Abstract:
A method of securing communications in an RFID system including a reader and an RF tag having a memory configured to store information comprises sending, from the reader, a message to the tag; the tag, in response to the message, generating a challenge value and sending the challenge value to the reader; in any order: the reader performing a mathematical operation on the challenge value based upon a key value to generate a challenge reply and sending the challenge reply to the tag and the tag independently computing a challenge response based on the key value and mathematical operation; the tag comparing the challenge response computed by the tag with the challenge reply sent by the reader; and the tag authenticating the reader if the challenge response matches the challenge reply. An RFID system including an RFID tag and a reader are configured to perform the above steps.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates to a remote communication system. More particularly, the invention relates to a radio frequency (RF) identification system and methods for secure communications between a RFID reader and RFID tags.  
         BACKGROUND OF THE INVENTION  
         [0002]    Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology, which is employed in many industries. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles.  
           [0003]    RF identification (RFID) tag systems have been developed to identify, monitor or control remote objects. As shown in FIG. 1, a basic RFID system  10  includes an interrogator  18  and transponders (commonly called RF tags)  16 . The interrogator  18  includes a transceiver with decoder  14  and an antenna  12 . The tag  16  includes an antenna  24 . In operation, the antenna  12  emits and receives electromagnetic radio signals generated by the transceiver  14  to activate the tag  16  and receive signals from the tag. When the tag  16  is activated, data can be read from or written to the tag.  
           [0004]    In some applications, the transceiver and antenna  12  are components of an interrogator (or reader)  18 , which can be configured either as a hand-held or a fixed-mount device. The interrogator  18  emits the radio signals  20  in range from one inch to one hundred feet or more, depending upon its power output and the radio frequency used. When an RF tag  16  passes through the electromagnetic radio waves  20 , the tag detects the signal  20  and is activated. Data encoded in the tag  16  is then transmitted by a modulated data signal  22  through an antenna  24  to the interrogator  18  for subsequent processing.  
           [0005]    An advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. Tags can be read through a variety of substances such as snow, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RF tags can also be read at remarkable speeds, in most cases responding in less than one hundred milliseconds.  
           [0006]    There are three main categories of RFID tag systems. These are systems that employ beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in fundamentally different ways. The invention described below in the Detailed Description can be embodied in any of these types of systems.  
           [0007]    The beam-powered RFID tag is often referred to as a passive device because it derives the energy needed for its operation from the radio frequency energy beamed at it. The tag rectifies the field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to change its reflectivity that is seen at the interrogator to develop a communication link. Here, the battery is the only source of the tag&#39;s operational power. Finally, in the active RFID tag, both the tag and reader have transceivers to communicate and are powered by a battery.  
           [0008]    A typical RF tag system  10  will contain at least one tag  16  and one interrogator  18 . The range of communication for such tags varies according to the transmission power of the interrogator  18  and the tag  16 . Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach in excess of 100 meters in range, depending on the frequency and environmental characteristics.  
           [0009]    Conventional RF tag systems utilize continuous wave backscatter to communicate data from the tag  16  to the interrogator  18 . More specifically, the interrogator  18  transmits a continuous-wave radio signal to the tag  16 , which modulates the signal  20  using modulated backscattering wherein the electrical characteristics of the antenna  24  are altered by a modulating signal from the tag that reflects a modulated signal  22  back to the interrogator  18 . The modulated signal  22  is encoded with information from the tag  16 . The interrogator  18  then demodulates the modulated signal  22  and decodes the information.  
           [0010]    Conventional continuous wave backscatter RF tag systems utilizing passive (no battery) RF tags require adequate power from the signal  20  to power the internal circuitry in the tag  16  used to modulate the signal back to the interrogator  18 . While this is successful for tags that are located in close proximity to an interrogator, for example less than three meters, this may be insufficient range for some applications, for example greater than 100 meters.  
           [0011]    A problem in RFID systems is in authentication and secure communications between the reader and RF tags in the field of view of the reader.  
         SUMMARY OF THE INVENTION  
         [0012]    One aspect of the invention provides a method and apparatus for authenticating reader or tags over a public, open channel. Another aspect of the invention provides a method and apparatus that is more robust against spoofing and other illicit access attempts than password-based methods.  
           [0013]    The invention provides a method of securing communications in an RFID system including a reader and an RF tag having a memory configured to store information, the method comprising: sending, from the reader, a message to the tag; the tag, in response to the message, generating a challenge value and sending the challenge value to the reader; in any order: the reader performing a mathematical operation on the challenge value based upon a key value to generate a challenge reply and sending the challenge reply to the tag and the tag independently computing a challenge response based on the key value and mathematical operation; the tag comparing the challenge response computed by the tag with the challenge reply send by the reader; and the tag authenticating the reader if the challenge response matches the challenge reply.  
           [0014]    Another aspect of the invention provides a method of securing communications in an RFID system including a reader and an RF tag having a memory configured to store information, the method comprising: sending, from the tag, a message to the reader; the reader, in response to the message, generating a challenge value and sending the challenge value to the tag; in any order: the tag performing a mathematical operation on the challenge value based upon a key value to generate a challenge reply and sending the challenge reply to the reader and the reader independently computing a challenge response based on the key value and mathematical operation; the reader comparing the challenge response computed by the reader with the challenge reply sent by the tag; and the reader authenticating the tag if the challenge response matches the challenge reply.  
           [0015]    An RFID communications system comprising: an RF tag having a memory configured to store information; and a reader configured to send a message to the tag; the tag being configured to, in response to the message, generate a challenge value and send the challenge value to the reader; the reader being configured to perform a mathematical operation on the challenge value based upon a key value to generate a challenge reply and send the challenge reply to the tag; the tag being further configured to independently compute a challenge response based on the key value and mathematical operation, to compare the challenge response computed by the tag with the challenge reply sent by the reader, and to authenticate the reader if the challenge response matches the challenge reply.  
           [0016]    An RFID communications system comprising: an RF tag having a memory configured to store information; and a reader configured for RF communication with the tag, the tag being configured to send a message to the reader; the reader being configured to, in response to the message, generate a challenge value and send the challenge value to the tag; the tag being configured to perform a mathematical operation on the challenge value based upon a key value to generate a challenge reply and send the challenge reply to the reader; the reader being further configured to independently compute a challenge response based on the key value and mathematical operation, to compare the challenge response computed by the reader with the challenge reply sent by the tag, and to authenticate the tag if the challenge response matches the challenge reply.  
           [0017]    A method of securing communications in an RFID system including a reader and an RF tag having a memory configured to store information, the method comprising the following steps, in order: sending, from the tag, a message to the reader; the reader, in response to the message, generating a challenge value and sending the challenge value to the tag; the reader independently computing a challenge response based on the key value and mathematical operation; the tag performing a mathematical operation on the challenge value based upon a key value to generate a challenge reply and sending the challenge reply to the reader; the reader directly comparing the challenge response computed by the reader with the challenge reply sent by the tag, without a need to first perform a mathematical operation on the challenge reply; and the reader authenticating the tag if the challenge response matches the challenge reply.  
           [0018]    An RFID communications system comprising: an RF tag having a memory configured to store information; and a reader configured for RF communication with the tag, the tag being configured to send a message to the reader, the reader being further configured to, in response to the message: generate a challenge value and send the challenge value to the tag, the tag being further configured to perform a mathematical operation on the challenge value based upon a key value to generate a challenge reply and send the challenge reply to the reader; independently compute a challenge response based on the key value and mathematical operation prior to receiving the challenge reply from the tag; directly compare the challenge response computed by the reader with the challenge reply sent by the tag without a need to first perform a mathematical operation on the challenge reply sent by the tag; and authenticate the tag if the challenge response matches the challenge reply. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Preferred embodiments of the invention are described below with reference to the following accompanying drawings.  
         [0020]    [0020]FIG. 1 is a block diagram of a conventional RFID communication system, including a tag and reader in which the invention could be incorporated.  
         [0021]    [0021]FIG. 2 is a block diagram of an RFID communication system, including a tag and reader, embodying various aspects of the invention.  
         [0022]    [0022]FIG. 3 is a flowchart illustrating steps performed by the tag and reader of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Attention is directed to the following commonly assigned applications, which are incorporated herein by reference: U.S. Patent Application Attorney Docket 13094-B (BA4-094) entitled “Radio Frequency Identification Device Communications Systems, Wireless Communication Devices, Wireless Communication Systems, Backscatter Communication Methods, A Radio Frequency Identification Device and A Radio Frequency Identification Device Communication Method” by inventors Mike A. Hughes and Richard M. Pratt; U.S. Patent Application Attorney Docket 12961-B (BA4-095), entitled “Method of Simultaneously Reading Multiple Radio Frequency Tags, RF Tag, and RF Reader”, by inventors Emre Ertin, Richard M. Pratt, Mike A. Hughes, Kevin L. Priddy, and Wayne M. Lechelt; U.S. Patent Application Attorney Docket 13095-B (BA4-096), entitled “RFID System and Method Including Tag ID Compression”, by inventors Mike A. Hughes and Richard M. Pratt; U.S. Patent Application Attorney Docket 13096-B (BA4-097), entitled “System and Method to Identify Multiple RFID Tags”, by inventors Mike A. Hughes and Richard M. Pratt; U.S. Patent Application Attorney Docket 13154-B (BA4-098), entitled “Wireless Communication Devices, Radio Frequency Identification Devices, Backscatter Communication Device Wake-Up Methods, Communication Device Wake-Up Methods and A Radio Frequency Identification Device Wake-Up Method”, by inventors Richard Pratt and Mike Hughes; U.S. Patent Application Attorney Docket 13218-B (BA4-099), entitled “Wireless Communication Systems, Radio Frequency Identification Devices, Methods of Enhancing a Communications Range of a Radio Frequency Identification Device, and Wireless Communication Methods”, by inventors Richard Pratt and Steven B. Thompson; U.S. Patent Application Attorney Docket 13219-B (BA4-100), entitled “Wireless Communications Devices, Methods of Processing a Wireless Communication Signal, Wireless Communication Synchronization Methods and a Radio Frequency Identification Device Communication Method”, by inventors Richard M. Pratt and Steven B. Thompson U.S. Patent Application Attorney Docket 13252-B (BA4-101), entitled “Wireless Communications Systems, Radio Frequency Identification Devices, Wireless Communications Methods, and Radio Frequency Identification Device Communications Methods”, by inventors Richard Pratt and Steven B. Thompson; U.S. patent application Ser. No. 09/589,001, filed Jun. 6, 2000, entitled “Remote Communication System and Method”, by inventors R. W. Gilbert, G. A. Anderson, K. D. Steele, and C. L. Carrender; U.S. patent application Ser. No. 09/802,408; filed Mar. 9, 2001, entitled “Multi-Level RF Identification System”; by inventors R. W. Gilbert, G. A. Anderson, and K. D. Steele; U.S. patent application Ser. No. 09/833,465, filed Apr. 11, 2001, entitled “System and Method for Controlling Remote Device”, by inventors C. L. Carrender, R. W. Gilbert, J. W. Scott, and D. Clark; U.S. patent application Ser. No. 09/588,997, filed Jun. 6, 2000, entitled “Phase Modulation in RF Tag”, by inventors R. W. Gilbert and C. L. Carrender; U.S. patent application Ser. No. 09/589,000, filed Jun. 6, 2000, entitled “Multi-Frequency Communication System and Method”, by inventors R. W. Gilbert and C. L. Carrender; U.S. patent application Ser. No. 09/588,998; filed Jun. 6, 2000, entitled “Distance/Ranging by Determination of RF Phase Delta”, by inventor C. L. Carrender; U.S. patent application Ser. No. 09/797,539, filed Feb. 28, 2001, entitled “Antenna Matching Circuit”, by inventor C. L. Carrender; U.S. patent application Ser. No. 09/833,391, filed Apr. 11, 2001, entitled “Frequency Hopping RFID Reader”, by inventor C. L. Carrender.  
         [0024]    As shown in FIG. 2, an embodiment of the present invention is directed to an RF communication system  30  that employs backscatter signals. The RF communication system  30  includes a reader or interrogator  32  that includes an antenna  34  through which the reader can transmit an interrogation signal  36  to an RF tag  44 . The RF tag modulates the continuous wave interrogation signal  36  to produce a backscatter response signal  40  that is transmitted back to the interrogator  32 . The signal  40  can include an identification code stored in memory  50 , or other data. While FIG. 2 shows only two tags  44 , there would typically be multiple tags  44  in use, capable of communicating with the reader  32 .  
         [0025]    In the embodiment shown in FIG. 2, the RF tag  44  includes an antenna  42  coupled to a modulator defined by processor  48 . The tag  44  includes a switch coupled between the antenna  42  and processor  48 . In the embodiment of FIG. 2, the switch is included in the processor  48 . Alternatively, the switch can be a switch external to the processor  48 , such as an n-channel MOS transistor, a p-channel MOS transistor, a bi-polar transistor, or any of numerous other types of switches.  
         [0026]    In FIG. 2, a modulating signal from the processor  48  is input to the antenna  42  to cause the antenna to alternately reflect or not reflect. One item that can be transmitted from the tag to the reader is an identification code that is stored in memory  50  of the RF tag  44 . In one embodiment, after receiving a command, the reader sends a carrier wave or interrogation signal  36  that is received by the antenna  42 , and that signal is selectively reflected or not reflected back by the antenna  42  by the tag  44  shorting or not shorting dipole halves of the antenna  42  to produce portions of the response signal  40  (backscatter communications). Other communication methods are possible.  
         [0027]    It will be appreciated that the depiction of the RF tag  44  in FIG. 2 is one embodiment only; RFID tags are well-known in the art. For example, U.S. Pat. No. 4,075,632 to Baldwin et al., which is incorporated herein by reference, discusses in detail circuit structures that could be used to produce the RF tag  44 , if modified as described below.  
         [0028]    Similarly, the internal structures of the interrogator  32  are not shown in FIG. 2. For example, the interrogator  32  can be the receiver described in U.S. Pat. No. 4,360,810 to Landt, which is incorporated herein by reference, modified as described below.  
         [0029]    In the illustrated embodiment, the reader  32  includes a processor  54 , which in turn includes an arithmetic logic unit (ALU)  56  and a random number generator  58 . The processor  54  further includes a memory  64 ; alternatively, the reader  32  includes a memory separate from the processor  54 . The memory  64  stores a secret key value  68 , the use of which will be described in greater detail below. The memory  50  of each tag  44  also stores a secret key value  66  which, in the illustrated embodiment, is the same value as the key value  68 .  
         [0030]    The processor  54  includes other features typically included in processors of the type typically employed in RFID readers. In one embodiment, the random number generator  58  actually generates pseudo random numbers; i.e., the numbers generated may follow a pattern that could be recreated. While the random number generator  58  is shown as being separate from the ALU  56 , in one embodiment, the random number generator  58  is defined in part by the ALU  56 . Similarly, each of the RF tags  44  is shown as including an arithmetic logic unit (ALU)  60  and a random number generator  62 . Depending on whether the tags  44  or the reader  32  will be the authenticator (as will be described below), the random number generator may be omitted from either the RF tags  44  or from the reader  32 .  
         [0031]    [0031]FIG. 3 is a flowchart illustrating steps performed by the tag and reader of FIG. 2. Aspects of the invention that will be described in connection with FIG. 3 can be embodied in methods and apparatus employing tags of varying levels of sophistication. Multiple IF frequencies or read-while-write capability are not required. An advantage is that the amount of communication between the reader and the tags is minimized. This is useful to increase the speed of discovering the identities of tags.  
         [0032]    Aspects of the invention described in connection with FIG. 3 also provide authentication (identity verification) over a public, open channel. This method can be used by the tag to authenticate the reader, or by the reader to authenticate the tag.  
         [0033]    All participants in the authentication perform long, logical mathematical operations. More particularly, in step S 1 , the Requestor device requesting access (can be either a tag or the reader) sends a message to the Authenticator. In one embodiment, the Requestor is a tag  44  and the Authenticator is the reader  32 . In another embodiment, the Requestor is the reader  32  and the Authenticator is a tag  44 .  
         [0034]    In step S 2 , the Authenticator generates an “Access Challenge” number. More particularly, in one embodiment, the “Access Challenge” number is a pseudo random number generated by the random number generator  58  or  62 . Still more particularly, in one embodiment, the “Access Challenge” value is a long binary value, which is randomly generated. The length can be 128 bits, 256 bits, 512 bits, or any desired value selected depending on the desired security level versus cost and processing time.  
         [0035]    In step S 3 , the Authenticator sends the “Access Challenge” number or value to the Requestor.  
         [0036]    In step S 4 , the Requestor performs a long mathematical operation (or series of mathematical operations) on the “Access Challenge” number based upon the secret key value  66  or  68  (see FIG. 2) to define a “Challenge Response.” 
         [0037]    In step S 5 , the Requestor replies to the Authenticator with a “Challenge Response”.  
         [0038]    In step S 6 , the Authenticator independently computes its own “Challenge Response” by performing the same mathematical operation of step S 4  that the Requestor performed and by using the same key value that the Requestor performed. Step S 6  could be performed after step S 1  and before step S 2 , simultaneously with step S 2 , or at any time between step S 1  and step S 7 .  
         [0039]    In step S 7 , the Authenticator compares the received “Challenge Response” from the Requestor matches the value computed by the Authenticator. If not, the Authenticator denies access in step S 8 . If so, the Authenticator grants access in step S 9 .  
         [0040]    In step S 10 , the Authenticator sends a message to the Requestor informing the Requestor that access is granted. After step S 10 , normal communications occur in step S 11 , including transmission of information stored in the Authenticator to the Requestor (e.g., identification information, account information, financial information, etc.).  
         [0041]    This is similar to the approach used in prior hardware based “dongle” designs, which were used in the past to prevent software piracy, except in a completely different field of use and without requiring a traditional dongle.  
         [0042]    The advantages of this approach include:  
         [0043]    (1) The “Access Challenge” number is a generally random number;  
         [0044]    (2) The response to the Access Challenge is dependent upon the Challenge value, but is convoluted to obscure the key value.  
         [0045]    (3) Multiple key values are possible, which can uniquely identify the user.  
         [0046]    (4) The length of the keys and challenge values can be configured to the security requirement at hand. Tradeoffs can be made between level of security and system cost. Greater security requires longer keys, longer messages, and more processor power.  
         [0047]    (5) The approach of FIG. 3 is used to generate encryption values for a data stream, in an alternative embodiment. The challenge response is not transmitted, but is used to encrypt the data stream.  
         [0048]    This approach can be implemented in a passive or semi-passive RFID application to provide a level of security and/or data encryption not presently available. The passive or semi-passive RFID application will require that the Authenticator initiate all communication processes with the Requestors. Various embodiments of the invention could be employed in security and inventory management applications.  
         [0049]    In another embodiment, public key encryption is used. Reader authentication occurs as follows. The reader requests to read or write tag data. The tag responds with a challenge value (random number). The reader encrypts the challenge value using its private key. The reader sends the encrypted challenge response to the tag. The tag uses the reader public key to decrypt the challenge response. The tag compares the results to the original challenge value and, if there is a match, the reader is authenticated.  
         [0050]    The process for a tag authentication is as follows. The reader issues a request to a tag including a non-encrypted challenge value. The tag encrypts the challenge value using its private key, creating a challenge response. The tag sends the challenge response to the reader (no key is sent). The reader decrypts the challenge response using the known tag&#39;s public key. If the reader-computed result matches the original challenge value, the tag is authenticated.  
         [0051]    In another embodiment related to those just described, both a tag and a reader function to authenticate the other. For example, a reader sends a non-encrypted first challenge value to a tag. The tag encrypts the first challenge value using a tag private key and sends it to the reader, as a first response, along with a non-encrypted second challenge value. The reader decrypts the first response using a tag public key, and compares the result with the non-encrypted first challenge value. If the comparison is valid, then the tag is determined to be authentic. The reader then encrypts the second challenge value using a reader private key and sends it to the tag as a second response. The tag then decrypts the second response using a reader public key, and compares the result with the non-encrypted second challenge value. If the comparison in valid, then the reader is also determined to be authentic. The tag and the reader may now continue with the exchange of data or commands.  
         [0052]    In the embodiments just described above, it is assumed that both the reader and the tag(s) know the public key or keys of the other, in advance of the authentication process. This is a valid assumption if both (all) belong to the same application. In addition, multiple keys (public and private) may be used to realize corresponding level of security. Generally, the longer (i.e., more complex) the key length, the greater the level of security within the exchange. In the example just described, the non-encrypted first and second challenge values may be the same; responses would be different due to different private keys. Other embodiments using public and private keys are also possible.  
         [0053]    Inventory management incorporates a wide variety of situations where RF tags can be used. These situations include the simple inventory/locating task of critical or high value items in storage, transport, or final use locations. Integrating authentication capability into the tags as described above would greatly increase the security of data contained within the tag and the item to which it is attached.  
         [0054]    Any design of RF tag can be used for security purposes. The addition of authentication capability described above provides an additional mechanism against spoofing. The security needs are many, with potential theft being high on the list. The traditional means of detecting when a theft has occurred is to track inventory. Items can be identified as they are removed from a monitored area or when subsequent inventories are made of the storage location.  
         [0055]    Thus, methods and apparatus for authenticating reader or tags over a public, open channel have been provided. A system has been provided that is more robust against spoofing and other illicit access attempts than password-based methods.  
         [0056]    Implementation requires minor changes to existing designs and is, for some embodiments, a matter of implementation in the RF tags&#39; and readers&#39; microprocessors.  
         [0057]    In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.