Encryption for modulated backscatter systems

Abstract: The present invention enables a Tag and Interrogator to exchange proprietary information using Modulated BackScatter (MBS) technology. This invention discloses a method for encrypting the RFID user's PIN and thereby making the interception and the subsequent illegal use of RFID accounts at least as difficult as a present day ATM cards. For example, the encryption method can be based upon the US Digital Encryption Standard (DES), either first or third level. The Tag's personal encryption key is only known by the financial database and the RFID Tag. This method can be applied to any type of financial, debt, identification, or credit card.

Description: This invention relates to radio communication systems and to methods of radio communication. Related subject matter is disclosed in the following U.S. patents and patent applications: "Shielding Technology In Modulated Backscatter System", Serial No. 08/777,770; "QPSK Modulated Backscatter System", Serial No. 08/775,694 (EP-A-0 851 639); "Modulated Backscatter Location System", Serial No. 08/777,643; "Antenna Array In An RDID System", Serial No. 08/775,217; "Subcarrier Frequency Division Multiplexing Of Modulated Backscatter Signals", Serial No. 08/777,834 (EP-A-0 853 392); "IQ Combiner Technology In Modulated Backscatter System", U.S. Patent No. 5784686; "In-Building Personal Pager and Identifier", Serial No. 08/775,738; "In-Building Modulated Backscatter System", Serial No. 775,701 (EP-A-0 851 239); "Inexpensive Modulated Backscatter Reflector", Serial No. 08/774,499 (EP-A-0 851 599); "Passenger, Baggage, And Cargo Reconciliation System", Serial No. 08/782,026 (EP-A-0 851 377); "Modulated Backscatter Sensor System", 08/777,771 (EP-A- 0 853 245); Serial No. 08/504,188, entitled "Modulated Backscatter Communications System Having An Extended Range"; Serial No. 08/492,173, (EP-A-0 750 200) entitled "Dual Mode Modulated Backscatter System"; Serial No. 08/492,174 (EP-A-0 750 201) entitled "Full Duplex Modulated Backscatter System"; Serial No. 08/571,004 (EP-A-0 779 520) entitled "Enhanced Uplink Modulated Backscatter System"; and Serial No. 08/777,771 entitled "Modulated Backscatter Sensor System". Background of the Invention Radio Frequency IDentification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. The Interrogator first transmits an amplitude modulated signal to the Tag. Then, the Interrogator transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag then modulates the CW signal, using Modulated BackScattering (MBS), where the antenna is electrically switched, by the Tag's modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation; thereby encoding the Tag's information onto the CW radio signal. The Interrogator demodulates the incoming modulated radio signal and decodes the Tag's information message. Over the next decade an enormous growth in the number and proliferation of RFID applications is expected. Many emerging applications will be of a financial nature and will require, for example, that a user's Personal Identification Number (PIN) cannot be intercepted by a hostile eavesdropper. The cellular phone industry is presently under siege by network pirates and industry losses are quoted to be approaching one billion dollars annually. So as not to repeat this particular failing of the cellular industry, RFID system designers should consider network security a top priority. There are three major security issues when transferring data in a wireless system: 1. A legitimate Tag and a legitimate Interrogator are involved in a communication session transferring sensitive data that a hostile eavesdropper would like to intercept. 2. A legitimate Interrogator would be queried by a fraudulent Tag trying to obtain service, such as to acquire data stored in the network or in an Application Processor. (This is similar to the case of someone stealing cellular air time.) 3. A legitimate Tag would be queried by a fraudulent Interrogator trying to acquire data stored in the Tag's memory (like stealing the PIN number from a cellular telephone). This invention discloses a method for encrypting both the data in a Tag's memory and the data stored in an Application Processor, where by transferring only ciphered data between network endpoints, one can thwart all three security breaches outlined above. This method encrypts the RFID user's PIN and therefore makes the interception and the subsequent illegal use of RFID data at least as difficult as for present day ATM cards. The encryption method can be based upon the US Digital Encryption Standard (DES), either first or third level. The Tag's personal encryption key is only known by the financial database and the RFID Tag. This method can be applied to any type of financial, debt, identification or credit card. EP-A-0577328 discloses a secure toll payment system realized by transmitting a changeable encryption code from roadside equipment at a toll plaza to a moving vehicle. Thereafter, the moving vehicle uses the encryption code to encrypt payment information according to the Data Encryption Standard algorithm. The moving vehicle transmits the encrypted payment information to the roadside equipment which performs a credit or debit transaction. Because the encryption code changes from time to time, so, too, does the nature of the signal which is transmitted by the vehicle: fraud, based on electronic eavesdropping, is substantially eliminated. The encryption code comprises an 8-bit random number and a time/date number. Vehicle-mounted apparatus includes a transponder unit and a portable smart card which inserts therein. The roadside equipment includes a roadside reader that includes a pair of spaced-apart antennas that are sequentially located along an express payment lane at a toll plaza, and a computer, or Server, which controls them. The Server uses the smart card ID number to look up its secret code, or it generated the secret code using the card ID number and a secret algorithm. The secret code, the encryption code, and the money balance of the smart card are combined and processed using the DES algorithm to generate an encrypted number (P') to be compared with an encrypted number (P) generated by the smart card. WO-A-91/17515 discloses an electronic identification system comprising a transmitter for generating an electromagnetic excitation signal, one or more portable transponders for storing variable identification data, and for transmitting an information signal containing the identification data upon entering the field. Transmission of the information signal is independent of the excitation signal in both time and frequency. A radio frequency receiver is provided for receiving the information signal and in response generating an output signal representing the variable identification data contained in the information signal. The system allows an excitation frequency to be selected for optimum performance given circumstances dictated by each application; and an independent response channel can be selected according to the requirements of each installation or application, thereby allowing response signals to be transmitted by transponders at a frequency which differs from the frequency of an interfering signal. EP-A-0670558 discloses a modulated backscatter communication system (e.g., an Electronic Shelf Label (ESL) system) that includes a central node (e.g., a ceiling node and one or more remote nodes (e.g., ESL tags). The central node transmits a downlink radio signal which is reflectively modulated at a remote node to produce a modulated reflected radio signal. At the remote node, a precise-frequency subcarrier signal is modulated with an uplink information signal to produce a modulated signal which is used to produce the modulated reflected radio signal. At the central node, the modulated reflected radio signal is received, detected and narrowband filtered to obtain the modulated subcarrier signal which is then demodulated to obtain the uplink information signal, with great immunity to close to carrier downlink radio noise and fluorescent light backscatter interference. US-A-5,485,520 discloses one or more roadside collection stations (RCS) that communicate over a short-range, high speed bidirectional microwave communication link with one or more in-vehicle units (IVU) associated with one or more respective corresponding vehicles in one or more traffic lanes of a highway. At least two up-link (IVU to RCS) communication sessions and at least one downlink (RCS to IVU) communication session are transacted in real time during the limited duration of an RCS communication footprint as the vehicle travels along its lane past a highway toll plaza. Especially efficient data formatting and processing is utilized so as to permit, during this brief interval, computation of the requisite toll amount and a fully verified and cryptographically secured (preferably anonymous) debiting of a smart card containing electronic money. Preferably an untraceable electronic check is communicated in a cryptographically sealed envelope with opener. Transaction linkage data is utilized in each phase of the complete toll payment transaction to facilitate simultaneous multi-lane RCS/IVU operation. A plaza computer local area network and downlink plaza controller is also used to facilitate simultaneous multi-lane transactions. According to one aspect of this invention there is provided a radio communication system as claimed in claim 1. According to another aspect of this invention there is provided a method of radio communication as claimed in claim 10. A MBS radio communication system comprises an Interrogator which generates a first modulated signal by modulating a first information signal onto a radio carrier signal. The Interrogator transmits the first modulated signal to at least one remote Tag of the system. The remote Tag receives and processes the received first modulated signal. A second information signal backscatter-modulates the reflection of the first modulated signal, the reflected signal being a second modulated signal. The Interrogator receives and demodulates the second modulated signal to obtain the second information signal. In one embodiment, demodulation utilizes a homodyne detector and the first modulated signal as the local oscillator source for the homodyne detector. In another embodiment, the second information signal is modulated onto a subcarrier, which is then backscatter-modulated onto the first modulated signal. The Interrogator communicates the second information signal to an Application Processor; the second information signal contains private information known only to the Tag and to the Application Processor. Both the Tag and Application Processor use digital encryption techniques to cipher the end-to-end communications of the private information. The present disclosure outlines three levels of security for which, depending on the RFID application, an Application Processor, at least one Interrogator, and at least one Tag can exchange information: The first level is the "Normal" mode, where a Tag and Interrogator exchange the RFID of the Tag. The second level is to transmit and detect a "secure" RFID between a Tag and Application Processor, using the Interrogator as a "wireless-to-wiredline" converter. The third level is the transfer of secure messages between a Tag and Application Processor. This can include downloading new information to a Tag; this is a "Read/Write" Tag where the data stored in memory is sensitive, i.e., cash stored on a debit card. Brief Description of the Drawing In the drawing, FIG. 1 shows a block diagram of an illustrative Radio Frequency Identification (RFID) system; FIG. 2 shows a block diagram of an illustrative Interrogator Unit used in the RFID system of FIG. 1; FIG. 3 shows a block diagram of a Tag Unit used in the RFID system of FIG. 1; FIG. 4 shows the flow of information between the Tag, Interrogator and the Application Processor vs. time during a secure message transfer; FIG. 5 shows a block diagram of how a Tag's processor and the Application Processor processor calculate the random query response; FIG. 6 shows the flow of information between the Applications Processor, the Interrogator, and at least one Tag, where the Applications Processor is writing new information to the Tag's memory. Detailed Description One class of RFID applications involves using RFID technology to read information from a Tag affixed to a container or pallet. In this application, the container is moved across the reading field of an Interrogator. The reading field is defined as that volume of space within which a successful transaction can take place. While the Tag is in the reading field, the Interrogator and Tag must complete their information exchange before the Tag moves out of the Interrogation field. Since the Tag is moving through the reading field, the RFID system has only a limited amount of time to successfully complete the transaction. Normal Mode With reference to FIG. 1, there is shown an overall block diagram of an illustrative RFID system useful for describing the application of the present invention. An Application Processor 101 communicates over Local Area Network (LAN) 102 to a plurality of Interrogators 103-104. The Application Processor and the Interrogator functions may be present in the same device, or separate. In this disclosure we refer to the logical functions performed by each device. The Interrogators may then each communicate with one or more of the Tags 105-107. For example, the Interrogator 103 receives an information signal, typically from an Application Processor 101. The Interrogator 103 takes this information signal and Processor 200 (FIG. 2) properly formats a downlink message (Information Signal 200a) to be sent to the Tag. With joint reference to FIGS. 1 and 2, Radio Signal Source 201 generates a radio signal, the Modulator 202 modulates the Information Signal 200a onto the radio signal, and the Transmitter 203 sends this modulated signal via Antenna 204, illustratively using amplitude modulation, to a Tag. The reason amplitude modulation is a common choice is that the Tag can demodulate such a signal with a single, inexpensive nonlinear device (such as a diode). In the Tag 105 (see FIG. 3), the Antenna 301 (frequently a loop or patch antenna) receives the modulated signal. This signal is demodulated, directly to baseband, using the Detector/Modulator 302, which, illustratively, could be a single Schottky diode. The diode should be appropriately biased with the proper current level in order to match the impedance of the diode and the Antenna 301 such that losses of the radio signal are minimized. The result of the diode detector is essentially a demodulation of the incoming signal directly to baseband. The Information Signal 200a is then amplified, by Amplifier 303, and synchronization recovered in Clock and Frame Recovery Circuit 304. The Clock Recovery Circuit 304 can be enhanced by having the Interrogator send the amplitude modulated signal using Manchester encoding. The resulting information is sent to a Processor 305. The Processor 305 is typically an inexpensive 8-bit microprocessor; the Clock Recovery Circuit 304 can be implemented in a ASIC (Application Specific Integrated Circuit) which works together with Processor 305. The Processor 305 generates an Information Signal 306 to be sent from the Tag 105 back to the Interrogator (such as Interrogator 103). This Information Signal 306 (under control of Clock Recovery and Frame synchronization 304) is sent to a Modulator Control Circuit 307, which uses the Information Signal 306 to modulate a subcarrier frequency generated by the subcarrier Frequency Source 308. The Frequency Source 308 could be a crystal oscillator separate from the Processor 305, or it could be a frequency source derived from signals present inside the Processor 305, such a a divisor of the primary clock frequency of the Processor. The Modulated Subcarrier Signal 311 is used by Detector/Modulator 302 to modulate the modulated signal received from Tag 105 to produce a modulated backscatter signal, also referred to herein as a reflected signal. This is accomplished by switching on and off the Schottky diode using the Modulated Subcarrier Signal 311, thereby changing the reflectance of Antenna 301. A Battery 310 or other power supply provides power to the circuitry of Tag 105. Secure Mode The present invention enables a Tag and Interrogator to exchange proprietary information using MBS technology. FIG. 4 shows the exchange of information over time between a Tag, Interrogator and the Application Processor using digital encryption techniques. The Interrogation of a Tag can be initiated merely by a Tag coming into Radio Frequency (RF) proximity of the Interrogator, depending on the exact service or specific application. In any case, the Tag senses the Interrogator's signal 401 and broadcasts its RFID 402. The Interrogator, which has no knowledge of the Tag's PIN or encryption key, sends the information along to the Application Processor, as identified by the type of ID number presented by the Tag. The Interrogator also generates a random number which it passes to both the Tag and Application Processor. The random number is chosen by the Interrogator 103 and is previously unknown to both the Tag and Application Processor. The Application Processor in turn looks up the appropriate identification information and, using the encryption information associated with the Tag's ID information, generates a response that is functionally based upon the RFID user's PIN (encryption key) and the random number sent by the Interrogator 405. Subsequently the Interrogator has also sent the same random number back to the Tag 404 and expects the same functionally based response 406. In this scenario, further transactions are allowed to continue between the Tag and Application Processor, through the Interrogator, only if the Processor and Tag return the same response to the random query. In this case, the Interrogator notifies the Tag and the Application Processor whether the transaction is accepted or rejected 409, 410. Note that in this scenario, no information about the Tag's PIN or encryption key has been transmitted anywhere in the network. The encryption key is only known by the Tag and Application Processor, which are distributed when the Tag is issued Also, all data is encrypted before transmission on any part of the network. The only information that is not protected is the Tag's ID number, which by itself is not useful to an eavesdropper and is treated as public information. Therefore, all three security issues outlined above are addressed by this method of user authentication and subsequent data transmission. The security provided by the encryption method outlined here is that the response to the Interrogator's random number is based on a "one-way function" which produces the appropriate response. A one-way function is characterized by the fact that an eavesdropper that sees the output of the function cannot reconstruct the input (the secret PIN, in this case) even knowing the particular function. Both the Application Processor and the Tag use the same function, which is determined by the encryption key, to cipher and decipher the information data. FIG. 5a shows a Tag, using a one way function, calculating the response to a random query. In FIG. 5b, the "first round" of a DES encryption process is shown. In a DES level one code, there are 16 coding rounds to encrypt 64 bits of data. The exact function is unique to each user's database entry since it depends on the particular user's encryption key. For example, with today's technology, a 64 bit PIN can be encrypted using a DES level one function by a 8 bit microprocessor in approximately 1 ms. Thus it is a cost effective way to implement digital encryption and still be able to manufacture simple and cheap RFID Tags. It is important to design system security that will be effective over the entire lifetime of the RFID system. As pointed out above, cellular network designers did not foresee the technical advancements and cost reductions of RF measurement equipment and the proliferation of cheap, powerful and portable computers. Therefore, one issue that must be addressed is what level of security can the MBS system provide against future attacks? Another implementation issue that will greatly effect the time response of the RFID system are the network and processing delays of the RF air interface and the connection to the authentication/transaction database. Secure Messages The previous example (FIG. 4) demonstrates how a Tag can exchange sensitive information with an Application Processor. FIG. 6 shows how the Application Processor can change the information stored on a Read/Write RFID Tag. After the completion of the Tag authentication procedures described above, the Application Processor may find it necessary to change the information stored on the Tag. (For example, the remaining total on a debit card or the air waybill on an airline cargo container.) In this case the Application Processor will digitally encrypt all the command messages and information messages with the "encryption key" of the particular Tag. After completing all of the data transmission, the Tag will decrypt all of the new data and store it in memory. However, the RF channel is an unreliable medium and will generally create errors in the data transmission stream. Since only the endpoints of the network have the ability to decipher the information, unnecessary delay would be added if data, corrupted by wireless transmission, would need to be retransmitted between the endpoints. Since the Interrogator only has knowledge of the Tag's ID and not the PIN or encryption key, it needs a mechanism to ensure the Tag has received all the information correctly when transmitting over the radio channel. Therefore, the Interrogator should add error detection and/or forward error correction coding to the transmitted data. This will allow any needed retransmissions to be only between the Tag and Interrogator, thereby reducing retransmission latency. For example, the Interrogator can calculate and add a CRC byte or bytes to the end of each transmitted packet, allowing both error detection and limited error correction. The Tag will in turn calculate the CRC for each received message packet and only ask for retransmission of corrupted packets. Therefore, it is possible to send encrypted data between the Application Processor and a Tag without the need to pass encryption keys (PINs) over the wireless air interface. Since all data is encrypted before transmission to the Interrogator over the wired line, this method provides end-to-end security for the MBS system. This leads to two immediate advantages: First, different applications can use different encryption algorithms, which can depend on the level of security the application needs or the maximum delay time an application can tolerate during information exchange. Since only the end points cipher, the data can be a priori encrypted and stored in memory thereby reducing latency. Second, as computers and microprocessors, the tools used by hostile eavesdroppers, become more powerful, the encryption algorithms can gracefully be updated by each generation release of the MBS system. Since only the Tag and the particular Application Processor entry for the Tag need to be updated, there is no need to modify any other parts of the MBS system. Using the above techniques as an example, an inexpensive, short-range, bi-directional digitally encrypted radio communications channel is implemented. Implementation is made inexpensive by using, e.g., such Tag components as a Schottky diode, an amplifier to boost the signal strength, bit and frame synchronization circuits, an inexpensive 8-bit microprocessor, subcarrier generation circuits, and a battery. Most of these items are already manufactured in quantities of millions for other applications, and thus are not overly expensive. The present invention outlines three levels of security for which, depending on the RFID application, an Application Processor, at least one Interrogator and at least one Tag can exchange information: The first level is the "Normal" mode, where a Tag and Interrogator exchange the RFID of the Tag. The second level is to transmit and detect a "secure" RFID between a Tag and Application Processor, using the Interrogator as a "wireless-to-wiredline" converter. The third level is the transfer of secure messages between a Tag and Application Processor. This can include downloading new information to a Tag; this is a "Read/Write" Tag where the data stored in memory is sensitive, such as cash stored on a debit card. What has been described is merely illustrative of the present invention. Other arrangements and methods can be implemented by those skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims: A radio communications system comprising an interrogator (103) adapted to transmit a radio signal, and a tag (105) adapted to receive the radio signals from the interrogator, wherein the tag (105) comprises a backscatter modulator (302) in receiving relationship to a source of a unique identifier, the backscatter modulator (302) being adapted to modulate a reflection of the radio signal from the interrogator (103) using the unique identifier, thereby forming a reflected radio signal modulated by the unique identifier, and the interrogator (103) comprises a demodulator (208) in receiving relationship to the reflected radio signal modulated by the unique identifier, such that the unique identifier is recoverable from the reflected radio signal modulated by the unique identifier. CHARACTERIZED IN THAT: the interrogator (103) comprises means for generating a random challenge and for transmitting the random challenge and the unique identifier to an application processor (101), a modulator (202) adapted to modulate the radio signal using the random challenge, and means (203, 204) for transmitting a radio signal modulated by the random challenge to the tag; the interrogator (103) further comprises means for receiving a first encrypted response from the application processor (101), wherein the first encrypted response is based on the random challenge and a predetermined encryption key retrieved by the application processor on the basis of the unique identifier; the tag (105) comprises means for generating a second encrypted response using at least the random challenge and said predetermined encryption key; the backscatter modulator (302) is adapted to modulate the reflection of the radio signal using the second encrypted response, thereby generating a reflected radio signal modulated by the second encrypted response; the demodulator (208) is adapted to demodulate the reflected radio signal modulated by the second encrypted response to recover the second encrypted response; and the interrogator (103) comprises means (200) for comparing the first and second encrypted responses, and means (200) for accepting or rejecting the tag based on results of such comparisons. A system as claimed in claim 1, wherein the application processor (101) comprises means for generating the first encrypted response using at least the random challenge and the unique identifier, and means for transmitting the first encrypted response to the interrogator (103). A system as claimed in claim 1 or 2, wherein: the tag (105) comprises means (308) for generating a subcarrier signal, and means (307) for modulating the subcarrier signal using the unique identifier, thereby forming a subcarrier signal modulated by the unique identifier; and the backscatter modulator (302) is adapted to modulate the reflection of the radio signal using the subcarrier signal modulated by the unique identifier. A system as claimed in claim 1 or 2, wherein: the application processor (101) comprises means for transmitting an encrypted information signal to the interrogator (103); the interrogator (103) is in receiving relationship to the application processor (101) with respect to the encrypted information; the interrogator (103) comprises means for adding the unique identifier to the encrypted information signal received from the application processor (101), thereby forming an addressed information signal; the interrogator (101) comprises means (200) for adding cyclic redundant check coding to the addressed information signal, thereby forming a message signal; the modulator (202) is adapted to modulate the radio signal with the message signal, thereby forming a radio signal modulated by the message signal, there being provided means (203, 204) for transmitting the radio signal modulated by the message signal to the tag (105); and the tag (105) comprises means for receiving (301) and demodulating (302) the radio signal modulated by the message signal, thereby recovering the message signal, means (305) for calculating and removing the cyclic redundant check code from the message signal, thereby recovering the addressed information signal, means (305) for removing the unique identifier from the addressed information signal, thereby recovering the encrypted information signal, and means for storing the encrypted information signal. A system as claimed in claim 4, wherein the tag (105) comprises means (305) for decrypting the encrypted information signal, thereby recovering unencrypted information, and a storage medium for the unencrypted information. A system as claimed in claim 4, wherein: the tag (105) comprises means (305) for determining the presence or absence of transmission errors based upon the cyclic redundant check coding, and means (305) for generating a negative acknowledgement signal in the event of such transmission errors; the backscatter modulator (302) is adapted to modulate the reflection of the radio signal using the negative acknowledgement signal, thereby forming a reflected radio signal modulated by the negative acknowledgement signal; the demodulator (208) is adapted to demodulate the reflected radio signal modulated by the negative acknowledgement signal, thereby recovering the negative acknowledgement signal; and the interrogator (103) comprises means (200) for deciding, based on content of the negative acknowledgement signal, whether re-transmission should occur. A system as claimed in claim 1 or 2, wherein: the backscatter modulator (302) is in receiving relationship to a source (305) of an encrypted signal and is adapted to modulate the reflection of the radio signal from the interrogator (103) using the encrypted signal from the source, thereby forming a reflected radio signal modulated by the encrypted signal; the demodulator (208) is in receiving relationship to the reflected radio signal modulated by the encrypted signal, such that the encrypted signal is obtained by the demodulator; and the interrogator (103) comprises means for transmitting the encrypted signal to the application processor (101). A system as claimed in claim 7, wherein: the tag (105) comprises a generator (308) of a subcarrier signal, and there is provided a subcarrier modulator (307) adapted to modulate the subcarrier signal using the encrypted signal, thereby forming a subcarrier signal modulated by the encrypted signal; and the backscatter modulator (302) is adapted to modulate the reflection of the radio signal using the subcarrier signal modulated by the encrypted signal. A system as claimed in claim 7, wherein the application processor (101) comprises means for deciphering the encrypted signal, thereby generating an information signal, and means for storing the information signal. A method of radio communication comprising: transmitting a radio signal from an interrogator to a tag (401); in the tag, modulating a reflection of the radio signal using, at least in part, a unique identifier, thereby forming a reflected radio signal modulated by the unique identifier (402); in the interrogator, receiving and demodulating the reflected radio signal modulated by the unique identifier, thereby recovering the unique identifier; and CHARACTERIZED BY: in the interrogator, generating a random challenge, and transmitting the random challenge and the unique identifier to an application processor (403); in the interrogator, modulating the radio signal using the random challenge, thereby forming a radio signal modulated by the random challenge, and transmitting the radio signal modulated by the random challenge to the tag (404); in the interrogator (103), receiving a first encrypted response from the application processor wherein the first encrypted response is based on the random challenge and a predetermined encryption key retrieved by the application processor on the basis of the unique identifier, in the tag (105), generating a second encrypted response using at least the random challenge and said predetermined encryption key, and modulating the reflection of the radio signal using the second encrypted response, thereby forming a reflected radio signal modulated by the second encrypted response (406); demodulating in the interrogator the reflected radio signal modulated by the second encrypted response to recover the second encrypted response, and comparing the first and second encrypted responses; and indicating acceptance or rejection of the tag based on the result of the comparing step (409, 410). A method as claimed in claim 10, comprising: in the application processor (101), generating the first encrypted response using at least the random challenge and the unique identifier: and transmitting the first encrypted response to the interrogator (405). A method as claimed in claim 10 or 11, comprising: in the tag, generating a subcarrier signal, and modulating the subcarrier signal using the unique identifier, thereby forming a subcarrier signal modulated by the unique identifier; and wherein in the tag, the modulating of the reflection of the radio signal using, at least in part, the unique identifier is carried out by using the subcarrier signal modulated by the unique identifier. A method as claimed in claim 10 or 11, comprising: transmitting an encrypted information signal from the application processor to the interrogator; in the interrogator, adding the unique identifier to the encrypted information signal, thereby forming an addressed information signal; adding cyclic redundant check coding to the addressed information signal, thereby forming a message signal; modulating the radio signal with the message signal, thereby forming a radio signal modulated by the message signal: transmitting the radio signal modulated by the message signal to the tag; in the tag, demodulating the radio signal modulated by the message signal, thereby recovering the message signal; in the tag, calculating and removing the cyclic redundant check code from the message signal, thereby recovering the addressed information signal; in the tag, removing the unique identifier from the addressed information signal, thereby recovering the encrypted information signal; and in the tag, storing the encrypted information signal; A method as claimed in claim 13, comprising: in the tag, decrypting the encrypted information signal, thereby forming unencrypted information; and storing the unencrypted information. A method as claimed in claim 13, comprising: in the tag, determining the presence or absence of transmission errors based upon the cyclic redundant check coding, and generating a negative acknowledgement signal in the event of the transmission errors; in the tag, modulating the reflection of the radio signal using the negative acknowledgement signal, thereby forming a reflected radio signal modulated by the negative acknowledgement signal; in the interrogator, receiving and demodulating the reflected radio signal modulated by the negative acknowledgement signal, thereby recovering the negative acknowledgement signal; deciding, based on the contents of the negative acknowledgement signal, whether re-transmission should occur; modulating the radio signal with the message signal, thereby forming the radio signal modulated by the message signal; and transmitting the radio signal modulated by the message signal to the tag. A method as claimed in claim 10 or 11, comprising: in the tag, modulating the reflection of the radio signal using an encrypted signal, thereby forming a reflected radio signal modulated by the encrypted signal; in the interrogator, receiving and demodulating the reflected radio signal modulated by the encrypted signal, thereby obtaining the encrypted signal; and transmitting the encrypted signal to the application processor. A method as claimed in claim 16, comprising: in the tag, generating a subcarrier signal, and modulating the subcarrier signal using the encrypted signal, thereby forming a subcarrier signal modulated by the encrypted signal; and wherein in the tag, the modulating of the reflection of the radio signal using the encrypted signal is carried out using the subcarrier signal modulated by the encrypted signal. A method as claimed in claim 16, comprising, in the application processor: deciphering the encrypted signal, thereby generating an information signal; and storing the information signal.

Inventors: MACLELLAN JOHN AUSTIN, VANNUCCI GIOVANNI, MACLELLAN, JOHN AUSTIN, VANNUCCI, GIOVANNI
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1997
Main classification: G06K7
IPC codes: H04B1, H04B7, G01S13, H04L9, H04B5