Abstract:
A method for data communication in a cryptographic system containing a plurality of entities is specified, in which the entities are arranged in a hierarchical structure. If a current entity in the structure is altered, those entities which are connected directly to the current entity&#39;s hierarchically superordinate entity are notified of the alteration.

Description:
The present application hereby claims priority under 35 U.S.C. Section 119 on German patent application number DE 10115600.6 filed on Mar. 29, 2001, the entire contents of which are hereby incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The invention generally relates to a method and an arrangement for data communication in a cryptographic system containing a plurality of entities. 
   BACKGROUND OF THE INVENTION 
   Methods for key distribution and key agreement are known generally (see [1]). One particular problem there is that keys need to be distributed, exchanged or agreed over an (insecure) communications path. To allow this, the following requirements are of particular significance: 
   1. Confidentiality 
   It is necessary to ensure that the exchanged key is accessible only to the authorized subscribers and processes. Secret keys need to be kept secret during their generation, distribution, storage and—where possible—even during implementation. 
   2. Identification of Data Intactness 
   It is necessary to take measures to ensure that the exchanged keys are available to the authorized subscribers in an unaltered and error-free state. If a transmission channel is subject to a high level of interference, error-correcting methods may be necessary. 
   3. Identification of Repetition and Delays 
   One risk is that keys which have already been used will be used a second time, because it may then not even be possible to distinguish the next communication from an earlier one. This risk exists particularly if a key exchange protocol has been subjected to tapping. Accordingly, particularly delays during key distribution can be regarded as suspicious. 
   4. Authentication of the Origin of the Key or Subkey 
   Key agreement without authentication may be pointless, because this might be done with a potential hacker. This is prevented by virtue of additional authentication subsequently being carried out using keys which have already been exchanged or agreed securely beforehand. 
   5. Acknowledgement of Receipt and Verification of the Agreed Key 
   The acknowledgement of receipt is intended to prove to the sender that the rightful recipient has received the key correctly. Since the exchanged keys are frequently not used directly, but rather serve as subkeys, references, etc., dynamically agreed keys need to be tested before they are used. This verification can be carried out explicitly by reciprocal transformation of prescribed data or implicitly by redundancy added to the protocol elements of the exchange protocol. 
   The result of this list of requirements, which is not conclusive (inclusive), is that, when they are observed, key distribution which can be implemented with a high level of security is possible. 
   A particular peculiarity of today&#39;s electronic systems is that they are implemented in distributed form. Consequently, a plurality of computers (also: entities, processes, processors, nodes, subscribers) are amalgamated in a network, with the computers being able to communicate with one another. Within the context of key distribution, it is also known practice for the subscribers in the network to be provided with a hierarchical structure. In this context, a particularly popular structure is a tree structure including a root node, branches and nodes, with the nodes, which themselves have no nodes on a lower level, being referred to as leaves of the tree structure. 
   If a method for key distribution is applied to a hierarchical structure of nodes, in particular to a tree structure, then the alteration of a node needs to involve negotiation of at least one new key for the entire system, that is to say the entire tree, and the new key needs to be communicated to all the nodes of the tree. In this context, a particular drawback is that every node receives a new key and that the same key is always used between two respective nodes. Even if just one particular key (or a symmetrical key pair) is used between two respective nodes, it is a drawback that received data need to be recoded separately for each key and recipient. 
   SUMMARY OF THE INVENTION 
   An object of an embodiment of the invention is to present an efficient and economical method for key distribution which avoids at least one of the drawbacks described above. 
   An object may be achieved by specifying a method for data communication in a cryptographic system containing a plurality of entities, in which the entities are arranged in a structure. If a current entity in the structure is altered, those entities which are connected directly to the current entity&#39;s hierarchically superordinate entity can be notified of the alteration. 
   This advantageously ensures that an association of entities is formed which includes part of the hierarchical structure and allows separate key distribution for this part. 
   One development is that the data communication includes a method for key distribution. 
   Another development is that the plurality of entities are nodes or subscribers to the data communication. 
   A further development is that the plurality of entities are amalgamated in a network. 
   Another development is that the hierarchical structure is a tree structure. 
   One particular development is that the alteration of the current entity includes one of the following options:
         a) the current entity is added;   b) the current entity is removed;   c) at least one property of the current entity is altered.       

   Another development is that the notification of alteration involves a modified cryptographic key being transmitted. A further development is that the method for implementing multicast services can be used. This means that a sender simultaneously transmits to a plurality of recipients data encrypted in the same manner, with each recipient being able to perform decryption using the key information associated with the sender. 
   In addition, an object can be achieved by specifying an arrangement for data communication in a cryptographic system containing a plurality of entities, in which a processor unit is provided which is set up such that
         a) the entities are arranged in a hierarchical structure;   b) if a current entity in the structure is altered, those entities which are connected directly to the current entity&#39;s hierarchically superordinate entity can be notified of the alteration.       

   Such an arrangement is particularly suitable for carrying out the inventive method or one of its developments explained above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated and explained with reference to the figures below, in which 
       FIG. 1  shows a sketch with a hierarchical structure comprising a plurality of nodes; 
       FIG. 2  shows a sketch with a hierarchical tree structure and group keys; 
       FIG. 3  shows a sketch to illustrate addition of a further node; 
       FIG. 4  shows a sketch of a hierarchical structure with steps in a method for data distribution; 
       FIG. 5  shows a processor unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a sketch with a hierarchical structure including a plurality of nodes. In this case, by way of example, a root node K 1  is shown which is connected to a node K 2  via an edge and to a node K 3  via an edge. The node K 2  is in turn connected to hierarchically subordinate nodes K 4 , K 5  and K 6  (in each case via an edge). Similarly, the node K 3  is connected to nodes K 7 , K 8  and K 9  via a respective edge. Between the node K 1  and the node K 2 , there is a symmetrical key S 1  for data encryption. Similarly, there is a key S 2  between the nodes K 1  and K 3 , a key S 6  between the nodes K 3  and K 7 , a key S 7  between the nodes K 3  and K 8 , and a key S 8  between the nodes K 3  and K 9 . In addition, there is a key S 3  between the nodes K 2  and K 4 , a key S 4  between the nodes K 2  and K 5 , and a key S 5  between the nodes K 2  and K 6 . 
   The symmetrical keys S 1  to S 8  can, in particular, also be in the form of a symmetrical key pair for data encryption between two respective nodes. The key pair ensures that an asymmetric encryption method can be carried out between two respective nodes. 
   In the embodiment shown in  FIG. 1 , a particular drawback is that a message which needs to be transmitted to another node, as a current node&#39;s directly adjacent node, needs to be repeatedly recoded. In this respect, a “multicast data transfer”, that is to say notification of a plurality of nodes without separate respective encryption, is not possible. 
     FIG. 2  shows a sketch with a hierarchical tree structure and group keys, where this structure supports a multicast data transfer, in particular. 
   The nodes K 1  to K 9  are arranged in accordance with  FIG. 1 . In this context, each node is a possible initiator for key distribution. The key distribution can be initiated when particular data within the tree structure, be it for the nodes or the structure, change or when the keys need to be renewed at a predetermined time. In particular, addition or removal of a node can involve a change being made to the tree structure such that a new key distribution results. 
   Each node negotiates a method for key distribution with the node to which it is directly connected. The initiator is, in particular, that node to which a plurality of nodes have been connected. For the method for key distribution, each initiator node negotiates with the directly connected nodes a (common) group key which is used for protecting the data, in particular the integrity and confidentiality thereof. 
   One advantage is that each node needs to reencrypt the received data only once after decryption—before forwarding them. For the aforementioned multicast data transfer, it is thus possible for each node to process the data only once and to forward them to a multiplicity of addressees. Another advantage is that a hacking attempt which involves feigning a false identity for a node (masquerade) is not possible, since each node has its own key for encryption. In  FIG. 2 , there is a group key GS 1  for the node K 1 , a group key GS 2  for the node K 2 , and a group key GS 3  for the node K 3 . There is a group key GS 4  to GS 9  for each of the subordinate nodes K 4  to K 9 . The abovementioned multicast data transfer is provided, for example, such that the node K 3  receives data and can forward them to all the nodes K 1 , K 7 , K 8  and K 9  connected to it at once, in which case it need recode the received data only once. If, by way of example, the node K 3  receives data from the node K 1 , then these data have been encrypted using the group key GS 1 , and the node K 3  converts the data, that is to say decrypts the data and encrypts them again using the group key KS 3 . It then transmits the newly encrypted data to the nodes K 7  to K 9 . 
   If a new node is then added, the group key needs to be negotiated again only for a tree section, that is to say for a group (see groups  201 ,  202  or  203  in  FIGS. 2 to 4 ), since the tree section changes for the group. This advantageously means that not every node in the entire hierarchical structure, in this case the entire tree, is affected, but rather only those nodes of a group in which the change is made. Such a change can involve, by way of example, the addition of a new node, the removal of an already existing node, or the changing of particular parameters for a node (or for a plurality of nodes). 
   The advantages of said solution are, in particular, that the node need recode the data only once, and multicast data transfer can also be ensured using protected data links. New keys are renegotiated only for part of the entire hierarchical structure when a node is altered (added, removed, changed). In addition, the method for key distribution (key management) is economically distributed over a plurality of nodes. Finally, internal hacking attempts, i.e. hacking attempts from nodes in the structure, are not possible because each node has its own key for data protection. 
   Optionally, the method for key distribution can also be organized on a hierarchical basis. In this case, it is particularly important for the node initiating the method for key distribution to have a superordinate node to which it is directly connected. The initiator negotiates a security union with the subordinate nodes which are directly connected to it. Optionally, the initiator can also agree the security conditions with the superordinate nodes, said security conditions serving as a basis for the method for key distribution with the subordinate node. Alternatively, the initiator can also determine the security conditions independently of the other nodes and can use them in the method for key distribution (key management). In this case, the method for key distribution (key management) is distributed over a plurality of subordinate nodes by the root node on an administrative basis, as a result of which the root node is relieved of load, that is to say the work for the method for key distribution is distributed over a plurality of nodes. 
   In the manner of  FIG. 2 ,  FIG. 3  again shows the hierarchical structure comprising the nodes K 1  to K 9 . A new feature in this case is a node K 10  which is arranged below the node K 3 . This addition of the node K 10  means that new keys need to be distributed within the security union  203  if the addition of the node K 10  changes anything about the properties of the security union. 
   In the present case of  FIG. 3 , a new key is negotiated for the security union  203  around the node K 3 , said new key then being transmitted in encrypted form to the nodes which are connected directly to the node K 3 . This new key is the group key GS 3 ′ and is transmitted to the nodes K 1 , K 7 , K 8 , K 9  and K 10 . The nodes K 2 , K 4 , K 5  and K 6  remain completely unaffected by the renegotiation of the group key GS 3 ′ and hence by the addition of the node K 10 . 
     FIG. 4  shows a hierarchical structure in accordance with  FIG. 3 , with an illustration being given of how a message is transmitted from a node K 7  to all the other nodes in the hierarchical structure. If the node K 7  sends data to all the other nodes in the tree structure, then the node K 3 , following decryption, needs to encrypt the data once again and can then transmit them to the nodes K 8 , K 9 , K 10  and K 1 . The node K 1  needs to recode the data once and transmit them to the node K 2 , which again recodes the data once and distributes them to the nodes K 4  to K 6  at once. The different data streams have been provided with the reference symbols  401  to  404  in  FIG. 4 . The data stream  401  is recoded by the node K 3  into the data stream  402 . This is recoded by the node K 1  into the data stream  403 , where it is recoded into the data stream  404  in the node K 2 . The particular advantage is that, from the node K 3 , the nodes K 8 , K 9 , K 10  and K 1  can be addressed at once during the multicast data transfer and, in particular, can be supplied with the same data stream  402 . The situation is similar for transmission of the same data stream  404  to the nodes K 4  to K 6 . 
     FIG. 5  shows a processor unit PRZE. The processor unit PRZE can be used to carry out the above-mentioned methodology and can include a processor CPU and a memory MEM. It can further include an input/output interface IOS, which can be used in various ways via an interface IFC: a graphical interface can be used to display an output on a monitor MON and/or to output it on a printer PRT, or any other type of output device. An input can be made using a mouse MAS or a keyboard TAST, or any other type of input device. The processor unit PRZE also can include a data bus BUS connecting the memory MEM, the processor CPU and the input/output interface IOS, for example. Additional components can also be connected to the data bus BUS, e.g. an additional memory, a data store (hard disk) or a scanner, etc. 
   The following publications have been cited within the scope of this document:
     [1] Christoph Ruland: Informationssicherheit in Datennetzen [Information Security in Data Networks], DATACOM-Verlag, Bergheim, 1993, pages 155 ff.   

   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.