Abstract:
A system and method to provide secure access across the untrusted PSTN is described. The system and method utilizes telephony resources that can be initiated by a security policy defining actions to be taken based upon at least one attribute of the call, providing multi-tiered policy-based enforcement capabilities and visibility into security events.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/210,347 entitled TELEPHONY SECURITY SYSTEM filed Dec. 11, 1998, and is related to U.S. patent application Ser. No. 09/457,494 entitled A TIGHTLY INTEGRATED COOPERATIVE TELECOMMUNICATIONS FIREWALL AND SCANNER WITH DISTRIBUTED CAPABILITIES filed Dec. 8, 1999, both assigned to the assignee of the present application and incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to telecommunications access control systems and more particularly, to a system and method whereby a virtual private telephone network is autonomously constructed between at least two in-line devices. 
     BACKGROUND OF THE INVENTION 
     Historically, government and business entities could be reasonably confident that their sensitive information communicated by telephone, fax, or modem was confidential, and that no one would monitor or eavesdrop on their plans and strategies. This is no longer true. In the past several years, information assets have become increasingly vulnerable to interception while in transit between the intended parties, as interception and penetration technologies have multiplied. 
     A wide range of communications, from those concerning military, government, and law enforcement actions, to contract negotiations, legal actions and personnel issues all require confidentiality, as do communications concerning new-product development, strategic planning, financial transactions or any competition-sensitive matter. They often require discussions over the telephone, faxes, videoconferences, data transmission and other electronic communication. As businesses depend on their communications systems more and more, those systems are delivering ever-greater volumes of information, much of it proprietary and extremely valuable to competitors. 
     It&#39;s not just business competitors that companies have to be concerned about. Risks are particularly high for businesses with operations outside the United States. Many nations are defining their national security as economic security, and they&#39;re putting their intelligence agencies into the business of industrial and economic espionage. Some foreign intelligence agencies actively and aggressively spy on businesses to collect technology and proprietary information. 
     The increasing prevalence of digital communications systems has led to the widespread use of digital encryption systems by governments and businesses concerned with communications security. These systems have taken several forms, from data Virtual Private Networks (VPN), to secure voice/data terminals. 
     Communications and computer systems move massive amounts of information quickly and routinely. With voice, fax, data and video to choose from, businesses are communicating in all these modes via the untrusted Public Switched Telephone Network (PSTN). Unfortunately, whereas a data VPN protects information traveling over the Internet, a data VPN is not designed to protect voice, fax, modem, and video calls over the untrusted PSTN. 
     While IP-based VPN technology is automated and widely available, solutions for creating safe tunnels through the PSTN are more manual, requiring user participation at both ends to make a call secure. Such is the case with the use of secure voice/data terminals, such as Secure Telephone Units (STU-IIIs), Secure Telephone Equipment (STE), and hand-held telephony encryption devices. 
     When used, secure voice/data terminals effectively protect sensitive voice and data calls. However, their design and typical deployment can be self-defeating. For example, to enter secure mode on a STU-III or STE device, both call parties must retrieve a physical encryption key from a safe storage location and insert it into their individual device each time a call is placed or received. Also, STU-III and STE devices are expensive, so they are typically located within a department or work center, but not at each work station. If a STU-III or STE call is not scheduled ahead of time, the caller may have to wait while the person they are calling is brought to the phone—with a key. 
     If the secure voice/data terminal is installed on an analog line, transmission speed and voice recognition quality is low. Slow speed may be tolerated for secure data transfer, but it can make secure voice communication difficult and frustrating. Good speed and voice quality is attainable on ISDN or T-1 lines, but replacement of analog lines is expensive and many organizations prefer to keep their existing equipment. 
     The inconvenience, frustration, and poor voice quality of using manually activated secure voice/data terminals can motivate individuals to “talk around” the sensitive material on non-secure phones. Although the confidential information is not directly spoken, these vague conversations can be pieced together to get a fair idea of the information that was supposed to be protected. Use of secure voice/data terminals for the communication of sensitive information can be mandated by policy, but there is no way to properly enforce such a requirement. 
     Additionally, secure voice/data terminals secure only one line per device. As point-to-point devices, secure voice/data terminals cannot protect the vast majority of calls occurring between users who do not have access to the equipment. And while there are policies that specifically prohibit it, sensitive material can be inadvertently discussed on non-secure phones and distributed across the untrusted PSTN. 
     Secure voice/data terminals cannot implement an enterprise-wide, multi-tiered policy-based enforcement of a corporate security policy establishing a basic security structure across an enterprise, dictated from the top of the tier downward. Neither can secure voice/data terminals implement an enterprise-wide, multi-tiered policy-based enforcement of selective event logging and consolidated reporting to be relayed up the tier. 
     Secure voice/data terminals cannot provide the capability of “live” viewing of all secure call actions performed by the device. 
     Lastly, secure voice/data terminals cannot provide call event logs, detailing secure calls. Therefore, a consolidated detailed and summary report can not be produced for use by security personnel and management in assessing the organization&#39;s security posture. 
     Clearly, there is a need for a system and method to provide secure access across the untrusted PSTN through telephony resources that can be initiated by a security policy defining actions to be taken based upon at least one attribute of the call, providing multi-tiered policy-based enforcement capabilities and visibility into security events. 
     SUMMARY OF THE INVENTION 
     A system and method to provide secure access across the untrusted PSTN is described. The system and method utilizes telephony resources that can be initiated by a security policy defining actions to be taken based upon at least one attribute of the call, providing multi-tiered policy-based enforcement capabilities and visibility into security events. 
     Some advantages of the system and method are: its completely operator-transparent; its less expensive; it does not require static secret keys—creates a new key each session; it does not require manual keys; it&#39;s a secure transport of modem, fax, and voice; its unaffected by transcoding; there is a separate message channel from the data so the message and data can be sent concurrently; there is automatic policy enforcement; the policy is implemented by call type and it accommodates a multi-tier policy enforcement. 
    
    
     Therefore, in accordance with the previous summary, objects, features and advantages of the present invention will become apparent to one skilled in the art from the subsequent description and the appended claims taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of an exemplary telecommunications Virtual Private Switched Telephone Network (VPSTN)  100  of the present invention; 
     FIG. 2 is a schematic block diagram of a VPSTN DS-0 sample  200 ; 
     FIGS. 3-4 are flowcharts of portions of the method of one embodiment; 
     FIGS. 5A and 5B are a schematic block diagram of an exemplary telecom appliance; 
     FIGS. 6A and 6B are a process flow diagram  208  illustrating the compression and encryption process; 
     FIGS. 7-17 are diagrams of different portions of the system; 
     FIGS. 18A and 18B are flowcharts of a SIPO converter of the system; FIGS. 18A and 18B show a process flow diagram illustrating the decryption and decompression process; and FIGS. 7-17 are diagrams of different portions of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention can be described with several examples given below. It is understood, however, that the examples below are not necessarily limitations to the present invention, but are used to describe typical embodiments of operation. 
     FIG. 1 is a schematic block diagram of an exemplary telecommunications Virtual Private Switched Telephone Network (VPSTN)  100  of the present invention, similar to the telecommunications firewall implemented as shown and described in U.S. patent application Ser. No. 09/210,347. VPSTN  100  can be combined with the telecommunications firewall to act as a VPSTN  100  and a firewall simultaneously, or to result in a mixture of capabilities of each device. 
     VPSTN  100  includes a plurality of Telephony Appliances (TA)  102  and  104 , management servers  106  and  108 , and clients  110  and  112 , all interconnected by a Local Area Network (LAN), Wide Area Network (WAN) or the Internet for interaction as described below. 
     The VPSTN  100  provides secure communication between two geographically separate, even globally distributed locations. The TA  102  or  104  is installed in-line on a Digital Signal level 1 (DS-1) circuit. The capacity (quantity and speed of channels) on a DS-1 varies relative to global location. For instance, a T1 or J1 circuit, used in North America and Japan respectively, operates at 1,544,000 bits per second (bps) and carries 24 time-division-multiplexed (TDM) DS-0 channels. Additionally, in North America, an Integrated Services Digital Network Primary Rate Interface (ISDN PRI) circuit may carry either 23 TDM DS-0 channels with one signaling channel, or 24 TDM DS-0 channels. In Europe, an E1 circuit operates at 2,048,000 bps and carries 30 TDM DS-0 channels in addition to 2 signaling channels. A DS-0 channel operates at 64,000 bps, [the worldwide standard speed for digitizing one voice conversation using Pulse Code Modulation (PCM) and sampling the voice 8,000 times per second and encoding the result in an 8-bit code (8×8000=64,000 bps)]. An additional variation relative to global location is the difference in the form of PCM encoding. Typically, mu-law is the standard used in North American and Japanese telephone networks, and A-law is used in European networks. Transcoding, or modifying the data stream from mu-law to A-law so that it can be carried via a different network may cause the PCM value to change. Regardless of whether the circuit type (T1, J1, ISDN PRI, E1, etc.) that connects the VPSTN  100  with the PSTN is the same on both sides of the PSTN (i.e., T1 to PSTN to T1, as may occur with calls conducted within North America), or is some combination of circuit types (i.e., T1 to PSTN to E1, as would occur with an international call), all operations are transparent to the individuals placing and receiving the call. 
     The TA  102  is installed in-series on a DS-1 span between a Public Branch eXchange (PBX)  114  and a Public Switched Telephone Network (PSTN)  116 . The TA  104  is installed in-series on the DS-1 span between the PSTN  116  and a PBX  118 . The TA  102  has two input and two output ports, specifically, a PBX-in port  120 , a PSTN-out port  122 , a PSTN-in port  124 , and a PBX-out port  126 . Similarly, the TA  104  has two input and two output ports, specifically, a PSTN-in port  128 , a PBX-out port  130 , a PBX-in port  132 , and a PSTN-out port  134 . 
     The client  110  and  112  is a point of user-interface for configuring a security policy, displaying and viewing real-time alerts, viewing real-time event logs, printing event logs and consolidated reports, and other operational features of the VPSTN  100 . 
     A security policy is a sequential listing of rules that define whether certain calls to or from an extension will be allowed, denied (hung-up), conducted in secure mode, monitored for content, logged, and if other actions such as sending a warning tone or message, or sending notifications by real-time alerts, pager or email are required. 
     The management server  106  and  108  receive the security policy and push a copy of the security policy to the TA  102  and  104  respectively. The TA  102  and  104  receive the security policy, and as appropriate, monitor incoming and outgoing calls, allow, deny, or otherwise manipulate calls, including conducting calls in secure mode, all in accordance with the security policy and based on a plurality of call attributes, including call content-type (voice, fax, modem, VTC, etc.). 
     Also in FIG. 1, numerals  136  and  138  designate end-user stations, representing as examples, one or more modems  140  and  142 , fax machines  144  and  146 , and telephones  148  and  150 , which may send or receive calls over the VPSTN  100 . The modems  140  and  142  may be connected to a desktop or portable personal computer. Individual extensions  152  and  154  connect the end-user stations  136  and  138  to the PBX  114  and  118  respectively. 
     For clarity and simplicity of explanation, FIG.  1  and subsequent figures show a complete DS-1 circuit (specifically, all  32  DS-0 channels on an E1) connected between the TA  102 , the PSTN  116  and the TA  104 , although typically, the DS-0 channels that make up the DS-1 trunk may be individually switched by the PSTN  116  to different locations relevant to call destination. All of the DS-0 channels on the DS-1 are shown to be processed using the present invention, although a security policy can be configured such that the present invention is selectively applied based on call attributes such as source and destination number, call content type, etc. Additionally, in the examples provided, voice is the media transported although the present invention also provides secure transport for a plurality of media in addition to voice, such as least fax, modem and VTC. 
     Additionally, the system and method supports a multi-tiered security policy. For example, a corporate-dictated security policy will contain basic rules for the Security Rule database. These rules are classified as either “Required” or “Optional”. Each level of the hierarchical environment must adhere to a required rule, but can choose to ignore optional rules. Each level of the tier is capable of making their local rules and the rules for the tiers below it more stringent than the corporate-dictated rules, but can not make the rules more lax. In this way, a basic security structure is ensured across the enterprise. 
     The corporate-dictated security policy contains basic security rules that dictate what information will be reported upward, thereby providing visibility into only the most important local security events at the corporate level. Just as the corporate-dictated rules send security guidelines that may become more stringent as they are passed downward, the policy institutes an information filter that becomes more selective as email, logs and reports, etc., are routed upward. The tasks in the “Tracks” column of the corporate-dictated rule (such as email notification, pager notification, logging of events, etc.), that are of interest at a local level but are not of interest at higher levels, are designated to be filtered out if notification of a rule firing is to be routed up the tier. All logging is real-time, both at the location where the event occurs and at upper levels of the organization that, in accordance with the security policy, may or may not require notification of the event. 
     FIG. 2 is a schematic block diagram of a VPSTN DS-0 sample  200  of the present invention. The DS-0 is the atomic level (the lowest level) of a standard telephone call, regardless of whether the call is voice, fax, modem or VTC). As previously mentioned, the DS-0 operates at 64,000 bps. The present invention subdivides the VPSTN DS-0 sample  200  into three subrate channels. The term subrate is used because each of the three channels operate below the full DS-0 rate of 64,000 bps. The three subrate channels include a bearer channel  202 , a Encrypted Packet (EP) boundary channel  204 , and a message channel  206 . The bearer channel  202  operates at a DS-0 subrate of 40,000 bps (5-bits per sample). The EP boundary channel  204  and message channel  206  each operate at a subrate of 8,000 bps (1-bit per sample). The three subrate channels add up to a rate of 56 (40+8+8) Kbps. The remaining 8 Kbps, is used for a Least Significant Bit (LSB)  208  position. The LSB  208  is set high during transmission and is discarded after it is received. 
     The three subrate channels are assigned bit positions within each VPSTN DS-0 sample sample  200 . The bearer channel  202  is assigned bit positions  3 ,  4 ,  5 ,  6 , and  7 . The EP boundary channel  204  is assigned bit position  2 , and the message channel  206  is assigned bit position  1 . 
     The bearer channel  202  carries the audio signal in a compressed format. The ITU-T G.726 Recommendation [Adaptive Differential Pulse Code Modulation (ADPCM)] in 5-bit mode is used to compress the audio signal. In 5-bit mode (which operates at 40K bps), the voice quality is equal to that of an uncompressed Pulse Code Modulated (PCM) DS-0 at 64 Kbps (toll quality). The 5-bit ADPCM mode was designed specifically to allow voice-band data modems to be transported using ADPCM at modem speeds greater than 4800 baud. The ITU has conducted extensive tests and found that 5-bit ADPCM G.726 allows voice-band data modems to operate at speeds up to 19,200 baud. Therefore, using the VPSTN  100  may cause a V.90 or V.34 modem to connect at a slower speed than would be possible on a DS-0 not using the VPSTN  100 . Moreover, because Group 3 fax transmissions operate at speeds less than 19,200 baud, using the VPSTN  100  should not impact fax transmission speeds. 
     The EP boundary channel  204  is used to create encryption packets made up of five 64-bit words (blocks). A 64-bit block size allows a 64-bit encryption/decryption engine to process the 64-bit blocks. An encryption packet of five 64-bit blocks are 8 milliseconds in length ({fraction (1/125)} of a second). The EP boundary is not relative to framing, such as the D3/D4 or ESF framing performed by the PSTN. 
     The message channel  206  is used to send messages between the TA  102  and  104 . An extensible protocol such as the IETF&#39;s Session Initiation Protocol (SIP) is used to send ASCII text-based message packets over the 8,000 bps channel in alignment with the encryption packet boundary established for the bearer channel  202 . Messages are used to setup a secure call, exchange and negotiate TA capabilities, exchange encryption keys, report errors, and control the call session. The message channel  206  remains active throughout the duration of a call, and is used to initiate or discontinue secure mode while a call is in progress. The 64-bit message packet may be subdivided into fields. The fields may contain the packet header, TA identification, message sequence numbers, timestamps, checksums, etc. 
     The LSB  208  of the VPSTN DS-0 sample  200  is discarded on receive channels and set high (1) on transmit channels. The LSB  208  data is not used because the PSTN  116  may cause some LSB  208  values to change during transport. Changes in the value of the LSB  208  can be caused by robbed-bit signaling, transcoding (mu-law to A-law to mu-law), or digital Packet Assembler/Disassembler (PAD) circuits. 
     FIG. 3 is a process flow diagram illustrating the VPSTN process  300  whereby a voice call is conducted in secure mode. Imagine the following example. The President of a bank in the United States places a call from the telephone  148 , to the Comptroller of the bank&#39;s branch office in “Country X”, who receives the call on the telephone  150 . The corporate security policy held by the TA  102  includes the following rule: “Encrypt all outgoing voice, fax, modem and VTC traffic, from all extensions, at any time, on any day, to destination numbers in the Country X group. If call can not be made secure, allow the call, play a warning message, email notification and log the call.” Adherence to this rule is required. Since the failure to secure a call is an indication of the security posture, it is of interest to the upper echelon. As notification of the failure to conduct the secure call is made at each upper level of the hierarchy, the system logs the event for report generation, but filters the task of email notification from the upper level. The system generates email notification of the failure to secure the call and sends it only to local and Country X security personnel (call source and call destination). 
     The VPSTN  100  autonomously sets up and conducts a secure audio call, transparent to both the President initiating the call and the Comptroller receiving the call. In accordance with the security policy, the VPSTN  100  logs the event, and generates alerts or notifications as required. 
     In step  302 , (reference will also be made to the elements within FIG. 1 for this example) the PSTN  116  uses the normal, non-secure telecommunications processes for connecting two terminals (phone sets). When the rule requiring secure communication with the Country X phone number fires, the TA  102  contacts the TA  104  to establish whether and under what conditions the call between the two locations can be conducted in secure mode. 
     The session&#39;s secret key is exchanged between the TA  102  and the TA  104  in step  304 . A unique secret key, generated for each session by the call-originating TA, is exchanged and used by both the TA  102  and  104  for encryption and decryption of each direction&#39;s bearer channel  202 . The exchange of the session secret key is performed using Public Key Exchange (PKE) on the message channel  206 . Steps  302  and  304  take place in less than three seconds. During that time, the TA  102  plays a tone or some other audio message to the phone sets, and is heard by both parties involved with the call. 
     In step  306 , the PBX-in port  120  receives the non-secure DS-1 from the PBX  114 . The TA  102  manipulates, compresses and encrypts the non-secure data bit stream, thereby generating the secure VPSTN DS-0 sample  200  bit stream. The PSTN-out port  122  transmits the secure DS-1 to the PSTN  116 , where it is switched to the PBX  118 . 
     In step  308 , the PSTN-in port  128  receives the secure VPSTN DS-0 channels from the PSTN  116 . The TA  104  manipulates, decrypts and decompresses the secure data stream, thereby restoring the non-secure DS-0 data stream that was previously compressed and encrypted in step  306 . The PBX-out port  130  transmits the non-secure DS-1 data stream to the PBX  118 , which transmits the signal to the telephone  150 . 
     While not shown, it is understood that the VPSTN  100  is capable of operating in a continuous loop, synchronously handling the flow of both the receiving and transmitting DS-0 channels. The process loop continues until the call is “hung up”. The PSTN  116  tearsdown the call using the normal telecommunications processes for disconnecting the two phone sets, as shown in steps  310  and  312 . 
     In step  314 , the call event is logged, and any other actions required by the security policy, such as generation of notifications are executed. 
     FIGS. 4A and 4B show a process flow diagram illustrating the secure call setup process  302  (of FIG. 3) whereby secure mode capabilities between the call source and destination are established prior to exchange of the session secret key. In step  400 , (reference will also be made to the elements in FIG. 1 for this flowchart) an audio connection is established between the telephone  148 , PBX  114 , PSTN  116 , PBX  118 , and telephone  150  in the normal, non-secure method used for connecting two phone set on the PSTN  116 . Once the audio connection is established, two non-secure DS-0 channels flow in a full duplex manner between the two phone sets. 
     In step  402 , if a security rule requiring the call to be conducted in secure mode does not fire, the call continues to be conducted in the normal, non-secure method used by the PSTN  116 , as described in step  404 . If in step  402 , at least one call attribute (such as source number, destination number, call content-type, time of call, etc.) fires a security rule that requires the call to be conducted in secure mode, the TA  102  responds accordingly to setup a secure call with the TA  104 . 
     In step  406 , shortly after audio establishment between the two telephones  148  and  150 , the TA  102  sends a SIP “invite” message packet over the message channel  206  to the TA  104 , and waits for a response. The invite message indicates that the TA  102  is attempting to initiate a secure call with the TA  104 . The invite message also indicates the capabilities of the TA  102 , such as compression and encryption options. 
     In step  408 , if the TA  104  is not VPSTN-capable, the TA  102  times-out while waiting for an acknowledge message from the TA  104 . If the TA  102  times-out in step  408 , the TA  102  discontinues the secure call setup process  302 , and respond to the failure to setup a secure call. 
     In step  410 , the security policy may require one or more of the following responses by the TA  102  and management server  106  if the secure call setup process  302  is discontinued: terminate the call; allow the call to continue in non-secure mode; provide a warning tone or message indicating to the call parties that the call is not secure; log the event; or send notifications to appropriate personnel at the call source and destination. 
     If the TA  104  is VPSTN-capable, it receives the SIP invite message and sends a SIP “acknowledge” message over the transmit message channel  206  in step  408 . 
     In step  412 , additional message packets are exchanged to coordinate capabilities such as the encryption algorithm and compression algorithm that should be used for this session. 
     In step  414 , the TA  102  disables the PSTN echo suppressor. The echo suppressor must be disabled because it hinders full duplex transmission of data. Full duplex transmission is necessary for encrypted data blocks to be synchronously transmitted and received by both the TA  102  and  104 . The TA  102  sends a message packet to the TA  104  to indicate that a echo suppressor disabler tone (typically equal to 2025 Hz), will be generated over the DS-0 for the next x seconds. When the TA  102  receives an acknowledge message from the TA  104 , the TA  102  sends the disabler tone. 
     After the disabler tone playback period, the TA  102  and TA  104  exchange messages to determine the line impairments of the two DS-0 channels flowing between the TA  102  and  104  in step  416 . The TA  102  sends a “known” frame over the bearer channel  202 , the content of which is known by both the TA  102  and  104 . For example, the known frame may consist of a sequential count of 0 through 63. The TA  104  compares the received “known” with an unmodified known frame and determines if line impairments changed some of the bearer channel “known” frame bit values along the way. 
     If in step  418 , the TA  104  determines that bits have changed value during transmission, the bearer channel  202  cannot support the VPSTN process  300 . If this is the case, in step  420 , the TA  104  sends a message packet telling the TA  102  to discontinue the secure call setup process  302 . Upon receipt of the discontinue message, the TA  102  and management server  106  respond to the failure to conduct the call in secure mode (terminate call, allow call, provide warning tone or message, log the event, send notifications, etc.), in accordance with the security policy and as described in step  410 . 
     If in step  418 , the TA  104  determines that bit values have not changed during transmission, the line impairments test is repeated on the return DS-0 channel. In step  422 , the TA  104  sends a “known” frame over the bearer channel  202  to the TA  102 . The TA  102  compares the received “known” frame with the unmodified known frame and determines if bit values changed. 
     If in step  424 , the TA  102  determines that bit values have changed during the transmission, the TA  102  discontinues the secure call setup process  302 . The TA  102  and management server  106  respond to the failure to conduct the call in secure mode in the manner defined by the security policy (terminate call, allow call, provide warning tone or message, log the event, send notifications, etc.), as described in step  410 . If the TA  102  determines that bit values have not changed, the TA  102  and  104  exchange the call session secret key in step  304 . 
     FIGS. 5A and 5B are a schematic block diagram of an exemplary telecom appliance. The TA  102  consists primarily of two input ports  120  and  124 , two output ports  122  and  126 , a pair of Line Interface Circuitry (LIC)  502  and  504  and framers  510  and  512  for the receive circuit, a pair of LIC  506 , and  508  and framers  514  and  516  for the transmit circuit, a Field Programmable Gate Array (FPGA)  518  which contains the components for manipulating, compressing and decompressing the DS-0 channels, an encryption/decryption processor  520  which accesses the FPGA  518  via a memory bus  522 , a Central Processing Unit (CPU)  524 , CPU memory  526 , and a PCI bus  528  which interconnects the encryption/decryption processor  520  and the CPU  524 . 
     The FPGA  518  components make up a transmit circuit which receives non-secure DS-0 channels from the PBX  114 , compresses and encrypts the data stream, and transmits the secure DS-0 channels to the PSTN  116 , and a receive circuit which receives secure DS-0 channels from the PSTN  116 , decrypts the data stream, and transmits the non-secure DS-0 channels to the PBX. 
     The FPGA transmit circuit includes a Serial-In Parallel-Out (SIPO) converter  530  which converts the bit stream received from the framer  510  to a word stream, a 32-channel ADPCM codec  532  which uses input from a 5-bit channel counter  534  and a ADPCM clock  536  to compress the 8-bit word stream to a 5-bit word stream, and a 1-to-32 demultiplexer  538 , which separates the 5-bit ADPCM word stream into an individual word stream for each DS-0 channel ( 0 - 31 ). 
     Each FPGA  518  transmit circuit contains 32 arrays of channel-dedicated components, which includes a word counter  539 , a switch  540 ,  545 ,  547 , and  549 , a SIPO shift register bank  542  and  544 , a Parallel-In Serial-Out (PISO) shift register bank  546  and  548 , and a PISO converter  550 . 
     The word counter  539  counts the number of 5-bit ADPCM words passing through the switch  540 . After every 64 th  word, the word counter causes the switch  540  and the switch  545  to change the direction of data flow between the pair of SIPO shift register banks  542  and  544 . The encryption/decryption processor  520  accesses the data in one of the pair of SIPO shift register banks  542  or  544 , depending on the position of the switch  545 . The switch  545  allows one bank to fill while the encryption/decryption processor  520  processes the data already in the other bank. Simultaneously, the word counter causes the switch  547 , and  549  to change the direction of data flow between a pair of PISO shift register banks  546  and  548 . The encryption/decryption processor  520  deposits the encrypted data in one of the pair of PISO shift register banks  546  or  548 , depending on the position of the switch  547 . The switch  547  allows one bank to fill while the other bank empties through the switch  549 , into the PISO converter  550 . The 32 channels converge at a multiplexer  552 , which uses time-division-multiplexing (TDM) to create a single bit stream. 
     The FPGA  518  receive circuit includes a SIPO converter  554  which converts the bit stream received from the framer  516  to a word stream, a 1-to-32 demultiplexer  556  which separates the 8-bit encrypted word stream into an individual encrypted word stream for each DS-0 channel ( 0  through  31 ). 
     Each FPGA  518  receive circuit contains 32 arrays of channel-dedicated components, which includes a Binary Pattern Correlator (BPC)  574 , First-In First-Out (FIFO) buffer  576 , a switch  568 ,  569 ,  578 , and  579 , a SIPO shift register bank  570  and  572 , and a PISO shift register bank  580  and  582 . 
     The BPC  574  determines if the SIPO shift register bank  570  or  572  contains a complete encryption packet, and when the bank is full, the BPC  574  causes the switch  568  and the switch  569  to change the direction of data flow between the pair of SIPO shift register banks  570  and  572 . The encryption/decryption processor  520  accesses the data in one of the pair of SIPO shift register banks  570  or  572 , depending on the position of the switch  569 . The switch  569  allows one bank to fill while the encryption/decryption processor  520  processes the data already in the other bank and message data to be routed to the FIFO buffer  576 . Simultaneously, BPC  574  causes the switch  578 , and  579  to change the direction of data flow between a pair of PISO shift register banks  580  and  582 . The encryption/decryption processor  520  deposits the decrypted data in one of the pair of PISO shift register banks  580  or  582 , depending on the position of the switch  578 . The switch  578  allows one bank to fill while the other bank empties through the switch  579 , into a channel selector  584 . 
     The 32 DS-0 channels converge at the channel selector  584  A channel counter  586  keeps track of the channel number for the channel selector  584  and an ADPCM clock  588  clocks the ADPCM core. The channel selector  584  multiplexes the separate word streams into a single word stream and routes it to a ADPCM codec  590 . The ADPCM codec  590  converts the 5-bit word stream an 8-bit word stream. 
     FIGS. 6A and 6B are a process flow diagram  208  illustrating the compression and encryption process  306 , whereby a non-secure DS-1 is processed for secure transport. In step  600 , upon entering the TA  102 , the non-secure DS-1 is routed through the LIC  502 , to the framer  510 . 
     In step  602  (and shown in FIG.  7 ), the framer  510  receives a non-secure DS-1  702 , extracts a data signal  704 , frame signal  706  and bit-clock signal  708  from the serial data stream, and places the signals on a TDM highway  710 . The TDM highway has  32  timeslot channels clocked at 2.048 Mbps, and consists of the data signal  704 , frame signal  706  and bit-clock signal  708 . 
     The data signal  704  carries the DS-0 data bit stream. The frame signal  706  indicates the beginning of the first 8-bit timeslot, sets the 8-bit timeslot boundaries and operates at 8 KHz. The bit-clock signal  708  synchronizes the DS-0 data bit stream and operates at 2.048 MHz. If the PBX-in link is a T1 or J1, 24 DS-0 channels are placed in timeslots  0  through  23 , while the remaining 8 timeslots remain empty (set to some value). If the PBX-in link is an E1, the 30 DS-0 channels are placed in their respective timeslots, while timeslot  0  and  16  are reserved for signaling. 
     In step  604  (and shown in FIG.  8 ), the 32-channel TDM highway  710  routes the data, frame and bit-clock signals  706 ,  708 , and  710  to the SIPO converter  530 , which converts the serial bit stream to an 8-bit word stream  802 . An 8-bit sample is output 256,000 times per second (one every 3.9 microseconds). 
     In step  606  (and shown in FIG.  9 ), the TDM highway  710  routes the 8-bit word stream  802  to the 32 channel ADPCM codec  532 , the frame and word-clock signals  708  and  804  to the 5-bit channel counter  534 , and bit-clock signal  708  to the ADPCM clock  536 . The ADPCM codec  532  converts the 8-bit word stream  802  into a 5-bit ADPCM word stream  902 . The 5-bit channel counter  534  keeps track of the channel number. The ADPCM clock  536  operates at a rate of 4.096 MHz, 16 times the algorithm processing rate of 256,000 bytes per second. 
     In step  608  (and shown in FIG.  10 ), the TDM highway  710  routes the 5-bit ADPCM word stream  902  and channel number information to the 1-to-32 demultiplexer  538 , which separates the TDM 5-bit ADPCM word stream  902  into an individual non-TDM 5-bit ADPCM word stream  1002 - 1064  for each DS-0 channel ( 0  through  31 ). 
     In step  610  (and shown in FIGS.  11  and  12 ), the 5-bit ADPCM word stream  1002  from channel n is routed through its own channel-dedicated switch  540 , into one of a pair of channel-dedicated 64-bit SIPO shift register banks  542  and  544 . Simultaneously, each of the 5-bit ADPCM word streams  1004 - 1064  are routed through their own channel-dedicated switch  540  to their own channel-dedicated 64-bit SIPO shift register banks  542  and  544 . 
     The word counter  539  receives the word-clock signal  804  and counts the number of 5-bit ADPCM words. The word counter  529  causes the switch  540  to change the direction of data flow, switching between the channel-dedicated SIPO shift register banks  542  and  544  after every 64th word. The switch  540  is switched synchronously with the switches  545 ,  547 , and  549  by the word counter  539 . The five 64-bit SIPO shift registers  1202 - 1210  in the first SIPO shift register bank  542  fills with 64 words before switch  540  moves and the second SIPO shift register bank  544  begins to fill. The encryption/decryption processor  520  processes the first bank while the second bank fills. The 64-bit SIPO shift registers bank  542  or  544  load with 64 5-bit ADPCM words every 8 milliseconds (8,000/64=125 times per second). 
     As shown in FIG. 12, each SIPO shift register bank  542  and  544  contains five 64-bit SIPO shift registers  1202 - 1210 . Each of the five SIPO shift registers in a bank is dedicated to one of the five bits in the 5-bit ADPCM word stream  1002 . The SIPO shift register  1202  receives bit  4 , the MSB. The SIPO shift register  1204 - 1208  receives bits  31 . The SIPO shift register  1210  receives bit  0 , the LSB. 
     The 64-bit SIPO shift registers  1202 - 1210  allow the data to be formatted into a 64-bit plaintext block  1212 , required by the encryption/decryption processor  520  for the encryption algorithm. Given that there are 32 channels and each channel has two banks of five 64-bit SIPO shift registers  1202 - 1210 , this equals a total of 320 (32×2×5) SIPO shift registers. The parallel output ports for each SIPO shift register is mapped in the memory space of the encryption processor  520 . The block address  1214  for each SIPO shift register is mapped into an address space of  2560  (320×8) bytes in the encryption/decryption processor  520  memory map. Any of the SIPO shift registers can be randomly accessed in the same manner as a RAM array by the encryption/decryption processor  520  and are read-only memory to the encryption/decryption processor  520 . 
     When the word counter  539  causes the switch  540  to change direction of data flow after the 64 th  word enters the SIPO shift register bank  542 , as described previously in step  610 , the switch  545  also moves to change direction of data flow. In step  612  (and shown in FIGS.  5 A and  13 ), the switch  545  allows the encryption/decryption processor  520  access to process the 64-bit plaintext block  1212  from each of the five 64-bit SIPO shift registers  1202 - 1210  within 8 milliseconds of the bank being filled. The output encrypted packet  1302  is 8 milliseconds ({fraction (1/125)} of a second) in length and includes five 64-bit cyphertext (encrypted) blocks  1304 . 
     It is understood that the encryption/decryption processor  520  processes  625  (5×125) 64-bit plaintext blocks  1212  per second for each DS-0 channel that requires encryption. If all the DS-0 channels in a T1 or J1 require secure communication, the encryption/decryption processor  520  processes 15,000 (24×625) 64-bit plaintext blocks  1212  per second. This rate means the encryption/decryption processor  520  processes the single 64-bit plaintext block  1212  in less than 66.7 microseconds. If all the DS-0 channels in an E1 require secure communication the encryption processor  520  processes 18,750 (30×5×125) 64-bit plaintext blocks  1212  per second. This rate means the encryption processor  520  processes the single 64-bit plaintext block  1212  in less than 53.3 microseconds. Additionally, if the encryption processor  520  is handling four E1 spans, and every DS-0 must be secured, the processor handles 75,000 (4×18,750) 64-bit plaintext blocks  1212  per second, equal to a block every 13.3 microseconds. 
     In step  614  (and shown in FIGS.  5 A and  14 ), the switch  547  directs the data flow from the encryption/decryption processor  520  that loads the five 64-bit cyphertext (encrypted) blocks  1304  and block address  1214  into one of two channel-dedicated 64-bit PISO shift register banks  546  or  548 . Each PISO shift register bank  546  or  548  is made up of seven PISO shift registers  1402 - 1414 . Five of the seven PISO shift registers in each bank, specifically the PISO shift registers  1402 - 1410 , are assigned to hold a 64-bit encrypted block  1304  in a one-to-one association with the five SIPO shift registers  1202 - 1210  previously mentioned with reference to step  610 . The 5-bit encrypted word stream  1416  output from the five PISO shift registers  1402 - 1410  will be carried on the bearer channel  202 . 
     The sixth register, the PISO shift register  1412 , receives a 64-bit Encryption Packet (EP) boundary pattern  1418 , which is uploaded from the CPU  524 . The bit stream output from the PISO shift register  1412  is carried on the EP boundary channel  204 . The EP boundary pattern  1418  is a constant 64-bit pattern that uses the BPC  574  to perform the encryption packet boundary function. The EP boundary pattern  1418  may be set to even or odd parity of the five bearer channels bits. The blocking is accomplished by alternating between even and odd parity between successive blocks. Parity blocking provides the ability to determine bit errors in the bearer channel  202  and signal an alarm when an error is discovered. 
     The seventh register, the PISO shift register  1414 , receives a 64-bit message packet  1420 , which is also uploaded from the CPU  524 . As previously discussed, messages are exchanged between the TA  102  and the TA  104  to setup a secure call, exchange and negotiate TA capabilities, exchange session secret keys, report errors, etc. The bit stream output from the PISO shift register  1414  is carried on the message channel  206 . 
     An LSB serial bit stream  1422  is uploaded from the CPU  524 . The LSB  208  is always set high in order to increase one&#39;s density on the DS-1 span. 
     In step  616  (and shown in FIG.  15 ), the 5-bit encrypted word stream  1416 , the 64-bit EP boundary pattern bit stream  1418 , the 64-bit message packet bit stream  1420  and the LSB bit stream  1422  for each channel are routed to the channel-dedicated PISO converter  550  that outputs a serial stream of the VPSTN DS-0 sample  200 , at 64,000 bps, which makes up a secure DS-0 bit stream  1502 . 
     In step  618  (and shown in FIG.  16 ), each separate secure DS-0 bit stream  1502 - 1564  (channel  0 - 31 ) is routed to the TDM multiplexer  552  and multiplexed onto a single 2.048 Mbps TDM highway  1602  as a secure data signal  1604 . The timeslot of each encrypted DS-0 channel on the outgoing TDM highway  1602  is the same timeslot used by that non-secure DS-0 channel on the incoming TDM highway  710  previously mentioned with reference to step  602 . In addition to the secure data signal  1604 , the TDM multiplexer  552  also places the frame signal  1606  and bit-clock signal  1608  on the TDM highway  1602 . 
     In step  620  (and shown in FIG.  17 ), the TDM highway  1602  routes the secure data, framing and bit-clock signals  1604 ,  1606  and  1608  to the framer  512 . The PSTN-out port  122  transmits the secure DS-1  1702  to the PSTN  116 , where each DS-0 is switched to one or more destinations. In most cases the 24 or 30 encrypted DS-0s in a T1, J1 or E1 span will be routed to multiple locations. However, for the following discussion related with FIG.  18  and the decryption and decompression process, assume that the entire DS-1 span is switched between the TA  102  and the TA  104 . 
     FIGS. 18A and 18B show a process flow diagram illustrating the decryption and decompression process  308 , whereby secure DS-0 channels are restored to their original non-secure state. 
     In step  1800 , the TA  104  receives the secure DS-1  1702  from the PSTN  144  on the net-in port  128 . Upon entering the TA  104 , the secure DS-1  1702  is routed through the LIC  508  to the framer  516 . 
     In step  1802  (and shown in FIG.  19 ), the framer  516  extracts the secure data signal  1604  (which carries the secure DS-0 bit stream  1502 - 1564 ), the frame signal  1606  and bit-clock signal  1608  from the serial data stream and places the signals on a TDM highway  1902 . The TDM highway  1902  has 32 timeslot channels clocked at 2.048 Mbps. 
     In step  1804  (and shown in FIG.  20 ), the 32-channel TDM highway  1902  routes the secure data, frame and bit-clock signals  1604 ,  1606 , and  1608  to the SIPO converter  554 , which converts the encrypted serial bit stream to an 8-bit encrypted word stream  2002 . The 8-bit encrypted word stream  2002  is comprised of the 8-bit VPSTN DS-0 sample  200  which is output 256,000 times per second (one every 3.9 microseconds). 
     In step  1806  (and shown in FIG.  21 ), the TDM highway  1902  routes the 8-bit encrypted word stream  2002 , frame signal  1606  and word-clock signal  2004  to the 1-to-32 demultiplexer  556 . The 1-to-32 demultiplexer  556  separates the 32-TDM 8-bit encrypted word stream  2002  into an individual non-TDM 8-bit encrypted word stream  2102 - 2164  for each DS-0 channel ( 0  through  31 ). 
     In step  1808  (and shown in FIGS.  22  and  23 ), the 8-bit encrypted word stream  2102  for channel n is routed through its own channel-dedicated switch  568 . The LSB  208  of the 8-bit encrypted word stream  2102  is discarded. Simultaneously, each of the 8-bit encrypted word stream  2104 - 2164  are routed thorough their own channel-dedicated switch  568  to their own channel-dedicated SIPO shift register bank  570  or  572 . The switch  568  directs data flow of a resulting 7-bit encrypted word stream  2202  into one of a pair of channel-dedicated 64-bit SIPO shift register bank  570  or  572 . The switch  568  is switched synchronously with the switches  569 ,  578  and  579  by the BPC  574 . 
     As shown in FIG. 23, each SIPO shift register bank  570  and  572  contains seven 64-bit SIPO shift registers  2302 - 2314 . Each of the seven SIPO shift registers in a bank is dedicated to one of the seven bits in the 7-bit encrypted word stream  2202 . The SIPO shift register  2302 - 2310  receives bit 7-bit  3  respectively, the contents of the bearer channel  202 . The SIPO shift register  2312  receives bit  2 , the contents of the EP boundary channel  204 . The SIPO shift register  2314  receives bit  1 , the contents of the message channel  206 . 
     The 64-bit SIPO shift register  2302 - 2314  allow the 7-bit encrypted word stream  2202  to be formatted into the five 64-bit encrypted blocks  1304  for the encryption/decryption processor  520 . Given that there are 32 channels and each channel has two banks of seven 64-bit SIPO shift registers  2302 - 2314 , this equals a total of  488  (32×2×7) SIPO shift registers. The serial bit-position streams are clocked into the serial-inputs of the 64-bit SIPO shift registers  2302 - 2314 . 
     In step  1810  (and shown in FIG.  24 ), the BPC  574  accesses the 64-bit block in the SIPO shift register  2312  and uses it in a correlation process to detect the boundaries of the encryption packet  1302 , thereby verifying that all five of the 64-bit encrypted blocks  1304  are fully loaded into the 64-bit SIPO shift register bank  570 . 
     The BPC  574  is a digital correlator that includes the 64-bit SIPO shift register  2312 , a 64-bit reference pattern register  2402 , a 64-bit mask register  2404  and a correlation array  2406 . To determine the number of matches in the 64-bit SIPO shift register  2312  data stream, the BPC  574  compares the data in the SIPO shift register  2312  with the digital pattern in the 64-bit reference pattern register  2402  on every clock rising edge. The 64-bit mask register  2404  allows certain bits within the SIPO shift register  2312  data stream to be either exact matches with the reference pattern, or to be considered as inconsequential. The number of matches is calculated on each rising bit-clock, and the correlation sum is compared to a programmable threshold. The threshold determines the probability of detection and the false alarm rate. The 64-bit pattern and mask registers  2402  and  2404  are memory mapped in the encryption/decryption processor  520  memory address space. 
     Continuing with step  1810 , when the BPC  574  determines that the current content of the 64-bit SIPO shift register  2312  indicates that the 64-bit SIPO shift register bank holds the complete encryption packet  1302 , the BPC  574  causes the switches  568   569 ,  578 , and  579  to synchronously change the direction of data flow between the 64-bit SIPO shift register bank  570  and  572  and the PISO shift register bank  580  and  582  respectively. When the switch  568  moves, the second SIPO shift register bank  572  begins filling. When the switch  569  moves, the encryption/decryption processor  520  has access to process the encryption packet  1302  that just completed filling the SIPO shift register bank  570 . In this way, the first 64-bit SIPO shift register bank  570  fills before the second 64-bit SIPO shift register bank  572  begins filling, and the encryption/decryption processor  520  processes the encryption packet  1302  in the first bank while the second bank fills. The 64-bit SIPO shift registers bank  570  or  572  fills with the encryption packet  1302  every 8 milliseconds (8,000/64=125 times per second). 
     In step  1812  (and shown in FIG.  25 ), the encryption/decryption processor  520  decrypts the five 64-bit blocks  1304  (encryption packet  1302 ) contained in the five 64-bit SIPO shift registers  2302 - 2310 , thereby restoring the five 64-bit plaintext (ADPCM) blocks  1212 . The parallel output ports for each of the 488 SIPO shift registers is mapped in the memory space of the encryption/decryption processor  520 . The block address  2510  requires  3584  (448×8) bytes in the memory map. Any of the SIPO shift registers can be randomly accessed in the same manner as a RAM array by the encryption/decryption processor  520 . The 64-bit SIPO shift registers  2302 - 2310  are read-only memory to the encryption/decryption processor  520 . 
     In step  1814  (and shown in FIG. 26) the data stream in the 64-bit SIPO shift register  2314  loads into a first-in first-out (FIFO) memory buffer  576  when the BPC  574  causes the switch  569  to change direction as previously mentioned with reference to step  1810 . The CPU  524  asynchronously reads the messages carried on the message channel  206 . 
     In step  1816  (and shown in FIGS.  27  and  28 ), the 64-bit ADPCM blocks  1212  and block address  2800  output from the encryption/decryption processor  722  are routed through the channel-dedicated switch  578 , which directs data flow into one of a pair of channel-dedicated 64-bit PISO shift register banks  580  or  582 . Each PISO shift register bank  580  and  582  is made up of five PISO shift registers  2802 - 2810 , which fill with 64-bit ADPCM blocks  1212  and output a 5-bit ADPCM word stream  1002 . 
     In step  1818  (and shown in FIG. 29) the 5-bit ADPCM word stream  1002 - 1064  from each DS-0 channel ( 0 - 31 ) is routed to the channel selector  584 . The channel counter  586  receives the frame signal  1606  and bit-clock signal  1608  and keeps track of the channel number for the channel selector  584 . The channel selector  584  receives the 5-bit ADPCM word streams  1002 - 1064  and time-division-multiplexes the 5-bit ADPCM words from each channel into a TDM 5-bit ADPCM word stream  2902  and places it on a TDM highway  2904 . The TDM highway  2904  routes the TDM 5-bit ADPCM word stream  2902  to the ADPCM codec  590 . The ADPCM codec  590  receives input from the ADPCM clock  588  and converts the TDM 5-bit ADPCM word stream  2904  to the TDM 8-bit mu-law PCM word stream  802 . 
     In step  1820  (and shown in FIG.  30 ), the TDM highway  2904  routes the TDM 8-bit mu-law PCM word stream  802 , frame signal  1606  and bit-clock signal  1608  to the PISO converter  592 . The PISO converter  592  places the output data signal  704 , frame signal  1606  and bit-clock signal  1608  on the TDM highway  2904 . 
     In step  1822  (and shown in FIG.  31 ), the TDM highway  2904  routes the data signal  704 , frame signal  1606  and bit-clock signal  1608  to the framer  514 . The PBX-out port  130  transmits the non-secure DS-1  702  to the PBX  118 . 
     The embodiments shown herein are intended to illustrate rather than to limit the invention, it being appreciated that variations may be made without departing from the spirit of the scope of the invention. It is understood that several modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.