminal to a key distribution center; And receiving a key distributed according to a tree structure for group key provision determined based on resource information from a key distribution center.

According to another aspect of the present invention, a terminal apparatus includes a processor; A memory for storing keys necessary for the multicast service; And a communication unit for performing communication via a network, wherein the processor transmits a participation request message including resource information of a user terminal device included in the memory to a key distribution center, And stores the received key in the memory according to a tree structure for providing a group key determined based on the received key.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. In addition, the terms described below are defined in consideration of the functions of the present invention, which may vary depending on the intention of the user, the operator, or the custom. Therefore, the definition should be based on the contents throughout this specification.

1 is a diagram showing an example of a logical key hierarchy.

In order to provide a multicast service, broadcast encryption (Broadcast Encryption) is used to effectively transmit information only to desired users of a key distribution center (i.e., broadcasting center) as a sender among all users. The most important nature of BE is revocation or exclusion of unwanted users (for example, illegal users or expired users).

BE consider transmission overhead, storage overhead, and computation overhead. The amount of transmission means the amount of transmission of a message to be transmitted on the transmission side and the amount of storage is the amount of the secret key that the user terminal device has to store and the amount of computation is the amount of data to be transmitted by the user terminal device in order to obtain the data decryption key Means the amount of calculation required.

As a key management method for providing a multicast service, a logical key hierarchy is used. LKH is a key update method in which a group key is transmitted to several users (or members) at a time using a logical hierarchical layer called a KEK (Key Encryption Key). This reduces the number of messages required because each group key can be passed to multiple users at a time, compared to sending keys to users, respectively.

In such a tree structure, each node corresponds to one group key, and each user-node corresponds to one member. It is assumed that each member knows the key from its member node to the root and can not know the other keys. The nodes (node keys) of the tree are divided into internal nodes and member nodes (member unique keys). The lowest nodes in the tree correspond to one member terminal.

1, a TEK (Traffic Encryption Key) is a data encryption key, a KEK (Key Encryption Key) is a group key as an internal key for securely transmitting a TEK, an IK (Individual Key) is a lowest node key corresponding to a member Means the unique key of the member.

Figure 1 shows a key update when the user leaves one. Suppose one user on the far left leaves the group. Because the outgoing user knows the existing TEK and KEK1, we need to update both keys. However, for the two users on the right, only the TEK needs to be updated, and for the one user on the left, both TEK and KEK1 must be updated. The TEK encrypts with each group key, and the group key (KEK1) to be updated is encrypted using the private key of the non-user.

In this tree-based group key management scheme, TEKs and KEKs that encrypt multicast data are stored in the user terminal device. Typically, these keys are stored in a small memory card, for example a secure memory of a Universal Subscriber Identity Module (USIM), which stores the subscriber identification information of the user terminal device.

At this time, when the number of users is N and the number of KEKs is (Log ₂ N-1), a binary structure optimized for the existing communication amount defined by the number of key re-issuing messages is used. However, considering that the USIM card's secure memory size is several hundreds of kilobytes, and other security related data and program codes must also be stored, the size of the secure memory is not sufficient. In particular, if multiple multicast services are used at the same time, the user should have TEK and KEK separately for each service.

The number of decryption operations performed by the USIM every time the TEK is updated is Log ₂ N. For example, if there are 65536 users, 16 decodes must be performed. If the time required to decode 256 bytes is about 3.7 ms, the delay time can not be ignored in real-time applications. For example, in case of VoIP, the maximum allowable delay time is about 50 ms.

Also, if there are multiple key distribution centers (KDCs), new TEKs and KEKs must be received at the handoff between the key distribution centers, so the delay time to receive the service becomes longer when the handoff delay time is considered. Therefore, there is a need for a group key management method that considers the amount of computation and storage of the secure memory of the user terminal, and the amount of communication between the user and the key distribution center.

2 is a diagram illustrating a k-level CCST tree according to an embodiment of the present invention.

The tree for group key management according to an exemplary embodiment of the present invention is a tree structure capable of controlling the amount of computation and storage amount of the secure memory of the user terminal, and the tradeoff of communication overhead between the user and the key distribution center (KDC). The tree structure according to an embodiment of the present invention is referred to as a k-level CCST (Communication-Computation-Storage Tradeoff) tree.

The overall level of the k-level CCST tree is k + 1. The k-level CCST tree has the form of a flat tree in which the number of branches distributed to the top nodes in the key distribution center is variable, and the number of branches distributed to the nodes in the lower layer of the remaining hierarchical nodes is 2 Structure. That is, unlike the conventional binary tree structure, the CCST tree structure according to the embodiment of the present invention is not proportional to the number of users who receive the group key using the number of hierarchies and the tree structure.

According to an embodiment of the present invention, when a participation request message is sent to a key distribution center (KDC), users transmit their own resource information including the available size of the current secure memory region and the delay time information required for decrypting the group key do.

The key distribution center (KDC) receiving the participation request determines the maximum allowable level (k ') that the user terminal device to participate in the service can accommodate in consideration of the amount of storage, the amount of calculation, and the amount of communication of the secure memory. According to an embodiment of the present invention, the maximum allowable level (k ') is less than a level determined based on the available memory space of the secure memory of the user terminal devices, and the user terminal device, for example, The maximum allowable communication amount can be determined to be smaller than the level that can be decrypted during the maximum allowed communication amount,

Also, according to an embodiment of the present invention, the amount of calculation in CCST is determined by the product of the unit decoding delay time and k, and the amount of storage can be determined by the product of k and key size. In CCST, traffic can be determined by averaging the key rekeying messages.

If the allowable level k 'is greater than or equal to the existing level k, which is the level of the binary structure included in the current tree structure, the new user can be accommodated while maintaining the existing level k, If not, it can be accepted or rejected according to the policy of the Key Distribution Center (KDC).

Also, if the allowable level (k ') is greater than or equal to the existing level (k), check whether the current CCST tree is a complete k-level CCST tree. A complete CCST tree means a tree structure in which a group key can no longer be distributed to a user terminal device to be newly joined while maintaining the current hierarchical level. In the case of a complete k-level CCST tree, if a new user is added as a member, one of the existing members will have (k + 2) keys and the decryption time will increase accordingly.

To this end, the key distribution center (KDC) should broadcast a message to the users currently participating in the service to delete the KEKs at the top level in the current tree, and send a join response message to the new user. By doing so, the number of keys that members need to hold can be kept no more than k. If it is not a complete k-level CCST tree, simply send a join response message to the new member.

When the allowable level k 'is smaller than the existing level, the key distribution center accepts the existing level k by changing it to the allowable level k' of the user terminal to be newly joined, May refuse to participate in the service. (K ') from the bottom of the tree in case of a complete k-level CCST tree when accepting the key (k') from the bottom of the tree, You must clear the above key.

According to an embodiment of the present invention, when a member having a low-performance secure memory leaves the group and the CCST level may become high, or when too many members leave and the total level of the CCST becomes lower than (k + 1) During the reissue period, the KEKs that need to be added to increase the level can be encrypted and transmitted to all members.

When the group key is distributed using the CCST structure according to an embodiment of the present invention, the group key of the highest level layer of the tree can be deleted or generated to maintain the binary level k at a certain level, The number of group keys to be stored can be kept constant. Therefore, the amount of traffic and the amount of computation of the user terminal can be reduced while the traffic is maintained at a level similar to that of the binary tree.

3A is a diagram illustrating a key management method based on a CCST tree structure according to an embodiment of the present invention.

FIG. 3A shows a case where the level (k) of the current tree is 2. FIG. Also, in FIG. 3A, it is assumed that the maximum allowable level (k ') of participating members is greater than k.

When the U8 a new user requests a service part in an existing tree structure 310, a key distribution center (KDC) is assigned the KEK (K ₂ and K _6), and the TEK to the user U8, such as the tree structure 320 service So that the user U8 can receive the service.

In addition, when a new user U9 requests a service participation in the tree structure 320, the key distribution center (KDC) can not accommodate a new user U9 in the current tree structure 320 without raising the tree level , Transform the tree structure. The form of a tree that can not distribute the group key to a new user, such as the tree structure 320, is referred to as a complete k-level CCST tree.

In the tree structure 320, the existing level k is 2, and the key distribution center KDC, as shown in the tree structure 330, And broadcasts a message to the user, i.e., the members, to delete the top keys K ₁ and K ₂ . The members receiving the delete message can delete the group keys designated in the delete message and continue to receive the service using the remaining keys.

Then, the new user is to U9 allowed to participate in the service to distribute a group key (K ₃ and K _7). That is, in the tree structure 330, the nodes located at the uppermost layer are deleted and changed to the tree structure 340.

Thus, according to one embodiment of the present invention, The center KDC accommodates the new user U9 without increasing the size of the binary tree level so that the new user U9 can receive the service without increasing the number of group keys that the users participating in the service should have.

FIG. 3B is a diagram illustrating a key management method based on a CCST tree structure according to another embodiment of the present invention.

When the user U9 leaves the service group in the tree structure 350 and the user U9 is a low-performance user terminal device and other users can operate in a tree structure higher in level than the existing tree structure, the CCST May be increased. Accordingly, the key distribution center (KDC) can form a tree structure 360 by encrypting and transmitting group keys (K ₁ and K ₂ ) to be added to increase the level of the CCST to all the members. That is, a new layer may be created in the top layer in the tree structure 350 and transformed into a tree structure 360.

4A is a diagram illustrating a key management protocol according to an embodiment of the present invention.

4A shows a key tree management protocol until the tree becomes a complete k-level CCST tree. The user terminal device to participate in the service transmits the join request message including the available size of the memory and the unit decoding delay time. Then, the key distribution center (KDC) transmits a participation permission response message including the KEK (1), ... KEK (i), and TEK to the user. Here, 1? I? K.

3A, the KDC includes a first-level group key KEK (1), a second-level KEK (2), and a TEK from the top of the tree in the participation permission response message in the tree structure 320 in the tree structure 310 .

4B is a diagram illustrating a key management protocol according to another embodiment of the present invention.

4B shows a key tree management protocol when the tree becomes a complete k-level CCST tree. The user who wants to participate in the service sends a participation request message including the available size of the memory and the unit decoding delay time. Then, the key distribution center (KDC) can keep the level of the CCST tree constant by broadcasting a message to the currently participating user terminal devices to delete the highest level KEKs. Then, the key distribution center (KDC) sends a participation acceptance response message including the KEK (j + 1), ..., KEK (j + k), and TEK to the user terminal device to participate in the service, Can be accepted. Where j is an integer greater than one.

3A, the key distribution center (KDC) is a user terminal device to which a key distribution center (KDC) transmits a second level KEK (2), a second level KEK A Join Accept Response message including the third level KEK (3) and the TEK.

5 is a flowchart illustrating a method of managing a group key in a key distribution center according to an exemplary embodiment of the present invention.

The key distribution center (KDC) receives a group participation request message including resource information of the user terminal device from a user terminal device to be newly joined (S 510). Here, the resource information of the user terminal includes the available memory size for storing the group keys and the delay time information for decrypting the group key.

The key distribution center (KDC) determines a tree structure for providing a group key based on the resource information (S 520). According to an embodiment of the present invention, a key distribution center (KDC) calculates a storage amount, a calculation amount, and a communication amount of a user terminal device based on resource information, and calculates a hierarchy of tree structure (K '). Then, the key distribution center (KDC) determines the tree structure based on the allowable level (k ').

In addition, the tree structure according to an embodiment of the present invention can be modified by deletion or generation of nodes located in the highest hierarchical layer. On the other hand, when the determined tree structure is a complete tree structure in which the group key can not be distributed to the user terminal, the tree structure can be modified by deleting the keys located at the highest hierarchical level in the determined tree structure.

Then, the key distribution center distributes the group key to the user terminal device based on the determined tree structure (S 530).

6 is a flowchart illustrating a method of managing a group key in a key distribution center according to another embodiment of the present invention.

The key distribution center (KDC) receives the participation request message including the resource information of the user terminal from the new user terminal (S610).

The key distribution center (KDC) calculates the amount of storage, the amount of traffic and the amount of traffic of the user terminal based on the resource information, and calculates the allowable level of the hierarchy of the newly requested tree structure k ') (S612).

The key distribution center (KDC) compares the allowable level (k ') with the level (k) of the binary structure of the current tree (S 614).

If the allowable level k 'is equal to or higher than the level k, it is confirmed whether it is a complete k-level CCST tree (S 616). In the case of a complete K-level CCST tree, the deletion message of the group key from the bottom of the tree to the k-th layer is broadcasted to the current user terminal devices participating in the service (S 618). Then, a joining permission message including TEKs and KEKs is transmitted to the new user terminal device (S620).

On the other hand, if it is not a complete K-level CCST tree, a new user terminal device can be accommodated in the currently determined CCST tree, and thus a join permission message including TEKs and KEKs is transmitted to the new user terminal device (S620).

When the allowable level k 'is smaller than the level k, the allowable level k' is compared with the level k of the binary structure of the current tree S 614. When the device participates in the tree structure of the current state, it means that it is difficult to receive a normal service in consideration of the storage amount, calculation amount, and communication amount of a new user terminal device. Accordingly, the key distribution center (KDC) determines whether to accept a new user terminal device requesting participation according to a predetermined policy (S622). If it is determined not to accept the new user terminal, the key distribution center (KDC) sends a reject message to inform the new user terminal device that it can not join the group (S632).

When it is decided to accommodate a new user terminal, the key distribution center (KDC) forms a tree structure based on the number of layers corresponding to the allowable level (k '). To this end, the key distribution center sets level k to an acceptable level k '(S 624). That is, the hierarchy of the current tree structure is made hierarchical to a level that can be accommodated by a new user terminal.

At this time, it is confirmed whether the determined tree structure is a complete k-level CCST tree (S 626). In the case of a complete k-level CCST tree, the key distribution center (KDC) broadcasts a deletion message for deleting group keys located at a level higher than the k'th level from the bottom in the CCST tree structure to user terminal devices included in the current group (S628). By doing so, a tree structure in which the level at the k < th > level or more of the tree structure is reduced is determined. Then, the key distribution center (KDC) transmits a participation grant message including the group keys necessary for receiving the service in the determined tree structure to the new user terminal device (S620).

On the other hand, if it is determined that the determined tree structure is a complete k-level CCST tree (S 626), and if the determined k-level CCST tree is not a complete k-level CCST tree, group keys higher than the k'th level from the bottom of the tree structure are deleted S 630). By doing so, a tree structure in which the hierarchical level higher than the k'th level of the tree structure is reduced is determined. Then, the key distribution center (KDC) transmits a join permission message including the group keys necessary for receiving the service in the tree structure formed in the new user terminal (S620).

In FIG. 6, the key is deleted (S 618, S 628, S 630). However, the hierarchy of the group keys to be deleted may differ from case to case.

7 is a flowchart illustrating a method of managing a group key in a user terminal according to an exemplary embodiment of the present invention.

A user terminal according to an embodiment of the present invention includes a processor (not shown) for controlling overall operation, a memory (not shown) for storing keys necessary for the multicast service, and a communication unit (Not shown).

A processor (not shown) of the user terminal controls a communication unit (not shown) to transmit a group join request message including resource information of the user terminal to the key distribution center KD 710.

The communication unit of the user terminal receives the group key distributed according to the tree structure for providing the group key determined based on the resource information from the key distribution center (KDC) (S720). The processor stores the group key in memory. The user terminal device can decrypt the content using the received group key and participate in the multicast service.

On the other hand, upon receiving a message to delete the group keys of the upper layer specified in the tree structure for group key distribution from the key distribution center (KDC), the processor of the user terminal deletes the group keys indicated by the message in the memory. Thereafter, the content is decrypted using the deleted and remaining group keys.

Also, upon receiving a group key generation message including a group key of a higher layer specified in the tree structure for group key distribution from the key distribution center (KDC), the user terminal device stores the group key included in the message In addition, the contents can be decrypted using the keys.

According to an embodiment of the present invention, when forming a tree structure for distributing group keys, it is possible to efficiently distribute the group key to the user terminal device considering the amount of traffic, the amount of calculation, and the amount of storage. Therefore, the level of the tree structure for group key distribution can be varied even in an environment where user terminal devices having various performances exist, so that a multicast service can be efficiently provided to user terminal devices having different performances.

The key management method according to an embodiment of the present invention can be applied to provide a multicast service in various communication environments such as WiBro, WiMAX, and WSN.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Therefore, the scope of the present invention should not be limited to the above-described embodiments, but should be construed to include various embodiments within the scope of the claims.

1 is a diagram showing an example of a logical key hierarchy,

FIG. 2 is a diagram illustrating a k-level CCST tree according to an embodiment of the present invention,

3A is a diagram illustrating a key management method based on a CCST tree structure according to an embodiment of the present invention,

FIG. 3B is a diagram illustrating a key management method based on a CCST tree structure according to another embodiment of the present invention,

4A is a diagram illustrating a key management protocol according to an exemplary embodiment of the present invention,

4B is a diagram illustrating a key management protocol according to another embodiment of the present invention,

5 is a flowchart illustrating a method of managing a group key in a key distribution center according to an exemplary embodiment of the present invention,

6 is a flowchart illustrating a method of managing a group key in a key distribution center according to another embodiment of the present invention,

7 is a flowchart illustrating a method of managing a group key in a user terminal according to an exemplary embodiment of the present invention.

Claims (14)

A method for managing group keys in a key distribution center, The key distribution center receiving a participation request message including resource information of the user terminal from a user terminal; The key distribution center determining a tree structure for providing a group key based on the resource information; And And distributing the group key to the user terminal device based on the tree structure determined by the key distribution center. The method according to claim 1, Wherein the resource information includes an available memory size for storing a group key and delay time information for decrypting the group key. The method according to claim 1, Wherein the tree structure is a flat structure in which the number of branches distributed to nodes in the highest hierarchical layer is variable and the number of branches distributed to lower hierarchical nodes in the remaining hierarchical layers is a binary structure of 2 . The method according to claim 1, Wherein the tree structure is modified by deletion or generation of nodes located at the highest hierarchical level. The method according to claim 1, The step of determining the tree structure comprises: Calculating a storage amount, a calculation amount, and a communication amount of the user terminal based on the resource information, determining an allowable level (k ') of a hierarchy of tree structure acceptable to the user terminal according to the calculation result, And the tree structure is determined based on the allowable level (k '). 6. The method of claim 5, In the step of determining the tree structure, Wherein when the determined tree structure is a complete tree structure in which the group key can not be distributed to the user terminal device, the tree structure is modified by deleting the keys located in the highest hierarchical layer in the determined tree structure How to manage. 6. The method of claim 5, The step of determining the tree structure comprises: When the determined tree structure is a complete tree structure in which the group key can not be distributed to the user terminal device when the allowable level k 'is equal to or higher than the level k of the binary structure included in the current tree structure, And broadcasting the deletion message for deleting the keys located at the highest layer in the determined tree structure to the user terminal devices participating in the current service. 6. The method of claim 5, If the allowable level k 'is smaller than the level k of the binary structure included in the current tree structure, And the tree structure is modified based on the number of layers corresponding to the allowable level (k '). A method for managing a group key in a user terminal apparatus including a processor, a memory for storing keys required for a multicast service, and a communication unit for performing communication via a network, Controlling the communication unit such that the processor transmits a participation request message including resource information of a user terminal device included in the memory to a key distribution center; And And the processor receives the key distributed from the key distribution center via the communication unit according to a tree structure for providing a group key determined based on the resource information. 10. The method of claim 9, Wherein the resource information includes an available memory size for storing a group key and delay time information for decrypting the group key. 10. The method of claim 9, Wherein the group key management unit deletes the group keys indicated by the message from the memory when the processor receives a message to delete the group keys of the upper layer specified in the tree structure for group key distribution from the key distribution center . A processor; A memory for storing keys necessary for the multicast service; And And a communication unit for performing communication via a network, The processor controls the communication unit to transmit a participation request message including resource information of a user terminal device included in the memory to a key distribution center, Wherein the key distribution center stores the key in the memory when a key distributed according to a tree structure for providing a group key determined based on the resource information is received from the key distribution center. 13. The method of claim 12, Wherein the resource information includes an available memory size for storing a group key and a delay time for decoding the group key. 13. The method of claim 12, Wherein the processor deletes the group keys indicated in the message from the memory when a message to delete the group keys of the upper layer specified in the tree structure for group key distribution from the key distribution center is received.

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