Patent Publication Number: US-11651088-B2

Title: Protecting a vehicle bus using timing-based rules

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of U.S. application Ser. No. 17/408,527, filed Aug. 23, 2021, which is Continuation of U.S. application Ser. No. 16/702,617, filed Dec. 4, 2019 (issued as U.S. Pat. No. 11,120,149), which is Continuation of U.S. application Ser. No. 15/924,223, filed Mar. 18, 2018 (issued as U.S. Pat. No. 10,534,922), which is Continuation of U.S. application Ser. No. 15/704,018, filed Sep. 14, 2017 (issue as U.S. Pat. No. 9,965,636), which is a Continuation of U.S. application Ser. No. 14/376,827, filed Aug. 5, 2014, and issued as U.S. Pat. No. 9,881,165 on Jan. 30, 2018, which is a U.S. national phase of International Application No. PCT/IL2013/050290, filed Mar. 28, 2013, which claims priority from U.S. Provisional Application No. 61/617,188 filed Mar. 29, 2012, which are all hereby incorporated herein in their entirety including all tables, figures, and claims. 
    
    
     TECHNICAL FIELD 
     The present invention relates to security systems and methods in general, and in particular to protecting a vehicle&#39;s electronic system or industrial control systems from cyber threats. 
     DEFINITIONS AND BACKGROUND ART 
     Definitions, Terms, Elements 
     Electronic Control Unit 
     The term “Electronic Control Unit” (ECU) denotes herein any electronic system within a vehicle with processing capabilities (e.g. a radio system is an ECU while a wiper controlled by a relay is not). An electronic control unit is a type of electronic component within a vehicle electronic system. 
     Some ECUs include an external communication interface, i.e. an interface to communicate with components outside the vehicle&#39;s electronic system including outside the vehicle itself. ECU also stands for “Engine Control Unit” which is a special case of an Electronic Control Unit. 
     Bus 
     A bus (also referred to as communications bus) is a shared wired or wireless communication channel over which different components transfer data from one to another. 
     Controller Area Network Bus 
     Controller Area Network (CAN or CAN bus) is a vehicle bus standard designed to allow electronic systems to communicate with each other within a vehicle without a host computer (no master required on the bus). ECUs in a vehicle usually communicate by accessing a CAN bus. CAN bus is also used in systems that are not a vehicle, such as Industrial Control Systems, and the invention encompasses uses of CAN bus or any similar bus in any system. For simplifying the description, most examples will refer to CAN bus and a vehicle. 
     Filter Element 
     Filter element denotes an element with two interfaces that upon receiving a message either discards it, changes it or passes it according to various conditions e.g. message ID value. The filter element is the part of the security system of the invention which is in charge of the logic of the filtering, e.g. classifying, analyzing and acting upon the messages received. The filter element can be either a hardware module, a software module, or a hardware and software module. The filter element may contain an additional logic module for supporting more actions, such as generating messages itself, maintaining an inner state, or any other action. 
     Proxy Element 
     The term proxy element as referred to herein denotes an element with at least one communication interface that holds the current state according to past communication. The proxy element can send messages to its interface(s) according to its current state, the current input (e.g. a message) and the time (e.g. an independent process that sends keep-alive messages periodically). This element is usually used to allow two parties to communicate with each other indirectly. 
     Attack Vectors 
     An attack vector is a path or means by which an attacker can gain access to a computerized device in order to deliver a payload which will cause a malicious outcome. An automobile has numerous attack vectors, including supply chain, physical access to the automotive communication bus, physically replacing one of the vehicle&#39;s ECUs, using one of the ECUs&#39; standard connections to the external world etc. 
     The disclosure assumes that most of the threats originate from ECU&#39;s connections to the external world. The disclosure assumes each of the ECUs, except the suggested security system (or device), is potentially vulnerable to attacks that might execute malicious code on it and may gain control over it. The attack on each ECU may be achieved using any of its data connections (physical or wireless). 
     Security System 
     Security system denotes a system (that may be implemented also as a device) for protecting an electronic component or bus within a vehicle electronic system or other industrial control system, embodiments of which are described in this disclosure. In some embodiments the security system is a stand-alone system as described in the STAND-ALONE SYSTEM and the GATEWAY SYSTEM sections. In some embodiments the security system is integrated inside another system as described in the INTEGRATED SYSTEM section. The security system can also be denoted by “communication filter/proxy”. 
     BACKGROUND 
     Hacking Threat 
     Automobiles are becoming more sophisticated and increasingly use computerized technology (ECU—electronic control unit) to control critical functions and components such as brakes and airbags functionality. While the computerized technology enhances the performance of the vehicle, compromising the operation of one of these safety-critical ECUs may cause severe damage to the vehicle, its passengers and potentially even the surroundings if the vehicle is involved in an accident with other vehicle(s) or pedestrians. 
     These ECUs are usually connected via a non-secure manner such as through CAN bus. Taking control of the vehicle&#39;s communication bus can result in compromising the safety critical ECUs (see “Experimental Security Analysis of a Modern Automobile” by KOSCHER et al., 2010 IEEE Symposium on Security and Privacy. 
     Some of the ECUs which are connected to the vehicle&#39;s communication bus have external connections, such as the telematics computer and the infotainment system. It is possible to compromise one of these ECUs using a cyber-attack. The compromised ECU serves as an entry point to deploy the aforementioned attack, see “Comprehensive Experimental Analyses of Automotive Attack Surfaces”, Checkoway et al., USENIX Security, Aug. 10-12, 2011. 
       FIG.  1    and  FIG.  2    present a simple vehicle&#39;s communication network of the art consisting of a single bus. 
       FIG.  1    shows a vehicle electronic system  101  comprising a plurality of ECU&#39;s  75  connected to a vehicle communication bus  105 . The ECU&#39;s  75  communicate with each other over the communication bus  105 . A vehicle electronic system  101  may contain multiple communication buses  105 , each connected to one or more ECU&#39;s  75 . 
       FIG.  2    displays a more detailed view of a vehicle electronic system  101  comprising a plurality of ECU&#39;s with external connections  104 , safety critical ECU&#39;s  100  and other ECU&#39;s  106  (without external connection and non-safety critical). 
     ECU&#39;s with external connections  104  include (but are not limited to) telematics  102 , infotainment system  112 , Tire Pressure Monitoring System (TPMS)  113 , vehicle to vehicle (V2V) or vehicle to infrastructure (V 2 I) communication ECU  114  and any combination of ECUs with an external connection not specifically mentioned  109 . 
     Safety critical ECU&#39;s  100  include (but are not limited to) the engine control unit  108 , brake control module (ABS/ESC etc.)  115 , airbag control unit (ACU)  116 , transmission control unit (TCU)  117 , and any combination of safety critical ECUs not specifically mentioned  110 . 
     Other ECU&#39;s  106  denote the set of ECUs that do not have an external connection and are not safety critical, which include the convenience control unit (CCU)  118  and any combination of ECUs which fall under this category but are not specifically mentioned  111 . All the ECUs  75  communicate using the same shared communication bus  105 . There is an external connection to the electronic system  101  using the telematics ECU  102 . The telematics ECU  102  communicates using the illustrated wireless connection  202  with a wireless transceiver  201 . 
     A vehicle electronic system  101  can be attacked where an external communication source (transceiver)  201  establishes a communication line  202  to an ECU, in this example the Telematics ECU  102 . 
     Vehicle theft using CAN bus manipulation becomes more and more popular. 
     Another financial threat that worries OEMs and Tier-1 suppliers is unauthorized ECU  75  replacement. The owner of a vehicle may replace an existing ECU  75  with an unauthentic/unoriginal one, for several reasons: 
     The ECU  75  needed to be replaced for maintenance reasons and the unauthorized replacement is cheaper (the vehicle owner may or may not be aware that the ECU  75  is not authentic); or 
     The replacement gives the vehicle more capabilities similarly to chip tuning (e.g. remove limitations from the engine giving more horsepower—although it is not in the engine&#39;s specification making it more prone to malfunction and/or making it unsafe). 
     The damage to the OEMs and Tier-1 suppliers is both because their original equipment isn&#39;t bought, and because the unauthorized replacement might damage the vehicle which is still under warranty which they have to cover. 
     Vehicle&#39;s Communication Bus 
     A vehicle&#39;s communication bus  105  is an internal communication network that interconnects components inside a vehicle. Examples of protocols include CAN, Local Interconnect Network (LIN), FlexRay, Vehicle Area Network (VAN) and others. 
       FIG.  1    and  FIG.  2    present a simple vehicle&#39;s electronic system of the art consisting of a single bus. 
       FIG.  3    and  FIG.  4    present a vehicle&#39;s electronic system of the art consisting of several buses  105  or bus segments  105 . Each bus segment  105  may consist of a different type of communication protocol or of the same communication protocol but possibly with a different configuration. 
       FIG.  3    illustrates an example of a vehicle&#39;s internal electronic system, comprising of several ECUs  102 ,  112  and  75  connected to a slow communication bus  301  and several ECUs  117 ,  116 ,  115 ,  113 ,  108  and  75  connected to a fast communication bus  305 . A bridge or a gateway  302  connects the two buses  301  and  305 . 
       FIG.  4    is a more general example of the vehicle&#39;s internal electronic system  101  illustrated in  FIG.  3   , where three communication busses  105  are connected by a gateway or bridge  302 . Each communication bus  105  has a set of ECUs  75  connected to it. 
     CAN Bus Gateway/Bridge 
     A gateway or bridge  302  connects several bus segments  105  and allows messages to pass between them. A bridge  302  is described, for example, in U.S. Pat. No. 6,292,862 entitled “BRIDGE MODULE”. A gateway  302  is described, for example, in US Patent Application No. 2009/0198856, entitled “GATEWAY FOR DATA BUS SYSTEM”. 
     Gateways and bridges  302  are designed to transfer messages between bus segments  105  in a reliable manner but are not designed from a cyber-security perspective. One perspective of cyber-security-directed design, as opposed to reliability-directed design, is message filtering. Usually a bridge or a gateway  302  will not discard messages out of the concern that these messages will be needed and their absence will cause harm. Some gateway  302  designs exhibit monitoring abilities of selected messages such the one described in US Patent Application No. 2009/0198856. When monitoring the communications, selected messages are sent to a monitoring interface (described below). The selective monitoring is often referred to as filtering; however this type of filtering does not interfere with the original communication. 
     CAN Bus Monitoring 
     A monitor is a device delivering messages being sent on a bus  105  (or their properties) to a diagnostic device. A monitor is either a standalone device or a module/part in another device such as a gateway  302 . A standalone monitor is described in US Patent Application No. 2006/0182040, entitled “DEVICE AND METHOD FOR DIAGNOSIS ON MULTI-CHANNEL-CAN-APPLICATION”. Some of these monitors selectively monitor messages but do not intervene with the communication on the bus  105 . 
     Bus Encryption 
     Encryption is a common method to address authentication problems. Encryption methods for CAN bus  105  are described in US Patent Application No. 2011/0093639, entitled “SECURE COMMUNICATIONS BETWEEN AND VERIFICATION OF AUTHORIZED CAN DEVICES” and US Patent Application No. 2009/0169007, entitled “CONTROL AREA NETWORK DATA ENCRYPTION SYSTEM AND METHOD”. 
     While encryption can be the basis for authentication, from a system perspective it is not a viable solution for the automotive environment. The automotive environment consists of many vendors and devices. These devices are usually simple and have little processing power. 
     For an effective encryption scheme, either a key exchange or specific preloaded keys are required. These are quite complicated processes given the limitations of the automotive industry and the devices described above. 
     The CAN bus  105  is usually quite slow and encryption demands additional bandwidth which may slow down the communication even further, and could impact overall system performance. 
     Firewall 
       FIG.  5    is a schematic drawing of a Firewall integration of the art. A firewall is a device, or set of devices, designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass. 
       FIG.  5    illustrates two networks A and B  501  separated by a firewall  503 . Each network  501  has at least one computer  500  connected to it. 
     A firewall  503  is usually designed for Internet Protocol (IP)-based networks  501  and uses the IP and Transmission Control Protocol (TCP) characteristics of the communication. Currently, there are no firewalls  503  intended for CAN bus  105 . CAN messages differ from IP messages in many aspects such as size, headers, content etc. 
     The assumptions of IT firewall  503  designers differ from the assumptions required in designing a protection for a safety critical system. A false positive or a false negative identification of a message in the IT arena usually does not have a direct physical impact (unlike the industrial control systems (ICS) arena where computers control physical processes such as temperature, pressure, engines etc.). An automotive involves human lives and ECUs  75  directly influence the automobile&#39;s functionality; therefore a traditional firewall  503  is not applicable in this case. 
     Firewall  503  implementation for a native CAN BUS communication bus  105  does not exist. In general, Industrial Control Systems (ICS) security solutions lack filters and firewalls  503  and usually exhibit a diode (one way communication) and network separation solution. These solutions are not viable for an automotive since an automotive requires two-way communications. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a security system. 
     It is another object of the present invention to provide a security system to protect an electronic system against malicious messages. 
     It is a further object of the present invention to provide a security system to protect an electronic system of a vehicle against malicious messages. 
     It is yet another object of the present invention to provide a security system to protect an electronic system of any transportation means against malicious messages. 
     It is yet a further object of the present invention to provide a security system to protect an industrial control system against malicious messages. 
     It is yet another object of the present invention to provide a security system to protect any electronic system comprising a CAN Bus against malicious messages. 
     The present invention relates to a security system by which computer-based equipment, information and services are protected from unintended or unauthorized access, change, malfunction or destruction. 
     The present invention thus relates to a security system for protecting a vehicle electronic system from malicious messages, comprising: 
     (i) one or more message receiving units, each message receiving unit connected to one or more communication buses via one or more ports, for receiving all messages sent from a source communication bus to a destination communication bus, prior to their arrival to said destination bus; 
     (ii) one or more message classification units which receive messages from the one or more message receiving units and classify them according to the port upon which the message was received, and according to at least one message property; 
     (iii) one or more message analyzer units which analyze a message according to its classification from the one or more message classification units and decide whether to transfer the message to the appropriate transmission unit as is, block the message or perform at least one of the following actions: (1) transfer a modified version of the message to the appropriate transmission unit; (2) limit the rate that such messages can be delivered to the appropriate transmission unit to predetermined value per time unit; (3) add a signature to the message and transfer it to the appropriate transmission unit; (4) verify that the message has arrived with a valid signature as a condition to transfer to its appropriate transmission unit; or (5) transfer the message to the appropriate transmission unit only after performing an authentication procedure; and 
     (iv) one or more message transmission units which receive from the one or more message analyzer units messages to be transmitted to a destination bus, and transmit said messages to the destination bus. 
     In some embodiments, the vehicle is a car, a truck, a motorcycle, a train, a tank, an airplane, a missile, a spaceship, a rocket or a robot. 
     In some embodiments, the malicious messages are valid messages that are sent in high volume in order to saturate a system resource and make it unavailable or slow to respond to legitimate messages. 
     In some embodiments, the source communication bus and the destination communication bus are the same communication bus. 
     In some embodiments, the system comprises at least two bus interfaces and can filter messages in each direction, from any communication bus to any communication bus. 
     In some embodiments, the one or more message analyzer units and the one or more message classification units are replaced by a proxy. 
     In some embodiments, the at least one message property comprises message ID, a message data field, or the message length. 
     In some embodiments, the at least one bus is a CAN BUS. 
     In some embodiments, the modified version of the message to its destination bus is an encrypted message or a message with an addition of a signature. Alternatively, a modified message can be both encrypted and with a signature. 
     In some embodiments, the message analyzer unit applies more than one rule on a received message. 
     In some embodiments, the system activities are logged for statistics purposes. 
     In some embodiments, the system is integrated with an ECU or a stand-alone system coupled to at least one ECU or a stand-alone system not coupled to any ECU. 
     In some embodiments, the security system is used for preventing vehicle theft, chip tuning or any unauthorized intervention in the vehicle operation. 
     In some embodiments, a plurality of ECU&#39;s with external connections are protected by said security system. 
     In some embodiments, all ECU&#39;s with external connections are protected by said security system. 
     In some embodiments, at least one security system is placed on the communication path between at least one ECU with an external connection and at least one safety critical ECU. 
     In some embodiments, a security system is placed on the communication path between each ECU with an external connection and each safety critical ECU. 
     In another aspect, the present invention further relates to a security system for protecting an industrial control system&#39;s electronic system from malicious messages, comprising: 
     (i) one or more message receiving units, each message receiving unit connected to one or more communication buses via one or more ports, for receiving all messages sent from a source communication bus to a destination communication bus, prior to their arrival to said destination communication bus; 
     (ii) one or more message classification units which receive messages from the one or more message receiving units and classify them according to the port upon which the message was received, and according to at least one message property; 
     (iii) one or more message analyzer units which analyze a message according to its classification from the one or more message classification units and decide whether to transfer the message to appropriate transmission unit as is, block the message or perform at least one of the following actions: (1) transfer a modified version of the message to the appropriate transmission unit; (2) limit the rate that such messages can be delivered to the appropriate transmission unit to predetermined value per time unit; (3) add a signature to the message and transfer it to the appropriate transmission unit; (4) verify that the message has arrived with a valid signature as a condition to transfer it to the appropriate transmission unit; or (5) transfer the message to the appropriate transmission unit only after performing an authentication procedure; 
     (iv) one or more message transmission units which receive from the one or more message analyzer units messages to be transmitted to a destination bus, and transmit said messages to the destination bus. 
     In some embodiments, at least one bus is a MODBUS, MIL-STD-1553 or MIL-STD-1773 ARINC. 
     In yet another aspect, the present invention relates to a security method for protecting a vehicle electronic system from malicious messages, comprising: 
     (i) intercepting messages sent from a source communication bus to a destination communication bus via one or more ports, prior to their arrival to said destination communication bus; 
     (ii) classifying said messages according to the port upon which the message was received, and according to at least one message property; 
     (iii) analyzing said messages according to their classification and deciding whether to transfer the message to its destination communication bus as is, block the message or perform at least one of the following actions: (1) transfer a modified version of the message to its destination communication bus; (2) limit the rate that such messages can be transferred to a destination communication bus to predetermined value per time unit; (3) add a signature to the message and transfer it to its destination communication bus; (4) verify that the message has arrived with a valid signature as a condition to transfer to its destination communication bus; or (5) transfer the message to its destination communication bus only after performing an authentication procedure; 
     (iv) transmitting messages to their destination communication bus based on the analysis of step (iii). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates an example of a vehicle&#39;s electronic system of the art, comprising of ECUs and a communication bus; 
         FIG.  2    illustrates a more detailed example of a vehicle&#39;s electronic system of the art, comprising of ECUs with external communications, safety critical ECU&#39;s and other ECU&#39;s, all ECU&#39;s connected to a communication bus. An external connection is established to the electronic system from an external communication source to the telematics ECU; 
         FIG.  3    illustrates an example of a vehicle&#39;s internal electronic system of the art, comprising of ECUs connected to two communication buses with different speeds, and a bridge or a gateway connecting the two buses; 
         FIG.  4    is a more general example of the vehicle&#39;s internal electronic system of the art illustrated in  FIG.  3    wherein a gateway or a bridge are connected to multiple communication buses; 
         FIG.  5    illustrates two networks of the art separated by a firewall; 
         FIG.  6    is an illustration of the electronic system illustrated in  FIG.  2    wherein all the ECU&#39;s with an external connection are protected by standalone security systems (communication filter/proxy), according to an embodiment of the invention; 
         FIG.  7    exemplifies the integration of the security system (communication filter/proxy) as a gateway between three communication buses, according to an embodiment of the present invention; 
         FIG.  8    illustrates the integration of the security system (communication filter/proxy) connected serially to an existing gateway, according to an embodiment of the present invention; 
         FIG.  9    illustrates the integration of the security system (communication filter/proxy) as an integrated security system inside an ECU, according to an embodiment of the present invention; 
         FIG.  10    illustrates a general message flow between two communication buses connected to a security system (communication filter/proxy), and a connection to a configuration and diagnostics computer, according to an embodiment of the present invention; 
         FIG.  11    is a top view illustration of the various modules of one embodiment of the security system (communication filter/proxy), according to an embodiment of the present invention; 
         FIG.  12    is flowchart illustration of one example of message handling done by the embodiment illustrated in  FIG.  11   , according to an embodiment of the present invention; 
         FIG.  13    illustrates a basic interface&#39;s message handler without a configuration interface. This is an example of an integrated message receiving &amp; transmission unit, without the configuration interface, according to an embodiment of the present invention; 
         FIG.  14    illustrates an interface&#39;s message handler, which also functions as the message receiving and transmission units. It is an example of an integrated message receiving &amp; transmission unit, according to an embodiment of the present invention; 
         FIG.  15    is a flowchart illustrating an example of the message handling in a message handler such as illustrated in  FIG.  14   , according to an embodiment of the present invention; 
         FIG.  16    is a block diagram illustrating one example of a filter/proxy element (combined message classification/message analyzer unit), according to an embodiment of the present invention; 
         FIG.  17    is a block diagram illustrating one example of the filter/proxy element (combined message classification/message analyzer unit) illustrated in  FIG.  16   , according to an embodiment of the present invention; 
         FIG.  18    is a flowchart illustration of one example of a message handling logic implemented by a filter/proxy element (combined message classification/message analyzer unit) illustrated in  FIG.  17   , according to an embodiment of the present invention; 
         FIG.  19    is a block diagram illustration of one example of a timing rule. A timing rule is a rule that can be integrated in a filter/proxy element (in the message analyzer unit) as one of the rules used, according to an embodiment of the present invention; 
         FIG.  20    is a flowchart illustration of one example of message handled by a timing rule illustrated in  FIG.  19   , according to an embodiment of the present invention; 
         FIG.  21    is a block diagram illustration of one example of a proxy element (in the security system), according to an embodiment of the present invention; 
         FIG.  22    is a block diagram illustration of one example of a link emulator such as Link emulator A of  FIG.  21   , according to an embodiment of the present invention; 
         FIG.  23    is a block diagram illustration of one example of the proxy element illustrated in  FIG.  21   . This illustration exemplifies the use of a security system containing a proxy to protect a vehicle from attacks origination from the infotainment system, according to an embodiment of the present invention; 
         FIG.  24    is a block diagram illustrating one example of a security system of the invention consisting of one message receiving unit, one message classification unit, one message analyzer unit and one message transmission unit; and 
         FIG.  25    illustrates an example of two ECUs with integrated security systems of the invention comprising authentication units, and connected on the same communication bus. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     In the following detailed description of various embodiments, reference is made to the accompanying drawings that form a part thereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     The figures described herein illustrate blocks. Each block can represent any combination of hardware, software and/or firmware which performs the functions as defined and explained herein. 
     Modern vehicles increasingly use more efficient computerized, electronic components and sub-systems instead of mechanical parts. Such systems are controlled by ECUs  75 , which are connected through one or more communication buses  105 . In case that more than one communication bus  105  exists, the buses  105  are usually connected using bridges or gateways  302 . Some of these ECUs  75  controlled systems are safety critical systems  100 , such as the engine control unit  108  or the brake control module  115 , and some are less critical or non-critical systems, such as infotainment systems  112  and wireless tire pressure sensors  113 . Some of the systems mentioned above are ECU&#39;s with external interfaces  104 , for example, the tire pressure sensors  113  communicate wirelessly with a receiver on the bus  105 , the radio  112  has wireless (radio, Radio Data System (RDS), etc.) and local (e.g. media files) interfaces and the telematics  102  (e.g. On-Star.™.) has a cellular interface  202 . 
     Though these interconnected computerized systems  75  offer the user increased performance of the vehicle and additional services, there is an inherit danger in such architecture wherein anyone who gets access to a communication bus  105  of the vehicle may maliciously interfere with the proper operation of the systems  75  communicating over the bus  105 , among them the safety critical systems  100 . There are many ways such attacks can be accomplished once access is gained to a communication bus  105 . Some examples include: attacking any system  75  directly; sending messages constantly over the bus  105  preventing others from communicating (denial of service); impersonating to other devices  75  sending false messages; sending messages that will have a harmful effect (press the brakes, disable the Anti-Lock Braking System  115 , etc.); sending messages to limit the functionality of a part  75  (limit speed etc.), etc. In today&#39;s vehicle architecture, there is no secured isolation between the safety critical systems  100  and the other systems  104  and  106 . 
     Some embodiments of the present invention relate to a cyber-security-directed design which selectively intervenes in the communication path in order to prevent the arrival of malicious messages at ECUs  75  (in particular at the safety critical ECUs  100 ). The security perspective suggests that more damage can be caused by passing a potentially unwanted message than by blocking or changing it. However, there may be reliability implications. In a cyber-security-directed design, reliability issues can be solved using methods described herein. 
     In some embodiments, the security system of the invention includes a filter which prevents illegal messages sent by any system or device  75  communicating over the communications bus  105  from reaching their destination. The filter of the invention may, at its discretion according to preconfigured rules, change the content of the messages or limit the rate such messages can be delivered, by buffering the messages and sending them only in preconfigured intervals. The rules in the filter of the invention, which determine which messages are allowed and which are not allowed and the rate of the messages, can be configured using an external interface of the filter. The security system can be located, for example, between each system component which has an external interface  109  and the communication bus  105 , protecting the bus  105  and the electronic devices  75  connected to it from the component  109 . 
     In some embodiments, the security system of the invention has at least two bus  105  interfaces and can filter messages in each direction. The filtering is done in any appropriate way, for instance, according to the message&#39;s properties (such as message headers, data, etc.) and/or according to inner state properties of the security system (such as the physical interface through which the message was sent, the timings, etc.) or any combination of the above. 
     In some embodiments, the security system has proxy capabilities. A proxy saves the state of the communication protocols over one or more of its physical interfaces. It also independently manages the communication protocol over each of its interfaces (such as sending keep-alive messages to the radio without involving other components  75 ). 
     In some embodiments, the security system has gateway  302  capabilities. It can connect two or more communication buses  105  which may have different physical properties. 
     In some embodiments, the security system can save its configurations in a non-volatile memory, and the configurations and the non-volatile memory may be updated from an external source. 
     In some embodiments, the security system may save statistics, monitoring data etc. internally for later usage, for example, when such data is read externally later. 
     In some embodiments, the security system can internally update the non-volatile memory contents. 
     In some embodiments, the security system can be integrated inside current ECUs  75 , between the physical driver and the logical part of the ECU  75 , saving the need for additional physical interfaces for the security system. In other embodiments, the security system can be a stand-alone security system. The stand-alone security system of the invention can be coupled to a single ECU  75 , coupled to a plurality of ECU&#39;s  75  or not coupled to any ECU  75 . 
     In some embodiments, the security system can be integrated into a system containing one or more communication buses  105  and ECUs  75 . It can learn the communication properties of the different parts  75  of the system, build filtering rules in an autonomic fashion, and filter when the learning phase is over. 
     Some embodiments may include a combination of any of the aforementioned embodiments. 
     Other aspects of the currently disclosed subject matter will become apparent by consideration of the detailed description and the accompanying drawings. 
     System Integration and Placement 
     There are several potential embodiments of the invention from a system integration point of view. In some embodiments, the security system will act as a protection system or device between at least two communication buses  105  or components  75 . 
     Attacking an automobile (without physically tampering or pre-installing a backdoor) requires logical access to its electronic components  75 . The suggested integration positions of the security system of the invention, according to some embodiments, prevent an improper logical access originating from external interfaces  202  from reaching safety critical components  100 . Therefore, the integration of the security system can protect from life threatening cyber-attacks. Assuming the security system is configured correctly, a potentially hermetic protection is achieved. 
     In some embodiments, when dealing with chip tuning, unauthorized ECU  75  replacement and vehicle theft, the security system of the invention can be coupled with ECUs  75  that need to be protected and/or authenticated (e.g. antitheft ECU  75 , immobilizer  110 , engine control unit  108 , etc.) 
     If the security system has a configuration port, in some embodiments the configuration port will not be connected to any untrusted communication buses  105 . 
     In some embodiments, the configuration port can be connected in-band, i.e. to one or more of the communication buses  105 , given it is protected in some manner. In some embodiments, this in-band configuration is optional and can be disabled after the initial configuration stage (e.g. during vehicle manufacturing or assembly) is completed. In some embodiments, special configuration messages sent over the communication bus  105 , will be transferred to the configuration interface for processing, and will cause a change in the configuration. 
     Standalone Device 
       FIG.  6    illustrates stand-alone security system integration, according to some embodiments.  FIG.  6    is an illustration of the electronic system  101  illustrated in  FIG.  2    protected by standalone security systems (referred to as communication filter/proxy devices)  703  of the invention. All ECUs with external communication interfaces  104  are protected by stand-alone communication filter/proxy protection devices  703 . The ECUs  104  are connected to the security system  703  via an interface  119 . The interface  119  can be any communication interface including a communication bus  105 . 
     In some embodiments, the security system  703  is a stand-alone system or device. The security system  703  has at least two communication interfaces and may additionally have a configuration port. 
     In some embodiments, the security system  703  is placed between an ECU that has an external interface  109  (physical or wireless e.g. radio or telematics) and the communication bus  105 . Each ECU which has an external connection  109  may be serially connected to the security system  703  in order to protect other ECUs  75  from the communication originating from it. 
     In some embodiments, the security system  703  is integrated with ECUs that don&#39;t have an external interface  106  in order to deal with threats such as vehicle theft, chip tuning, unauthorized ECU  75  replacement etc. 
     In some embodiments, the power supply to the security system  703  is either external or originates from the communication interfaces (a dedicated line or bootstrapped from the communication bus  105 ). 
     Optionally, the stand-alone security system  703  can electrically drive the communication bus  105  attached to it (e.g. provide negative voltage and termination on a CAN bus  105 ). This option may be configurable for each of the communication ports, depending on the implementation. This option emulates the physical properties of the bus  105  towards any segment it is connected to. In case of retrofitting; it saves the need to install an additional physical driver to the bus  105 . 
     In some embodiments, this integration allows for the retrofitting of an automobile, without replacing existing ECUs  75 . 
     In some embodiments, when each security system  703  generally handles only one ECU  75 , its configuration and operation is rather simple, and it can be implemented with simple hardware architecture (compared to other alternatives). 
     Gateway Device 
     In some embodiments, the security system  703  is a stand-alone system or device. The security system  703  has at least two communication interfaces and may additionally have a configuration port. 
     In some embodiments, the security system either replaces an existing gateway/bridge  302 , as shown in  FIG.  7   , or is integrated between a gateway/bridge  302  and one of their connected communication buses  105  as shown in  FIG.  8   . 
     Optionally, the security system  703  can electrically drive the communication bus  105  similarly to the stand-alone security system or device  703  described above. 
     In some embodiments, if the security system  703  replaces an existing gateway/bridge  302 , it will also function as one (e.g. converting protocols, connecting the buses  105 ). 
     In order for this type of integration to be effective, an appropriate architecture of the automobiles&#39; communication buses  105  must be implemented. In some embodiments, the design may have to include a security system of the invention  703  in every path between a safety critical ECU  110  and an ECU with an external interface  109  (e.g. all the ECUs with an external connection  104  are connected to a single segment  105 , separated from the other ECUs  75  by a security system  703 ). 
     In some embodiments, such integration allows for the retrofitting of an automobile without replacing existing ECUs  75 . 
     In some embodiments, each security system  703  has to handle the communication of several ECUs  75  connected to the bus. Therefore, the configuration and implementation is potentially more complex, and more complicated hardware is required. Such design may require an integration of a single security system  703 . On one hand, such a security system  703  may be more complex and expensive. On the other hand, it substitutes multiple simpler security systems  703 ; therefore it may be financially worthwhile. Additionally, the design requires a single point of configuration which may become more convenient for design and maintenance. 
     Integrated Security System 
     In some embodiments, the security system  703  is integrated inside an ECU  905  as depicted in  FIG.  9   . The security system  703  has at least two communication ports  901  and  903 , one  903  connected to the physical layer driver  904  and the other  901  connected to the rest of the ECU&#39;s logic  900  (e.g. ECU&#39;s controller) using its native physical layer (e.g. Complementary Metal Oxide (CMOS) or Transistor Transistor Logic (TTL)). The physical layer driver  904  is connected to the communication bus  105 . 
     In some embodiments, the integration of the security system  703  inside an existing ECU  905  will save many components (e.g. power supply, mechanical casing, physical drivers etc.) thus making the design cheaper and more robust. 
     In some embodiments, the security system  703  will be integrated in the same ECUs  905  (those with external interfaces  104 ) and with the same configurations as in the case of a stand-alone security system  703 . 
     In some embodiments, integration will enable a supplier of ECUs  75  to integrate a security solution  703  done by a trusted  3 rd party, thus providing a complete and secure ECU  905 . 
     In some embodiments, this solution will not allow for retrofitting into existing ECUs  75 . However, it will be viable for new designs. This solution embodies all of the advantages of the stand-alone security system  703 . 
     When referring to an ECU  75  coupled with a security system it may also refer to an ECU with an integrated security system as in the case of  905 . 
     Internal Design 
     For clarity reasons, the security system&#39;s  703  core which is responsible for the security aspects of the security system  703  (e.g. filtering or serving as a proxy) is referred to in some embodiments described herein as “filter element” or “proxy element”. However, it is possible that an element designated as “filter element” may also provide proxy functionality, and/or an element designated as “proxy element” may also provide filter functionality. All these variations and combinations are encompassed by the present invention. 
     Additionally, for simplicity&#39;s sake the message flow is depicted in some embodiments herein as if a simple rule based filter is used, although a more complex rule based filter can be applied and is encompassed by the present invention. For example, multiple rules can be applied to the same message. 
     Top View 
       FIG.  10    illustrates the security system&#39;s  703  general overview, in which the filter/proxy element  1304  (which functions as the message classification unit and the message analyzer unit) connects to two communication buses  105  using two message handlers  1801 , according to some embodiments of the present invention. In some embodiments, the filter/proxy element  1304  receives messages from one of these buses  105  through an input buffer  1302  (of the message handler  1801 ), filters these messages, and sends the filtered messages through the appropriate output buffer  1303  (of the message handler  1801 ), to the other communication bus  105 . The security system  703  can also serve as a filtering gateway between two different buses  105  and do any necessary conversions (such as protocol conversions) between the buses  105 , as seen in  FIG.  7   . 
     In some embodiments, as seen in  FIG.  10    the security system  703  can be configured by an external device (e.g. configuration/diagnostics computer)  1408 , through an out of band (OOB) interface such as a serial connection (e.g. RS-232). The configuration affects the security system&#39;s  703  behavior, the messages it lets through, changes or blocks, and any other of its configurable properties. The new configuration can be saved so next time the security system  703  resets, the new configuration will run at startup. 
     The message handler  1801  includes (1) a message receiving unit for receiving a message to its input buffer  1302  from the communication bus  105 ; and (2) a message transmission unit for transmitting a message from its output buffer  1303  to the communication bus  105 . 
       FIG.  11    illustrates the security system&#39;s  703  top view in more detail than illustrated in  FIG.  10   , according to some embodiments of the present invention. Messages arriving into message handler  1801  (described in more detail in  FIG.  14   , and also functions as the message receiving unit and the message transmission unit) through the physical interface I/O  1800 , or any other interface of it, are sent to the proper interface. If sent to the configuration interface, it will be handled by the configuration, statistics and control module  1807  which can handle it in any configured way (e.g. send it through the OOB interface I/O  1806  out of the system for logging, inspection, or any other purpose). If the message is sent to the filter/proxy element  1304 , it inspects it and decides whether to send it to the destination interface or not. If the message is to be sent to the destination interface by the filter/proxy element  1304  (combined message classification unit and message analyzer unit), it is sent to the appropriate message handler  1801 . The message handler  1801 , handles the message in some embodiments as depicted in  FIG.  14    and  FIG.  15   , and sends it to the proper destination interface (e.g. physical interface I/O B  1800 ). The interfaces  1808  between the message handlers  1801  and the filter/proxy element  1304  are named “proxy interface” and they fit both filters and/or proxies (they can also be referred to as “filter interface” or “filter and/or proxy interface”). 
     Some embodiments of the described process are illustrated in  FIG.  12   . In step  3200 , a message is received in the message receiving unit of the message handler&#39;s  1801  interface, e.g by its physical interface, and is sent to one or more of its interfaces in step  3201 . If the message is to be sent to the physical interface, it&#39;s sent to its physical interface in step  3202 . If the message is to be sent to the configuration interface  1602 , it is handled by the configuration interface  1605  according to its functionality, e.g. printed on the screen of the operator, written to a log, etc. in step  3204 . If the message is to be sent to the filter/proxy interface  1604 , it is sent to the appropriate filter element  1304 , and is classified by its message classification unit in step  3203 , which sends it to the filter element&#39;s message analyzer unit. The filter element  1304  then checks the legality of the message (by the message analyzer) in step  3205 . If the message is illegal, it will be discarded in step  3206 . If the message is legal, it is sent to its destination (which can be the opposite message handler  1801 ) in step  3207 . 
     Message Handler 
       FIG.  13    depicts the simple form of the message handler mechanism  1801  each (filter/proxy element  1304 ) interface has, according to some embodiments of the present invention. In some embodiments, each message arriving from a physical interface  1800  is processed by a message receiving unit of a message handler  1801  and sent to the input buffer  1302  of the filter/proxy element  1304 . Each message which originates from another interface&#39;s message handler and is destined to interface  1800  and allowed by the filter/proxy element  1304  is sent to the appropriate output buffer  1303 . From the output buffer  1303  it is sent to the message transmission unit of the message handler  1801  and sent out to the physical interface  1800 . The message receiving unit and the message transmission unit can be either separate units, or integrated together into a message handler  1801  for more efficient two-way communications with a communication bus  105 . 
       FIG.  14    describes a more complex form of the interface&#39;s message handler  1801  than described in  FIG.  13   , according to some embodiments of the present invention. Each message arriving from a physical interface I/O  1800  to the physical interface  1602  through the transceiver  1301  goes into the routing component  1603 . The routing component can determine the message&#39;s inner headers (such as message source or any other information about the message) and then decides towards which destination to send the message, according to its routing algorithm. The possible destinations include, but are not limited to, zero or more of the interfaces illustrated in the figure through their respective input buffers, such as: the physical interface  1602 , the filter/proxy interface  1604  or the configuration module interface  1605 . There can be any number of other such interfaces as well. The message is sent to the proper interface which handles it. The routing component  1603  can be configured through the configuration module  1807  (the configuration dataflow is not explicitly drawn), to change its behavior, such as changing its routing tables or routing algorithm. Messages arriving from the filter/proxy element  1304  through its  110  interface  1808  are sent to the routing component which sends them to the proper interface as described above. Messages arriving at the configuration module interface  1605  from the routing component are sent to the configuration module  1807  through the configuration output buffer  1607  and the external interface transceiver  1608 . The external interface transceiver  1608  can be implemented as a software and/or hardware module. In some embodiments, the external interface transceiver  1608  is optional and can be omitted. The configuration module  1807  handles messages in various ways, e.g. print on the operator screen (if such thing exists), and can be used for any purpose, e.g. inspection, sending messages, controlling or debugging the system.  FIG.  15    describes the message flow in the interface&#39;s message handler  1801 , according to some embodiments. A message is received by the message receiving unit in one of the message handler&#39;s interfaces in step  3200 . The message headers of the message can then be determined in step  3301 , and the message&#39;s appropriate routing is decided in step  3302 . The message is then sent to its destination interface in step  3303 . The classifier and analyzer are part of the filter/proxy element  1304 , so only if the message is directed to the filter/proxy element  1304  they will handle it. The other options routing a message are to the physical interface  1602  or the configuration interface  1605 . Since the classifier and analyzer are not part of the message handler,  1801  they are not described here. The interface&#39;s message handler  1801  may collect and save any statistics information about the system and the messages being sent to it or from it (e.g. the number of messages that were received from or sent to each interface). 
     In some embodiments, each filter element  1304  is coupled with at least 2 such message handlers  1801  and each proxy element  1304  is coupled with at least one such message handler  1801 , one for each interface that they are connected to. 
     Configuration Module 
     The “configuration module”  1807  denotes the “configuration, statistics and control module”  1807  (some embodiments being illustrated in  FIG.  11   ). 
     In some embodiments, the configuration module  1807  is connected to the Interface&#39;s messages handler  1801  using two types of connections: a messages connection  1609  and a configuration connection  1810 . The configuration module  1807  can send or receive messages to/from the interface&#39;s message handler  1801  through the messages connection  1609 . The configuration module  1807  is connected to the filter/proxy element  1304  using a configuration connection  1810 . The configuration module  1807  controls the configuration of the filter/proxy element  1304  and the message handlers  1801  through the configuration connection  1810 , changing their behavior, logging their activities, and/or any other configurable change they support. This module  1807  is controlled externally using an OOB interface (external interface I/O)  1806 , which can be any data interface (e.g. Universal Asynchronous Receiver Transmitter (UART) interface). The configuration module  1807  can also have a non-volatile memory  1805  connected to it (e.g. flash memory). This memory  1805  stores data which is used by the configuration module  1807 . Such data can include, but is not limited to, different system configurations to be loaded into the system components (e.g. the filter elements  1304  and the interfaces&#39; message handlers  1801 ), and statistical information. It might also, but not necessarily, be possible to manipulate this memory  1805 , through the OOB interface  1806  or directly. Such manipulation may include, but is not limited to, deleting the memory, copying it, dumping it, copying new information into it, etc. 
     In some embodiments, the configuration module  1807  can be connected in-band, i.e. to one or more of the communication buses  105 , given it is protected in some manner. In some embodiments, this in-band configuration is optional and can be disabled after the initial configuration stage (e.g. during vehicle manufacturing or assembly) is completed. 
     Filter/Proxy Element 
       FIG.  16    illustrates a simple example of a filter/proxy element (combined message classification and message analyzer units)  1304 , built from two interface filtering components  2001 , according to some embodiments of the present invention. Each interface filter component  2001  filters messages arriving from its input interface (message receiving unit) and sends them after filtering to its output interface (message transmission unit). A more detailed example of  2001  is illustrated in  FIG.  17   , according to some embodiments. A message arrives from the proxy interface input  2000  of the message handler  1801 , and goes into the rule selector  2100  of the message classification unit (also referred to as classifier), which according to the message properties (such as headers, source, destination, data, or any other properties) sends it to the proper rule  2102  in the message analyzer unit. If no proper rule is found, the rule selector rejects the message according to its policy (possible policies are described below). The appropriate rule  2102  (of the plurality of rules  2102 ) which receives the message checks it more thoroughly and decides whether the message should be allowed or not, or should be modified. The action upon the result of a rule  2102  is part of the message analyzer&#39;s unit. If the message should be allowed, the rule  2102  passes the message to the proxy interface output  2002  connected to the message transmission unit of the message handler  1801 . If the message should be changed, the rule  2102  (of the analyzer) can make the necessary changes and pass the message to the proxy interface output  2002  connected to the message transmission unit. In some embodiments, if the message should not be allowed, the rule selector  2100  is notified and it chooses the next proper rule  2102  for the message or rejects the message according to its policy. If no more proper rules  2102  are found, the rule selector  2100  acts according to its policy in such case. The rule selector  2100  policy may include, but is not limited to, discarding the message, notifying the sender, or performing any preconfigured action. A rule  2102  can be of any type and can also be timing rule as will be described below. A rule  2102  may require that a message is signed, that a message signature is verified, or conditional transmission of a message as described in the authentication module section below. It should be clear that the term rule  2102  encompasses any combination of a plurality of rules  2102 , thus more than a single rule  2102  can apply to any one message. The number of rules  2102  is not limited and can vary. In some embodiments, the configuration module  1807  may also control the adding or removing of rules  2102  dynamically. A rule  2102  can contain any filtering logic to decide whether a message is legal or not. Such logic may include but is not limited to, properties of the message, message&#39;s headers, message&#39;s content, message length, the filter state, timings of the message or any other parameters or properties or any combination of these properties, in a whitelist or blacklist manner. An example of a filtering logic can be checking that the message destination is ‘y’, the ID of the message is between ‘xx’ to ‘zz’, the message data length is 3 and the first two bytes of the message are ‘aa’ and ‘bb’. 
       FIG.  18    illustrates an example of the filter logic and the message flow described, according to some embodiments. The message is received in the proxy interface input in step  3000  and is delivered to the message classification&#39;s unit rule selector  2100 , which selects the next appropriate filter rule  2102  to filter the message with in step  3001 . If no appropriate rule  2102  was found, the message can be discarded in step  3206 . If a rule  2102  was found, the message is checked by the rule  2102  (by the message analyzer unit) for its legality according the rule  2102  in step  3003 . If the message is not legal according to the rule  2102 , it returns to the rule selector  2100  to select the next appropriate rule  2102  in step  3001 . In some embodiments, if the message is legal according to the rule  2102 , it is sent to the proxy interface output  2002  in step  3207 . 
     Timing Rules 
       FIG.  19    describes a timing rule  2300 , which is a type of rule  2102  that can be added to the rules  2102  list (e.g. as rule  2102 ) in the filter element  1304  described above, according to some embodiments of the present invention. The difference between a timing rule  2300  and a regular rule  2102  is that the timing rule  2300  does not only filter the incoming messages, but it also applies rate limit according to a policy which can also include traffic shaping of the communication, for example, sending messages to the proxy interface  1808  in predefined timings (leaky bucket), thus preventing denial of service (DOS) attacks. When the rule selector  2100  sends the received message to a timing rule  2300 , the filtering logic  2301  works as in a regular rule  2102 . In case the message is allowed, it is sent through interface  2302  to the rule output buffer  2303 , in which it is buffered and waiting to be sent to the proxy interface output  2002 . When the right timing arrives, the timing function  2305  checks whether there are messages waiting in the output buffer  2303 , and if so, pulls a message out through interface  2304  and sends it to the proxy interface output  2002 . In case the message is illegal, the filtering logic  2301  will reject the message. In any case, be it a legal or illegal message, the rule selector  2100  can be notified of the operation&#39;s result. 
       FIG.  20    illustrates an example of the timing rule  2300  logic and the message flow described, according to some embodiments. The message is received in the proxy interface input  2002  in step  3000  and is being filtered as in a regular (not timing) rule  2102  in step  3101 ) If the message is illegal, it is discarded in step  3102 . In case the message is legal it is stored in the output buffer of the timing rule  2300  and waiting to be sent in step  3103 . When the time arrives, the timing task of the rule  2300  transfers the message waiting in the output buffer to the transmission unit to be sent to its destination in step  3104 . The advantage of using a timing rule  2300  is preventing DOS attacks. Examples of such DOS attacks include, but are not limited to, rapid message sending and planed timing of message sending. Additionally this type of rule can help deal with malfunctions sending many messages over the communication bus  105  which leads to DOS. Another advantage is the ability of such filter to bridge between two buses  105  with different capabilities of handling messages pace. This type of filter  703  is quite simple to configure compared to other stateful filters and can handle many threats. 
     Proxy 
       FIG.  21    illustrates the proxy element  2500  design, according to some embodiments of the present invention. The proxy element  2500  is connected to one or more message handlers  1801  through interfaces  1808 . Each proxy element  2500  is composed of link emulators (one for each interface)  2501 , and one state filter and updater  2502 . A proxy element  2500  emulates the operation of one bus  105  segment towards the other without allowing direct communication between the segments. All the messages transferred towards any segment using a proxy element  2500  are created by the proxy element  2500  using its state machines and rules (unlike a conventional filter that allows messages that are not blocked to pass). 
     In some embodiments, it is possible to use a proxy element  2500  connected only to one message handler  1801 , in case there is a need to emulate a disconnected side as if it was connected. An example to such case can be assembling a radio which needs a connection to the vehicle without making the connection, by emulating such connection using a proxy element  2500 . 
       FIG.  22    illustrates one embodiment of a Link Emulator  2501  which emulates a communication protocol between two or more participants toward the participants that are connected to its emulated side (e.g. if TCP is the protocol, the emulator will send Ack (Acknowledge) messages for each message received), according to some embodiments. The link emulator  2501  manages and saves a state of the communication (e.g. if TCP is the protocol, the emulator will save a window of the Acknowledged messages). The state that the link emulator stores may include any data and meta-data related to received and sent messages. The link emulator  2501  can update the state filter and updater passively or actively with its current state. The communication will be affected by the link emulator&#39;s state, the received messages and the time. 
     In some embodiments, the link emulator  2501  illustrated in  FIG.  22    consists of a protocol abstraction layer/high level driver  2601 , a state machine  2602  and state/configuration data module  2603 . A protocol abstraction layer  2601  acts as an abstraction layer of the communication protocols the proxy element  2500  handles. It communicates in a relatively simple manner with the state machine  2602 , by sending messages metadata, status and commands through interface  2604 . The state machine  2602  implements the logic the link emulator  2501  includes. The logic the state machine  2602  implements includes, but is not limited to, updating the state, immediately responding to events etc. (e.g. upon receiving a TCP message it updates the window stored in  2603 , changing the state and sends an Ack message through the protocol abstraction layer  2601 ). The state/configuration data module  2603  stores the current state of the link emulator  2501  and can be accessed both by the state machine  2602  and the state filter and updater  2502 . The state machine  2602  and the state/configuration data  2603  modules communicate by sending states to each other through the state interface  2605 . 
     In some embodiments, the state filter and updater  2502  reads the state from each link emulator  2501  using the read-state link  2504  passively or actively and according to the proxy logic it configures the state on each of the link emulators  2501  through the configuration link  2503 . In some embodiments, messages are never directly transferred between link emulators  2501 ; the only communication between link emulators  2501  is by using a state update. The state filter  2502  will enable only legitimate states to pass between link emulators  2501 . 
     In some embodiments, the logic of the proxy element  2500  is either hardcoded or configurable. 
     Existing conventional stateful filters have an internal state machine that tries to emulate the state of the transferred messages and when the state machine discovers an anomaly, messages are discarded. When the filter&#39;s state machine is different than the state machine used by the communicating parties an inconsistent state may occur between the filter and the communicating parties, allowing forbidden communication to pass (e.g. different TCP timeout configuration). In some embodiments, allowing only a state to pass between the link emulators and correctly designing the proxy, can solve the aforementioned problem. 
     A proxy application example according to some embodiments of the currently disclosed subject matter is now described: 
     A radio  112  often uses the vehicle&#39;s integrated display to display information (e.g. radio station frequency). The example assumes a normal radio-vehicle communication is between the radio and the display system. The display system sends its model type and repeatedly sends a keep-alive message. The radio  112  communicates with the display system, queries the model type and sends display data according to the display&#39;s capabilities. 
     A proxy element application  2700 , as seen in  FIG.  23   , consists of a link emulator  2701  towards the vehicle  2708 , a link emulator  2703  toward the radio  112  and a state filter and updater  2502 , according to some embodiments. The proxy element is only a part of the security system  703 , which was omitted from the figure for the sake of clarity. 
     The link emulator  2701  towards the vehicle  2708  is connected between the vehicle  2708  and the state filter and updater  2502 . It holds the formatted text (display data)  2704  designated for the display and the display type (state)  2705 . At startup, this link emulator towards the vehicle  2701  queries the display system for its type, stores the information in  2705  and sends it to the state filter and updater  2502 . The link emulator towards the vehicle  2701  is in operational mode if it holds a valid display data and a valid display type. When in operational mode the link emulator  2701  sends messages containing display data on every display data change according to the display type. 
     The link emulator towards the radio  2703  is connected between the radio  112  and the state filter and updater  2502 . It contains the same type of registers as the link emulator toward the vehicle  2701 . At startup, the link emulator  2703  waits to receive a display type from the state filter and updater. Once the link emulator  2703  has a valid display type in  2707  it responds to queries for the display type received from the radio. The link emulator  2703  sends repeated keep-alive messages to the radio. The link emulator  2703  stores the data from the display messages received from the radio in the display data register  2706 . If the display data register  2706  is changed, the link emulator  2703  sends the new state to the state filter and updater  2502 . 
     The state filter and updater  2502  receives the display type from the link emulator toward the vehicle  2701 . If the display type is valid, the state filter and updater  2502  sends it to the link emulator towards the radio  2703 . The state filter and updater  2502  receives the display data from the link emulator towards the radio  2703 . If the display data is valid, it sends the data to the link emulator towards the vehicle  2701 . 
     It must be understood that the rules  2102  described above are merely an example of possible types of rules  2102 , and rules  2102  can also be any kind of other rules  2102 , or any combination of them. There is a possibility to combine rules  2102  in a row, such that the output message of one rule  2102  will be sent as an input to the next rule  2102 . A rule  2102  can also change the message properties, content, or any other data related to the message, before sending it to its output interface  2002 . Any person skilled in the art and reading the current specification will immediately be able to advise different types and combination of rules  2102 , and all these rules  2102  are encompassed by the present invention. 
     Critical ECUs with External Communication 
     Some previously described embodiments related to protecting safety critical ECUs  100  which do not have any external communication interfaces. However, in embodiments where safety critical ECUs  100  have external communication interface(s) the security system  703  can also be used to protect ECUs  75  as described in this section. 
     In some embodiments, safety critical ECUs  100  have an external communication interface (e.g. some vehicle to vehicle (V2V) communication ECU are able to command the vehicle to brake). Such an ECU could send critical messages (i.e. having effect on the vehicle&#39;s behavior) and non-critical messages (e.g. traffic information). The non-critical messages can be filtered in the same manner as described in previous sections. 
     In some embodiments, some ECUs responsible for safety  100  (e.g. Electronic stability control (ESC)  110  or Mobileye) have the ability to supervise messages arriving from the driver (e.g. an ESC ECU monitors the brake pedal and prevents skidding because of braking). Such an ECU  110  can supervise specific critical messages, and prevent harm caused by these messages. These messages are denoted by ECM (external critical messages). 
     In some embodiments, the security system  703  can allow the relevant ECM (i.e. ECM supported by the critical external ECU) to pass towards the vehicle&#39;s inner bus  105  as long as these messages are supervised effectively by a safety ECU. In this manner critical and potentially lifesaving critical messages are supported securely by the vehicles electronic system  101 . Such a security system  703  can also be used in case there is no relevant safety ECU, but in such case it will be possible to attack the vehicle using the allowed critical messages. 
     In some embodiments where an ECU with an external communication has the ability to send critical messages to the communication bus  105 , the driver should have the ability to manually override or disable the messages from this ECU. 
     MODBUS and Other Control Protocols 
     MODBUS is a protocol extensively used in industrial control systems. Similarly to CAN bus  105 , it is a simple protocol used by controllers. Additionally, several other control protocols with similar characteristics exist such as FlexRay, VAN bus, LIN bus etc. The embodiments described above for CAN bus can be applicable for other communication protocols, such as MODBUS, mutatis mutandis. 
     In some embodiments, the main difference between the implementation of a filter and/or proxy  703  for CAN bus  105  and any other protocol is the physical layer and the specific filter logic. Different protocols have different message characteristics thus requiring different type of filtering (e.g. a MODBUS filter takes a special notice to the function code field). The proxy logic may be different but the proxy concept is the same: The link emulator  2501  has to handle communication with specific communication protocol (e.g. message handling and specific state machine  2602  for the protocol). The state filter and updater  2502  filters and updates the state as the CAN bus  105  proxy state filter and updater. 
     MODBUS is built as master-slave architecture, meaning there is one master and one or more slaves connected to the bus  105 . The master can send a request (e.g. read or write data command) to one or more slaves, and the relevant slaves should act according to the request and send their response to the master over the bus  105 . 
     In some embodiments, a proxy  703  protecting such communication bus  105  may save the sent request properties, and allow only the relevant response to pass towards the master (e.g. the request and response can be characterized by their function code). 
     In some embodiments, the said proxy  703  can also generate the received request and/or response by itself according to the messages it receives, and send the generated message instead of the original message. 
     In some embodiments, a proxy  703  may block requests originating from any components which should not function as the master on the bus. 
     Authentication Unit 
     In one embodiment of the present invention, the security system  703  also functions as an authentication unit. The authentication unit can be another module of the security system  703  of the invention. 
     The authentication unit of the invention is responsible for verifying that communication is performed with authentic counterparts inside or outside the vehicle&#39;s electronic system  101 . Authentication units can be integrated with ECU&#39;s  75  and in particular with ECU&#39;s  75  that don&#39;t have an external communication interface. For better security, an authentication unit can be coupled to every safety critical ECU  110 , every valuable ECU  75  and every ECU with an external communication interface  109 . 
     The authentication unit can employ one or more mechanisms for authentication of a communication source or destination. In some embodiments, authentication units can be the source or destination of messages. These mechanisms are, for example, authentication of a source or destination element (that sends or receives messages); conditional transmission of messages based on a successful authentication; and signature and/or signature verification of messages. 
     In some embodiments, the authentication unit can also encrypt and/or decrypt messages. This type of encryption can be used for secrecy, integrity, authenticity etc. 
     Authentication—any authentication unit (stand-alone or coupled to or integrated with an ECU  905 ) can perform an authentication procedure with any other authentication unit (stand-alone or coupled to or integrated with an ECU  905 ). In some embodiments, a stand-alone authentication unit can be coupled with a bus  105 . In some embodiments, a stand-alone authentication unit can be coupled with one or more ECU&#39;s  75 . In some embodiments, the authentication unit is integrated with one ECU  905  (i.e. the authentication unit is included inside the ECU  905  as part of the security system  703 ). In some embodiments, the authentication unit is a stand-alone security system  703  that is not coupled with any ECU  75  when proving its existence itself is meaningful, for example, for proving that a general sub-system was provided by a valid supplier. In some embodiments, referring to authenticating an ECU  75  means authenticating the authentication unit coupled with it. 
     In some embodiments, each authentication unit is configured with a list of all the authentication units in the system  101  with which authentication is required. Each authentication unit can periodically initiate an authentication process with any other authentication unit. The period after which the authentication must be renewed can be fixed or variable per authentication unit. The authentication process can involve a challenge message from one authentication unit to another. The receiving authentication unit then responds to the challenge with a response (typically encrypted). The authentication unit that has sent the challenge message verifies the response and if correct, marks that authentication unit in the list as authenticated. If the response is not correct, the challenge may be repeated one or more times, after which that authentication unit will be marked in the list as unidentified (not authenticated). 
     The challenge and response messages flow between authentication units as regular messages in the vehicle electronic system. These messages are received by a receiving unit. The classification unit classifies them as challenge/response messages and sends them to the message analyzer unit which handles them. 
     The message analyzer unit is capable of initiating challenge messages when it is necessary to authenticate an ECU  75  before delivering a message to it or considering a message from it. 
     In some embodiments, the authentication process can also be initiated by an authentication unit, when the authentication unit or the ECU  75  to which it is coupled, are programmed to periodically authenticate ECU&#39;s  75  on its authentication list. 
     In some embodiments, the authentication process can be one-way or two-way. In a one-way authentication process, each authentication unit sends a challenge to the other. That is, authentication unit A challenges authentication unit B, and authentication unit B challenges authentication unit A. In a two-way authentication process, the challenge message sent by authentication unit A to authentication unit B is sufficient for authenticating authentication unit A, and authentication unit B does not need to issue its own challenge message to authentication unit A. 
     In some embodiments, the authentication process can be multi-way, that is authentication unit A broadcasts a challenge and/or response message that reaches a plurality of authentication units over one or more communication buses  105 . 
     Conditional Message Transmission Based on Authentication—the message analyzer unit can be configured to transmit a message only if an authentication requirement is fulfilled. Examples of authentication requirements include but are not limited to: that the source authentication unit is authenticated; that the destination ECU  75  is authenticated; that any other ECU  75  (not source or destination) is authenticated; that any combination of ECU&#39;s  75  are authenticated etc. The authentication requirement can be against the source, destination or any other ECU  75 . 
     When a message that requires authentication arrives to the message classification unit, the message is classified as requiring authentication against ECU X  75 , and the message is sent to the message analyzer unit. The message analyzer unit verifies if ECU X  75  is authenticated. If ECU X  75  is authenticated, the message analyzer unit continues to process the message. If ECU X  75  is not authenticated, the message analyzer unit can decide either to discard the message or to issue a challenge message to ECU X  75  to see if it authenticates itself. 
     It should be emphasized that the authentication requirement does not have to involve necessarily the source or destination ECU  75 . For example, when ECU  1   75  sends a message to ECU  2   75 , it may be required that ECU  1   75  is authenticated with ECU  7   75  before the message can be transmitted to ECU  2   75 . 
     Signature and Verification—One of the actions that the message analyzer unit can perform relates to the signature of messages. When a message arrives with a signature, the analyzer unit can verify that signature is valid. The analyzer unit can also add a signature to a message before transferring it to the transmission unit. 
       FIG.  24    illustrates a basic one way communication filter/proxy security system  703 . A message received by the physical port input  1800  is inserted into the message receiving unit in message handler  1801 . The message receiving unit transmits the message through the proxy interface input  2000  to the message classification unit (classifier)  2100  in the one way filter/proxy element  1304 . The classifier  2100  classifies the message and sends the message and the message classification to the action selector  2801  in the message analyzer unit  2800 , which chooses the proper action according to the classification. The message analyzer unit  2800  performs an action  2102  (without loss of generality) on the message according the classification and sends a message (if needed) through the proxy interface output  2002  to the message transmission unit  1801 . The message transmission unit in the message handler  1801  transmits the message to the physical port output  1800 . 
       FIG.  25    illustrates one embodiment of the process of verifying/signing messages between communication filter/proxy (security systems) A and B  905 . A message is sent by an ECU A logic  900  to a communication filter/proxy A  905  (containing rules for an authentication unit  2900 , which illustrates the group of rules  2102  in charge of the authentication and signature processes). The ECU logic  900  is basically part of or all of the processing mechanisms of the ECU  905  except for the physical layer driver/transceiver  904  which is in charge of physically sending the communication signals to the outside world, which is a communication bus  105 . The message arrives to the classification unit  2100  which classifies the message as “signature required against communication filter/proxy B”  905 . The analyzer unit  2800  receives the message and proceeds to sign it against communication filter/proxy B  905 . The signature process involves modifying the original message by adding a signature to it (in a predetermined format). The analyzer unit  2800  can further process the message (in addition to the signature) in accordance with the classification instructions received and general rules  2102  of the analyzer  2800 . The analyzer unit  2800  will then send the message to the message transmission unit in the message handler  1801  which will send the message to its destination  904 , which is a physical layer driver (port) of A. 
     The message will then arrive to its destination port  904  in ECU B  905 , and will be transferred to the receiving unit in the message handler  1801  in communication filter/proxy B  703  and from there to the classification unit  2100 . The classification unit  2100  classifies the message as requiring signature verification against communication filter/proxy A  703 . When the analyzer unit  2800  receives the message, it verifies that the signature is authentic. If the signature is verified, the original message is extracted from the signed message and transferred to the relevant message transmission unit in the message handler  1801  which delivers the message to the logic  900  of B. 
     In some embodiments, if the signature verification has failed, the analyzer unit  2800  will ignore (discard) the message and no further action will be taken on the message. In some embodiments, the result of the signature verification will be logged. 
     There can be many implementations of adding and verifying a signature and they are all encompassed by the present invention. One such example can be: calculating a hash value on the content of the message data, and then encrypting the result using a shared key between the parties. The signature is done by adding the encrypted result to the message, and the verification is done by doing that process and comparing the result to the embedded sent signature. If both results are equal--the message&#39;s signature is valid. 
     In some embodiments, one or more communication filter/proxy security systems  703  of the invention can be implemented outside the vehicle computerized system  101 . 
     In some embodiments, for efficiency considerations, the signature addition and/or verification does not need to occur with all messages, but only with messages that were classified as such by the classification unit. 
     The classification unit  2100  takes into consideration the format requirements of each message, including maximum allowed length, so that when adding a signature the message is still valid and can be transmitted and read properly. The system should be consistent with the protocol even when modifying messages. That means that not all messages will be signed for example if the signature will make the message size exceed the protocol&#39;s limit. 
     If, for example, ECUs A and B  75  exchange messages of different types and size, even if only one type of messages malfunctions, the entire system may malfunction, a situation that must be avoided. Accordingly, the classification unit  2100  is configured so that signed messages meet all the format requirements of regular (unsigned) messages, and thus can be transmitted and read correctly.