Abstract:
A system is disclosed for secure communication between an interrogator and an RFID tag. The system includes means for singulating the tag in a population of RFID tags and means for extracting from the tag, identity data adapted to uniquely identify the tag. The system further includes means for securely communicating the identity data to a secure database, means for providing authentication data by the database and means for securely communicating the authenticating data to the interrogator. The system also includes means for providing a further communication between the tag and the interrogator, and wherein at least one stream of data between the tag and the interrogator includes random data generated via a random physical process. The tag and database may each include means for maintaining a count of secure authentications. The count may be separately maintained by the tag and database and may be incremented following each secure authentication. A method for secure communication between an interrogator and an RFID tag is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of International Application No. PCT/AU02/01671, filed Dec. 10, 2002, the disclosure of which is hereby incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates to an object management system wherein information bearing electronically coded radio frequency identification (RFID) tags are attached to objects which are to be identified, sorted, controlled and/or audited. In particular the present invention relates to a system for authenticating RFID tags including the information that is contained in the tags. 
   BACKGROUND OF THE INVENTION 
   The object management system of the present invention includes information passing between an interrogator which creates an electromagnetic interrogation field, and the electronically coded tags, which respond by issuing a reply signal that is detected by the interrogator, decoded and consequently supplied to other apparatus in the sorting, controlling or auditing process. The objects to which the tags are attached may be animate or inanimate. In some variants of the system the interrogation medium may be other than electromagnetic, such as optical and/or acoustic. 
   Typically each tag in a population of such tags may have an identity that is defined by a unique number or code that is assigned to each tag, in a global numbering scheme. The tags may also carry other fixed or variable data. Communications between the interrogator and tags is via a radio-frequency electromagnetic link that is inherently insecure and susceptible to eavesdropping, or the insertion of bogus signals. 
   Under normal operation the tags may be passive, i.e. they may have no internal energy source and may obtain energy for their reply from the interrogation field, or they may be active and may contain an internal energy source, for example a battery. Such tags respond only when they are within or have recently passed through the interrogation field. The interrogation field may include functions such as signalling to an active tag when to commence a reply or series of replies or selecting a single tag among a population of such tags, or in the case of passive tags may provide energy, a portion of which may be used in constructing the reply. 
   One example of an insecure electronic tag reading system is illustrated in  FIG. 1 . In  FIG. 1  an interrogator  11 , containing a transmitter and receiver, both operating under a controller, communicate via electromagnetic means with a code responding electronic tag  10 . This system has a disadvantage in that information passing between tag  10  and interrogator  11  is directly related to information stored within tag  10 . A further disadvantage is that the process of communication between tag  10  and interrogator  11  is susceptible to eavesdropping. Because such communication is normally carried out by electromagnetic waves, a clandestine receiver located nearby may make a record of the communication and deduce the data content of a legitimate tag. Knowledge of such data content may subsequently allow counterfeit tags to be manufactured by an unscrupulous party or parties. Such tags may appear legitimate because they can generate data content that is indistinguishable from genuine tags. Eavesdropping may take place either on interrogator to tag communication or tag to interrogator communication. Because of a substantial difference in signal levels involved, communication in the direction from interrogator to tag is much more vulnerable to eavesdropping than is communication in the reverse direction. 
   In some systems it is important to guard against eavesdropping in one, other or both directions or even to conceal the fact that an information extraction process is under way. Guarding against eavesdropping is particularly important when private information is being extracted from the tag. 
   Communication between the interrogator and tag is frequently via an exchange of messages in a half duplex mode, but in some systems single bits of data may alternately be sent between interrogator and tag. In this case it is common to regard the process of extraction of data from the tag as equivalent to exploration of a binary tree as illustrated in  FIG. 2 . In  FIG. 2  different bits of a tag identity or tag internal data correspond to different levels of the tree, and a single tag with particular data corresponds to a particular path through the tree. Different paths through the tree correspond to different tags with different data. 
   As discussed above, it is desirable to ensure that tags are authentic, and not substitute tags which produce easily predictable responses of normal unencoded identification tags. An ability to provide such assurances may be required in product authentication, baggage reconciliation, secure entry systems and the like. 
   In a number of situations it may also be important that the flow of information between the tag and the interrogator is not meaningful to an eavesdropper. This may include situations where economic or military advantage can be gained from such information becoming known, or when owners of goods with attached tags desire to keep their ownership private. Hence, it is desirable to guard against eavesdropping on the process of communication between an electronic tag and its interrogator. 
   One defence against eavesdropping employs encryption of data passing between interrogator and tag. However, installation of complex circuits with encryption engines in the tag poses excessive demands on tags designs, which should be maintained as simple as possible for reason of costs. Moreover, even if such encryption engines are used, available encryption algorithms may still allow determined analysts to determine the parameters of those algorithms from eavesdropping operations. 
   SUMMARY OF THE INVENTION 
   The present invention provides a system that may determine the identity or data of a tag in a manner that defeats efforts at eavesdropping on the electromagnetic communication link. The system of the present invention may determine that the tag is genuine and not counterfeit. The system of the present invention may provide a relatively high level of security that is comparable to systems that make use of non re-used truly random codes. The system of the present invention may produce these results with a relatively simple and low cost tag. The system may also be capable of disguising the fact that an information extraction process is in progress. 
   The system of the present invention may, with addition to a tag of a simple and relatively small size writeable memory and acceptance of a limitation that there may be a limited number of authentications between operations of recharging the tag in a secure environment, provide an authentication system that matches the security of a one-time code. The system may also be used to extract in a secure way variable data from RFID tags. As part of the system, the interrogator may interact not only with the tag but also through secure communications with a secure database containing for each tag, security information used in the authentication process (refer  FIG. 3 ). 
   Prior to a tag being put into service, one or more random codes may be generated for each tag by a truly random physical process. The random codes may be used to provide authentication test keys or numbers. The random codes may be loaded in a secure way into both the database and each tag. In the database, the random codes for each tag may be associated with an unencrypted tag serial number, or a separate but randomly chosen number that may be read from the tag by conventional tag interrogation processes. 
   In one embodiment communication between an interrogator and a single tag may be achieved through spatial separation between tags and their placement in close proximity to the interrogator. 
   The authentication system of the present invention may achieve extraordinary levels of security without a requirement to install within the tags complex circuits of encryption engines. The system may therefore be suitable for installation in relatively low cost RFID tags. The extraordinary levels of security are available because the system makes use of utterly random codes generated by a truly random physical process. The codes therefore will not be repeated more often than random numbers generated from truly random physical processes will be repeated. 
   According to the present invention there is provided a system for secure communication between an interrogator and an RFID tag, said system including: 
   means for singulating said tag in a population of RFID tags; 
   means for extracting from said tag, identity data adapted to uniquely identify said tag; 
   means for securely communicating said identity data to a secure database; 
   means for providing authentication data by said database; 
   means for securely communicating said authenticating data to said interrogator; and 
   means for providing a further communication between said tag and said interrogator, wherein at least one stream of data between said tag and said interrogator includes random data generated via a random physical process. 
   The tag and the database may each include means for maintaining a count of secure authentications. The count may be separately maintained by the tag and the database and may be incremented following each secure authentication. 
   According to a further aspect of the present invention there is provided a method for secure communication between an interrogator and an RFID tag, said method including: singulating said tag from a population of RFID tags; extracting from said tag, identity data adapted to uniquely identify said tag; 
   securely communicating said identity data to a secure database; 
   providing authentication data by said database; 
   securely communicating said authentication data to said interrogator; and 
   providing a further communication between said tag and said interrogator, wherein at least one stream of data between said tag and said interrogator includes random data generated via a random physical process. 
   The method may include the step of maintaining a count of secure authentications. The count may be separately maintained by the tag and the database and may be incremented following each secure authentication. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: 
       FIG. 1  shows a conventional electronic tag reading system; 
       FIG. 2  shows how interrogation of an electronic tag may be viewed as an exploration of a binary tree; 
       FIG. 3  shows an electronic tag reading system augmented by communication with a secure database; 
       FIG. 4  shows one form of architecture of a securely authenticable tag; 
       FIG. 5  shows a memory structure of a securely authenticable tag; 
       FIG. 6  shows a tag reply generator in a securely authenticable tag; and 
       FIG. 7  shows a secret scrambling string used for modulating a tag reply. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a tag reading system that is inherently insecure. It has the disadvantage that eavesdropping on the process of communication between electronic tag  10  and its interrogator  11 , which is normally carried out by electromagnetic waves, allows a clandestine receiver that may be located nearby to make a record of the communication, and deduce the data content of a legitimate tag, thus allowing apparently legitimate tags to be manufactured by unscrupulous parties. 
     FIG. 3  shows one embodiment of a tag reading system that has been made secure. In operation of the system shown in  FIG. 3 , interrogator  20  seeks the identity of tag  21  over an insecure radio frequency communications link represented by bold arrows  22 ,  23 . Tag  21  responds to interrogator  20  with its identity from tag identity register  40  (refer  FIG. 5 ) over the insecure radio frequency link. Interrogator  20  sends the identity of tag  21  to secure database  24  over preferably secure data link  25 . For some transmissions a non-secure data link may be used. The data stored in tag identity register  40  may include a fixed and/or variable data string and may include encrypted data and/or data stored in tag data register  41  (refer  FIG. 5 ). 
   Database  24  uses its data on tag identity, its history of authentications, and stored authentication test keys to select a test key to be sent to tag  21 . The selection may be sequential or non-sequential and may be based on records of the number of prior authentications which are maintained independently but in synchronism by database  24  and tag  21 . In some embodiments a genuine test key sent to the tag may be mixed with a non-authentic test key such as before or after the genuine test key is sent to the tag. 
   The selected authentication test key is sent from database  24  to interrogator  20  over the preferably secure data link  25 . Interrogator  20  then sends the test key to tag  21  over insecure radio link  22 . Tag  21  produces an authentication reply to interrogator  20  over insecure radio link  23 . 
     FIG. 4  shows details of tag architecture incorporated in tag  21 . Tag  21  includes common receiving/transmitting antenna  30  connected to receiver  31  via rectifier  32 . An output of receiver  31  is operably connected to authentication/reply circuit  33 . Authentication/reply circuit  33  includes memory  34  and reply generator  35 . Reply generator  35  is operably connected to modulator  36 . Modulator  36  is arranged such that it influences the impedance presented to antenna  30  via rectifier  32 . 
     FIG. 5  shows the memory structure associated with memory  34  of tag  21 . Memory  34  includes a tag identity register  40 , a tag data register  41 , an authentication test keys register  42 , an authentication reply codes register  43 , a singulation string register  44  and a scrambling string register  45 . The tag identify, tag data, singulation string and scrambling string registers  40 ,  41 ,  44  and  45  may each include one row of data containing  64  bits. The test keys register  42  and the reply codes register  43  may each include  16  rows of data each containing  64  bits. 
     FIG. 6  shows details of authentication/reply circuit  33  in tag  21 . Authentication/reply circuit  33  includes test unit  50  receiving data from receiver  31 . Test unit  50  is operably connected to data selector  51  for selecting data from authentication reply memory  52  or from random reply generator  53  according to whether an authentic or a not-authentic reply signal respectively, is to be sent to modulator  36  and subsequently to interrogator  20 . Test unit  50  receives from authentication test memory  54 , which includes test keys register  42 , a current test key determined by a count of authentications maintained in events counter  55 . 
   The response is generated by the following rules. If a test key received by the tag matches a test key stored in memory register  42  at a location (eg. row) determined by a count of authentications maintained by the tag, an authentication reply code is selected from a corresponding location in authentication reply codes register  43  included in authentication reply memory  52 . 
   If the test key received by tag  21  does not match the test key stored in memory register  42  at the location determined by the count of authentications maintained by the tag, the authentication response of the tag is produced by random reply generator  53 . 
   In the case of a genuine authentication, the count of tag authentications maintained by events counter  55  and a separate count of authentications maintained by database  24  are each incremented. For this purpose interrogation power to tag  21  may be maintained at an adequate level and for an adequate time to allow a non-volatile memory in the tag associated with events counter  55  that maintains a count of tag authentications to be re-written with its incremented value. In a preferred realisation of the system, this count may be updated before an authentication reply (authentic or not-authentic) is provided by tag  21 . Database  24  and tag  21  may signal between them the count or number of authentications. 
   The authentication reply is sent to secure database  24  which may check the reply of the tag against a selected row in the record of expected tag replies which is maintained in database  24 , the selection depending on the count of authentications maintained by database  24 , and may generate an authentic or a not-authentic signal. 
   The authentic or not-authentic signal is transmitted to interrogator  20  over secure data link  25 . Interrogator  20  signals identity of tag  21  and sends an authentic or not-authentic signal to user  26  or whatever agent uses the output of interrogator  20 . In some circumstances the authentic or not-authentic signal may be sent to an entity other than interrogator  20 . 
   In other circumstances it may be desirable to modify the contents of memories  52 ,  54  in tag  21  from a site that is remote from database  24 . This may be accomplished if communication between interrogator  20  and tag  21  can be made secure. One way to establish secure communication may be to provide a closed or electromagnetically shielded communication chamber around interrogator  20  from which electromagnetic waves that communicate to and from tag  21  do not radiate to the outside world, and to place tag  21  inside the closed chamber for the duration of recharging its memory contents. 
   In such a system interrogator  20 , with assistance of secure database  24 , may explore correctness of several entries in the authentication memory of tag  21  before signalling to tag  21  that its authentication memory may be written. 
   To support that exploration, events counter  55  is initialised to zero each time tag  21  receives power, and is incremented each time a successful authentication occurs during a period of continuous tag powering, until a predetermined final value is reached, whereupon a register that permits writing to the memory of tag  21  is enabled. The authentication memory of tag  21  and authentication count number may then be re-written by processes familiar to those skilled in the art of electronic tag design. 
   In another embodiment, communication with a single tag may be achieved by initially communicating with a population of tags, and then singulating a single tag by various techniques known in the industry as tag selection or singulation. In one of those techniques, transmission, without interruption, of a selection or singulation string, may take place. After the selection string is transmitted, it may be compared in the tags with an internal singulation string, and only a tag in which a match is obtained will take part in further communication. In another such technique, known as tree scanning, as illustrated in part in  FIG. 2 , the interrogator may transmit bit by bit a singulation string, and may receive responses from tags. The transmitted data may be matched against a singulation string in the tags, and tags which have a mismatch in their singulation string and that transmitted by the interrogator become progressively unselected, until only a single tag is selected. 
   In common embodiments, a unique tag identity may be used as the singulation string. Authentication test keys and/or tag data may additionally or alternatively be used in singulation. The interrogator may at the first authentication operation read the unencrypted tag identification number or singulation string, so it knows which tag is being processed. 
   When a high level of security is desired, singulation that uses interrogator transmissions related to tag identity might be undesirable. In such cases, the tag may contain a singulation string, not related to its identity, used in a tree scanning process. The singulation string may be originally programmed into the tag, or may be automatically generated within it. The tags may echo the singulation string to the interrogator, but such echoes are relatively weak and are less susceptible to eavesdropping than are interrogator transmissions, and are in any case not meaningful to an eavesdropper except that they may indicate that a singulation is in progress. 
   During singulation, the singulation string may in part be provided by the tag, and followed by the interrogator, that is, the tag leads the way down the tree scan, and the interrogator follows. Alternatively, the singulation string may be provided by the interrogator, that is, the interrogator points the way down the tree scan, and the tag follows as long as singulation bits match. In both cases, with a suitable design of tag that ignores certain interrogator signals, the interrogator may transmit incorrect singulation information so as to disguise the fact that genuine singulation is in progress, and thus which tag replies were the correct ones. Even though non re-use of singulation or response data gives great security, this procedure has an advantage of adding further confusion to an eavesdropping process. 
   For greater security, the tag may contain a number of singulation strings that are not re-used. The singulation string may serve as a key to a secure database containing the tag identity and the correct tag reply to an authentication inquiry. For greater security the tag may contain a number of different correct tag replies that are not re-used. When the tag is singulated by the appropriate singulation string, and provides one of the correct tag replies, and those elements are compared in the secure database, the tags may be regarded as authentic. 
   If there is not a match between singulation bits transmitted to the tag and the appropriate set of singulation bits occurring within in the tag, the tag response may be a random response of the same length as the authentication response. After consulting the secure database, and identifying which tag is being dealt with, the interrogator may send one or more data streams to the tag. One of the data streams should match the first of a series of tag authentication test keys stored in memory register  42 . 
   For an interrogation in which there is a match of transmitted data to tag authentication test key, the tag may respond with a return authentication code known only to the database. There may then be an incrementation in the tag and in the secure database of the content of non-volatile counters, which determine which of several authentication test keys is next in force. 
   For interrogations which do not so match, such as may occur when an non-authentic tag is interrogated, or a non-authentic interrogator performs the interrogation, the tag may respond with a random code of the same length and general structure as an authentic response. 
   In this way, eavesdropping on the transaction may not provide any clue as to the next correct authentication test key, or next tag authentication response. All an eavesdropper will detect is a sequence of apparently random transactions. 
   In a variation permitting tag identity or data to be disguised, the memory may contain, as shown in  FIG. 7 , in addition to its secret singulation string and secret authentication string a secret scrambling string. Using appropriate variations on the connections shown in  FIG. 7 , the secret scrambling string may be used to modulate (digitally, an XOR operation) the tag reply when tag identity or data is sought. In one embodiment, the authentication string may be used as the scrambling string, or as an input to a pseudo random string generation process, another input being the number of genuine tag authentications, the count being maintained separately within the tag and within the secure database. 
   The use of a scrambling string may ensure that no aspect of interrogator transmission or tag response is of significance to an eavesdropper. It has an advantage in that variable data present in the tag, but not yet present in the database, may be extracted to the database in a totally secure way. 
   Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.