Abstract:
A method and apparatus to allow a key manager node in a network to initiate the process of changing a group key for all nodes in a multicasting group. In the described embodiment, the key manager node initiates changing the group key by setting an indicator in a multicast packet. The indicator indicates that each of the nodes in the multicast group should obtain a new group key from the key manager node. The key manager node sets the indicator whenever the key manager node determines that the nodes in the group need to change their key. The nodes in the multicast group then obtain a key from the key manager node. In one embodiment of the present invention, the key manager node sends the group key to the members of the group and, once all nodes in the group have received their key, sends an indicator that the group members should start using the new keys. In another embodiment, the key manager node sends the new key to the group, along with instructions specifying when the new key is to take effect. For example, the new key can take effect at a certain time or when a certain packet number is received. In another embodiment, each receiver in the group uses both the new key and the old key for a predetermined time period or until all group members have received the key.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to networking and, specifically, to a method and apparatus allowing a key manager node in a network to initiate a process of changing a group key for multiple members of a group in the network. 
     Internet Protocol (IP) multicasting is useful for disseminating data to a large group of receivers in a network. Multicasting of data is a form of network communication in which a transmitting node (a “sender”) sends the data via a single message to multiple destinations at once. The multiple destinations are the recipients (“receivers”) of the message. Other methods of network communication include broadcast, in which a sender transmits to all possible recipients, and unicast, in which the sender transmits only to one specific recipient. Multicast is described in more detail in T. A. Maufer,  Deploying IP Multicast in the Enterprise,  Prentice Hall PTR, 1998, which is herein incorporated by reference in its entirety to the extent that it does not conflict with the invention as described herein. A multicast sender may send a message to a selected group of receivers in a multicast group. A multicast group includes at least one sender that transmits data to nodes on a particular multicast address. A multicast group also includes one or more receivers. A receiver is a node that listens on a particular address in the network. Receivers become members of the group because they are interested in receiving messages. A node may be both a sender and a receiver of data to and from other nodes. 
     In certain conventional multicast systems, a sender distributes a group key to all nodes in the multicast group. Each member in the multicast group receives the same group key. This group key may be used by the one or more senders to encrypt data and by the receivers to decrypt the data sent to the group or to decrypt other, individual keys sent to the group members. When a member leaves a group or is no longer trusted, it is necessary to change the group key so that the former member will not be able to decrypt information encrypted with the group key. It is also wise for the sender to change the group key periodically in case the key has been compromised. It is also wise to change the group key if enough time has passed since the group key was last distributed that the group key could be compromised. 
     Some conventional multicasting systems, such as the “Enclave” system developed by Li Gong (as described in L. Gong, “Enclaves: Enabling Secure Collaboration over the Internet”. IEEE Journal on Selected Areas in Communications, 15(3):567-575, April 1997) allow the sender to distribute a new key (encrypted separately for each member) directly via multicasting. Unfortunately, this method does not scale to large numbers of members, since the amount of data multicast to all members grows as the number of members grows. 
     As another example, the SKIP (Simple Key Management for Internet Protocols) protocol distributes keys that are deemed valid for a certain predetermined time period and updates these keys by a unicast. This distribution method causes a problem when a member leaves the group, since the member still has access to the group key until that group key expires. SKIP does not allow for quick key change when a member leaves the group or is suspected to be compromised. 
     SUMMARY OF THE INVENTION 
     Described embodiments of the present invention allow a key manager node in a network to initiate the process of changing a group key for all nodes in a multicasting group. A “key manager” is the network entity in charge of key distribution and management. In the described embodiment, the key manager node initiates changing the group key by setting an indicator (called a “key change indicator”) in a multicast packet. The key change indicator indicates that each of the nodes in the multicast group should obtain the new group key. The key manager sets the indicator whenever the key manager determines that the nodes in the group need to change their key. The members in the multicast group then obtain the new group key from the key manager via an appropriate key distribution process. 
     Various embodiments use one of several methods described herein to perform key distribution. In certain embodiments, the group members individually request a new group key. In other embodiments, the key manager transmits the key to the group members using another appropriate mechanism. In one embodiment of the present invention, the key manager distributes the group key to the members of the group in response to a request from each member. Once all group members have received the new group key (or a timeout has occurred), the key manager sends an indicator that the group members should start using the new group key. In another embodiment, the key manager sends the new group key to the group members, along with instructions specifying when the new key is to take effect. For example, the new key can take effect at a certain time or for all received packets having a packet number higher than a certain packet number. In another embodiment, each receiver in the group uses both the new group key and the old group key for a predetermined time period or until all group members have received the key, while each sender in the group receives an indication from the key manager that it should switch to the new group key. In still other embodiments, the key manager unicasts or multicasts the new group key to the group members without receiving a request. 
     Various embodiments implement the key change indicator in different ways. As discussed above, the key change indicator can be a flag formed of one or more bits in a packet. The key change indicator can also be an indicator in the data, such as a control character. The key change indicator can also be a separate type of packet or message. Similarly, the indicator that the group should start using the new key, which is used in certain embodiments, can also be a flag, a control character, a type of packet or message, or any other appropriate type of indicator. 
     The group key used in the described embodiments of the present invention is a shared secret key, as is known to persons of ordinary skill in the art. An example of such a shared secret key encryption method is the DES encryption method. 
     In accordance with the purpose of the invention, as embodied and broadly described herein, the invention relates to at least a method of changing a group key, comprising the steps performed by a node including a key manager function in a system for processing data, of: sending an indicator to each member of a group that it is time to change the group key; and distributing a new group key to at least one member of the group. 
     In further accordance with the purpose of the invention, as embodied and broadly described herein, the invention relates to a method of changing a group key, comprising the steps performed by a system for processing data, of: sending, by a key manager node, an indicator to each member of a group that it is time to change the group key; and distributing, by the key manager node, a new group key to at least one member of the group. 
     In further accordance with the purpose of the invention, as embodied and broadly described herein, the invention relates to method of changing a group key, comprising the steps performed by a member of a group in a system for processing data, of: receiving, by the member of the group, an indicator that it is time to change the group key; sending, by the member of the group, in response to the indicator, a request for a new group key; and receiving, after the sending step, the new group key. 
     Advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims and equivalents. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG.  1 ( a ) is a block diagram of sending packets by a sender in a conventional multicast network. 
     FIG.  1 ( b ) is a block diagram of a multicast network wherein a key manager node sends a key change indicator in accordance with an embodiment of the present invention. 
     FIG.  1 ( c ) is a block diagram of a multicast network wherein a key manager node sends a key change indicator in accordance with an embodiment of the present invention. 
     FIG.  2 ( a ) is a diagram of a key manager data processing system in accordance with an embodiment of the present invention. 
     FIG.  2 ( b ) is a diagram of a group member data processing system in accordance with an embodiment of the present invention. 
     FIG. 3 is a flow chart showing steps performed by a first embodiment of the present invention to disseminate a group key. 
     FIG. 4 is a flow chart showing steps performed by a second embodiment of the present invention to disseminate a group key. 
     FIG. 5 is a flow chart showing steps performed by a third embodiment of the present invention to disseminate a group key. 
     FIG. 6 is a flow chart showing steps performed by a fourth embodiment of the present invention to disseminate a group key. 
     FIG. 7 is a flow chart showing steps performed by a fifth embodiment of the present invention to disseminate a group key. 
     FIG. 8 is a flow chart showing steps performed by a sixth embodiment of the present invention to disseminate a group key. 
     FIG. 9 is a diagram showing an example of a packet format including a key change indicator. 
     FIGS.  10 ( a )- 10 ( c ) are diagrams showing various examples of a packet used to send the new group key to a member of the group. 
     FIG. 11 is a diagram showing an example of a packet format including an indicator that the group key should begin to be used by group members. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to several embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever practicable, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     I. General Discussion 
     FIG.  1 ( a ) is a block diagram showing a sender multicasting packets in a multicast network. FIG.  1 ( a ) shows a general diagram of how data is sent from a sender node  104  to the nodes in its multicast group. The nodes in the multicast group of FIG.  1 ( a ) include nodes  105 ,  106 ,  108 ,  110 , and  112 . As shown, the nodes to which the sender sends data can be other senders and receivers. In general, sender  104  sends a multicast packet as indicated by line  116 . The sender encrypts using the group key. The nodes decrypt using the group key. Any of sender nodes  104 ,  105 ,  110  can send data in the manner shown in FIG.  1 ( a ). The nodes of FIG.  1 ( a ) may be connected by any type of appropriate communication medium, such as an Ethernet, the Internet, wireless communications, cellular communications, etc. 
     FIG.  1 ( b ) is a block diagram of an embodiment of a network in accordance with the present invention wherein key manager node  102  sends a key change indicator to the nodes of the multicast group in accordance with an embodiment of the present invention. A key manager  102  is the network entity in charge of key distribution. As shown in the Figure, key manager  102  multicasts a key change indicator to each node in the multicast group. This multicast is herein referred to as “key change indication.” Each member of the multicast group unicasts back a request for the new group key. Although not shown in FIG.  1 ( b ), key manager  102  then sends the new group key to the group members using any of several methods, examples of which are shown in FIGS. 3-8 below. This is herein referred to as “key distribution.” 
     FIG.  1 ( c ) is a block diagram of another embodiment of a network in accordance with the present invention wherein key manager node  102  sends a key change indicator to the nodes of the multicast group in accordance with an embodiment of the present invention. This multicast is herein referred to as “key change indication.” In this embodiment, the key manager further distributes the group key to the group using an appropriate multicast key distribution mechanism (key change). 
     As shown in FIGS.  1 ( b ) and  1 ( c ), the multicast group to which key manager  102  sends a key change indicator can include both senders and receivers. In some embodiments, key manager  102  is a separate node. In other embodiments, any node capable of sending (such as sender  104 ) can also act as the key manager. Still other embodiments can include more than one key manager. Having more than one key manager provides fault tolerance and robustness in the system, since it allows a key manager to fail without shutting down the key distribution mechanism. If there is more than one key manager  102 , the key managers need to communicate with each other to prevent more than one key manager from trying to change the group key at a given time. 
     The group key used in the described embodiments of the present invention is a shared secret key, as is known to persons of ordinary skill in the art. An example of such a shared secret key encryption method is the DES encryption method. The shared secret key method can also be used to distribute other kinds of keys to a group. 
     FIG.  2 ( a ) is a diagram of a key manager data processing system in accordance with an embodiment of the present invention. It will be understood that the term “data processing system” covers any type of system that processes data, such as a cellular telephone, a handheld processing unit, a network computer, a personal digital assistant, an internet appliance, etc. Key manager  102  includes processor  202  and storage area (such as a memory)  204 . Storage area  204  includes at least key manager software  210  to accomplish multicasting and/or unicasting. Key manager software  210  can also send and receive unicast transmissions to and from the nodes in its group. Storage area  204  in key manager  102  further includes at least one group key  212  and data  214 , such as a number of requests for the new group key currently received by the key manager. 
     FIG.  2 ( b ) is a diagram of a group member node data processing system  106  in accordance with an embodiment of the present invention. In the following discussion, the term “group member” applies to a node that receives a key change indicator from the key manager  102 . Some of the “group members” can also be senders of data themselves under other circumstances, while other group members are only receivers of data from the senders. Group member  106  is shown as including processor  252  and storage area (such as a memory)  254 . Storage area  254  in group member  106  includes at least receiving/sending software  260 . Receiving/sending software  260  can receive multicast and unicast transmissions from the key manager and can also send unicast transmissions to key manager  102 . Storage area  254  in group member  106  further includes at least one new group key  262  and at least one old group key  264 . In certain embodiments, storage area  254  includes a set of previously distributed group keys. 
     Although, in FIGS.  2 ( a ) and  2 ( b ), each of key manager  102  and group member  106  is shown in a separate data processing system/network element, it should be understood that one or more of elements  102  and  106  (and/or one or more of the other group members) also can be resident on the same data processing system/network element. Furthermore, the functionality of elements  102  and  106  can be distributed between additional data processing systems, nodes, or network elements (not shown) without departing from the spirit and scope of the present invention. 
     Each key manager  102  and group member  106  preferably includes a respective input device  220 ,  270  such as a keyboard, a touchpad, or a mouse that receives input from a user or other appropriate source. Each key manager  102  and group member  106  also preferably includes a respective output device  222 ,  272  such as a display screen, a printer, etc. that outputs information to the user or other appropriate destination. In addition, each key manager  102  and each group member  106  preferably includes a respective computer readable medium input device  224 ,  274 , which is capable of reading a computer readable medium  226 ,  276 . 
     A person of ordinary skill in the art will understand that the systems of FIGS.  2 ( a ) and  2 ( b ) may also contain additional information, such as input/output lines; input devices, such as a keyboard, a mouse, and a voice input device; and additional display devices. The systems of FIGS.  2 ( a ) and  2 ( b ) may also include application programs, operating systems, data, etc., which are not shown in the figure for the sake of clarity. It also will be understood that the systems of FIGS.  2 ( a ) and  2 ( b ) can also include numerous elements not shown, such as disk drives, keyboards, display devices, network connections, additional memory, additional CPUs, additional processors, LANs, input/output lines, etc. 
     In the following discussion, it will be understood that the steps of methods and flow charts discussed preferably are performed by processor  202 ,  252  (or other appropriate processors) executing instructions stored in respective storage areas  204 ,  254  (or other appropriate storage areas). Specifically, the steps described herein are performed by one of key manager software  210  and receiving/sending software  260  in, respectively, key manager  102  and group member  106 . It will also be understood that the invention is not limited to any particular implementation or programming technique and that the invention may be implemented using any appropriate techniques for implementing the functionality described herein. The invention is not limited to any particular programming language, operating system, or network protocol. 
     Some or all of the instructions and data structures in storage areas  204 ,  254  may be read into memory from computer-readable media  226 ,  276 . Execution of sequences of instructions contained in the storage areas causes processors  202 ,  252  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, preferred embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to a processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as a storage device. Volatile media includes dynamic memory. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise a bus within a computer. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications, or electrical signals transmitted over a computer network. 
     Common forms of computer-readable media include, for example a floppy disk, a flexible disk, a hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertapes, any other physical medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to a processor for execution. For example, the instructions of key manager software  210  or group member  106  may initially be carried on a magnetic disk or a tape. The instructions are loaded into storage area  204 ,  254 . Alternately, instructions can be sent over a telephone line using a modem. A modem local to the computer system can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector coupled to a bus can receive the data carried in the infra-red signal and place the data on the bus. The bus carries data to main memory, from which a processor retrieves and executes the instructions. The instructions received by main memory may optionally be stored on a storage device either before or after execution by a processor. The connection between key manager  102  and group member  106  is generally designated  211 ,  261  and can be any appropriate connection. 
     The following paragraphs describe several ways of disseminating the group key to a multicast group. Both the senders and receivers of the multicast group need the group key in order to encrypt data to be sent (by a sender) and to decrypt data received (by a receiver). 
     II. Key Management 
     FIGS. 3-8 show several embodiments of a key change indication step, in which the key manager indicates that the group key is going to be changed, and a key change step in which the new group key is distributed (or begun to be used, if it was previously distributed). It will be understood that the following embodiments are described for the purpose of example only and are not intended to limit the present invention. 
     FIG. 3 is a flow chart  300  showing steps performed by a first embodiment of the present invention to disseminate a group key. Steps performed by key manager  102  are shown on the left of the figure. Steps performed by a member in a multicast group, e.g., group member  106 , are shown on the right of the figure. In step  302 , key manager  102  determines whether there is a need to change the group key. A need to change the group key may arise, for example, if one of the members has dropped out of the group and is no longer qualified to send or receive multicast data. As another example, key manager  102  may change the group key at regular predetermined intervals. 
     If there is a need to change the group key, key manager  102  sets the key change indicator in step  304 . This key change indicator can be, for example, one or more bits in the multicast packet. An example of such bit or bits is shown in FIG. 9, which shows a key change indicator  904  in the flags field  902  of a multicast packet. Key change indicator  904  can be a flag in a packet. The key change indicator can also be an indicator in the data, such as a control character. The key change indicator can also be a separate type of packet or message. For example, key manager  102  can send a type of packet that is only sent when it is time to change the group key. In step  304 , key manager  102  sends the new key group key available indicator in an outgoing, signed packet using multicast transmission. This packet is preferably signed by key manager  102 . Having key manager  102  sign the packet using public key encryption or other appropriate technique prevents unauthorized nodes from trying to change the group key. 
     In step  306 , group member  106  receives the packet and verifies the signature to ensure that the packet was signed by a legitimate key manager. If, in step  308 , group member  106  determines that there is a key change indicator in the packet (or that the key change has been indicated in some other way), group member  106  preferably unicasts a signed request for the new group key to the key manager  102  (as shown in FIG.  1 ( b )). Requiring the group member to sign its request ensures that the key manager will be able to tell whether the request comes from a legitimate group member. Note that each group member sends a separate request for the new group key. 
     In step  312 , key manager  102  receives the request for a new group key and verifies the signature. In step  314 , key manager  102  preferably unicasts the new group key to the requesting group member via any appropriate key distribution mechanism. In step  316 , group member  106  receives the new group key and holds the new group key in its storage area. It will be understood that the key distribution mechanism can encrypt the group key when it is sent between the key manager and group member if such encryption is appropriate. Examples of a packet including a group key are shown in FIGS.  10 ( a )- 10 ( b ), as discussed below in connection with that Figure. 
     In step  318  of FIG. 3, if key manager  102  determines that all members of the multicast group have requested and been sent the new group key (or a timeout has occurred in step  319 ), key manager  102  multicasts an indicator to the group members to tell them to start using the new group key. This indicator can be signed the key manager. Requiring that the key manager  102  sign the indicator allows the group members to determine whether the indicator comes from a legitimate key manager. In step  322 , group member  106  receives and verifies the indicator to start using the new group key. Group member  106  then starts using the new group key to receive messages from other nodes in the group. If the group member receiving the new group key can send data (e.g., sender  104  of FIG.  1 ( b )), then the node also starts using the new group key at this time to encrypt data to be sent. The indicator that the group member should start using the new key can be a flag, a control character, a type of packet or message, or any other appropriate type of indicator. 
     FIG. 4 is a flow chart  400  showing steps performed by a second embodiment of the present invention to disseminate a group key. Steps  402  through  412  are similar to steps  302  through  312  of FIG.  3  and will not be described in detail. In step  414 , key manager  102  preferably unicasts the new group key to the requesting group member via any appropriate key distribution mechanism. The unicast of step  414  also includes a time value. This time value represents the time at which the switch to the new group key is to occur. An example of a packet including the key and a time value is shown in FIG.  10 ( b ), as discussed below in connection with that Figure. In step  416 , group member  106  receives the new group key and the time value and holds the new group key and the time value in its storage area. It will be understood that the key distribution mechanism encrypts the group key and/or time value when they are sent between the key manager and group member if such encryption is appropriate. 
     In the embodiment of FIG. 4, if group member  106  determines in step  418  that the time to change the group key has arrived, group member  106  then starts using the new group key to verify received messages in step  420 . If the node receiving the new group key can send data (e.g., sender  104  of FIG.  1 ( b )), then the node also starts using the new group key at this time to encrypt data to be sent. FIG. 5 is a flow chart  500  showing steps performed by a third embodiment of the present invention to disseminate a group key. Steps  502  through  512  are similar to steps  302  through  312  of FIG.  3  and will not be described in detail. In step  514 , key manager  102  preferably unicasts the new group key to the requesting receiver via any appropriate key distribution mechanism. The unicast of step  514  also includes a packet number. This packet number is the number of the first packet that the key manager will encrypt using the new group key. In this embodiment, packets are numbered sequentially by the sender. An example of a packet including a key and a packet number that will start using the new key is shown in FIG.  10 ( c ). 
     In step  516 , group member  106  receives the new group key and the packet number and holds them in its storage area. It will be understood that the key distribution mechanism can encrypt the group key and/or packet number when they are sent between the key manager and group member if such encryption is appropriate. The number of the first packet that will be sent using the new group key may be determined, for example, based on criteria such as the last packet number sent, the rate at which the packets are expected to be sent, and the amount of time required to distribute the new key. Thus, for example: 
     
       
         new packet#=current packet#+(rate*time*1.5),  
       
     
     where 1.5 is a predetermined margin of error. 
     In the embodiment of FIG. 5, in step  518 , if group member  106  determines that it has received a packet having the packet number received in step  516  (or a higher packet number), group member  106  then starts using the new group key to verify received messages. Group member  106  may need to save both the new and the old key for some predetermined time, since packets often arrive out of order. If the node receiving the new group key can send data (e.g., sender  104  of FIG.  1 ( b )), then the node starts using the new group key to encrypt data to be sent when it sends the packet having the received packet number or higher. If there is more than one sender in the group, the key manager needs to coordinate with the senders to make sure that all the senders have received (and will use) the new key and packet number. 
     FIG. 6 is a flow chart  600  showing steps performed by a fourth embodiment of the present invention to disseminate a group key. Steps  602  through  612  are similar to steps  302  through  312  of FIG.  3  and will not be described in detail. In step  614 , key manager  102  preferably unicasts the new group key to the requesting group member via any appropriate key distribution mechanism. In step  616 , group member  106  receives the new group key and holds the new group key in its storage area. It will be understood that the key distribution mechanism can encrypt the group key when it is sent between the key manager and group member if such encryption is appropriate. 
     In the embodiment of FIG. 6, group member  106  uses both the old group key and the new group key for a predetermined time period. In step  618 , group member  106  decrypts received data using both keys. Steps  620 ,  622 , and  624  use the data that is correct. For example, all data may have a known value in a known location. Whichever decrypted data contains the known value will be the correct data. In some embodiments, the predetermined time period is determined by the key manager and sent to the group members, either for each key change or at some previous time. In other embodiments, the predetermined time is determined by each group member, and may be the same or different for each member in a group. If the node receiving the new group key can also send data (e.g., sender  104  of FIG.  1 ( b )), then the node also starts using the new group key at this time to encrypt data to be sent. 
     FIG. 7 is a flow chart  730  showing steps performed by a fifth embodiment of the present invention to disseminate a group key. Steps  732  through  736  are similar to steps  302  through  306  of FIG.  3  and will not be described in detail. In this embodiment, a set of group keys has been previously distributed to the group members. When a group member receives a key change indicator, the group member starts using the next group key in the set. As shown in FIG. 7, some embodiments may use the new group key and the old group key for a predetermined period of time, as described, for example, above. 
     FIG. 8 is a flow chart  860  showing steps performed by a sixth embodiment of the present invention to disseminate a group key. Steps  862  through  866  are similar to steps  302  through  306  of FIG.  3  and will not be described in detail. When a group member receives a key change indicator, the group member is alerted to wait for a new group key, which is sent by the key manager. This key can be distributed, for example, via multicasting or unicasting. 
     FIG. 9 is a diagram showing an example of a packet format including a key change indicator  904 , indicating that the group key will be changed. The key change indicator of FIG. 9 is provided by way of example only. Any appropriate indicator can be used as a key change indicator and the key change indicator can be located in any appropriate part of the multicast packet. The key change indicator  904  can be, for example, a flag in a packet. The key change indicator  904  can also be indicator in the data, such as a control character. The key change indicator  904  can also be a separate type of packet or message. 
     FIGS.  10 ( a )- 10 ( c ) are diagrams showing examples of a packet sending the new key indicator to a multicast group. FIG.  10 ( a ) shows a packet containing, among other information, a new group key  1002 , such as would be sent in step  314  of FIG.  3 . FIG.  10 ( b ) shows a packet containing, among other information, a new group key  1012  and a time value  1014  representing a time to start using the new group key  1012 , such as would be sent in step  414  of FIG.  4 . FIG.  10 ( c ) shows a packet containing, among other information, a new group key  1024  and a first packet number  1026  that will start using the new group key, such as would be sent in step  514  of FIG. 5. A packet having the format of FIG.  10 ( c ) has a packet number  1022  in each packet. 
     FIG. 11 is a diagram showing an example of a packet format including an indicator  1102  to indicate that the group members can begin to use the new group key. The packet of FIG. 11 would be sent, for example, in step  320  of FIG.  3 . Any appropriate indicator can be used. Similarly to the key change indicator  902 , the indicator  1102  that the group can/should start using the new group key can also be a flag, a control character, a type of packet or message, or any other appropriate type of indicator. 
     In summary, the present invention provides a mechanism for a multicast key manager to change a group key used by all members in a group. In one embodiment, each of the group members then request the new group key. Once the key manager determines that all the group members have the group key, the key manager multicasts an indicator to all group members to start using the new group key to decrypt/encrypt multicasts to the group. In another embodiment, the key manager sends a time indicating at what time the new group key will become effective. In another embodiment, the key manager sends the first packet number for which the new group key will become effective. In yet another embodiment, the group members use both the old and new group keys for a predetermined period of time. In another embodiment, the group members already have a set of group keys and change to the next key in the set. In another embodiment, the key manager distributes a new group key without needing to receive requests from the group members. 
     While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. For example, although the invention is described above in the context of a multicast group, the invention can also be used in any situation where a key manager needs to be able to control when a key is sent to a group of receivers. As another example, the present invention can send different keys to different senders and can then send the keys for each of the senders to all receivers. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims and equivalents.