Patent Publication Number: US-6708275-B1

Title: Process and device for transmitting confidential connection establishment and service information between subscriber terminals and one or more digital exchanges

Description:
FIELD OF THE INVENTION 
     The present invention concerns a process for the transmission of confidential and authentic connection establishment and/or service information between subscriber-side terminals and one or more digital exchanges of a digital communication network, in particular an ISDN network, as well as a device for the transmission of confidential connection establishment and service information. 
     RELATED TECHNOLOGY 
     Digital telecommunication networks are known that feature a plurality of subscriber connections and digital exchanges. Since a subscriber connection transmits all dialing and service information unencoded, i.e., as plain text to a digital exchange, intruders or eavesdroppers can simply tap into this confidential information on the subscriber lines by cutting into existing communication paths or intruding into the existing lines at different points. An intruder, once having obtained said dialing and service information, can use the exchange at the expense of the subscriber, even without being authorized to do so. 
     In the article “INTEGRATING CRYPTOGRAPHY IN ISDN,” published in “ADVANCES IN CRYPTOLOGY”, Santa Barbara, Aug. 16-20, 1987, Conf. No. 7, Jan. 1, 1987, Pomerance C., pp. 9-18, XP000130200, K. Presttun discloses a cryptographically secured ISDN communication system with a plurality of data terminals connected to the ISDN via a network terminator, as well as with a plurality of exchanges associated with an authentication service device. A security module, used only for coding and decoding useful data transmitted over the B channel of an ISDN connection, is implemented in each data terminal or network terminator. The D channel of the ISDN connection is used to distribute security keys among the data terminals and to manage security. Coding of connection establishment and service information is not disclosed in that document. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the invention is to make misuse of the exchange by unauthorized intruders at the expense of authorized subscriber connection owners difficult or even impossible. 
     This object is achieved by the invention through the steps of process claim  1  and the features of device claims  6  and  8 . 
     The invention is implemented in a digital communication network, in particular an ISDN. Such a digital communication network includes, as is known, a plurality of exchanges, at least one network terminator installed at the subscriber, to which at least one data terminal, such as telephone sets, personal computers, or fax machines, can be connected. Undesired use by an intruder is prevented by providing at least one first security device at a subscriber connection owner; in addition, at least one second security device is provided in at least one exchange, with the first and second security devices assigned to a specific subscriber being capable of encoding and/or decoding and exchanging, via a separate control channel of the digital communication network, connection establishment and/or service information. Each of said security devices has a security module capable of receiving an identification carrier. 
     Connection-specific assemblies, each containing a second security device, are installed at each exchange. This embodiment is, however, expensive and complex, since the exchanges themselves must be rebuilt. 
     A more cost-effective method, which can be implemented in a simpler manner, comprises of installing an additional device, based on the existing digital exchanges, between the network terminators assigned to the exchange and the exchange. The respective second security device for each subscriber connection is installed in this additional device. 
     The first security device of a given connection owner is advantageously arranged in the network terminator itself corresponding to each subscriber connection. In this case a single security device is sufficient even if the owner of a subscriber line can connect up to eight data terminals to the network termination via an S 0  bus. It is perfectly possible to equip each data terminal of a given network termination with its own security device. Another alternative may consist of connecting a security device between each data terminal and its network terminator. It can be easily seen, however, that both of the latter implementation options are complex and costly, since each data terminal requires its own security device. The service information may include call forwarding and conferencing functions, for example. If the digital communication network is an ISDN, the transmission of confidential information between the subscriber-side first security device and the respective second security device in the exchange takes place via the D channel of the ISDN network. Each identification carrier can then store an individual cryptographic key that is specific to a given subscriber connection owner. The identification carrier may be a smart card that can be inserted by the owner of a subscriber connection in the first security device and by an employee of the network operator in the second security device. An advantageous alternative provides a software module as the identification carrier, which can be installed exchangeably in the respective security device. In an advantageous refinement, the identification carrier is a software module, which can be exchangeably installed in the respective security module. In an advantageous refinement, the first, subscriber-side, security device and the second security device of the exchange can perform subscriber authentication to the exchange. Alternatively, separate authentication devices can be installed for this function. 
     Protection for the exchange and the authorized subscriber connection owner can be enhanced by having the first and second security devices, assigned to a given subscriber, encode and decode, respectively, a user-to-user communication to be transmitted via the control channel of the digital communication network. 
     The subscriber-side security device of a given subscriber and the security device of the exchange preferably have the same subscriber-specific ID. In addition, they encode and decode the confidential information with the same subscriber-specific key. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is explained below with reference to the embodiments illustrated in the following figures, in which: 
     FIG. 1 schematically shows a section of a digital communication network shown as a communication path from a telephone set to a digital exchange where the present invention is implemented, 
     FIG. 2 shows one embodiment where the security device according to present invention is installed in an additional device on the exchange side, 
     FIG. 3 shows a detailed block diagram of the network terminator with the security device according to the present invention, and 
     FIG. 4 shows a detailed block diagram of a connection-specific assembly in the exchange with a built-in security module, 
     FIG. 5 shows the frame format of the ISDN bit streams. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows in a simplified form a part of a digital communication network, exemplified in the description that follows as an ISDN network. A telephone set  20  is illustrated as a data terminal on the subscriber side, which is connected to a network terminator  10  via an S 0  bus  25 . Network terminator  10 , also referred to as Net Terminator (NT), can be installed in the building or room of a subscriber connection owner. Up to eight data terminals, such as other telephone sets  20 , fax machines, or personal computers, can be connected to S 0  bus  25 . In the example illustrated, a subscriber-side security device  40 ,  50  according to this invention is built into network terminator  10 . The safety device includes a subscriber-specific identification carrier  50  and a safety module  40 , which can be permanently installed in network terminator  10  where an identification carrier  50  can be inserted. Identification carrier  50  can be a smart card or a software module. Security device  40  and identification carrier  50  are designed so that they can encode or decode confidential connection establishment or service information with a subscriber-specific or connection-specific key. This key can be stored in a storage module of the smart card of the connection owner. The output of network terminator  10  is connected to a corresponding ISDN exchange  30  via a twisted two-wire cable in a known manner. Of course, an ISDN network includes a plurality of network terminators  10  and a plurality of exchange systems  30 , which may be interconnected via twisted two-wire cables. As a possible alternative, conventional two-wire cables can be supplemented and replaced by fiber-optic cables, for example. Exchange  30  includes a plurality of assemblies  80  (FIG. 1 only illustrates a single connection-specific assembly  80 ), each assigned to certain subscriber connection owners. In the first embodiment according to this invention, a security device  60 ,  70  is arranged in each connection-specific assembly  80 . Like the subscriber-specific security device  40 ,  50 , security device  60 ,  70  in exchange  30  includes a module  60 , which can be permanently installed in assembly  80  and an identification carrier  70 , which can be inserted therein. It is assumed that identification carrier  70  in exchange  30  also contains the individual cryptographic key of subscriber-side security device  40 ,  50 . 
     FIG. 2 shows an alternative embodiment where additional device  100  is connected between network terminator  10  and exchange  30 . For greater clarity, FIG. 2 shows additional device  100  only with the built-in security device  60 ,  70 . Normally all security devices  60 ,  70  of subscribers or network terminators jointly served by exchange  30  are installed in additional device  100 . The corresponding subscriber lines are indicated in FIGS. 1 and 2. Again, identification carriers  70  can be inserted as smart cards from the outside by an operator or, in the implementation as a software module, can be installed in the respective security module  60 . The use of additional device  100  has the advantage that existing ISDN network exchanges can continue to be used without time-consuming, costly, and complex modifications of the exchange system for the encoded transmission of confidential information, for example, of the subscriber of telephone set  20  to exchange  30 . 
     FIG. 3 shows a simplified block diagram of the known network terminator  10 , where the security module  40  according to this invention, together with identification carrier  50 , are installed. On the subscriber side, network terminator  10  has a connection unit for S 0  bus  25 , to which up to eight data terminals  20  can be connected. Since the structure and the mode of operation of network terminator  10  are generally known, only the essential components are briefly explained below. Basically network terminator  10  has a send path and a receive path. The send path includes an encoder  210 , which, among other things, supplies the confidential information to be transmitted in the D channel of the ISDN network to an encoder in security device  40 ,  50 , and a multiplexer  200 , which combines the two B channels and the D channel to a continuous data stream in a time multiplexing operation. A suitable frame format consists of 48 bits per 250 ms, with only four D channel bits provided per frame. In other words, 16 kbits/sec are transmitted over the D channel (see FIG.  5 ). The send path then goes to a hook switch  170  via a transmitter  180 ; said hook switch sends the outgoing data stream to a two-wire cable that connects exchange  30  to network terminator  10 . Incoming data streams go through hook switch  170 , a receiver  160 , and a device  150 , which equalizes and amplifies the data stream received and recovers the clock signal from it. Then the data stream passes through a demultiplexer  140 , which decomposes the data stream again into the two B channels and the D channel. The demultiplexed data stream passes through decoder  130  and is then transmitted, according to a destination address, to telephone set  20 , for example, via the S 0  bus  25 . An echo compensation  190 , connected in parallel between transmitter  180  and receiver  160 , is used, among other things, for compensating outgoing messages sent to the receive path through hook switch  170  and receiver  160 . The heart of network terminator  10  is a controller  220 , which controls the management and mutual control of the individual components. The security module  40  according to this invention with the inserted identification carrier  50  is connected, for example, to controller  220 , encoder  210 , multiplexer  200 , demultiplexer  140 , and decoder  130 . Controller  220  is also responsible for activating or deactivating security device  40 ,  50  as required by the situation. 
     FIG. 4 shows an example of a block diagram of a subscriber-specific assembly  80 , installed in exchange  30 . The connection-specific assembly  80  basically forms the counterpart of network terminator  10 . Incoming data messages go, via the two-wire line, to a hook switch  230  and then pass through a demultiplexer  240 , a decoder  250 , and a D channel handler  260 . D channel handler  260  supplies control information to a central controller of exchange  30 . In the opposite direction, outgoing messages pass through an encoder  270 , a multiplexer  290 , and a hook switch  230  on the two-wire line to network terminator  10 . Also in connection-specific assembly  80 , a controller  280  is responsible for the management of and interaction between the individual components. Security module  60  is installed in connection-specific assembly  80 , according to this invention, with a smart card that can be inserted from the outside, or an installed software module  70 . Security device  60 ,  70 , which includes security module  60  and identification carrier  70 , is connected to encoder  270 , decoder  250 , D channel handler  260 , and controller  280 . As mentioned previously, authentication device  60 ,  70  can also be advantageously installed in additional device  110 , as shown in FIG.  2 . 
     It is convenient to install security device  40 ,  50  in network terminator  10  itself, since in this way only one security device  40 ,  50  is required, regardless of the number of data terminals  20  connected. However, it is also conceivable to arrange the subscriber-side security device  40 ,  50  in each data terminal  20 . Another alternative consists of providing a security device (not illustrated) between network terminator  10  and each connected data terminal  20 . For reasons of economics, it is convenient to install security devices  40 ,  50  in network terminator  10  itself, shown in FIG.  1 . Then identification carrier  50  can be installed in the form of a software module by the network operator when network terminator  10  is installed at the subscriber. If identification carrier  50  is a smart card, the subscriber can purchase this card, containing the subscriber&#39;s individual subscriber key, from the network operator. 
     The transmission of confidential connection establishment and/or service information between telephone set  20  and exchange  30  is now described in detail. 
     It is assumed that one subscriber-side security device  40 ,  50  is installed in network terminator  10  and a second security device  60 ,  70  is installed in assembly  80 , assigned to the subscriber of telephone set  20  in exchange  30 . According to the embodiment illustrated in FIG. 2, security device  60 ,  70  can also be installed in additional device  100 . The procedures described below occur basically in the same way in both cases. 
     Let us also assume that the subscriber-side identification carrier  50  and the exchange-side identification carrier  70  have the same ID, i.e., the identity of the subscriber of telephone set  20 . In addition, subscriber-side security device  40 ,  50  and exchange-side security device  60 ,  70  have the same cryptographic key associated with the subscriber of telephone set  20 . Both identification carriers  50 ,  70  are smart cards. 
     After security device  40 ,  50  has authenticated the subscriber of telephone set  20  to exchange  30  connected to security device  60 ,  70  installed in subscriber-specific assembly  80 —the authentication can be performed by separate authentication devices—all connection establishment and service information is encoded by the subscriber-side security device  40 ,  50  with the subscriber-specific key and transmitted to security device  60 ,  70  in the subscriber-specific assembly in the D channel via the ISDN network. Security device  60 ,  70  in exchange  30  decodes the received encoded connection establishment and/or service information with the common subscriber-specific key and sends the decoded information via the D channel handler to a central unit in exchange  30 . In response to the decoded connection establishment information, the desired connection to the subscriber dialed is established and, in response to the decoded service information the requested services are made available. This ensures that an unauthorized intruder will not be able to establish a connection at the expense of the owner of the connection of telephone set  20 . Different known encoding procedures can be used for encoding and decoding, such as, for example, the Rivest-Shamir-Adlerman method or the Data Encryption Standard method. 
     Let us now assume that instead of the smart card, a special encoding and decoding module  70  is installed in subscriber-specific assembly  80 . Prior to encoding the confidential connection establishment and service information and transmitting it via the ISDN network to exchange  30 , decoding module  70  is provided with the key assigned to the subscriber of telephone set  20  by a central or decentralized encryption center. The key can be transmitted to security device  60 ,  70  via a data medium, such as a diskette, or by electronic data transmission. Important and known security aspects must be taken into account, especially in confidential and authenticated information transmission, so the subscriber-specific key does not become known to unauthorized third parties. After security device  60 ,  70  of assembly  80  has received the subscriber-specific cryptographic key, the confidential connection establishment and service information is transmitted as described above. 
     An advantageous refinement of the invention provides that the control channel (D channel) of the digital communication network serves as a data transmission line for user-to-user transmission, specifically between data terminal  20  and exchange  30 . In this case the subscriber-side security device  40 ,  50  is capable of encoding, for example, data transmitted by a personal computer and transmitting it to security device  60 ,  70  of exchange  30 . Security device  60 ,  70 , which can be installed in subscriber-specific assembly  80  or in additional device  100 , can decode this data information and transmit it in conjunction with exchange  30 , for example, via the Datex P network to the subscriber dialed.