Patent Publication Number: US-7590848-B2

Title: System and method for authentication and fail-safe transmission of safety messages

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/355,282, filed Feb. 7, 2002 entitled SYSTEM AND METHOD FOR AUTHENTICATION AND SECURE TRANSMISSION OF SAFETY MESSAGES, which is incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates to the general field of safety busses and distributed safety systems and, in particular, to a system and method for fail-safe communication of safety messages among field devices. 
   BACKGROUND OF THE INVENTION 
   Within distributed safety systems, sensing devices will typically periodically issue safety messages to an associated actuator regarding the states of various sensors. Appropriate response to such safety messages is necessary to ensure optimal and safe operation. For example, in the event a safety message indicates a condition has arisen which may lead to catastrophic failure and unsafe operation unless corrected, it is necessary that the appropriate corrective action (e.g., valve shutoff) actually be taken. 
   In these distributed safety systems, certain bus integrity methods may be used in an attempt to ensure better or more reliable communication of the safety information over the applicable data bus. These methods have included various error checking and coding schemes for detecting and correcting data errors arising within the data communicated via the data bus. For example, a safety message may contain a check sum or cyclic redundancy code (CRC) to detect bit errors. In addition, while particular bus systems, such as the Process Field Bus (“PROFIBUS”) communication protocol and system, may employ various error coding methods in order to identify erroneous data, such systems are generally unsuitable for applications involving safety messages. 
   Moreover, the increasing automation of network-based industrial processes and control systems has rendered such systems vulnerable to attack by computer “hackers”, i.e., those individuals engaging in malicious code breaking. For example, it is conceivable that hackers may attempt to disrupt process operation by falsely emulating or interfering with the various safety messages transmitted among a distributed arrangement of sensors and actuators. In extreme circumstances, such interference could result in unsafe process operation and potentially dire attendant consequences. 
   SUMMARY OF THE INVENTION 
   In summary, the present invention pertains to a system and method for transmitting safety messages by way of communication channels containing non-safety-certified equipment. Consistent with the disclosed method, digital signatures and/or encryption may be used to authenticate both the origin and content of the transmitted safety messages. In particular, the present invention leverages digital signature technology and “watchdog” timers to ensure that safety messages are fail-safe, even when transmitted through non-safety-certified equipment. 
   The present invention relates to a method for fail-safe transmission of safety messages in a network environment. The method includes generating a safety message that indicates the state of a sensor. A digital signature is then generated to sign this safety message. The method further includes communicating the safety message and the digital signature between network nodes. Upon receipt, the safety message may be authenticated using the digital signature and watchdog timers. 
   In a particular implementation the present invention is directed to a system in which a sending field device creates a safety message, “signs” the message with a digital signature, and sends the message to another field device via a communications network. The receiving field device “verifies” the digital signature to authenticate both the origin and the content of the safety message. In addition, the receiving field device uses a watchdog timer to verify periodic reception of the safety messages. Creation, signing, and verification of the safety message are effected in safety-certified layers within the transmitting field device, even though the intervening communications network may consist of non-safety-certified commercial off the shelf (C.O.T.S.) elements. 
   This implementation may be exemplified by considering the case in which the transmitting field device comprises an intelligent pressure transducer and the receiving field device comprises an intelligent safety shutoff valve. In this case it is desired to shut off the valve if the monitored pressure exceeds some predefined limit. A safety application in the intelligent pressure transducer periodically sends a safety message indicating that the pressure is still within an acceptable range. A corresponding safety application in the intelligent shutoff valve expects to receive safety messages periodically. If the safety application associated with the valve does not receive a valid message within a predetermined timeout period maintained by a “watchdog timer”, then the valve shuts off. In accordance with the invention, the reliability of this process is enhanced through use of safety-certified elements within the pressure transducer and valve, even though the intervening communications network need not and generally will not be safety-certified. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature of the features of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of an exemplary industrial system in which a distributed safety system in accordance with the invention is implemented. 
       FIG. 2  depicts an intelligent sensor configured to send safety messages through a communications network to an intelligent actuator. 
       FIG. 3  is a block diagram representative of the operations performed during signature generation and signature verification in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a block diagram of an exemplary industrial system  100  in which a distributed safety system in accordance with the invention is implemented. The system  100  includes a plurality of intelligent sensors  120  in communication with a set of intelligent actuators  130  via a communications network  110 . In operation, each of the intelligent actuators  130  receives data in the form of “safety messages” from one or more of the intelligent sensors  120 . The applicable intelligent actuator  130  then responds by performing an appropriate action (e.g., opening/closing a valve or switch). 
   As is described below, the present invention contemplates using digital signatures and/or encryption, in conjunction with watchdog timers, to enhance the security and reliability of distributed safety systems. In accordance with this embodiment, a “safety layer” within an intelligent sensor  120  creates a safety message, “signs” the safety message to create a digital signature, and sends the message and digital signature to an intelligent actuator  130  via the communications network  110 . A corresponding safety layer within the intelligent actuator  130  “verifies” the digital signature to authenticate both the origin and the content of the safety message. In addition, the safety layer uses a watchdog timer to verify periodic reception of the safety messages. Advantageously, creation, signing, and verification of the safety message is performed in safety-certified layers of the applicable intelligent sensor  120  and intelligent actuator  130 , even though the communications network  110  may consist of non-safety-certified commercial off the shelf (C.O.T.S.) elements. 
   As an example, consider an embodiment in which the intelligent sensor  120  comprises an intelligent pressure transducer and the intelligent actuator  130  comprises an intelligent safety shutoff valve in the distributed safety system of  FIG. 1 . In this case it is desired to shut off the valve if the monitored pressure exceeds some predefined limit. A safety application in the intelligent pressure transducer periodically sends a safety message indicating that the pressure is still within an acceptable range. In this embodiment a corresponding safety application in the intelligent shutoff valve expects to receive safety messages periodically. If the safety application associated with the valve does not receive a valid message within a predetermined timeout period, then the valve shuts off. In accordance with the invention, the reliability of this process is enhanced through use of safety-certified elements within the pressure transducer and valve, even though the intervening communications network need not and generally will not be safety-certified. 
   Turning now to  FIG. 2 , there is shown a block diagrammatic representation of an exemplary implementation of an intelligent sensor  200  and an intelligent actuator  202  in accordance with the present invention. In the embodiment of  FIG. 2 , the intelligent sensor  200  includes a safety application  203   a  and safety layer  204   a  which collectively create “safety messages” indicative of the current state of the sensor  200 . Each such safety message is sent by the intelligent sensor  200  through a communications network  201  to the intelligent actuator  202 . In accordance with the invention, the safety layer  204   a  generates a digital signature  206  for the safety message or a message digest derived therefrom. The corresponding safety layer  204   b  of the actuator  202  “verifies”  207  the digital signature to authenticate both the origin and the content of the safety message. The safety layer  204   b  will also contain a watchdog timer  210  enabling verification that valid safety messages are periodically received. Each safety layer  204  generally implements one of a variety of encryption algorithms (described below), which are preferably stored in a non-volatile manner and permanently write-protected to discourage tampering. 
   In a particular embodiment, the intelligent sensor  200  could be implemented using, for example, an intelligent pressure, temperature or flow transducer, and the intelligent actuator  202  could be realized as a safety shutoff valve or switch. Intelligent field devices of this type may be realized using, for example, various I/A Series® devices available from the Invensys Foxboro unit of Invensys plc, as modified consistent with the teachings herein. The communications network  201  could be realized as an Ethernet network or as a F OUNDATION  Fieldbus network available from Invensys Foxboro. The F OUNDATION  Fieldbus is an all digital, serial, two-way communication system which interconnects field devices, such as transmitters, actuators, and controllers. It functions as a Local Area Network (LAN) with built-in capability to distribute control application across the network. 
   Although the embodiment of  FIG. 1  is specific to the context of intelligent sensors and actuators in order to facilitate explanation of the principles of the invention, in other embodiments the network node generating the safety message (i.e., the “source node”) and the network node receiving the safety message (i.e., the “destination node”) may be comprised of electronic devices (e.g., controllers, routers, workstations) lacking sensors or actuators. For example, in certain industrial or transportation applications the source node could include a switch or the like configured with appropriate transmission capabilities. Similarly, the destination node could comprise a controller or workstation outfitted with a conventional network interface. In addition, in certain embodiments safety messages may be transmitted from a source node through a communications network to a controller, and then forwarded from the controller to another network node. As may be appreciated by those skilled in the art, each of these embodiments is within the spirit and scope of the present invention described herein. 
   As shown in  FIG. 2 , in addition to the safety application  203   a  and the safety layer  204   a , the intelligent sensor  200  further includes a plurality of communication layers  205   a . The safety application  203   a , safety layer  204   a  and communication layers  205   a  may each be implemented in hardware, firmware, software, or some combination thereof. The intelligent actuator  202  similarly includes a plurality of communication layers  205   b  in addition to the safety application  203   b  and safety layer  204   b . Each of the layers within the actuator  202  may also be implemented in hardware, firmware, software, or some combination thereof. 
   As is indicated by  FIG. 2 , the various functional elements of the intelligent sensor  200  are bifurcated into a safety-certified portion  208   a  and a non-safety-certified portion  209   a . In this regard the safety-certified portion  208   a  includes a safety application  203   a  and safety layer  204   a , while the non-safety-certified portion  209   a  includes the communication layer  205   a . Similarly, the intelligent actuator  202  is bifurcated into a safety-certified portion  208   b  and a non-safety-certified portion  209   b . As shown, the safety-certified portion  208   b  includes a safety application  203   b  and safety layer  204   b , while the non-safety-certified portion  209   b  includes the communication layer  205   b.    
   As used herein, the term “safety-certified” indicates that the applicable layer or component has been certified by an authorized organization as being compliant with one or more pertinent international or industry standards. For example, the International Electrotechnical Commission (IEC, Geneva, Switzerland) has promulgated the IEC 61508 in support of the use of Safety Instrumented Systems (SISs) as a means of protecting against hazardous events. SISs are composed of sensors, logic solvers, and final control elements assembled for the purpose of transitioning a process to a “safe” or otherwise stable state when predetermined conditions are violated. Other terms commonly used to describe SISs include emergency shutdown systems, safety shutdown systems, and safety interlock systems. Various commercial organizations provide “safety-certified” certification marks and certificates evidencing compliance with applicable international standards, such as IEC 61508. As is discussed below, it is a feature of the present invention that the neither the elements of the communication layers  205   a ,  205   b , nor of the communication network  201 , are required to be safety-certified in order to ensure the authenticity of the safety messages produced by the intelligent sensor  200  and received by the intelligent actuator  202 . 
   During operation of the intelligent sensor  200 , the safety application  203   a  monitors its state and periodically produces a corresponding safety message. The safety layer  204   a  then adds various safety measures to the safety message. Such safety measures include a message sequence number, time stamp or the equivalent in order to ensure that successive safety message are distinguishable. This prevents a potentially malicious third party (e.g., a “hacker”) from simply copying one of the safety messages and sending the copy periodically. Additional measures may include, for example, source, destination, and CRC information. As indicated above, the safety layer  204   a  then “signs”  206  the safety message, or a message digest derived therefrom, in order to create an associated digital signature. 
   The communications network  201  transports each safety message and associated digital signature generated by the intelligent sensor  200  to the intelligent actuator  202 . In the exemplary embodiment the communications network  201  may be comprised of commercial-off-the-shelf (C.O.T.S.) equipment that is not safety-certified. As shown, the communications network  201  interfaces with communication layers  205   a  and  205   b  of the intelligent sensor  200  and intelligent actuator  202 , respectively, which are also not safety certified (i.e., are included within the non-safety-certified layers  209   a  and  209   b  of the intelligent sensor  200  and intelligent actuator  202 , respectively). 
   Upon receipt at the intelligent actuator  202  of a safety message and associated digital signature produced by the intelligent sensor  200 , the safety layer  204   b  verifies  207  the received digital signature to authenticate both the origin and the content of the safety message. In addition, the safety layer  204   b  verifies the safety measures, which may include sequence number, time stamp, source, destination, and CRC. The safety layer  204   b  will also contain one or more watchdog timers  210  facilitating detection of the loss of periodic receipt of safety messages. In the exemplary embodiment the safety application  203   b  of the intelligent actuator  202  monitors the received safety messages and performs some safety action (e.g., changes the ON/OFF state of a valve) if the safety messages indicate an unsafe condition. The safety application  203   b  is also configured to undertake some prescribed action if the safety application  203   b  does not receive a valid safety message within the required timeout period. 
   As is discussed below, signature generation  206  involves generating a digital signature by applying a private key of a private/public key pair associated with the intelligent sensor  200  to a condensed version of a safety message (i.e., a message digest). In order to preserve security, the private key is preferably kept in confidence and securely stored within the safety layer  204   a . The resulting digital signature and the safety message are then transmitted to the intelligent actuator  202  via the non-certified communications network  201 . Within the intelligent actuator  202 , a recovered message digest is computed using the safety message received via the communications network  201 . Using this recovered message digest and a public key of the intelligent sensor  200 , the signature verification module  207  generates another digital signature for comparison with the digital signature originally created by the intelligent sensor  200 . If these digital signatures are the same, the safety message received at the intelligent actuator  202  is presumed valid and may be processed accordingly; if not, the received safety message is deemed invalid or corrupted and discarded. 
   Signature Generation and Verification 
     FIG. 3  is a block diagram representative of the operations performed during signature generation  206  and signature verification  207 . In the exemplary embodiment of  FIG. 3 , signature generation  206  and signature verification  207  is conducted in accordance with the Digital Signature Algorithm (DSA) to generate and verify digital signatures based upon safety messages, respectively. In this regard the Digital Signature Standard (DSS), Federal Information &amp; Processing Standard Publication (FIPS) PUB  186 , specifies the Digital Signature Algorithm, which comprises a known public key algorithm used for digital signatures. Other cryptographic algorithms of potential utility in connection with the present invention are DES (Data Encryption Standard) and RSA. DES is a symmetric algorithm with a fixed key length, while RSA is a public key algorithm that can be used for both encryption and digital signatures. 
   Turning now to  FIG. 3 , the safety layer  204   a  of the sensor  200  generates and provides a safety message  302   a  to a secure hash algorithm (SHA)  303   a . The SHA  303   a  condenses the safety message  302   a  to a condensed version termed a message digest  304   a . This hash algorithm may comprise the Secure Hash Algorithm (SHA-1) as specified in the Secure Hash Standard (SHS), FIPS PUB 180-1, National Institute of Standards &amp; Technology, 1995, which is consistent with the Digital Signature Standard. As shown, a digital signature  307  is then generated on the basis of the private key  305  of the sensor  200  and the message digest  304  through execution of a DSA Sign Operation  306 . The digital signature  307   a  and safety message  302   a  are then transmitted to the intelligent actuator  202  via the communications network  201 . 
   As mentioned above,  FIG. 3  also illustratively represents the operations performed during signature verification  207  in the actuator  202 . This verification  207  involves verifying the digital signature generated during the signature generation  206  occurring within the intelligent sensor  200 . A secure hash algorithm  303   b  condenses the received message  302   b  to a recovered message digest  304   b . A DSA verify operation  308  then verifies the digital signature  307   b  given the message digest  304   b  and the public key  309  associated with the intelligent sensor  200 . The result  320  of the DSA verify operation  308  is either “signature verified” or “signature verification failed”, thereby indicating whether or not the received message  302   b  has been authenticated by virtue of its digital signature  307   b.    
   Simplified Exemplary Representation 
   In a particular exemplary embodiment, the present invention may be applied to the case in which the intelligent sensor  200  comprises a manual shutdown switch and the intelligent actuator  202  comprises an associated valve. In this embodiment the switch has two positions, RUN and SHUTDOWN. If the position of the switch is SHUTDOWN and the valve does not close, then potentially dangerous consequences may ensue. 
   During normal operation, the shutdown switch periodically sends an “encrypted watchdog” message (i.e., an encrypted safety message) to the valve, indicating that the switch is in the RUN position. The valve expects to periodically receive the encrypted watchdog message, and closes if the message is not received. The message is changed each time it is transmitted, perhaps by including a sequence number or a time stamp. Encryption of the watchdog message may be effected by, for example, using one of the encryption algorithms described above. 
   There are a variety of potential ways to maintain the private key used in encrypting the watchdog message in secrecy. One extreme approach might be to set private key for the switch at the time of its manufacture, and not allow (by quality control) the private key to be communicated from the applicable manufacturing facility. The valve is configured with the corresponding public key, which need not be kept in secrecy. 
   Of course, in alternative embodiments of the present invention more complicated logic may be employed to determine an appropriate course of action to be taken on the basis of encrypted watchdog messages generated consistent with the invention. For example, configurations could be provided in which the encrypted messages received from any of several sensors could cause closure of a valve, or in which messages from m out of n sensors could lead to closure of such a valve. Moreover, the safety messages generated by each intelligent sensor may be encrypted prior to transmission to an intelligent actuator. The encrypted safety messages received at each actuator would then be decrypted prior to being processed in the manner described above, thereby further discouraging tampering with or “hacking” of the transmitted safety messages. 
   Accordingly, a method has been described herein for transmitting safety messages by way of communication channels comprised of non-safety-certified equipment. Consistent with the disclosed method, digital signatures may be used to authenticate both the origin and content of the transmitted safety messages. In other embodiments data encryption may be employed instead of digital signatures in connection with message authentication. In yet other embodiments data encryption may be used in addition to digital signatures in order to effect such authentication. 
   The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well-known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.