Patent Publication Number: US-2005120248-A1

Title: Internet protocol telephony security architecture

Description:
This application claims priority from co-pending PCT Application No. PCT/US00/09318 filed on Apr. 7, 2000 entitled, “Built-in Manufacturer&#39;s Certificates for a Cable Telephony Adapter to Provide Device and Service Certification,” which claims priority from U.S. application Ser. No. 60/128,772 entitled, “Internet Protocol Telephony Security Architecture” filed on Apr. 9, 1999, as well as PCT Application No. PCT/US00/02174 filed on Jan. 28, 2000 entitled “Key Management for Telephone Calls to Protect Signaling and Call Packets Between CTA&#39;s,” all of which are hereby incorporated by reference for all that they disclose and for all purposes. 
    
    
     BACKGROUND  
      This invention relates generally to network security, and more particularly, to a system for providing key management between a server and a client, e.g., in a telephony or an IP telephony network.  
      In networks that are based on a client/server configuration, there is a need to establish a secure channel between the server and the clients. In addition, in networks that utilize a third party to certify a trust relationship, there is a need to provide an efficient mechanism that allows a key management message to be initiated by the server. In such networks that utilize a trusted third party for the server and client, the client can typically request an encrypted authentication token from the trusted third party that can be used to initiate key management with the specified server; however, the server will typically initiate the key management session directly with the client. It is less preferable for the server to obtain from the trusted third party encrypted authentication tokens for each of the clients. Such an approach would add overhead to a server, requiring it to maintain cryptographic state for each of the clients. If such a server were to fail, a backup server would be required to undergo a recovery procedure in which it has to obtain new authentication tokens for each of the clients. The clients need to be initialized during their provisioning phase to allow them to successfully authenticate to a trusted third party and obtain the encrypted authentication tokens. One proposed method for client initialization is disclosed in PCT Application No. PCT/US00/09318 entitled “BUILT-IN MANUFACTURER&#39;S CERTIFICATES FOR A CABLE TELEPHONY ADAPTER TO PROVIDE DEVICE AND SERVICE CERTIFICATION.” Nevertheless, a need exists to provide an efficient mechanism through which the server can initiate the key management session with the client, as opposed to a system in which only the client can initiate such a session.  
      One such client/server network is the client/server network that exists in IP telephony. In IP telephony systems, a cable telephony adapter (CTA) device can be used to allow a user to send and receive information in secure transactions over an IP telephony network. In typical operation, a series of signaling messages are exchanged that register the CTA device with the IP telephony network before a secure channel with another user can be established. Therefore, the CTA device needs to be authenticated by the IP telephony system. Otherwise, the process would be open to denial of service attacks—since some provisioning exchanges can be forged. In addition, it is desirable for the service provider to identify the CTA device—to make sure that only authorized devices are allowed in its IP Telephony network.  
     SUMMARY OF THE INVENTION  
      One embodiment of the invention comprises a system for providing key management in a client/server network. This embodiment of the invention utilizes a method to provide key management by providing a server; providing a client configured to be coupled to the server; providing a trusted third party configured to be coupled to the client; and allowing the server to initiate the key management session with the client.  
      One embodiment is operable as a method to generate a trigger message at the server; generate a nonce at the server; and, convey the trigger message and the nonce to the client. At the client, the client receives the trigger message and the nonce and responds by conveying a response message with a return nonce. The server can then determine that the response message is valid by comparing the values of the returned_nonce and the nonce that was generated by the server.  
      In addition, one embodiment can be implemented in code and by circuitry operable to produce the acts of the method.  
      A further understanding of the nature of the inventions disclosed herein will be realized by reference to the remaining portions of the specification and the attached drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a flow chart demonstrating an overview of one embodiment of the invention.  
       FIGS. 2A and 2B  show a more detailed flow chart demonstrating a key management session between a server and a client.  
       FIG. 3  shows steps of a key management session after the key management session is initiated.  
       FIG. 4  shows a general block diagram of a client/server/trusted third party network.  
       FIG. 5  shows a block diagram of an IP telephony network in which a cable telephony adapter, a signaling controller, and a key distribution center are coupled with one another.  
       FIG. 6  shows the implementation of the data structures for establishing a key management session as implemented by one embodiment of the invention. 
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS  
       FIG. 1  shows a flow chart demonstrating an overview of one embodiment of the invention. In flow chart  100 , a server is provided  104  and a client coupled to the server is also provided  108 . A trusted third party for the server and the client is provided  112  and the server is allowed to initiate a key management session with the client by utilizing a nonce  116 .  
      It should be understood that a server is a shared computer on a network, such as a signaling controller used in an IP telephony network. Furthermore, it should be understood that a client is a computer or device served by another network computing device, such as a cable telephony adapter (client) being served by a signaling controller (server) via an IP telephony system. In addition, it should be understood that a trusted third party for the server and the client is a device or computer utilized by at least two parties that facilitates cryptographic processes such as certifying the identity of one of the two parties to the other. Finally, it should be understood that a nonce is a number generated that is utilized only once. The use of a nonce helps to prevent an attacker from implementing a replay attack. Such a nonce can be generated randomly.  
      The method of  FIG. 1  can be better understood by reference to  FIG. 2A  and  FIG. 2B . In the method designated  200  in  FIG. 2A  and  FIG. 2B , a server such as a signaling controller in an IP telephony system is provided  204 . In addition, a client such as a cable telephony adapter in an IP telephony system is also provided  208 . A trusted third party for the client and server, such as a key distribution center in an IP telephony system, is provided  212 , as well. The server, client, and trusted third party are coupled to one another. Typically, the client initiates key management sessions with the server. However, there will be times when the server will need to initiate a key management session with the client. Rather than authenticating the trigger message (e.g. with a digital signature and certificate), the invention can utilize a nonce in the authentication of the subsequent AP Request message from the client. This embodiment of the invention does not prevent an adversary (impersonating a legitimate server) from sending an illicit trigger message to the client and fooling it into responding with an AP Request. Instead it provides that such an AP Request will be rejected by the legitimate server. This mechanism is designed to reduce the server&#39;s overhead of initiating key management exchanges with its clients, while still maintaining sufficient security. Thus, in  216  a trigger message is generated at the server to initiate a key management session. Then, a nonce is generated at the server  220  and the nonce and trigger message are coupled together and conveyed to the client  224 . The client receives the trigger message and the nonce  228 . Then the client designates the nonce as a returned_nonce  232 . In this way, the client can return the received nonce to the server for verification that the message is from the client. In  236 , a second nonce is generated at the client. The second nonce is for use by the server and client as part of the key management session being initiated. The client generates a response message to the trigger message that was received from the server  240 . Then the response message, the returned_nonce, and the second nonce are conveyed to the server  244 .  
      At the server, the value of the returned_nonce is compared to the value of the nonce which was generated at the server. If the values of the returned_nonce and the nonce stored at the server are equivalent, the key management session can proceed. However, if the value of the returned_nonce does not equal the value of the nonce stored at the server then a determination is made that the returned_nonce is actually a false nonce  252 . In such a case there is a possibility that the signal has been corrupted; or, there is a possibility that an attacker is trying to initiate a service attack. In a service attack, the attacker tries to fraudulently initiate a rekeying session in order to cause the server to utilize processor cycles which prevent the processor from utilizing those cycles for other operations. Thus the server would become less effective under such an attack than it would be under normal conditions. By repeating such an attack, an attacker can prevent the server from operating efficiently and thus can compromise the operation of the client server network, such as an IP telephony network. If the returned_nonce is determined to be not equivalent to the value of the nonce stored at the server, the response message sent with the returned_nonce is disregarded as being unauthenticated  256 . However, if the returned_nonce does equal the value of the nonce stored at the server, then the key management session continues  260 .  
       FIG. 3  shows additional steps in a typical key management session as highlighted by block  260  in  FIG. 2B . In  FIG. 3 , method  300  shows that an application (AP) REPLY is generated  364  by the server. The AP REPLY is conveyed to the client with the second nonce that was generated by the client  368 . The AP Request is an abbreviation for Application Request and AP Reply stands for Application Reply. For example, these two messages can be specified by the Kerberos Key Management standard (see IETF RFC 1510). As a further example, in the context of Kerberos, the second notice can be the client&#39;s time expressed in microseconds. When the AP REPLY and second nonce are received at the client, the client transmits a security association (SA) recovered message to the server  372 . This completes the applicable Kerberos key management session.  
       FIG. 4  shows a block diagram of a client/server/trusted third party network. A client  401  is coupled with a server  402 . In addition, the client is coupled with a trusted third party  404 . The trusted third party is also coupled with the server  402 .  FIG. 4  thus demonstrates the network within which one embodiment of the invention can be implemented.  
      In  FIG. 5  an IP telephony network implementing one embodiment of the invention is demonstrated. A client such as a cable telephony adapter  501  is coupled with a server, such as signaling controller  502 . Furthermore, the cable telephony adapter and signaling controller are also coupled to a trusted third party, illustrated as key distribution center  504 . Furthermore the signaling controller is coupled with the IP telephony network  508 . Such a network as that illustrated in  FIG. 5  would be useful for establishing an IP telephony call from a user who is coupled to the cable telephony adapter through the IP telephony network  508  to another user connected to a similar network. Thus the user can be authenticated as the calling party through the cable telephony adapter and signaling controller when the call is placed across the IP telephony network. Further details of such a network are illustrated in the references which were incorporated by reference.  
       FIG. 6  illustrates data structures for implementing a Kerberos key management session initiated by a server in a client/server network. In  FIG. 6 a  nonce number  1  is coupled with an initiation signal such as a trigger or wakeup message and the combined message is transmitted across an interface  601  to the client. The client stores nonce number  1 . It then adds nonce number  2  and an application request in data structure such as that shown in  FIG. 6 . This set of data is then transmitted across the interface back to the server. The server compares the value of received nonce number  1  with the value of nonce number  1  stored at the server so as to confirm the authenticity of the AP Request. Upon authenticating the AP Request, the server generates an AP Reply and couples it with nonce number  2  which was generated by the client. The combined nonce number  2  and AP Reply are then transmitted across the interface to the client. The client is able to verify the authenticity of the AP Reply by comparing the value of nonce number  2  received from the server with the value of nonce number  2  stored at the client. Upon authenticating the AP Reply, the client generates a Security Association (SA) recovered message and transmits that across the interface to the server. This Kerberos-based key management protocol is thereby implemented in an efficient way and furthermore allows the server to initiate the key management session with the use of only an additional nonce as overhead to the initiation message. Thus the method is highly efficient in that only a nonce need be used in the authentication process of the initiation message.  
      In addition to embodiments where the invention is accomplished by hardware, it is also noted that these embodiments can be accomplished through the use of an article of manufacture comprised of a computer usable medium having a computer readable program code embodied therein, which causes the enablement of the functions and/or fabrication of the hardware disclosed in this specification. For example, this might be accomplished through the use of hardware description language (HDL), register transfer language (RTL), VERILOG, VHDL, or similar programming tools, as one of ordinary skill in the art would understand. The book “A Verilog HDL Primer” by J. Bhasker, Star Galaxy Pr., 1997 provides greater detail on Verilog and HDL and is hereby incorporated by reference for all that it discloses for all purposes. It is therefore envisioned that the functions accomplished by the present invention as described above could be represented in a core which could be utilized in programming code and transformed to hardware as part of the production of integrated circuits. Therefore, it is desired that the embodiments expressed above also be considered protected by this patent in their program code means as well.  
      It is noted that embodiments of the invention can be accomplished by use of an electrical signal, such as a computer data signal embodied in a carrier wave, to convey the pertinent signals to a receiver. Thus, where code is illustrated as stored on a computer medium, it should also be understood to be conveyable as an electrical signal. Similarly, where a data structure is illustrated for a message, it should be understood to also be capable of being embodied in an electrical signal for transmission across a medium, such as the internet.  
      It is also noted that many of the structures and acts recited herein can be recited as means for performing a function or steps for performing a function, respectively. Therefore, it should be understood that such language is entitled to cover all such structures or acts disclosed within this specification and their equivalents, including the matter incorporated by reference.  
      It is thought that the apparatuses and methods of the embodiments of the present invention and many of its attendant advantages will be understood from this specification and it will be apparent that various changes may be made in the form, construction and arrangement of the parts thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form herein before described being merely exemplary embodiments thereof.