Abstract:
A method which comprises generating a first signature by encoding an identifier with a first additional data set at a first time instant; responding to a first read request from a tag reader by releasing the first signature; generating a second signature by encoding the identifier with a second additional data set at a second time instant, the second additional data set being different from the first additional data set; and responding to a second read request by releasing the second signature. Also, a method which comprises obtaining a signature from a contactlessly readable tag; decrypting the signature with a key to obtain a candidate identifier and a scrambling code associated with the signature; and validating the candidate identifier based on at least one of the scrambling code and the signature.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation, and claims the benefit under 35 USC 120, of International Application No. PCT/CA2007/002343 filed on Dec. 20, 2007 and hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to contact-less tags and, more specifically, to a contact-less tag having a signature as well as to applications using the properties of such a tag. 
       BACKGROUND 
       [0003]    Contact-less tags, such as radio frequency identification (RFID) tags, are becoming increasingly commonplace in various commercial applications, two non-limiting examples of which include access control and inventory management. 
         [0004]    An RFID tag affixed to an item stores a code (e.g., a bit pattern) that is output in contact-less fashion to a reader, either in response to a request from the reader or autonomously by the tag. The reader captures the bit pattern and then an action may be taken, depending on the commercial application at hand. For example, in an access control scenario, the captured bit pattern may reveal that the person presumed to be carrying the tag (by virtue of an association with the bit pattern) is—or is not—authorized to enter a building or operate a vehicle. In an inventory management scenario, the bit pattern may give an indication of items contained on a pallet, for example, which may result in certain decisions being taken regarding shipping or storage of these items. 
         [0005]    In both cases, the ease with which an RFID tag may be read by a reader enables rapid processing but also may lead to problems. In the access control scenario, for example, an RFID tag of an individual authorized to access certain property may be interrogated and then the bit pattern cloned for use by an impostor to gain what is in fact unauthorized access to such property. Similarly, in the inventory management scenario, an acquired knowledge of the bit pattern associated with a certain item may allow a malicious party to gain intelligence about inventory locations that the item&#39;s rightful owner (which may include the manufacturer all the way down to the retail customer) may wish to keep secret. 
         [0006]    In both of the above scenarios, it is apparent that what is relevant to a malicious party is the knowledge that a certain bit pattern output by a certain RFID tag will either give access to property or indicate the presence of a specific inventory item. Whether the bit pattern is itself an encrypted version of some original data is actually of no relevance to the malicious party. Thus, schemes based on straightforward encryption of the bit pattern do not mitigate the problems mentioned above. 
         [0007]    Against this background, there is clearly a need in the industry for a contact-less tag having improved properties. 
       SUMMARY OF THE INVENTION 
       [0008]    A first broad aspect of the present invention seeks to provide a method, which comprises generating a first signature by encoding an identifier with a first additional data set at a first time instant; responding to a first read request from a tag reader by releasing the first signature; generating a second signature by encoding the identifier with a second additional data set at a second time instant, the second additional data set being different from the first additional data set; and responding to a second read request by releasing the second signature. 
         [0009]    A second broad aspect of the present invention seeks to provide an apparatus, which comprises means for generating a first signature by encoding an identifier with an additional data set at a first time instant; means for responding to a first read request from a tag reader by releasing the first signature; means for generating a second signature by encoding the identifier with a second additional data set at a second time instant, the second additional data set being different from the first additional data set; and means for responding to a second read request from a tag reader by releasing the second signature. 
         [0010]    A third broad aspect of the present invention seeks to provide a computer-readable medium, which comprises computer-readable program code which, when interpreted by a computing apparatus, causes the computing apparatus to execute a method. The computer-readable program code comprises first computer-readable program code for causing the computing apparatus to generate a first signature by encoding an identifier with an additional data set at a first time instant; second computer-readable program code for causing the computing apparatus to respond to a first read request from a tag reader by releasing the first signature; third computer-readable program code for causing the computing apparatus to generate a second signature by encoding the identifier with a second additional data set at a second time instant, the second additional data set being different from the first additional data set; and fourth computer-readable program code for causing the computing apparatus to respond to a second read request from a tag reader by releasing the second signature. 
         [0011]    A fourth broad aspect of the present invention seeks to provide a device for use in contact-less communication with a reader, which comprises a memory configured to store a first signature generated by encoding an identifier with a first additional data set at a first time instant; and a controller configured to generate a new signature by encoding the identifier with a second additional data set at a second time instant, the second additional data set being different from the first additional data set. The controller is further configured to cause the new signature to be stored in the memory after the second time instant. 
         [0012]    A fifth broad aspect of the present invention seeks to provide a device for use in contact-less communication with a reader, which comprises a memory configured to store a signature that encodes a pre-determined identifier; a transceiver configured to contactlessly receive read requests from the reader and to contactlessly transmit responses thereto; a controller configured to respond to read requests received via the transceiver by releasing via the transceiver a current version of the signature stored in the memory, wherein the version of the signature stored in the memory varies over at least two time instants while continuing to encode the pre-determined identifier; and a power source for powering at least the controller. 
         [0013]    These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the accompanying drawings: 
           [0015]      FIG. 1  is a block diagram of a system comprising a reader and a tag, in accordance with a non-limiting embodiment of the present invention. 
           [0016]      FIG. 2  is a block diagram showing details of the tag, in accordance with a non-limiting embodiment of the present invention. 
           [0017]      FIG. 3  illustrates a decoding function implemented by a controller in the tag, for generation of a signature at two points in time. 
           [0018]      FIGS. 4A and 4B  depict two possible functional architectures for generation of a signature. 
           [0019]      FIG. 5  illustrates application of an embodiment of the present invention in an inventory management context. 
           [0020]      FIG. 6A  shows application of a non-limiting embodiment of the present invention in a validation context. 
           [0021]      FIG. 6B  is a block diagram of a multi-reader architecture, in accordance with a non-limiting embodiment of the present invention. 
           [0022]      FIG. 7A  is a flowchart showing operation of a processing entity of  FIG. 6  when considering tags whose signatures encode a variable scrambling code and that are encrypted using a common key that is known to the reader or can be determined from an index supplied with the signature. 
           [0023]      FIG. 7B  is a flowchart similar to that of  FIG. 7A , but where the common key is unknown to the reader. 
           [0024]      FIG. 8  shows application of a non-limiting embodiment of the present invention in an identification context when considering tags whose signatures are encrypted using a variable key. 
           [0025]      FIG. 9  is a flowchart showing operation of a processing entity of  FIG. 8  when considering tags whose signatures are encrypted using a variable key. 
       
    
    
       [0026]    It is to be expressly understood that the description and drawings are only for the purpose of illustration of certain embodiments of the invention and are an aid for understanding. They are not intended to be a definition of the limits of the invention. 
       DETAILED DESCRIPTION 
       [0027]    With reference to  FIG. 1 , there is shown a system comprising a reader  12  and a tag  14 . Communication between the reader  12  and the tag  14  occurs over a contact-less medium  16 . In a specific non-limiting embodiment, the contact-less medium  16  is a wireless medium that may include a spectrum of radio frequencies. Depending on the application at hand, the tag  14  could be affixed to: an item for sale, goods during transportation, a person&#39;s clothing, an animal, a piece of equipment (including communications equipment such as wireless communications equipment) and so on. For its part, the reader  12  can be fixed or mobile. In the fixed scenario, the reader  12  could be located at any desired position within a building, vehicle, warehouse, campus, etc. In the mobile scenario, the reader  12  could be implemented in a handheld or portable unit, for example. 
         [0028]      FIG. 2  shows details of the tag  14 , in accordance with a specific non-limiting embodiment of the present invention. The tag  14  comprises a memory  202 , a transceiver  204  (including an antenna), a controller  206  and a power source  208 . 
         [0029]    The memory  202  stores a current signature  212 . In addition, the memory  202  may store a program for execution by the controller  206 , including computer-readable program code for causing the controller  206  to execute various steps and achieve wide-ranging functionality. In a non-limiting embodiment, the current signature  212  can take the form of a bit pattern having a certain number of bits. In accordance with an embodiment of the present invention, the bit pattern exhibited by the current signature  212  is dynamic, that is to say the current signature  212  changes over time. 
         [0030]    The controller  206  executes various functions that allow communication to take place via the transceiver  204  between the tag  14  and an external reader such as the reader  12 . In what follows, communications will hereinafter be referred to as occurring with the reader  12  although it will be appreciated that the tag  14  may communicate similarly with other external readers that it encounters. 
         [0031]    As part of its functionality, the controller  206  is operative to retrieve the current signature  212  from the memory  202  and to release the current signature  212  via the transceiver  204 . Alternatively, depending on the computational capabilities of the controller  206 , the controller  206  can be operative to compute the current signature  212  on demand and to release via the transceiver  204  the current signature  212  so computed. 
         [0032]    It is recalled that in this embodiment, the current signature  212  is dynamic. Accordingly, the controller  206  is operative to communicate with the memory  202  in order to change the bit pattern of the current signature  212  stored in the memory  202 . This can be achieved by executing diverse functionality that will be described in greater detail later on, and which may include implementing functional elements such as an encryption engine  222 , a counter  230 , a pseudo-random number generator  240 , a geo-location module  250  and a clock module  260 , among others. 
         [0033]    The configuration of the power source  208  and its inter-relationship with the controller  206  depend on whether the tag  14  is categorized as “passive”, “active” or somewhere in between. Specifically, the tag  14  may be designed as “passive”, whereby transmissions of the current signature  212  via the transceiver  204  are effected in response to detection of a burst of energy via the transceiver  204 , such burst of energy typically coming from the reader  12  issuing a “read request”. In this case, the controller  206  only needs to be powered during the short time period following the detection of the burst. In fact, the burst itself can charge the power source  208  for a brief period, enough to allow the controller  206  to cause transmission of the current signature  212  via the transceiver  204  in response to the read request. The current signature  212  may be extracted from the memory  202  or it may be generated on demand, upon receipt of the read request. 
         [0034]    Alternatively, in some embodiments of an “active” tag, transmissions of the current signature  212  via the transceiver  204  are similarly effected in response to detection of a read request via the transceiver  204 . In this case, the availability of the power source  208  allows the controller  206  to transmit the current signature  212  at a longer range than for passive devices. Certain active tags also have the capability to switch into a passive mode of operation upon depletion of the power source  208 . In other embodiments of an active tag, transmissions of the current signature  212  are effected via the transceiver  204  at instances or intervals that are controlled by the controller  206 . This can be referred to as autonomous (or unsolicited) issuance of the current signature  212 . To this end, the controller  206  needs to be continuously powered from the power source  208 . 
         [0035]    Active and passive tags may have other features that will be known to those of skill in the art. 
         [0036]    In still other cases, the power source  208  (either continually storing a charge or accumulating a sensed charge) can be connected to the controller  206  via a switch  210 , which is optional. The switch  210  can be toggled between a first state during which an electrical connection is established between the power source  208  and the controller  206 , and a second state during which this electrical connection is broken. The switch  210  is biased in the second state, and can be placed into the first state. Toggling into the first state can be achieved by a burst of energy that is sensed at a sensor (not shown) or by use of an activation element. In various non-limiting embodiments, the activation element may be a touch-sensitive pad on a surface of the tag  14 , or a mechanical component (e.g., a button). Placing the switch  210  into the first state may also trigger the controller  260  to change the current signature  212  in the memory  202 . 
         [0037]    With reference now to  FIG. 3 , there is shown conceptually how the current signature  212  stored in the memory  202  may change over time. Specifically, different versions of the current signature  212  (denoted S A  and S B ) are generated by an encoding function  302  implemented by the controller  206 . For notational convenience, the current signature  212  is used to denote which of the two signatures S A , S B  is currently stored in the memory  202 . The encoding function  302  generates the signatures S A  and S B  by encoding a common “identifier” (denoted I D ) with a respective “additional data set” (denoted D A  and D B ) at respective time instants (denoted T A  and T B ). Thus, at T A , the signature S A  is generated by encoding the identifier I D  with the additional data set D A , whereas at T B , the signature S B  is generated by encoding the identifier I D  with the additional data set D B . While in this example, two time instants are shown and described, this is solely for simplicity, and it should be understood that in actuality, the current signature  212  may change many times. 
         [0038]    The identifier I D  is constant, and in one embodiment conveys information about the item, animal, vehicle, piece of equipment, etc., to which the tag  14  is affixed. Examples of such information include, without limitation: a serial number, a universal product code (UPC), a vehicle registration number (VIN) and a customized identifier. In another embodiment, the identifier I D  conveys information about an expected user of the vehicle, clothing or mobile communication device, computer, restricted access area, network, etc., to which the tag  14  is affixed. Examples of such information include, without limitation: a name, an ID number, a driver&#39;s license number, an account number and login credentials. 
         [0039]    In accordance with a non-limiting embodiment of the present invention, the additional data sets D A  and D B  are different, which makes both signatures S A , S B  different. In fact, the two signatures S A , S B  will appear scrambled relative to one another due to use of the encryption engine  222  within the encoding function  302 . More specifically, the signatures S A  and S B  can be generated from the additional data sets D A  and D B  in a variety of ways, two of which will be described herein below. 
         [0040]    First Approach 
         [0041]    In a first approach, described with reference to  FIG. 4A , the identifier I D  is encrypted by the encryption engine  222  with a dynamic key—represented by the additional data sets D A , D B  themselves, resulting in the two signatures S A , S B . The two signatures S A , S B  will be different because the additional data sets D A , D B  are different. In fact, they will appear scrambled relative to one another when observed by someone who has not applied a decryption process using a counterpart to the keys used by the encryption engine  222 . 
         [0042]    It will be noted that in order to make the first approach practical, the reader  12  needs to have knowledge of which key (i.e., which of the additional data sets D A , D B ) was used for encryption of a received one of the signatures S A , S B , in order to effect proper decryption and recover the identifier I D . For this purpose, in order to assist the reader  12  in identifying the correct key to be used for decryption, and with reference again to  FIG. 2 , the current signature  212  may be accompanied by an index  214  also stored in the memory  202 . The index  214  may point the reader  12  to the correct key to be used. The reader  12  may have access to a key database (not shown) for this purpose. 
         [0043]    For example, consider the case where the keys (in this case, the additional data sets D A , D B ) correspond to outputs of the pseudo-random number generator  240  having a seed known a priori to the tag  14  and to the reader  12 . Here, at T A , the index  214  may indicate the sequential position in the output of the pseudo-random number generator  240  that corresponds to the additional data set D A , while at T B , the index  214  may indicate the sequential position in the output of the pseudo-random number generator  240  that corresponds to the additional data set D B . The reader  12  can then easily find the value occupying the correct sequential position in the output of an identical local pseudo-random number generator and effect successful decryption of the received signature (S A  or S B ). 
         [0044]    Alternatively, the keys (in this case, the additional data sets D A , D B ) are provided by the reader  12 . This can be done where the reader  12  (or an entity associated therewith) decides that a change in the current signature  212  is required. As a variant, the reader  12  may issue a trigger which, when received by the controller  206 , causes the controller  206  to effect a change in the current signature  212 . In such cases, changes to the key (and thus to the current signature  212 ) are effected by the controller  206  in response to triggers received from the reader  12 . 
         [0045]    Second Approach 
         [0046]    For other applications, the approach of  FIG. 4B  may be useful. Here, the identifier I D  is augmented with differing scrambling codes (denoted C A  and C B ), and then encrypted by the encryption engine  222  with a common key (denoted K), thus producing the two signatures S A , S B . The “additional data set” D A  used for encryption at T A  is therefore composed of the key K and the scrambling code C A , while the “additional data set” D B  used for encryption at T B  is composed of the same key K and the scrambling code C B . The encryption process can be designed so that small differences (in terms of the number of bits where there is a difference) between the scrambling codes C A  and C B  will cause large differences (in terms of the number of bits where there is a difference) in the resultant signatures S A  and S B . Thus, the scrambling codes C A , C B  have the effect of scrambling (i.e., randomizing) the resultant signatures S A , S B . 
         [0047]    The controller  206  is responsible for determining which scrambling code is to be used to generate a particular signature at a particular time instant. The current version of the scrambling code can be stored in the memory  202  and is denoted  220  for convenience. It will be appreciated based on the above description that the scrambling code C A  corresponds to the current scrambling code  220  at T A  and that the scrambling code C B  corresponds to the current scrambling code  220  at T B . 
         [0048]    Continuing with the second approach, several classes of embodiments are contemplated for changing the current scrambling code  220 . In a first class of embodiments relevant to the approach of  FIG. 4B , the current scrambling code  220  is changed in a way that can be predicted by the reader  12 , that is to say, where the reader  12  (or an entity associated therewith) has knowledge of how each successive scrambling code is generated. 
         [0049]    For example, the current scrambling code  220  can be changed each time (or, generally, each N th  time where N≧1) that the controller  206  receives a read request or releases the current signature  212  in response to a read request. This can ensure that the current signature  212  is different each N th  time that the controller  206  receives a read request. Alternatively, the current scrambling code  220  is changed every the current scrambling code  220  can be changed every set period of time (ex. every N seconds, minutes, hours, days, etc.). The variations in the current scrambling code  220  may governed in a variety of ways that are predictable to the reader  12 . For example, the controller  206  may implement a counter  230 , whose output is incremented (by a step size that can equal unity or can be negative, for example) after each N th  time that the controller  206  responds to a read request received from a nearby reader (or each N seconds, etc.). If the current scrambling code  220  is set to correspond to the current output of the counter  230 , then the scrambling codes C A , C B  used to generate the two signatures S A , S B  will differ by the step size. 
         [0050]    Alternatively, the controller  206  may implement the aforesaid pseudo-random number generator  240 , which produces an output that depends on one or more previous values of the output and on a seed. If the current scrambling code  220  is set to correspond to the current output of the pseudo-random number generator  240 , then the scrambling codes C A , C B  used to generate the two signatures S A , S B  will differ in accordance with the characteristics of the pseudo-random number generator  240 . 
         [0051]    Other variants will become apparent to those of skill in the art without departing from the scope of the present invention. 
         [0052]    In a second class of embodiments relevant to the approach of  FIG. 4B , the additional data sets D A , D B  are not only predicted by the reader  12  but are actually controlled by the reader  12 . This can be useful where the reader  12  (or an entity associated therewith) decides that a change in the current signature  212  is required. Alternatively, and recognizing that the key K is common to both of the additional data sets D A , D B , the reader  12  could supply the unique portions of the additional data sets D A , D B , namely the scrambling codes C A , C B . 
         [0053]    As a variant, the reader  12  may simply issue a trigger which, when received by the controller  206 , causes the controller  206  to effect a change in the current signature  212 . In such cases, changes to the current signature  212  are effected by the controller  206  in response to triggers received from the reader  12 . 
         [0054]    In a third class of embodiments relevant to the approach of  FIG. 4B , it may be desired to change the signatures S A , S B  in a stochastic way, that is to say, without the need to follow an underlying pattern that could be predicted by the reader  12 . 
         [0055]    For example, the controller  206  may implement the aforementioned geo-location module  250 , which is configured to output a current spatial position of the tag  14  or of an item or person to which it is affixed. If the current scrambling code  220  is set to correspond to the current output of the geo-location module  250 , then the scrambling codes C A , C B  used to generate the two signatures S A , S B  will differ in a stochastic fashion. 
         [0056]    Alternatively, the controller  206  may implement a clock module  260 , which is configured to determine a current time. If the current scrambling code  220  is set to correspond to a value measured by the clock module  260  (e.g., number of milliseconds elapsed since midnight of the day before), then the scrambling codes C A , C B  used to generate the two signatures S A , S B  will differ in a stochastic fashion. 
         [0057]    While the above embodiments have focused on temporal variations in the current signature  212  stored in the memory  202  of the tag  14 , it is also within the scope of the present invention for the current signature  212  stored in the memory  202  of two different tags to be different at a common time instant (e.g., at a time when the tags are being read in bulk). This can be referred to as spatial scrambling. More particularly, with reference to  FIG. 5 , a plurality of tags  514  are affixed to a number of units  506  of a particular article. The units  506  may be arranged on a pallet  508 , on a shelf or in a container, for example. To take a simple non-limiting example, the article in question can be a pair of denim jeans of a certain brand, size, style and color. Of course, the article could be any other item of which multiple units are available, such as a consumer product, food product, vehicle, etc. Other possibilities that may appear to one of skill in the art are within the scope of the present invention. 
         [0058]    The tags  514  store respective signatures  510  that are each derived by encrypting an identifier  550  (common to the tags  514 ) and a respective one of a plurality of current scrambling codes  520  (different for the various tags  514 ) with a common key. The common identifier  550  can be used to identify the article in question (in this case, a pair of jeans of a particular brand, size, style, color, etc.). To ensure that the signatures  510  appear scrambled while nevertheless encrypting the common identifier  550 , approaches such as the following may be taken. 
         [0059]    In one non-limiting approach, a centralized entity generates unique current scrambling codes  520  and unique signatures  510  for each of the tags  514 . The tags  514  are pre-loaded with their respective unique signatures  510  before being affixed to the units  506 . In this approach, the unique signatures  510  are fixed, as a result of which the tags  514  can be greatly simplified since they do not need to perform any processing functions. Practically speaking, this allows a distributor to purchase a plurality of tags  514  that have been pre-loaded with unique signatures  510  in order to securely identify the units  506  of a particular article. 
         [0060]    In another non-limiting approach, the tags  514  may each operate a respective clock module which, though structurally identical, may output different results, due to differences in oscillation characteristics (e.g., the oscillation crystals used, etc.) This will result in differences between the current scrambling code produced based on an output of the clock module of one of the tags  514  and the current scrambling code produced based on an output of the clock module of another one of the tags  514 , albeit at the same time instant. 
         [0061]    In yet another non-limiting approach, different current scrambling codes  520  can be produced as a result of the tags  514  each operating a respective pseudo-random number generator using a different seed, which could be pre-loaded by the above mentioned centralized entity. 
         [0062]    Still other ways of making the current scrambling codes  520  different among the various tags  514  are within the scope of the present invention. 
         [0063]    It is noted that the signatures  510  will tend to be widely varying even if the differences in the current scrambling codes  520  used to generate them are small, this effect being due to application of an encryption process, even when a common key is used. In fact, to an observer not equipped with the complementary key for decryption (which may be the same as the common key in a symmetric encryption scenario), the signatures  510  corresponding to the various units  506  on the pallet  508  will appear scrambled. This provides protection against external observers (e.g., thieves, corporate intelligence investigators) who may have gathered knowledge of signatures output by one or more units of the article in the past (e.g., from a previous purchase—or knowledge of a previous shipment—of the same brand, size, style and color of jeans) and are now on the lookout for the presence of units of the same article on the pallet  508 . On the other hand, by using the appropriate key in order to decrypt any of the signatures  510 , then no matter how diverse one such signature is from another, the common identifier  550  will be revealed alongside a stochastically derived scrambling code. 
         [0064]    In order to allow the reader  12  to identify the appropriate key for decryption, each of the signatures  510  may be accompanied by the aforesaid index  214  stored in the memory  202 . The index  214  may point the reader  12  to the correct key for decryption. For example, the index  214  could be a piece of public information such as a manufacturer identification code or a product category, such information being common to the units  506  but sufficiently generic to be of little value to an outside observer. This will allow the reader  12  (or an entity associated therewith) to select the correct key for decryption by accessing a table of keys (not shown) on the basis of the index. Such an approach can be useful to accelerate the decryption process and reduce the incidence of false positives (successful but inadvertent decryption of the wrong identifier) when multiple keys are potentially available to the reader  12 . 
         [0065]    It should also be appreciated that the signatures  510  on the various tags  514  can, in addition, be designed to change in a dynamic fashion (as described earlier), thus providing, in addition to spatial scrambling of the signatures  510 , temporal scrambling of the signatures  510  that leads to even greater security vis-à-vis external observation. 
         [0066]    In view of the foregoing, it should thus be appreciated that a common identifier, which is encoded within a plurality of signatures that vary over space (for multiple tags) and/or time (for the same tag), can be extracted by the reader  12  (or an entity associated therewith) by utilizing the appropriate key for decryption. This allows the reader  12  (or an entity associated therewith) to perform
       (I) validation of the identifier based on the signature and/or the scrambling code; and/or   (II) an action related to identification, based on the identifier.       
 
         [0069]    Both of these scenarios, which are not mutually exclusive, are now described in some detail. 
         [0070]    In scenario (I), a dynamic scrambling code is used in the generation of a signature that continually encodes the same identifier, and it is of interest to recover the current scrambling code to detect a potential instance of tag cloning. Accordingly, with reference to  FIG. 6A , there is shown a system that is similar to the system of  FIG. 1 . In addition, the system of  FIG. 6A  comprises a processing entity  610  that implements a validation operation, as will be described herein below. In various embodiments, the processing entity  610  referred to above may be connected to the reader  12 , or it may be a remote entity. Such a remote entity may be reachable over a network, or it may be integrated with the reader  12 . The system of  FIG. 6A  also includes a storage entity, such as a database  602 , that is accessible to the processing entity  610  and stores a plurality of records  604 , each associated with a respective identifier. For the purposes of the present example, one can consider that each identifier for which there exists a record in the database  602  is indicative of a privilege to access certain property or make certain transactions, although other scenarios are possible without departing from the scope of the present invention. 
         [0071]    In accordance with one embodiment of the present invention, each of the records  604  also comprises a field  606  indicative of zero or more scrambling codes  608  that were encoded in signatures which were previously received and which encoded the respective identifier for that record. Thus, receipt of a particular signature that encodes the identifier in a given one of the records  604  as well as one of the scrambling code(s)  608  stored in the corresponding field  606  will indicate that the particular signature has been previously received and therefore its instant receipt may be indicative that a cloning attempt has been made. 
         [0072]    More specifically, with reference to the flowchart in  FIG. 7A , consider what happens following step  710  when a signature S X  is received at a particular time instant by the reader  12 . At the time of receipt, whether the signature S X  encodes any particular identifier or scrambling code is unknown to the reader  12 . At step  730 , an attempt to decrypt the signature S X  is made by the processing entity  610  using a decryption key K X . The decryption key K X  may be known in advance to the processing entity  610 . Alternatively, as shown in step  720 , the signature S X  may be accompanied by an index that allows the processing entity  610  to determine the appropriate decryption key K X . The result of the decryption attempt at step  730  is a candidate identifier I X  and a candidate scrambling code, denoted C X . 
         [0073]    At step  740 , the processing entity  610  consults the database  602  based on the candidate identifier I X  in an attempt to identify a corresponding record and extract therefrom a list of scrambling code(s) that have been received in the past in association with the candidate identifier I X . For the purposes of the present example, it is useful to assume that such a record exists (i.e., the “YES” branch is taken out of step  740 ), but if there is no such record, this may indicate that there is a high-level failure requiring further action. At step  750 , the processing entity  610  compares the candidate scrambling code C X  to the scrambling code(s)  608  in the field  606  of the record identified at step  740  and corresponding to identifier I X . 
         [0074]    If there is a match, this indicates that the scrambling code C X  has been used in the past in association with the identifier I X . Under certain conditions, this may lead the processing entity  610  to conclude that the validation operation was unsuccessful. 
         [0075]    For example, if the signature S X  was expected to change at least as often as every time that the tag on which it is stored was read, then the fact that the scrambling code C X  matches one of the scrambling code(s)  608  stored in the field  606  of the record corresponding to identifier I X  may lead the processing entity  610  to conclude that the validation operation was unsuccessful. Alternatively, if the signature S X  was expected to change every N th  time that the tag on which it is stored was read, then the processing entity  610  may look at how many of the scrambling code(s)  608  stored in the field  606  of the record corresponding to identifier I X  correspond to the scrambling code C X , and if this number is greater than or equal to N, this may lead the processing entity  610  to conclude that the validation operation was unsuccessful. Alternatively still, if the signature S X  was expected to change at least as often as every N seconds etc., then the processing entity  610  may look at how long ago it has been since a matching one of the scrambling code(s)  608  was first stored in the field  606  of the record corresponding to identifier I X , and if this time interval is greater than or equal to a pre-determined number of seconds, minutes, hours, days, etc., this may lead the processing entity  610  to conclude that the validation operation was unsuccessful. Where a conclusion is reached that the validation operation was unsuccessful, the privilege to access the property or make transactions may be revoked or at least questioned on the basis of suspected tag cloning. 
         [0076]    On the other hand, if there is no match between the scrambling code C X  and any of the scrambling code(s)  608  stored in the field  606  of the record corresponding to identifier I X , this may lead the processing entity  610  to conclude that the validation operation was potentially successful. In such a case, the default privilege to access the property or make transactions may be granted (or at least not revoked on the basis of suspected tag cloning). 
         [0077]    In accordance with an alternative embodiment of the present invention, the field  606  in the record associated with each particular identifier may be indicative of an “expected” scrambling code, i.e., the scrambling code that should (under valid circumstances) be encoded in a signature received from a tag that encodes the particular identifier. Alternatively, the field  606  in the record associated with each particular identifier may be indicative of an “expected” signature, i.e., the signature that should (under valid circumstances) be received from a tag that encodes the particular identifier. Thus, upon receipt of the signature S X , if it is found to correspond to the expected signature (or if the scrambling code C X  is found to correspond to the expected scrambling code), this may lead the processing entity  610  to conclude that the validation operation was potentially successful. On the other hand, if there is no match between the signature S X  and the expected signature stored in the database  602  (or between the scrambling code C X  and the expected scrambling code), this may lead the processing entity  610  to conclude that the validation operation was unsuccessful. 
         [0078]    It should be appreciated that in the above alternative embodiments, the processing entity  610  may obtain knowledge of the expected scrambling code or the expected signature by implementing plural pseudo-random number generators for each of the identifiers, analogous to the pseudo-random number generator  240  implemented by the controller  206  in a given tag  14 , which produces an output that depends on one or more previous values of the output and on a seed. Thus, the next output of the pseudo-random number generator implemented by the processing entity  610  for a given identifier allows the processing entity  610  to predict the scrambling code (or the signature) that should be received from a tag legitimately encoding the given identifier. In another embodiment, the processing entity  610  may know what is the expected scrambling code/signature because it has instructed the reader  12  to cause this expected scrambling code/signature to be stored in the memory of the tag. 
         [0079]    In accordance with an alternative embodiment of the present invention, the database  602  simply comprises a running list of all signatures that have been received in the past. Thus, upon receipt of the signature S X , if it is found to correspond to one of the signatures on the list, this may lead the processing entity  610  to conclude that the validation operation was unsuccessful. On the other hand, if there is no match between the signature S X  and any of the signatures stored in the database  602 , this may lead the processing entity  610  to conclude that the validation operation was potentially successful (or at least not unsuccessful). 
         [0080]    It should also be appreciated that having obtained the identifier I X , the processing entity  610  may also perform an action related to identification of an item associated with the particular tag that encoded the identifier I X . 
         [0081]    In a first example of an action related to identification, the processing entity  610  may simply note the fact that the item (bearing the identifier I X ) was encountered in a vicinity of the reader  12 . This information may be stored in a database (not shown) or sent as a message, for example. In an inventory management scenario, the processing entity  610  may consult an inventory list and “check off” the item as having been located, or may signal that the presence of a spurious item (that is not on the inventory list) has been detected. 
         [0082]    In another example of an action related to identification, the processing entity  610  may consult another database (not shown) in order to ascertain whether the identifier is on a list of identifiers associated with individuals/objects permitted to access, or prohibited from accessing, certain property. Examples of property include, without limitation: computing equipment, a computer network, a building, a portion of a building, an entrance, an exit and a vehicle. 
         [0083]    In another example of an action related to identification, the processing entity  610  may consult another database (not shown) in order to ascertain whether the identifier is on a list of identifiers associated with individuals permitted to effect, or prohibited from effecting, a transaction, which could be a financial transaction or a login to controlled online content, for example. 
         [0084]      FIG. 7B  shows a variant where multiple keys are possible but no index (or one that does not permit identification of the appropriate decryption key) is provided along with the signature S X . Specifically, taking the “NO” branch after step  750  does not conclude the validation operation. Rather, the validation operation goes through step  770  where a next key is selected and then the validation operation returns to step  730 , whereby steps  730  through  770  are re-executed until the earlier occurrence of (i) taking the “YES” branch at step  750  and (ii) exhaustion of all keys, which can result in the equivalent of taking the “NO” branch out of  740  (i.e., this may indicate that there is a high-level failure requiring further action). 
         [0085]    It should be appreciated that in the above embodiments, encryption and decryption can be effected using various techniques known in the art, including encryption using a symmetric key, an asymmetric key pair, a public/private key pair, etc., as well as in accordance with a variety of algorithms and protocols For example, RSA and ECC are suitable examples of asymmetric encryption algorithms, while AES, DES, and Blowfish are suitable examples of symmetric algorithms. Still other possibilities exist and are within the scope of the present invention. 
         [0086]    In the above example with reference to  FIGS. 6A ,  7 A and  7 B, although a single reader was described and illustrated, it should be appreciated that it is within the scope of the present invention to provide a multi-reader architecture, as shown in  FIG. 6B . A plurality of readers  1012  are connected to each other and to a centralized control entity  1010  by a network  1030 , which can be a public packet-switched network, a VLAN, a set of point-to-point links, etc. In such a case, the centralized control entity  1010  (e.g., a network controller) can implement the functionality of the processing entities  610 , including encryption and validation. To this end, the centralized control entity  1010  maintains a master database  1020 , which includes the equivalent of a consolidated version of various instances of the database  602  previously described as being associated with the reader  12  in the single-reader scenario. 
         [0087]    Thus, decryption and validation can be performed entirely in the centralized control entity  1010 . Alternatively, certain functionality (such as decryption) can be performed by the readers  1012  while other functionality (such as validation) can be performed by the centralized control entity  1010 . Still alternatively, the processing entities  610  can inter-operate amongst themselves in the absence of the centralized entity  1010 , thereby to implement decryption on a local basis, and the validation operation in a joint fashion. In such a distributed scenario, the master database  1020  can still be used, or the processing entities  610  can communicate with one another to share information in their respective databases  602 . 
         [0088]    In scenario (II), a dynamic key is used in the generation of a signature that encodes a constant identifier, and it is of interest to recover the underlying identifier despite the time-varying key. Accordingly, with reference now to  FIG. 8 , there is shown a system that is similar to the system of  FIG. 1 . In addition, the system of  FIG. 8  comprises a processing entity  810  that implements an identification operation, as will be described herein below. The processing entity  810  may be connected to the reader  12 , or it may be a remote entity. Such a remote entity may be reachable over a network, or it may be integrated with the reader  12 . It should be understood that the system in  FIG. 8  is being shown separately from the system in  FIG. 6 ; however, it is within the scope of the present invention to combine the functionality of both systems. 
         [0089]    With reference to the flowchart in  FIG. 9 , consider what happens following step  910  when a signature S Y  is received from a particular tag at a particular time instant by the reader  12 . The signature S Y  is assumed to have been generated by encrypting an identifier I Y  using an encryption key that varies in a dynamic fashion. To this end, the particular tag may have generated the dynamic encryption key based on, for example:
       the output of the aforementioned clock module  260  (e.g., in terms of seconds, minutes or hours of elapsed time since an event known also to the processing entity  810 );   the output of the aforementioned geo-location module  250 ;   an index;   a seed for use by a pseudo-random number generator.       
 
         [0094]    Still other possibilities are within the scope of the present invention. The decryption key can then be determined based on the above quantity. For example, the decryption key could be the above-mentioned output of the clock module or the geo-location module. Alternatively, the encryption key could be the output of a table or a pseudo-random number generator (both known to the processing entity  810 ) based on the above-mentioned seed, or at a position that corresponds to the above-mentioned index. In the latter case, the index or seed can be supplied along with the signature S Y . 
         [0095]    In accordance with the present embodiment, once the signature S Y  is read by the reader  12 , the processing entity  810  is expected to determine the appropriate decryption key, denoted K Y . Accordingly, at step  930 , the processing entity  810  first determines a dynamic parameter that will allow the decryption key K Y  to be determined. Examples of the dynamic parameter include:
       the output of a clock module (which attempts to emulate the aforementioned clock module  260 ) at the time of receipt of the signature S Y  (e.g., in terms of seconds, minutes or hours of elapsed time since a known event);   the output of a geo-location module (which can be similar to the aforementioned geo-location module  250 );   the index or seed provided along with the signature S Y .       
 
         [0099]    Next, at step  940 , the processing entity  810  obtains the decryption key K Y  based on the dynamic parameter determined at step  930 . For example, where the dynamic parameter corresponds to the output of a clock module or a geo-location module, the decryption key K Y  could be the dynamic parameter itself. Alternatively, where the dynamic parameter is an index or a seed, the decryption key K Y  could be the output of the aforementioned table or pseudo-random number generator known to the processing entity  810 , at a position that corresponds to the received index, or using the received seed. 
         [0100]    Once the decryption key has been obtained, the signature S Y  is decrypted at step  950  using the decryption key. This leads to extraction of the identifier I Y . It is noted that a scrambling code was not required in this embodiment, although its use is not disallowed. Having obtained the identifier I Y , the processing entity  810  proceeds to step  960 , where it performs an action related to identification of an item associated with the particular tag that encoded the identifier I Y . 
         [0101]    In a first example of an action related to identification, the processing entity  810  may simply note the fact that the item (bearing the identifier I y ) was encountered in a vicinity of the reader  12 . This information may be stored in a database (not shown) or sent as a message, for example. In an inventory management scenario, the processing entity  810  may consult an inventory list and “check off” the item as having been located, or may signal that the presence of a spurious item (that is not on the inventory list) has been detected. 
         [0102]    In another example of an action related to identification, the processing entity  810  may consult another database (not shown) in order to ascertain whether the identifier is on a list of identifiers associated with individuals/objects permitted to access, or prohibited from accessing, certain property. Examples of property include, without limitation: computing equipment, a computer network, a building, a building, a portion of a building, an entrance, an exit and a vehicle. 
         [0103]    In yet another example of an action related to identification, the processing entity  810  may consult another database (not shown) in order to ascertain whether the identifier is on a list of identifiers associated with individuals permitted to effect, or prohibited from effecting, a transaction, which could be a financial transaction or a login to controlled online content, for example. 
         [0104]    It should be appreciated that the processing entity  810  may also perform an action related to validation of the identifier I Y  in conjunction with the above action related to identification. Specifically, in accordance with one embodiment of the present invention, the processing entity may consult a variant of the aforementioned database  602 , where each of the records  604  now includes a field indicative of zero or more signatures which were previously received and which encoded the respective identifier for that record. Thus, receipt of a particular signature that encodes the identifier in a given one of the records  604  as well as one of the signature(s) stored in the corresponding field will indicate that the particular signature has been previously received and therefore its instant receipt may be indicative that a cloning attempt has been made. 
         [0105]    In the above example with reference to  FIGS. 8 and 9 , although a single reader was described and illustrated, it should be appreciated that it is within the scope of the present invention to provide a multi-reader architecture, as in  FIG. 6B . 
         [0106]    Also, those skilled in the art will appreciate that in some embodiments, the functionality of any or all of the processing entity  610 , the processing entity  810 , the reader  12  and the readers  1012  may be implemented using pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), electrically erasable programmable read-only memories (EEPROMs), etc.), or other related components. In other embodiments, the functionality of the entity in question may be achieved using a computing apparatus that has access to a code memory (not shown) which stores computer-readable program code for operation of the computing apparatus, in which case the computer-readable program code could be stored on a medium which is fixed, tangible and readable directly by the entity in question (e.g., removable diskette, CD-ROM, ROM, fixed disk, USB drive), or the computer-readable program code could be stored remotely but transmittable to the entity in question via a modem or other interface device (e.g., a communications adapter) connected to a network (including, without limitation, the Internet) over a transmission medium, which may be either a non-wireless medium (e.g., optical or analog communications lines) or a wireless medium (e.g., microwave, infrared or other transmission schemes) or a combination thereof. 
         [0107]    While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.