System for providing secure telecommunication access to a computer
BACKGROUND OF THE INVENTION 
     1. Related Applications 
     This application is a continuation-in-part of Applicants&#39; pending application Ser. No. 091,677 filed Aug. 31, 1987, now abandoned. 
     2. Field of the Invention 
     The invention relates to a system for providing secure telecommunication access to a computer connected thereto, the system comprising a telecommunication exchange, terminals connected to the exchange, and a switching network and central control unit in the exchange. The central control unit includes a control processor programmed to follow a security procedure for safeguarding access to the computer by a terminal requesting such access. The control processor includes a receive memory for receiving from a requesting terminal an identification code assigned to such terminal, and an authorization memory for storing the identification codes and corresponding addresses (such as telephone numbers) of terminals which are authorized to establish connection with the computer. The control processor is programmed to match a received identification code stored in the receive memory against the contents of authorized codes stored in the authorization memory, and actuates the switching network to transmit a ringing signal to the terminal address corresponding to such identification code if there is such a match. The control processor subsequently further actuates the switching network to establish a telecommunication line connection between the terminal 50 and the computer after such terminal engages the line in response responds to the ringing signal. 
     3. Description of the Related Art 
     A telecommunication system for providing secure access to a computer is described in the article entitled &#34;Call Back Schemes Ward Off Unwanted Access By Telephone&#34;, Electronics, Mar. 8, 1984, pp. 131-135. Such system comprises a telecommunication exchange and a separate security unit connected to the telecommunication line just before the access ports of the computer. 
     It is possible for a data terminal to be connected to a computer, even though the distance between the computer and the terminal may be large. This connection can be achieved via a telecommunication network (public or private), and a plurality of terminals can be connected to the computer via a telecommunication exchange comprised in such telecommunication network. 
     A problem which may arise, however, is that unauthorized users may try to gain access to the computer. A known security system is known under the name of &#34;call-back system&#34;. When implementing this system a person who wants to start using the computer dials a telephone number to an access port of the computer. Subsequently, this person provides the access port with an identification code, which the computer compares with the identification codes stored in its memory. If it appears that the provided identification code matches a stored identification code, the computer will select a telephone number which inside the computer memory is linked with the stored identification code. This will generally be the telephone number of the person who wants to start using the computer, thus of the requesting terminal, and the computer calls back such telephone number to establish connection to such terminal. 
     From the aforesaid publication it is known to insert between the exchange and the computer a separate security unit for performing the access port function by means of the call-back scheme. When implementing this scheme the user dials the telephone number of the subscriber line connected to the security unit, upon which a normal telephone connection between the user and the security unit is established. Then the identification code is transferred via the thus formed communication channel to the security unit which establishes through-connection to the computer. 
     Such known system has two disadvantages. A first disadvantage is that signaling information available in the exchange is not readily available in the security unit; consequently, this signaling information cannot be used to advantage for controlling access to the computer. A second disadvantage is that the known system requires a separate security unit, the complexity and price of which increases with an increasing number of users and computer ports. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a telecommunication system providing secure access to a computer, and wherein besides the usual network elements such as the exchange, terminals, and a computer, no additional equipment is required; security access control being achieved by utilizing the signaling information already available in the exchange. 
     To this end, a system in accordance with the invention is characterized in that the function of the security unit is performed by the control processor in the central control unit comprised in the exchange, such processor being programmed so that the security procedure must be completed before connection is established between a terminal and a telephone line giving such terminal access to the computer. 
     As the security function is performed within the exchange, safeguarding access to the computer can be provided in a modern electronic telecommunication exchange as a software program in the control processor. Thereby the control processor and the memory capacity already available in the exchange can be used to advantage, which is far less expensive than separate security equipment. The aforesaid security procedure prevents unauthorized callers who happen to learn of the subscriber number of one of the subscriber lines to the computer from dodging the access control by simply dialing the number of such line. 
     A further advantage of the system in accordance with the invention is that signaling information already available in the exchange is used for access control therein. Consequently, terminals connected to the exchange can employ either pulse dialing or tone dialing. In either case, access to the computer can be achieved by means of a code to be entered by the user. This is in contrast with the known system wherein the separate security unit can only receive tone dialing signals. 
     Modern electronic exchanges can offer the associated users rerouting facilities known as &#34;follow-me&#34;, &#34;call-forwarding&#34; or &#34;chief-secretary facilities&#34;. The user of a terminal (the original destination terminal) can then make known to the exchange that calls meant for him should be transferred to another terminal (the divert terminal). The exchange control processor reroutes a call intended for an original destination terminal to a divert terminal connected to the exchange. 
     To avoid calls processed by the exchange from being diverted to a non-authorized terminal, an exchange in accordance with the invention is characterized in that the control processor controls the switching network in the exchange to switch through the connection between the computer and the original destination terminal under the control of the security procedure irrespective of the selective rerouting. 
     With these measures and by combining signaling information indicating rerouting with the security information, it is avoided that a non-authorized terminal can gain access to the computer by the &#34;call-back&#34; method. This ability is not possible when a separate security unit is used for controlling connection to the computer. 
     An advantageous embodiment of the invention is characterized in that the identification code of a terminal forms part of the telephone number for requesting connection to the computer, and must be dialed by the user of the requesting terminal. 
     The aforesaid measures simplify requesting a connection to the computer. If such a facility with a separate security unit were to be inserted between the exchange and the computer, as many subscriber lines would be required between the exchange and the security unit as the number of authorized identification codes. 
     Besides the aforedescribed rerouting and inserting the identification code in the computer request number, the use of the information already available in the exchange offers still further possibilities. A first possibility is identification of the terminal by means of which an unauthorized person tries to gain access to the computer. This is particularly important in private exchanges if the terminal is connected to that exchange. A second possibility is using the so called &#34;repeated call-back&#34; facility, enabling repeated calling-back of the authorized terminal if it appears to be busy at a first calling-back. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete description of a system in accordance with the invention and its advantages is set forth below with reference to the accompanying Figures, in which: 
     FIG. 1 is a flow-chart of the request part of the &#34;call-back&#34; procedure in accordance with the invention; 
     FIG. 2 is a flow-chart of the call-back part of such procedure; and 
     FIG. 3 is a block diagram of a telephone exchange wherein the control processor in the central control unit of such exchange is programmed in accordance with the above flow-charts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The block diagram in FIG. 3 shows a telephone exchange in accordance with the invention. A plurality of terminals 1, 2, 3 are connected to the exchange 4. An external computer 5 is also connected to exchange 4. Each terminal is connected to a line circuit 6, 7, 8 in the exchange, which line circuits generate ringing current to the terminal connected thereto under the control of an activating signal. The external computer 5 is connected by a plurality of connecting lines to a plurality of line circuits 9, 10, 11, 12 which constitute the computer ports. The plurality of line circuits is divided into subsets respectively connected to peripheral modules 13, 14 in the exchange and which are connected to a switching network 15 and a central control unit 16. The central control unit 16 contains a receive memory 17 for storing the identification code received from a requesting terminal, an authorization memory 18 for storing a list of authorized identification codes and the telephone numbers of terminals having such codes, and a control processor 19. The control processor 19 executes all control tasks as described hereinafter with reference to FIGS. 1 and 2, in accordance with a program stored in a program store 20 therein. 
     The request part of the call-back procedure is shown in the flow-chart FIG. 1, the blocks therein having the following meanings: 
     
         ______________________________________                                    
Block                                                                     
Number Title        Meaning                                               
______________________________________                                    
1-2    START        begin of the call-back procedure                      
1-4    B            junction point with all further                       
                    blocks marked &#34;B&#34;                                     
1-6    INC.CALL     check whether a call is made over                     
                    an exchange subscriber line or over                   
                    a line connecting the exchange to                     
                    another exchange;                                     
1-8    DIG.DIALL    check whether the caller dials a                      
                    digit;                                                
1-10   PREF.COMPL.  check whether the first part of the                   
                    number to be dialled (the prefix)                     
                    is complete;                                          
1-12   EXIST.PREF.  check whether the digit combination                   
                    forming the prefix is a prefix                        
                    indicating that the caller wishes                     
                    to gain access to the computer;                       
1-14   NORM.PROC.   the exchange follows the normal                       
                    procedure for calls which are not                     
                    intended for the computer;                            
1-16   DIG.DIALL.   check whether the caller dials a                      
                    further digit;                                        
1-18   CODE COMPL.  check whether the second part of                      
                    the number to be dialled (the                         
                    identification code) is complete;                     
1-20   EXIST.CODE   check whether the received code is                    
                    a code existing in the                                
                    authorization memory;                                 
1-22   SEND REJ.    if the code is not authorized,                        
       TONE         send a reject tone to the caller                      
1-24   B            junction point with all other                         
                    blocks marked &#34;B&#34;;                                    
1-26   FETCH PROT.  the telephone number of the                           
       DEST.        terminal having the authorized                        
       ENG.LN.CRCT  identification code is being                          
       SEND CONF.   fetched; the line circuit of a                        
       TONE         subscriber line connected to the                      
                    computer is engaged and a confirm                     
                    tone is transmitted to inform the caller              
                    to release the line connection;                       
1-30   CALL.REL.    check whether the caller has                          
                    released the line connection;                         
1-32   t &gt; T1       check whether a time T1 has passed                    
                    after the start of a confirm tone;                    
1-34   REL.         all units used for handling the                       
       RESOURCES    request for a computer connection                     
                    are released, but the engaged line                    
                    circuit to the computer                               
                    remains engaged;                                      
1-36   C            junction point with all further                       
                    blocks marked &#34;C&#34;                                     
______________________________________                                    
 
    
     The request part of the &#34;call-back&#34; procedure starts with a check whether there is an incoming call (block 1-6). It is assumed that the number transmitted by the user to the exchange consists of two parts: a prefix indicating that the user wishes to gain access to the computer and an identification code in combination with the prefix indicating with which authorized terminal the connection is desired. 
     After the relevant digits are received it is checked whether it is a prefix for computer access (blocks 1-8, 1-10, 1-12). If this is not the case the normal standard procedure is followed for handling a call (1-14). If it is a prefix for computer access, the subsequent identification code (1-16, 1-18, 1-20) is checked. 
     If the received identification code does not match one of the stored identification codes, the request is not processed any further by the exchange; this can be made known to the sender by means of a reject signal (1-22). If the identification code is valid the exchange sends a confirm signal to the sender signifying that the line connection should be released. The telephone number of the sending terminal corresponding to the valid code will likewise be fetched and the line circuit of the desired computer port will be reserved for connection to such terminal (1-26). 
     After the connection to the sender has been released, either by the sender himself (1-30) or after a clock interval T1 in the exchange (1-32), all units associated with the exchange (hardware and software) which were used in the aforesaid procedure are released again. Only the already engaged line circuit to the computer remains in the engaged state. 
     Subsequently, the call-back part of the &#34;call-back&#34; procedure is executed. 
     In FIG. 2 is shown a flow-chart representing this procedure. The blocks in this Figure have the following meanings: 
     
         ______________________________________                                    
Block                                                                     
Number Title        Meaning                                               
______________________________________                                    
2-2    C            junction point with all further                       
                    blocks marked &#34;C&#34;;                                    
2-4    DEST.INT.    check whether the authorized                          
       PABX         terminal is connected direct to the                   
                    exchange;                                             
2-6    SET UP       attempt to set up an internal                         
       CONN.INT.    connection between the authorized                     
                    terminal and the already engaged                      
                    line circuit to the computer disregarding any         
                    call-rerouting for that terminal;                     
2-8    INT.CONN.    check whether the internal                            
       SUCCESF.     connection is set up successfully;                    
2-10   A            junction point with all further                       
                    blocks marked &#34;A&#34;;                                    
2-12   RING DEST.   send a ringing signal to the                          
                    authorized terminal;                                  
2-14   ANSWER       check whether the authorized                          
                    terminal answers the call                             
                    (i.e., engages its line);                             
2-16   t &gt; T2       check whether a time T2 has passed                    
                    after the start of the ringing                        
                    signal to the authorized terminal;                    
2-20   REL.         all units that were used for                          
       RESOURCES    calling the internal authorized                       
                    terminal are released.                                
2-22   B            junction point with all further                       
                    blocks marked &#34;B&#34;;                                    
2-24   SET UP       attempt to set up a connection                        
       CONN.EXT     between the (external) authorized                     
                    terminal and the already engaged                      
                    line circuit;                                         
2-26   EXT.CONN.    check whether the external                            
       SUCCESF.     connection has been set up                            
                    successfully;                                         
2-28   t &gt; T3 V     check whether a time T3 has passed                    
       BACKW.REL    after the external connection has                     
                    been set up successfully or whether                   
                    the connection was released during                    
                    calling;                                              
2-30   ANSW.FRM.    check whether the authorized                          
       DEST.        terminal answers the call                             
                    (i.e., engages its line);                             
2-32   RING PORT    the computer port is called by                        
                    means of a ringing signal;                            
2-34   ANSWER       check whether the computer port                       
                    answers the call;                                     
2-36   t &gt; T4       check whether a time T4 has passed                    
                    after the start of the ringing                        
                    signal to the computer port;                          
2-38   SEND B.T.    a busy tone is sent to the                            
                    authorized terminal;                                  
2-40   DEST.REL.    check whether the authorized                          
                    terminal has released its line                        
                    connection;                                           
2-42   t &gt; T5       check whether a time T5 has passed                    
                    after the start of the busy tone to                   
                    the authorized terminal;                              
2-44   REL.         all units that were used for                          
       RESOURCES    setting up a connection between the                   
                    computer port and the external                        
                    authorized terminal are released;                     
2-46   B            junction point with all further                       
                    blocks marked &#34;B&#34;;                                    
2-48   SW.THR.CONN. a communication channel is                            
                    established between the computer                      
                    port and the external authorized terminal;            
2-50   NORM.PABX.   the exchange starts following the                     
                    normal procedure for further                          
                    handling an existing communication                    
                    channel;                                              
2-52   REL.CONNECT. all exchange units that were used                     
                    to set up the connection are                          
                    released, except for the already                      
                    engaged line circuit of the computer;                 
2-54   MORE ATTPS.  check whether a new attempt should                    
                    be made to set up a connection;                       
2-56   REL.         all exchange units that were used                     
       RESOURCES    to set up the connection are                          
                    released;                                             
2-58   B            junction point with all further                       
                    blocks marked &#34;B&#34;;                                    
2-60   T &gt; T6       check whether a time exceeding                        
                    T6 has passed since the units                         
                    that were used to set up the                          
                    connection were released.                             
______________________________________