WO2007123224A1

WO2007123224A1 - Method of generating secure tickets for a new secure group, method of secure group communication, computing device having a recording medium, and network system

Info

Publication number: WO2007123224A1
Application number: PCT/JP2007/058692
Authority: WO
Inventors: Chuan-Feng Chiu
Original assignee: Panasonic Corporation
Priority date: 2006-04-20
Filing date: 2007-04-17
Publication date: 2007-11-01
Also published as: CN101060398A

Abstract

A method of generating secure tickets for a new secure group is characterized by that, when a group change occurs in an original secure group, such as joining or leaving of a secure group, a central server center will be notified to find a minimum number of keys that can protect all group members of the new secure group and to subsequently generate a corresponding number of secure tickets each of which contains a new secure group key. Finally, after encryption using corresponding keys, encrypted messages with each containing a corresponding secure ticket are sent to all members of the new secure group for secure update of the new secure group key. Therefore, the number of secure tickets created by the central server center can be lowered to reduce computational amount and to reduce loading.

Description

DESCRIPTION

METHOD OF GENERATING SECURE TICKETS

FOR A NEW SECURE GROUP, METHOD OF SECURE GROUP COMMUNICATION,

COMPUTING DEVICE HAVING A RECORDING MEDIUM,

AND NETWORK SYSTEM

Technical Field The invention relates to a method of group communication, and more particularly to a method and apparatus for creating secure tickets in dynamic group communication.

Background Art With the progress and convenience of communication services, services constructed on group communication, such as video conferences, internet telephones, chat rooms, etc., are increasing day by day. In group communication, ensuring privacy of communication is an important consideration, and therefore in order to establish the security of group communication, a secure group key is usually utilized to protect information transmitted in a group. This secure group key is generated by a key distribution center (hereinafter referred to as KDC) and is sent to group members.

Figure 1 shows a typical secure group communication system. The system 1 includes a KDC 11 and a plurality of group members A, B, C and D. Each of members A, B, C and D connects with others via the Internet 115. With further reference to Figure 2, in the following example, it is assumed that members A and B belong to a secure group, and members C and D belong to another secure group. When members A, B, C and D intend to form a new secure group, with member A as a group initiator (or group leader), member A will first issue a request to the KDC 11 to retrieve a new secure group key (step 100). When the KDC 11 receives this request, secure tickets T_A, T_B, TC and T₀ each of which contains a secure group key k_Aβco are created for the members A, B, C and D of the new secure group, respectively (step 101 ). After encrypting the secure tickets T_A, T_B, T₀ and T_D using existing corresponding keys W_A, K_B, k_c and kp, the KDC 11 transmits the same to member A (step 102), who then distributes them to the other members B, C and D (steps 103, 104, 105). After the secure ticket T_A is decrypted by member A using the key k_A, the secure group key k_ABC_D is stored by member A. Likewise, members B, C and D can decrypt the secure tickets T_{B 1}T_C and T_D using kβ, kc and ko respectively to derive the secure group key k_ABC_D as well.

After the aforementioned steps, a new secure group can be formed among members A, B, C and D based on the secure group key kABCD-

It is apparent from the foregoing that, when a group change occurs, such as when the original two groups are to form a new secure group, the KDC 11 must respectively create secure tickets T_A, T₆, TC and T_D each of which includes the key k_ABCD for members A, B, C and D of the new secure group. In the above example, if the number of members of the new secure group is 4 (members A, B, C and D), the KDC 11 must create 4 secure tickets and transmit the same to members A, B, C and D after encryption using corresponding keys kA, kβ, kc and kσ. In other words, when the number of members of the new secure group to be formed becomes larger, the number of secure tickets created by the KDC 11 becomes correspondingly more, and the loading of the KDC 11 becomes correspondingly heavier.

The above problem will also occur in the following situations of group change, including but not limited to the following situations: when a new member intends to join an original secure group, or when a member of an original secure group intends to leave the original secure group, the KDC 11 must create a new secure ticket for each member of the new secure group. For the KDC 11 , not only the computational amount grows larger, but the loading is also correspondingly heavier, thereby causing inefficiency.

Currently, there are a number of applications related to generation and distribution of group keys in group communication. For example, in U.S. Patent Application Publication No. 20050050004, receiving a group key from a neighboring wireless device, one can join a specific secure group. However, the problem with this application is that, when the number of members in the secure group is large, since members must have shared keys thereamong, each wireless device must store a large amount of secure keys. In other words, each member has to store the secure key of every other member and keys of all secure groups, thereby resulting in a storage burden of secure keys for wireless devices.

U.S. Patent No. 6240188B1 discloses another group key management system and method, and provides secure many-to- many communication. Group keys are transmitted and managed through a key tree structure. The problem of this patent is that: due to the hierarchy of the tree structure, when a new secure group is to be formed, a set of new group keys must be generated anew for the key tree structure. Therefore, for the central server that creates the group keys, not only its computational amount is larger, but its loading is also correspondingly larger.

U.S. Patent Application Publication No. 20050018853 provides a method of updating a secure group key in a group. The restriction in this application is the same as that of the previous case in that there is a hierarchal tree structure. Therefore, when several groups are to form a new secure group, a set of new group keys must be generated anew for the key tree structure. For the central server that creates the group keys, not only its computational amount is still larger, but its loading is also correspondingly larger.

D i s c l o s u re of I n v e n t i o n

Therefore, one object of the present invention is to provide a system and method for creating secure tickets using existing keys of original groups when a group change occurs, thereby reducing the number of secure tickets created, reducing the computational amount of a KDC, and reducing the loading of the KDC.

According to one aspect of the present invention, a method of generating secure tickets for a new secure group is provided, which is applied when a change occurs in a plurality of existing secure groups to form a new secure group. Each of the existing secure groups includes at least one member and has a corresponding dedicated group key. The method comprises the following steps when a group change notification is received. First, a new secure group key is generated according to the group change notification. Next, a combination that can form the new secure group in a minimum selection of the existing secure groups is found, and according to the relationship in the combination, secure tickets whose number corresponds to that of selected secure groups are generated, where each secure ticket has the new secure group key.

According to another aspect of the present invention, a computer-readable recording medium that can be disposed in a computing device so as to be read for executing the following steps is provided. First, according to a received group change notification, a new secure group key is generated. Next, according to pre-formed keys, a minimum number of secure tickets that can protect all group members of a new secure group are generated, wherein each secure ticket has the new secure group key. Finally, a plurality of encrypted messages each of which contains a corresponding secure ticket and is encrypted using a corresponding key are outputted for securely conducting update of the new secure group key among all members of the new secure group. According to yet another aspect of the present invention, a secure group network system constructed in an Internet environment is provided. The secure group network system includes at least one secure group and a central key distribution center. When a group change occurs to form a new secure group, a group change notification will be issued to the central key distribution center to generate a new secure group key applicable to all group members of the new secure group. Then, a minimum number of secure tickets that can protect all group members of the new secure group are generated, wherein each secure ticket has the new secure group key. Finally, a plurality of encrypted messages each of which contains a corresponding secure ticket and is encrypted using a corresponding key are respectively transmitted to all members of the new secure group for securely conducting update of the new secure group key. B r i ef D e s c r i p t i o n of D r aw i n g s

Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which:

Figure 1 is a system block diagram illustrating a conventional secure group communication system framework;

Figure 2 is a flowchart to illustrate relevant steps executed by the conventional system when a group change occurs;

Figure 3 is a system block diagram illustrating the preferred embodiment of a secure group communication system framework according to the present invention;

Figure 4 is a block diagram illustrating a central distribution server device in the preferred embodiment of this invention;

Figure 5 is a block diagram illustrating a communication device in the preferred embodiment of this invention;

Figure 6 is a flowchart to illustrate the steps of the first preferred embodiment of this invention; Figure 7 is a flowchart to illustrate detailed steps of step 403 in Figure 6;

Figure 8 is a flowchart to illustrate the steps of the second preferred embodiment of this invention;

Figure 9 is a flowchart to illustrate the steps of the third preferred embodiment of this invention;

Figure 10 is a flowchart to illustrate the steps of the fourth preferred embodiment of this invention;

Figure 11 is a schematic diagram to illustrate a form of data display in the first preferred embodiment of this invention;

Figure 12 is a schematic diagram to illustrate a form of data display in the second preferred embodiment of this invention;

Figure 13 is a schematic diagram to illustrate a form of data display in the third preferred embodiment of this invention; and

Figure 14 is a schematic diagram to illustrate a form of data display in the fourth preferred embodiment of this invention.

B e s t M o d e f o r C a r ry i n g O u t t h e I n v e n t i o n Before the present invention is described in greater detail, it should be noted that, in the following description, like elements are denoted by the same reference numbers. First Preferred Embodiment Referring to Figure 3, in the first preferred embodiment of the present invention, the framework of a secure group network system 8 includes a key distribution center (hereinafter referred to as KDC) 80 and a plurality of group members A, B, C and D. Each of members A, B, C and D connects with others via the Internet 115.

The term "secure group" as used herein may be a single member (such as member A) who has exchanged a key with the KDC 80 or a plurality of members who have a secure group key so as to be able to conduct secure communication thereamong.

Referring to Figure 4, the KDC 80 includes a central distribution server device 110 that has a network connection to the Internet 115 (see Figure 3). The central distribution server device 110 includes a security unit 200 for conducting encryption/decryption operations, a key database 203 for storing keys, a key generating unit 204 for generating new secure group keys, a secure ticket generating unit 202 for generating secure tickets, and a processing unit 201 for coordinating the operations of the aforementioned units. The aforementioned units can be software or firmware program stored in a computer-readable medium. When the program is read and executed by a conventional computer, a secure group key updating method based on the creation of secure tickets according to the present invention can be executed. Suitable computer- readable media include, but are not limited to, optical discs, floppy discs, hard discs, and computer memories. Functionality of these units will be further described herein with reference to the steps of execution.

Each of group members A, B, C and D in Figure 3 includes a communication device 30 (see Figure 5) having computing functionality. The internal block framework of each communication device 30 includes a security unit 300, a key database 303 and a processing unit 301. Functionality of these units will be further described herein with reference to the steps of execution.

Referring to Figure 4, Figure 5 and Figure 6, when member A and member B form a secure group, and member C and member D form another secure group, the key database 203 of the KDC 80 then has corresponding keys K_a, K_b, K_c, Kd, K_ab and K_Cd stored therein. When one secure group intends to join another secure group, i.e. , when members A, B, C and D are to form a new secure group, first, member C and member D will each issue a join request to the group initiator (member A in this embodiment, step 400) to notify the KDC 80 of their participation. Then, the KDC 80 will receive a group initiation request (step 401 ) sent by the group initiator, i.e. , member A, to notify the KDC 80 that a new secure group including members A, B, C and D is to be formed.

The group initiation request includes a list (not shown) of all group members of the new secure group which, in this embodiment, includes members A, B₁ C and D. After the KDC 80 receives the group initiation request, the processing unit 201 will instruct the key generating unit 204 to generate a new secure group key K_abcd applicable to all members in the member list (step 402).

Thereafter, in step 403, a minimum number of keys that can protect all group members of the new secure group are found. In particular, according to the member information of pre-formed secure groups (in this embodiment, there are two secure groups, one including members A and B, the other including members C and D), the processing unit 201 finds from the key database 203 the minimum number of keys: K_ab and K_Cd that can protect all members A, B, C and D of the new secure group.

With further reference to Figure 7, the detailed flow of step 403 includes the following steps: First, in step 4030, pre-formed secure group keys are discovered in the key database 203. In this embodiment, members of the pre-formed secure groups are members A and B and members C and D, respectively. Therefore, in step 4030, the key database 203 finds the corresponding secure group keys K_ab and K_Cd- Then, in step 4031 , an arrangement of the secure group keys thus found is performed according to the number of members protected by each secure group key so as to assist in selecting a minimum number of group keys. For example, in this embodiment, because the numbers of members included in the two keys are the same, the order of the keys after arrangement is still Kab followed by K_cd- Subsequently, the key database 203 is examined for data of the selected secure group keys to determine whether all members of the new secure group have been selected (steps 4032 and 4033). Execution of subsequent steps is continued if the result is yes. If the result is no, according to the arrangement, one secure group key is selected so that the accumulated number of members under protection most approximates that of the new secure group (step 4034), and execution of steps 4032 to 4034 is repeated until all members have been selected. In this embodiment, data of available secure group keys K_ab and K_Cd is examined in step 4032, showing that they are not selected, and it is determined in step 4033 that none of members A, B, C and D of the new secure group has been selected. Therefore, the flow proceeds to the step 4034 which chooses Kab protecting members A and B. Back to step 4032, it is known that K_Cd is not selected, and it is determined in step 4033 that members C and D of the new secure group have not been selected. Hence, the flow proceeds to the step 4034 which chooses K_Cd protecting members C and D. Last, in step 4032, it is known that K_ab and Kcd have been selected, and it is determined in step 4033 that all members A, B, C and D of the new secure group have been selected. Therefore, the flow proceeds to the subsequent step 404.

Referring again to Figure 6, after all secure keys are found, step 404 is executed to generate secure tickets each of which includes the new secure group key and whose number corresponds to that of the selected secure keys. In particular, after finding the minimum number of keys: K_ab and K_Cd, that can protect members A, B, C and D of the new secure group in step 403, the processing unit 201 will transmit the new secure group key K_abcd generated by the key generating unit 204 to the secure ticket generating unit 202. The secure ticket generating unit 202 generates two corresponding secure tickets T_ab and T_Cd, and embeds the new secure group key Kabcd therein. Finally, the processing unit 201 retrieves from the key database 203 the corresponding keys K_ab and K_Cd, and instructs the security unit 200 to form encrypted information K_ab(T_ab) and K_Cd(T_Cd) by encrypting the secure tickets T_ab and T_Cd respectively by the keys Kab and Kcd, which are transmitted to member A via the Internet 115 (step 405), whereby the new secure group key K_abod is securely transmitted to member A. At the same time, the KDC 80 will display the generated tickets and all group members corresponding thereto in an electronic data form (see Figure 11 ) on any device or medium capable of presenting data, such as an electronic display device or an electronic storage medium capable of recording data. After member A receives the encrypted information K_ab(T_ab) and Kcd(Tcd), the flow proceeds to step 406 where the new secure group key K_abcd is extracted from the secure ticket T_ab after the decryption of the encrypted information K_ab(T_ab) using the internally stored key K_ab, and the new secure group key K_abod is subsequently stored. In particular, after member A receives the encrypted information K_ab(T_ab), the security unit 300 of the communication device 30 will extract the corresponding key K_ab from the key database 303 and decrypt the encrypted information K_ab(T_ab) to extract the new secure group key K_abcd- The processing unit 301 then stores the new secure group key K_abcd in the key database 303. In step 407, member A who acts as the group initiator distributes the encrypted information K_ab(T_ab) to member B, and distributes the encrypted information K_cd(T_Cd) to member C and member D (steps 408 and 409).

Member B performs decryption using the key K_ab to extract from the secure ticket T_ab the new secure group key K_at_>cd, which is subsequently stored thereby (step 410). Likewise, after member C and member D receive the encrypted information containing the secure ticket T_Cd, they will perform decryption using the key K_Cd to extract the new secure group key K_abcd, which is subsequently stored thereby (steps 411 and 412). How these are accomplished in practice are the same as that in step 406 and will not be described further.

Accordingly, members A, B, C and D of the new secure group have completed the procedure for secure updating of the new secure group key K_abcd, and can subsequently conduct secure group communication. It is apparent from the foregoing that, in the present invention, a minimum number of tickets T_ab and T_Cd are generated corresponding to a minimum number of existing secure group keys Kab and Kcd that can protect all members of the secure group. Compared to the conventional technique, in which a corresponding secure ticket (T_A, TB, T_C, TD) is generated for the key (k_A, k_B, k_c, k_D) of each member, the computational amount of the KDC 80 can be dramatically lowered. The superior computational efficiency of the present invention will be more evident in situations where the number of group members is large. Second Preferred Embodiment

In the second preferred embodiment of this invention, the framework of the secure group network system 8 is the same as that of Figure 3, and the internal block frameworks of the KDC 80 and the communication device 30 included in each member are the same as those of the first preferred embodiment and will not be described further herein. The situation depicted in this embodiment differs from the first preferred embodiment primarily in that, at this time, the secure group includes members A, B, C and D, and the flow steps executed are those when a new member E intends to join the secure group.

Referring to Figure 8, in this embodiment, members A, B, C and D already formed a secure group at this time. When a new member E intends to join the group, member E will first issue a join request to the group initiator, who is exemplified herein as member A (step 501 ). After member A receives the join request, member A will transmit a group initiation request to the KDC 80 as a notification that members A, B, C, D and E are to form a new secure group (step 502). After the KDC 80 receives the group initiation request, the KDC 80 will first generate a new secure group key K_abcde applicable to all members in the member list (step 503), and according to current secure group data (members A, B, C and D), subsequently find a minimum number of keys, which are K_abcd and K_e in this embodiment, that can protect all members A, B, C, D and E included in the new secure group (step 504). The method of finding the minimum number of keys K_abcd and K_e is the same as step 4030 to step 4034 of the first preferred embodiment and will not be described further herein. Subsequently, the new secure group key Kabcde is embedded into correspondingly generated secure tickets T_abcd and T_e (step 505). Finally, the two secure tickets T_abcd and T_e are encrypted using the keys Kabcd and K_e respectively to form encrypted information Kabcd(Tabcd) and K_e(T_e), which are transmitted to member A (step 506). At the same time, the KDC 80 will display the generated tickets and all group members corresponding thereto on any device or medium capable of presenting data, such as an electronic display device or an electronic storage medium capable of recording data, as shown in Figure 12.

Since members A, B, C and D are in an existing secure group, members A, B, C and D all store the key K_at_>cd- After member A receives the encrypted information K_abcd(T_abcd), the encrypted information K_abcd(T_abcd) is first decrypted using the key K_abcd to extract from the secure ticket T_abcd the new secure group key K_abcde, which is stored by member A (step 507). Thereafter, member A distributes the encrypted information K_abcd(T_abcd) that includes the secure ticket T_abCd to member B, member C and member D (steps 508, 509 and 510). Member B, member C and member D likewise perform decryption of the encrypted information K_abcd(T_abod) using the key K_at_>Cd to extract from the secure ticket T_abcd the new secure group key K_abcde, which is subsequently stored thereby (steps 511 , 512, 513). In step 514, the encrypted information K_e(T_e) is transmitted to member E. After member E receives the encrypted information K_e(T_e), member E performs decryption of the encrypted information K_e(T_e) using the internally stored key K_e to extract from the secure ticket T_e the key Kabcde, which is subsequently stored thereby (step 515). At this time, members A, B, C, D and E all have the key Kabcde, meaning that the flow for secure update of the new secure group key is completed, and thus all members of the new secure group can begin to conduct secure group communication. Third Preferred Embodiment

In the third preferred embodiment of this invention, the framework of the secure group network system 8 is the same as that of Figure 3, and the internal block frameworks of the KDC 80 and the communication device 30 included in each member are the same as those of the first preferred embodiment. Member A and member B are originally included in a secure group. Member C and member D are included in another secure group. Moreover, members A, B, C and D as a whole are further included in another larger secure group. In the situation depicted in this embodiment, the flow steps executed are those when a member who is not the group initiator (exemplified as member B in this embodiment) intends to leave the original larger secure group.

Referring to Figure 9, in this embodiment, members A, B, C and D already formed a secure group at this time. When member B intends to leave the group, member B will first issue a leave request to member A (step 601 ). After member A receives the leave request, member A will transmit a group initiation request to the KDC 80 as a notification of forming a new secure group (step 602). After the KDC 80 receives the group initiation request, the KDC 80 will first generate a new secure group key K_aCd (step 603), and according to the members A, C and D of the secure group to be formed, subsequently search pre-formed secure group keys through the flow of the aforesaid steps 4030 to 4034 (Figure 7), thus finding a minimum number of keys, which are K_a and K_Cd in this embodiment, that can protect members A, C, D of the new secure group (step 604). The new secure group key K_aCd is then embedded into two correspondingly generated secure tickets T_a and T_Cd (step 605). The two secure tickets T_a and T_cd are encrypted using the key K₃ and key Kod respectively to form encrypted information K_a(T_a) and K_Cd(T_CCι), which are transmitted to member A (step 606). At the same time, the KDC 80 will display the generated tickets and all group members corresponding thereto on any device or medium capable of presenting data, such as an electronic display device or an electronic storage medium capable of recording data, as shown in Figure 13.

In step 607, after member A receives the encrypted information Ka(Ta) and K_Cd(T_Od), the encrypted information K_a(T_a) is decrypted using the internally stored key K_a to extract from the secure ticket T₃ the new secure group key K_acd, which is subsequently stored by member A.

Thereafter, in steps 608 and 609, member A distributes the encrypted information K_Cd(T_Cd) to member C and member D. When member C and member D receive the encrypted information K_Cd(T_Cd), member C and member D will decrypt the encrypted information Kcd(Tcd) using the corresponding key K_Cd to extract from the secure ticket Ted the new secure group key K_aCd, which is subsequently stored thereby (steps 610 and 611 ).

Accordingly, members A, C and D of the new secure group have completed the flow for secure update of the new secure group key K_ac_d, and all members of the new secure group can begin to conduct secure communication. Fourth Preferred Embodiment

In the fourth preferred embodiment of this invention, the framework of the secure group network system 8 is the same as that of Figure 3, and the internal block frameworks of the KDC 80 and the communication device 30 included in each member are the same as those of the first preferred embodiment. Member A and member B are originally included in a secure group. Member C and member D are included in another secure group. Moreover, members A, B, C and D as a whole are further included in another larger secure group. In the situation depicted in this embodiment, the flow steps of group change executed are those when a group initiator (exemplified as member A in this embodiment) intends to leave the original larger secure group. Referring to Figure 10, when member A intends to leave the original secure group, member A will first issue a leave request in step 701 to the KDC 80 so as to notify the KDC 80 of its leaving the original secure group and so as to designate a new group initiator. Aside from a command to request the generation of a new secure group key, the leave request further includes data of the newly designated group initiator. In this embodiment, the newly designated group initiator is member B. Then, in step 702, the KDC 80 generates a new secure group key Kbcd according to the command requesting the generation of the new secure group key, and according to the existing secure group keys, finds a minimum number of keys, which are Kb and K_Cd in this embodiment, needed to form a minimum number of secure tickets that can protect members B, C and D in the new secure group through the flow of the aforesaid steps 4030 to 4034 (see Figure 7) (step 703), and generates corresponding secure tickets Tb and T_ocj each of which includes the new secure group key K_bCd (step 704). The two secure tickets Tb and T_ed are encrypted using the key Kb and key K_Od respectively to form encrypted information Kb(Tb) and K_Cd(T_Cd), which are transmitted to member B (step 705). At the same time, the KDC 80 will display the generated tickets and all group members corresponding thereto on any device or medium capable of presenting data, such as an electronic display device or an electronic storage medium capable of recording data, as shown in Figure 14.

In step 706, after member B receives the encrypted information Kb(Tb) and K_Cd(T_Cd), the encrypted information Kb(Tb) is decrypted using the internally stored key Kb to extract from the secure ticket Tb the new secure group key Kbcd, which is subsequently stored by member B. The encrypted information Kcd(Tcd) is transmitted to member C and member D (step 707 and step 708). When member C and member D receive the encrypted information K_Od(T_Cd), member C and member D will decrypt the encrypted information K_Cd(T_Cd) using the internally stored key K_Cd to extract from the secure ticket T_Cd the new secure group key Kbcd, which is subsequently stored thereby (step 709 and step 710).

Accordingly, members B, C and D of the new secure group have completed the flow for secure update of the new secure group key Kbcd, and all members of the new secure group can begin to conduct secure communication.

In sum, in the method of the present invention, the secure keys of all members and secure groups will be stored in the KDC 80, and each member only stores the secure keys of involved secure groups and a personal key, which will not cause a storage burden of secure keys. Moreover, creating a minimum number of secure tickets that can protect all secure group members from originally existing secure group keys outperforms the prior art that requires the creation of a secure ticket for each member of a new secure group, where the number of created secure tickets and the computational amount of the KDC 80 can be reduced to lower the loading of the KDC 80 correspondingly, thereby achieving enhancement in efficiency of secure update of new secure group keys. While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.

Industrial Applicability

The present invention can be applied to a method of generating secure tickets for a new secure group, a method of secure group communication, a computing device having a recording medium, and a network system.

Claims

C LA I M S

1. A method of generating secure tickets for a new secure group, which is applied when a change occurs in a plurality of existing secure groups to form a new secure group, each of the existing secure groups including at least one member and having a corresponding dedicated group key; the method comprising the following steps when a group change notification is received:

(A) generating a new secure group key according to the received group change notification; and

(B) according to the existing secure groups, finding a combination that can form the new secure group in a minimum selection of the existing secure groups, and according to the relationship in the combination, generating secure tickets whose number corresponds to that of selected secure groups, wherein each of the secure tickets has the new secure group key.

2. The method of generating secure tickets for a new secure group as claimed in claim 1 , wherein, in step (A), the group change notification was issued by a member of an existing secure group who acts as a group initiator, the notification including a group initiation request with a new secure group member list upon which the new secure group key applicable to all members in the member list is generated.

3. The method of generating secure tickets for a new secure group as claimed in claim 2, wherein, prior to the transmission of the group initiation request in step (A), the method further comprises a step of receiving a group join request sent by a member intending to join the group to the group initiator.

4. The method of generating secure tickets for a new secure group as claimed in claim 2, wherein, prior to the transmission of the group initiation request in step (A), the method further comprises a step of receiving a group leave request sent by a member intending to leave an original secure group to the group initiator.

5. The method of generating secure tickets for a new secure group as claimed in claim 2, wherein, in step (A), the group change notification further includes a new group initiator designation message for designating a new group initiator.

6. The method of generating secure tickets for a new secure group as claimed in claim 5, wherein, in step (B), a plurality of messages each of which contains a corresponding secure ticket and is encrypted using a corresponding group key in the selected secure groups are generated, the encrypted messages being first delivered to the new group initiator and subsequently being distributed to other corresponding members of the new secure group, where the new secure group key can be obtained after decryption using the corresponding group key.

7. The method of generating secure tickets for a new secure group as claimed in claim 2, wherein, in step (B), a plurality of messages each of which contains a corresponding secure ticket and is encrypted using a corresponding group key in the selected secure groups are generated, the encrypted messages being first delivered to the group initiator and subsequently being distributed to other corresponding members of the new secure group, where the new secure group key can be obtained after decryption using the corresponding group key.

8. The method of generating secure tickets for a new secure group as claimed in claim 1 , wherein, in step (B), the generation of the secure tickets whose number corresponds to that of the selected secure groups includes the following sub-steps:

(B1 ) discovering existing group keys; (B2) performing an arrangement of the group keys according to the number of members protected by each group key;

(B3) determining if all members of the new secure group have already been included; generating secure tickets whose number corresponds to that of the selected secure groups, if the result is yes; executing step (B4) if the result is no; and

(B4) selecting a group key from existing group keys according to the arrangement so that the accumulated number of members under protection most approximates that of the new secure group, and returning to step (B3).

9. A method of secure group communication using pre-formed keys to create secure tickets, which is adapted for use in a network, the network having at least one secure group, the secure group including at least one member and having a corresponding dedicated key, the method including the following steps: (A) issuing a group change notification when a group change occurs in the secure groups, the group change notification being used to generate a new secure group key applicable to all group members of a new secure group;

(B) according to the pre-formed keys, generating a minimum number of secure tickets that can protect all group members of the new secure group, wherein each of the secure tickets has the new secure group key; and

(C) respectively transmitting to all members of the new secure group a plurality of messages each of which contains a corresponding secure ticket and is encrypted using a corresponding key, the encrypted messages being used to securely conduct update procedure of the new secure group key among all members of the new secure group.

10. The method of secure group communication using pre-formed keys to create secure tickets as claimed in claim 9, wherein one member is a group initiator, and the group change notification issued by the group initiator in step (A) includes a group initiation request that has a new secure group member list upon which the new secure group key applicable to all members in the member list is generated.

11 . The method of secure group communication using pre-formed keys to create secure tickets as claimed in claim 10, wherein, prior to the transmission of the group initiation request in step (A), the method further comprises a step of receiving a group join request sent by a member intending to join the group to the group initiator.

12. The method of secure group communication using pre-formed keys to create secure tickets as claimed in claim 10, wherein, prior to the transmission of the group initiation request in step (A), the method further comprises a step of receiving a group leave request sent by a member intending to leave an original secure group to the group initiator.

13. The method of secure group communication using pre-formed keys to create secure tickets as claimed in claim 10, wherein, in step (A), the group change notification further includes a new group initiator designation message for designating a new group initiator.

14. The method of secure group communication using pre-formed keys to create secure tickets as claimed in claim 9, wherein, in step (B), the generation of the minimum number of secure tickets that can protect all group members of the new secure group includes the following sub-steps:

(B1 ) discovering the pre-formed keys; (B2) performing an arrangement of the pre-formed keys according to the number of members protected by each key;

(B3) determining if all members of the new secure group have already been included; generating secure tickets whose number corresponds to that of the selected keys, if the result is yes; executing step (B4) if the result is no; and

(B4) selecting a key from the pre-formed keys according to the arrangement so that the accumulated number of members under protection most approximates that of the new secure group, and returning to step (B3).

15. The method of secure group communication using pre-formed keys to create secure tickets as claimed in claim 9, the secure group including a group initiator, wherein, in step (C), securely conducting the update procedure of the new secure group key among all members of the new secure group includes: transmitting the plurality of encrypted messages to the group initiator for subsequent distribution to other corresponding members of the new secure group, where the new secure group key can be obtained after decryption using the corresponding key.

16. A computing device having a recording medium for creating secure tickets using pre-formed keys to ensure security of group communication, said device executing the following steps:

(A) according to the receipt of a group change notification, generating a new secure group key applicable to all group members of a new secure group;

(C) respectively outputting to all members of the new secure group a plurality of messages each of which contains a corresponding secure ticket and is encrypted using a corresponding key, the encrypted messages being used to securely conduct update procedure of the new secure group key among all members of the new secure group.

17. The computing device having a recording medium as claimed in claim 16, wherein, in step (A), the group change notification includes a group initiation request having a new secure group member list upon which the new secure group key applicable to all members in the member list is generated.

18. The computing device having a recording medium as claimed in claim 16, wherein, in step (B), the generation of the minimum number of secure tickets that can protect all group members of the new secure group includes the following sub-steps: (B1 ) discovering the pre-formed keys;

(B2) performing an arrangement of the pre-formed keys according to the number of members protected by each key;

19. The computing device having a recording medium as claimed in claim 16, wherein, in step (B) for the generated minimum number of secure tickets that can protect all group members of the new secure group, information of all included group members and the secure tickets thus generated are displayed in an electronic data form on any electronic display device that can display data.

20. The computing device having a recording medium as claimed in claim 16, wherein, in step (B) for the generated minimum number of secure tickets that can protect all group members of the new secure group, information of all included group members and the secure tickets thus generated are stored in an electronic data form in an electronic storage medium.

21. A server device to be included in a secure group network system for establishing network connections among secure groups, said server device comprising: a key database for storing keys; a key generating unit for generating a new secure group key; a secure ticket generating unit for generating secure tickets; and a processing unit for coordinating the operations of the above units; said processing unit being used to receive a group change notification for instructing said key generating unit to generate a new secure group key; said processing unit subsequently finding from said key database a minimum number of keys that can protect all group members of a new secure group, and sending the new secure group key to said secure ticket generating unit for generating secure tickets whose number corresponds to the minimum number of keys and embedding the new secure group key in each secure ticket.

22. The server device as claimed in claim 21 , further comprising a security unit for sending out the secure tickets after encryption using a corresponding key.

23. The server device as claimed in claim 21 , wherein the group change notification includes a group initiation request containing a new secure group member list upon which said key generating unit depends to generate the new secure group key applicable to all members in the member list.

24. A secure group network system constructed in an Internet environment, and comprising: at least one secure group having a network connection to the

Internet; and a central key distribution center having a network connection to the Internet and having a key of the secure group; wherein, when a group change occurs in the secure group to form a new secure group, the secure group issues a group change notification to the central key distribution center for instructing the central key distribution center to generate a new secure group key applicable to all new group members of the new secure group, to generate a minimum number of secure tickets that can protect all group members of the new secure group, where each of the secure tickets contains the new secure group key and to respectively transmit to all members of the new secure group a plurality of messages each of which contains a corresponding secure ticket and that is encrypted using a corresponding key so that update of the new secure group key among all members of the secure group can be securely conducted.

25. The secure group network system as claimed in claim 24, wherein the secure group has a group initiator for issuing the group change notification, and the group change notification includes a group initiation request that has a new secure group member list.

26. The secure group network system as claimed in claim 25, wherein, prior to the transmission of the group initiation request, the group change notification further includes a group join request sent by a member intending to join the group to the group initiator.

27. The secure group network system as claimed in claim 25, wherein the group change further includes, prior to the transmission of the group initiation request, the group change notification further includes a leave request sent by a member intending to leave to the group initiator.

28. The secure group network system as claimed in claim 27, wherein the member intending to leave is the group initiator.

29. The secure group network system as claimed in claim 28, wherein the group change notification further includes a new group initiator designation message for designating a new group initiator.