Abstract:
Methods and apparatus are disclosed which provide a system for secure and reliable communication between client computers residing on separate private networks but connected via a public network such as the Internet. The communications described herein are designed to function even if a persistent link can not be established between the two computers. Further, the systems and apparatus described herein are designed to traverse any locally installed gateways or firewalls to obtain access to a remote destination.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/226,150 filed Aug. 16, 2000 and entitled Method and Apparatus for Secure Communication Over Unstable Public Connections, the entire content of such Application being expressly incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to the field of data communication between computer systems. More specifically, it relates to a method of communication between a local computer system potentially protected by a firewall and a remote computer system connected to the local system via a public network.  
           [0004]    2. Background Information  
           [0005]    In the field of communications many systems exist for passing data from one point to another. A typical communication system consists of several layers. A low-level layer might include software designed to drive hardware devices, such as modems or Ethernet interface cards. An example of a fully featured top-level software transport layer that is designed to provide reliable end to end communications is the TCP/IP protocol. Computer Networks, by Andrew S. Tanenbaum, printed by Prentice Hall PTR, Upper Saddle River, N.J. 1996, provides a more detailed view of computer networks, TCP/IP and the OSI model.  
           [0006]    This invention builds upon a number of established systems that can be readily understood by one skilled in the art. These systems are summarized as follows:  
           [0007]    Protocol encapsulation: This is a technique where high-level communication messages are packaged into the payload of a lower level communication system. One example of this is the manner in which TCP/IP messages are packaged into Ethernet packets for communication over a local area network (LAN). In a similar way TCP/IP can be packaged into Frame Relay packets for communication over wide area networks (WAN), or into serial streams for communication over networks such as the Internet. Protocol encapsulation can also be application-specific, as described in Batz et al., U.S. Pat. No. 5,918,022 entitled Protocol for Transporting Reservation System Data Over A TCP/IP Network. The present invention, while possessing some limitations, is intended for general use and is not necessarily tied to any specific application.  
           [0008]    TCP/IP: This basic communications medium is described in detail in the above referenced work by Tanenbaum and provides a reliable point-to-point communication system that applications can use to communicate. Protocol encapsulation methods have been written that can encapsulate TCP/IP requests into just about every conceivable low-level network transport, including Ethernet and PPP.  
           [0009]    HTTP and HTTPS: HTTP is a high-level protocol that builds upon TCP/IP and was designed specifically to carry content between Web sites and Web browsers. HTTPS is a secure implementation of HTTP that is used for transmitting sensitive data such as credit card details.  
           [0010]    HTTP firewalls and Proxies: With recent advances in electronic communications, corporations have begun to use public networks, specifically the internet, for internal communications, communications with clients, and for accessing public data stores such as third-party web sites. Corporations are normally connected to the Internet through dedicated communications links that are available on a permanent basis. However, Internet connectivity poses a great security risk to a corporation: any machine with a known address that can access the Internet is in turn accessible from any other machine on the Internet. To prevent unwanted third-party access, most corporations, and some individuals, deploy firewalls to secure their sites. A firewall is a computer software and hardware solution that allows communications to be originated only from within the secure site. For example, most firewalls allow outgoing HTTP traffic (Web page requests) and incoming replies to messages originated within the site (Web pages). Email is often allowed to pass directly into a secure site as it intended to be a passive form of communication. This ability to allow limited communication is often performed by a proxy. A proxy is a forwarding agent that receives a request for information from a computer within the secure site, passes it to a destination, and returns any responses to the originator. The combination of a firewall preventing access to machines within a secure site, and a proxy masking a secure machine&#39;s true identity, provide a level of security which most demand. Some corporations impose an even higher level of security by restricting, or denying completely, certain forms of outgoing communication. For example, many corporations permit only small amounts of data to be sent through their firewalls; this can be accomplished by denying HTTP POST requests and disabling all other upload protocols, such as FTP. More details can be found in Coley et al., U.S. Pat. No. 6,061,798 entitled, “Firewall System for Protecting Network Elements Connected To A Public Network.” 
           [0011]    Tunnels: With the deployment of firewalls and proxies it became impossible, or at least quite difficult, to provide a bi-directional communication system between a computer within a secure site and another computer on the Internet. Several solutions exist that require special bypasses or tunnels to be added to firewalls, but these typically require additional applications to be executed on the firewall host. This is at the least an inconvenience, and often prohibited due to security considerations. For more detail, see Jade et al, U.S. Pat. No. 6,061,797 entitled “Tunnels Outside Access To Computer Resources Through A Firewall”; Birrell et al, U.S. Pat. No. 5,805,803 entitled, “Secure Web Tunnel,” and Aziz et al., U.S. Pat. No. 5,548,646 entitled, “System For Signatureless Transmission And Reception Of Data Packets Between Computer Networks.” The present application describes a system that does not deploy anything on a firewall host, and yet allows reliable two-way communications between local and remote applications using only HTTP. As discussed above, HTTP requests are normally successfully proxied through firewalls.  
           [0012]    Encryption: While the present embodiment of the invention makes use of encryption to provide secure communications, it should be clear to one skilled in the art that any one of a number of available techniques could be used, and the invention is not dependent on the exact method used. It should also be apparent that a non secure embodiment of the invention is possible by not using encryption. For example, in one embodiment the process described in Hellman, et al., U.S. Pat. No. 4,200,770 entitled, “Cryptographic Apparatus and Method,” might be used.  
         SUMMARY OF THE PRESENT INVENTION  
         [0013]    Methods and apparatus are disclosed which provide a system for secure and reliable communication between a pair of client computers, or a plurality of client computers residing on separate private networks, and connected via a public network such as the Internet. The communications described herein are designed to function even if a persistent link can not be established between the communicating computers. Further, the system described herein is designed to traverse any locally installed gateways or firewalls to obtain communicative access to a remote destination. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 generally depicts an environment in which the present invention may be deployed;  
         [0015]    [0015]FIG. 2 is a diagram schematically illustrating the architecture of a client application and associated data processor in accordance with the present invention;  
         [0016]    [0016]FIG. 3 is a diagram schematically illustrating one embodiment of the data processor architecture used in accordance with the present invention;  
         [0017]    [0017]FIGS. 4 a  and  4   b  illustrate the composition of a data packet constructed and sent by a local computer in accordance with the present invention;  
         [0018]    [0018]FIG. 5 illustrates one embodiment of the composition of an aggregated data packet constructed and sent by a public computer as a reply to a message from a local computer in accordance with the present invention;  
         [0019]    [0019]FIG. 6 illustrates an embodiment of the composition of an HTTP POST encapsulated data packet for transmission from a local computer to a remote computer through a firewall in accordance with the present invention;  
         [0020]    [0020]FIG. 7 illustrates an embodiment of the composition of an HTTP GET encapsulated data packet for transmission from a local computer to a remote computer through a firewall in accordance with the present invention;  
         [0021]    [0021]FIG. 8 illustrates one embodiment of the composition of an individual reply data packet after it is received and unpacked by a local computer in accordance with the present invention;  
         [0022]    [0022]FIG. 9 depicts a flow chart generally illustrating an embodiment of the process by which data is processed, encapsulated and transmitted from a local computer in accordance with the present invention;  
         [0023]    [0023]FIG. 10 is a flow chart generally illustrating an embodiment of the process by which a local computer implementing the process of FIG. 9 splits a message originating at the local computer system into suitably-sized chunks and packages it according to FIG. 4 for transmission in accordance with the present invention;  
         [0024]    [0024]FIG. 11 depicts a flow chart generally illustrating an embodiment of the process reply substep of FIG. 9 wherein encapsulated data packets (FIGS. 6,7) received by a local computer are processed in accordance with the present invention;  
         [0025]    [0025]FIG. 12 is a flow chart generally illustrating an embodiment of the packet separating substep of FIG. 11 wherein payload message segments are extracted from an aggregated data packet (FIG. 5) and individual data packets (FIG. 8) are assembled in accordance with the present invention;  
         [0026]    [0026]FIG. 13 depicts a flow chart generally illustrating an embodiment of the process by which a public application sends data in accordance with the present invention;  
         [0027]    [0027]FIG. 14 depicts a flow chart generally illustrating an embodiment of the process by which a public computer system may request information from a local computer system;  
         [0028]    [0028]FIG. 15 is a flow chart generally illustrating an embodiment of the process by which a public computer receives and processes data in accordance with the present invention;  
         [0029]    [0029]FIG. 16 is a flow chart generally illustrating an embodiment of the process message step of FIG. 15 by which the public computer processes pending data into a reply to a received message in accordance with the present invention;  
         [0030]    [0030]FIG. 17 is a flow chart generally illustrating an embodiment of the recombine data substep of FIG. 15 by which a public computer recreates a message from its constituent chunks in accordance with the present invention;  
         [0031]    [0031]FIG. 18 is a flow chart generally illustrating an embodiment of the package data substep of FIG. 16 by which a public computer concatenates message segments into a composite payload message for transmission as a reply to a message received from a local computer in accordance with the present invention; 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    In the following description, various aspects of the present invention are described. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some or all aspects of the present invention. For the purposes of explanation, specific numbers, materials and configurations are set forth to provide a thorough understanding of the present invention. However, there it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details. In some instances, well known features are omitted or simplified in order not to obscure the present invention.  
         [0033]    Parts of the description are presented in terms of operations performed by a computer system, using terms such as data, values, characters, strings, numbers and the like, consistent with the manner commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. As is well understood by those skilled in the art, these quantities take the form of electrical, magnetic, or optical signals capable of being stored, transferred, combined, and otherwise manipulated through mechanical and electrical components of the computer system. The term computer system as used herein includes general purpose as well as special purpose data processing machines, systems, and the like, that are standalone, adjunct or embedded.  
         [0034]    Various operations are described as multiple discrete Steps in turn, in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, all operations need not be performed in the order of presentation.  
       Description of FIG.  1   
       [0035]    In FIG. 1, the environment in which the present invention may be deployed is shown. This environment is typically comprised of a local computer system  10 , which may include a local computer  11 , on which part of the present invention resides, connected by a private network  12 , through a firewall  16  to a public network  18 , such as the Internet. This connection may be unstable, in the sense that the data path may inadvertently be interrupted from time-to-time. Another part of the present invention resides on a public computer  20 , which may be a component of a remote computer system  21 , that is connected directly to the public network  18 .  
       Description of FIG.  2   
       [0036]    It is well known to those skilled in the art that the general architecture of client applications may consist of client application code, linked with third-party application libraries. In FIG. 2, the architecture utilized in accordance with the present invention is shown generally at  22 . As depicted, the client application  24  is linked via an Application Programming Interface (API  23 ) to a specially configured data processor  26 . As is also well known to those skilled in the art, the actual form of the API may be configured to provide an unlimited number of different views of the processor to fit pre-existing application code architectures. The processor  26  makes use of the HTTP protocol and the TCP/IP protocol described above.  
       Description of FIGS.  3  and  4   a.    
       [0037]    [0037]FIG. 3 depicts generally at  27  the overall architecture of the present invention. In one embodiment the processor  26  may be implemented in computer hardware. In another embodiment the processor  26  may be implemented as computer software. It should be clear to those skilled in the art that the processor  26  could also be a combination of both, without limitation as to which portion of the architecture is implemented in hardware or software.  
         [0038]    Data intended to be included in a transmittable Local Message is schematically represented by the block  50  in FIG. 4 a . This data enters and exits the processor  26  in the local computer through the connections  25  to the API buffers  31  and  34  on one side thereof, and after being encrypted, packaged, and encapsulated for transmission, the data leaves the processor through communications buffer  32  on the other side and enters the transporting network(s)  30 . Data entering the processor from the transport side is received by the buffer  36  and after the encapsulation is removed, is decrypted and unpackaged, and then placed in the API receive buffer  34 . In processor  26 , the API send buffer  31 , data send buffer  32 , API receive buffer  34  and data receive buffer  36  all provide temporary storage means for data in transit.  
         [0039]    An encryption unit  38  is responsible for encrypting and decrypting the message data. A packaging unit  40  operates under control of control logic  44  and is responsible for dividing the encrypted local message data into “chunks”  52  (FIG. 4 a ) of predetermined size, and for combining the chunks with identifying header data  54  (FIG. 4 b ) to form data packets  4 H, as will be described below. Packaging unit  40  also performs an unpacking operation with respect to received data. An addressing unit  42  is responsible for encapsulating the outgoing data packets to fit the transport protocol requirements for data transmission, and for stripping incoming encapsulated packets of their encapsulation, as will be described below.  
         [0040]    In accordance with the present invention, another processor  26 ′ resides on the public computer  20  (FIG. 1) and is substantially identical to that of the local computer described in the upper part of FIG. 3. Entities  31 ′ through  44 ′ are functionally identical to entities  31  through  44 .  
       Description of FIG.  4   b    
       [0041]    [0041]FIG. 4 b  illustrates one configuration of the components of a data packet transmitted from a local computer  11  to a public computer  20  after packaging but before encapsulation. It will be clear to those skilled in the art that the order in which the components of the packet are assembled is unimportant, as is the exact nature and number of the components. Component  4 A is an identification number unique to the local message, identifying the local message on both the local computer  11  and the public computer  20 .  4 B is the number of chunks in which the original local message is divided for transmission from the local computer  11  according to the present invention,.  4 C is the chunk number of this instance of the message as determined by a process explained below.  4 D is the identification (ID) of the sender of the particular message, and  4 E is the identification (ID) of the destination.  4 F specifies which remote message this local message is a reply to, if in fact it is a reply to a previously received message from the public computer  20 . If this message is not a reply, then this ID will be null.  4 G represents the payload of the data packet. As suggested above, the payload may be an entire message to be sent, or if the length of the message exceeds the limits of the firewall  16  (FIG. 1), a partial message, or chunk.  
       Description of FIG.  5   
       [0042]    [0042]FIG. 5 illustrates one configuration of either an Aggregated Data Packet in which is included either an original message or a reply message to be transmitted through the firewall from a public computer  20  to a local computer  11 . As described above, the Payload of this packet can also be an aggregation of multiple messages, or message segments, to be sent at the same time to the local computer. This packet is comprised of a header in which component  5 A specifies the Number of Messages, or “Payload Segments”, in the Aggregated Data Packet contained within the transmission, and  5 B and  5 C identify the Sender and the receiver (Destination) respectively. For each included Payload Segment, a Segment Identification Number  5 D, its Length  5 E, and the Identification Number  5 F of the Local Message to which it is potentially a reply, is specified. The Payload  5 G of this packet includes a concatenation of all of the Message Segments (of which three,  5 G 1 ,  5 G 2  &amp;  5 G 3  are shown) to be communicated by the packet.  
       Description of FIG.  6   
       [0043]    [0043]FIG. 6 shows the format of one embodiment of an encapsulated data packet to be sent from a local computer  11  to a public computer  20 . In this embodiment, it is assumed that the HTTP POST operation is allowed with regards to the security policy enforced at the site where the local computer resides. The HTTP Address  6 A contains the address of the public computer  20  written according to the HTTP syntax. The Header  6 B contains fields required by the HTTP protocol, such as the total message length in bytes. The payload  6 C is comprised of a data packet of the configuration illustrated in FIG. 4 b.    
       Description of FIG.  7   
       [0044]    [0044]FIG. 7 shows an alternative embodiment of an encapsulated message to be sent from a local computer  11  to a public computer  20 . In this embodiment, it is assumed that only HTTP GET operations are permitted with regards to the security policy enforced at the site where the local computer  11  resides. In this case, the entire Data Packet (or portions thereof) need to be transmitted as part of one or more Encapsulated Data Packets each having an HTTP address specified in a GET command. Such addresses are nonexistent, but the public computer knows how to decode these addresses into a useful message.  
       Description of FIG.  8   
       [0045]    [0045]FIG. 8 shows one embodiment of an Unpacked Data Packet  8 F in the form received by the local computer  11  after the Aggregated Data Packet (FIG. 5) is decomposed (as illustrated in FIG. 11 below) in accordance with the present invention. As depicted, the message is delivered to the client application  24  in a packet form including a Message (payload segment) ID  8 A, a Sender ID  8 B, a Destination ID  8 C, a Local Message ID  8 D to which this message is a Reply, and the Message Segment  8 E.  
       Description of FIG.  9   
       [0046]    Referring now to FIG. 9, as well as previously described figures, when a client application running on a local computer  11  of the local computer system  10  needs to transmit data (a message) to a remote public computer  20 , the application  24  (FIG. 2) in Steps  9 A and  9 C uses the associated API to deposit blocks of information in the API send buffer  31 , such information including the data to be communicated (“local message”), the sender address, the destination address, and the reply to message ID. A stimulus (Step  9 B) is then applied to the control logic  44  by the client application to abort the waiting (Step  9 J) and trigger data processing. A stimulus is a request to cut short the wait period ( 9 J). An example of such a request might be any internal or external event the occurrence of which triggers the immediate processing and sending of the data payload in buffer  9 C via  9 E- 9 I.  
         [0047]    In Step  9 E, the message data present in the API send buffer  32  is encrypted by the encryption unit  38 , using an appropriate encryption mechanism, to obtain encrypted data.  
         [0048]    In Step  9 F of the preferred embodiment, the packaging unit  40  splits the encrypted message data into small “chunks”, as illustrated above in FIG. 4 a  and described below with respect to FIG. 10, to accommodate the firewall restrictions of the communication path with regards to the permissible amount of data transmitted in a single message.  
       Description of FIG.  10   
       [0049]    Skipping ahead momentarily to FIG. 10 which illustrates in more detail the packaging process of Step  9 F, it will be noted that in Step  10 C the packaging unit  40  (FIG. 3) looks at the encrypted local message ( 10 A) and then, depending on the firewall imposed limit on the length of message allowed, calculates the number “N” of chunks necessary for the current block (FIG. 4 a ) of Local Message Data. For example, N=(Local Message size)/(maximum message size−header size) rounded up. The data is then split into data chunks, each chunk is numbered at step  10 E, and the Local Message ID  4 A and the Number of Chunks  4 B are prepended at Step  10 F. The packaging unit then increments the local message ID in Step  10 G and preprocesses the next message. More specifically, the packaging unit  40  assembles each chunk of the encrypted Local Message Data into a Data Packet  4 H including, as illustrated in FIG. 4 b,    
         [0050]    (1) the Local Message ID Number ( 4 A) common to all chunks of the same encrypted block of message data,  
         [0051]    (2) the Number N of Chunks ( 4 B) required to form the original encrypted block of message data, and  
         [0052]    (3) the current chunk sequence number (Chunk Number  4 C).  
         [0053]    Reverting now to FIG. 9, in Step  9 F, to complete the packet header  54  (FIG. 4 b ), the following addressing items are duplicated into each Data Packet  4 H:  
         [0054]    (4) the local computer&#39;s address (Sender ID  4 D);  
         [0055]    (5) the public computer&#39;s address (Destination ID  4 E); and  
         [0056]    (6) an identification of any message to which this data is a response, if applicable, (Reply To Remote Message ID  4 F).  
         [0057]    In an alternative embodiment of the present invention wherein a firewall  16  does not restrict the amount of data transmitted in a single message, packaging unit  40  augments the encrypted but undivided block of message data with a simpler header including:  
         [0058]    (1) the Local Message ID Number;  
         [0059]    (2) the local computer&#39;s identification (Sender ID);  
         [0060]    (3) the public computer&#39;s identification (Destination ID); and  
         [0061]    (4) an identification of the message to which this data is a response, if applicable, (Reply To Remote Message ID).  
         [0062]    In Step  9 G, the Data Packets are encapsulated into HTTP POST messages, or HTTP GET messages (depending on whether or not the security policy implemented by the firewall allows POST messages to traverse to the public network). If POST messages are allowed, the addressing unit  42  adds to the Data Packet an HTTP address and an HTTP header (as explained above with respect to FIG. 6). If POST messages are not allowed, the addressing unit inserts the Data Packet into one or more HTTP GET messages as described above and shown in FIG. 7.  
         [0063]    In Step  9 H, the Control logic  44  then deposits the resulting Encapsulated Data Packet into the send buffer  32  (FIG. 3) where it is made available for transmission to the public computer  20  via connections to transport  30 . Typically, this will establish a connection to the public computer (or the firewall if present) to which the message will be transmitted. The connection is then maintained until a reply is returned. This process can be carried out by any number of available web communication standard libraries.  
         [0064]    When a reply is received from the public computer  20  via the firewall  16 , the reply is processed in Step  9 I as further described below with respect to FIG. 11.  
       Description of FIG.  11   
       [0065]    [0065]FIG. 11 illustrates an embodiment of the program flow in accordance with the present invention which implements the processing of a reply to a message that was previously sent out to a public computer by a client application resident in the local computer. As in the processing and transmission of the messages originating at the local computer, the reply messages originating at the remote computer may also be encapsulated in an HTTP protocol package including HTTP header information describing the following content. When the reply message is received from the public computer  20  via the connections to transport  30  and over the established connection, the encapsulated message is placed in the receive buffer  36  (FIG. 3) as indicated at  11 A.  
         [0066]    In Step  11 B the encapsulation is stripped from the received data packet and discarded leaving the Aggregated Data Packet (FIG. 5). The Packet is tested at  11 C to determine whether or not it includes compound data, i.e., multiple Message Segments. If not, the payload is decrypted and processing continues. If the Packet is compound, then it is unpackaged as set forth in FIG. 12.  
       Description of FIG.  12   
       [0067]    [0067]FIG. 12 is a block diagram illustrating the Public Compound Reply message separation process invoked in Step  11 D. When a message is received from the remote public computer  20  in the form of an Aggregated Data Packet, illustrated in FIG. 5, the packaging unit  40  selects the first Message Segment ( 5 G 1  in FIG. 5) identified by the header component, Message Segment ID Number  5 D 1 . In Step  12 C, the packaging unit  40  forms a new data header by concatenating the Sender ID  5 B 1  and the Destination ID  5 C 1 . The packaging unit  40  then prepends (at  12 D) the Segment ID Number  5 D 1  and then at  12 E, appends the Reply to Local Message ID  5 F 1  to form the new header. It then appends the selected Message Segment  5 G 1  to the header to form an individual Reply Data Packet  8 F (as illustrated in FIG. 8).  
         [0068]    To recap the above, the Aggregated Data Packet is comprised of several individual component parts. In Step  11 D the packaging unit  40  unpacks the received Aggregated Data Packet and reconfigures it into a plurality of individual Reply Data Packets  8 F including:  
         [0069]    a header comprised of  
         [0070]    (1) a Message Segment ID Number ( 8 A);  
         [0071]    (2) a Sender ID ( 8 B);  
         [0072]    (3) a Destination ID ( 8 C); and  
         [0073]    (4) a Reply to Local Message ID ( 8 D); and a payload including  
         [0074]    (5) a Message Segment ( 8 E).  
         [0075]    Returning now to FIG. 11, in Step  11 E, the encryption unit  38  (FIG. 3) decrypts the Message Segment of each individual Packet and discards simple Acknowledgements ( 11 F) before depositing the Reply Data Packets into the API receive buffer  34  at Step  11 G. The control logic  44  then informs (at  11 H) the application  22 , via the connections to the API  23 , of the presence of the decrypted Reply Data Packet in the receive buffer.. The program flow then proceeds to the send sequence (Step  9 D of FIG. 9).  
         [0076]    It is well known to those skilled in the art that the remote public computer  20  cannot initiate a communication with a client, or local, computer  11  that is protected from the public network  18  by a firewall  16  using the HTTP communications protocol.. Therefore, all messages sent by the remote computer  20  to the local computer  11  must be in the form of responses to requests originated from the local computer  11 .  
         [0077]    Description of FIG. 13  
         [0078]    Accordingly, in order to send a properly formatted block of data (Aggregated Data Packet) to local computer  11 , the public computer  20  must first place the data block in its API send buffer  31 ′ as indicated in Step  13 B. It should be noted however, that this data is not sent immediately, but must wait for a communication from the local computer  11  before actual transmission back to the local computer.  
       Description of FIG.  14   
       [0079]    [0079]FIG. 14 is a flow diagram illustrating a situation wherein it is urgent that data stored in the API send buffer  31 ′ be sent without further delay. In such a case, the control logic  44 ′ (FIG. 3) generates a stimulus. In accordance with the present invention, the stimulus may, for example, be an e-mail message sent from the public computer  20  to the local computer  11  through usual e-mail communication channels which, incidentally, pass freely through the firewall. Upon arrival at the local computer  11 , the processing of the e-mail message will prompt the local computer that a message is waiting to be sent from the public computer  20 , and in response, a stimulus ( 14 G) will be generated causing immediate processing of the message in API Send Buffer  31 ′ (Step  13 B).. Otherwise, the control logic  44 ′ will cause the system to wait (Step  14 F) until a predefined period of time expires, at which time a stimulus is generated, as described above.  
       Description of FIG.  15   
       [0080]    When a message is received ( 15 A) by the public computer during the waiting period (FIG. 14), the processing of the received message is engaged, and the packaging unit  40 ′ strips the HTTP encapsulation from the received message (Step  15 B), and determines whether or not there are any complete messages presented. If so, the header data and message data are recombined in Step  15 E as is more clearly depicted in FIG. 17.  
       Description of FIG.  17   
       [0081]    Jumping ahead, FIG. 17 illustrates the message recombination process of Step  15 E. After receiving a message, and after the addressing unit  42 ′ has stripped the HTTP wrapper from the message, the packaging unit  40 ′ (in public computer  20 ) waits until it has received N chunks of data ( 17 C); N being specified in the message packet. Once all N chunks are received, the packaging unit forms a data header (Step  17 E) comprising:  
         [0082]    (1) Message Segment ID number;  
         [0083]    (2) Sender ID;  
         [0084]    (3) Destination ID and;  
         [0085]    (4) Reply to Local Message ID. Then it concatenates ( 17 F) all of the data chunks into one Message Segment ( 8 E in FIG. 8).  
         [0086]    Returning to Step  15 C in FIG. 15, wherein the packaging unit  40 ′ assesses the completeness of the message, it will be understood that the data segment can be complete message or a portion of a multi-part message as described above with respect to FIG. 10. If no complete message can be formed from the contents of the receive buffer  36 ′, the connection is closed and the wait is resumed for more incoming messages. As soon as a complete message can be formed, the packaging unit  40  recombines all chunks and forms an individual data packet (Step  15 E). The data part of the packet is then decrypted by the encryption unit  38 ′ (Step  15 F) and the control logic  44 ′ deposits the decrypted data packet in the API receive buffer  34 ′ (Step  15 G) and informs the application (Step  15 H) that a message is pending retrieval via connections  25 ′ to the API.  
         [0087]    To ensure that every message from the local computer  11  receives an answer, the control logic  44 ′ places an acknowledgement (ACK) in the API send buffer  31 ′ (Step  15 I), and in Step  15 K, processes the messages in the API send buffer as described above with respect to FIG. 16. The public computer then terminates the connection and resumes a wait for new messages as indicated by Step  15 D.  
       Description of FIG.  16   
       [0088]    To transmit pending data from the public computer  20  to a local computer  11  over a currently established communication channel, the encryption unit  38 ′ encrypts the data (Step  16 B) present in its API send buffer  31 ′. In Step  16 C, the packaging unit  40 ′ aggregates all encrypted segments of the message data in the API send buffer  31 ′ into a single payload, (as described more specifically below with respect to FIG. 18), and in Step  16 D adds address and other header data to develop an Aggregated Data Packet ( 5 H) as described above with respect to FIG. 5,. The control logic  44 ′ then deposits the Aggregated Data Packet in the send buffer  32 ′ and transmits it as a reply to the message last received from the local computer  11  (Step  16 E). The control logic  44 ′ then clears the API send buffer  31 ′, and at Step  16 F, returns to the receive sequence at Step  15 C (FIG. 15).  
       Description of FIG.  18   
       [0089]    [0089]FIG. 18 depicts the packaging process Step  16 C of combining multiple data segments ( 5 G of FIG. 5) and associated header data ( 5 A- 5 G) into one single message block (Aggregated Message Packet  5 H) to be transmitted from the public computer  20  to the local computer  11  as a reply message. In Step  18 C the packaging unit  40 ′ forms the packet header by concatenating the Number of Message Segments  5 A about to be sent, the Sender ID  5 B and the Destination ID  5 C. At Step  18 D, the packaging unit  40 ′ adds to the header in sequence, the Segment ID Number  5 D, the Segment Length  5 E, and the Reply to Local Message ID  5 F for each Message Segment  5 G about to be sent in this packet. At Step  18 E, the encrypted data forming each Message Segment to be transmitted is concatenated and added to the packet being formed, to eventually obtain the Aggregated Data Message  5 H.  
         [0090]    Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modification as fall within the true spirit and scope of the invention.