WO2009090602A1

WO2009090602A1 - Rf device emitting an rf signal and method for operating an rf device

Info

Publication number: WO2009090602A1
Application number: PCT/IB2009/050122
Authority: WO
Inventors: Peter Rombouts; Denis Noel
Original assignee: Nxp B.V.
Priority date: 2008-01-15
Filing date: 2009-01-13
Publication date: 2009-07-23

Abstract

An RF device (1) emitting an RF signal (RF0Ut) is provided. The device has a configurable structure (2, 3, 4) operable in a plurality of different configurations in which different device specific fingerprints are present in emitted RF signals (RF0Ut) to provide different device specific fingerprints at different points in time. Further, a method for operating an RF device (1) emitting RF signals (RF0Ut) is provided. The method comprises the step: controlling the RF device to operate, at different points in time, in a plurality of different configurations (2, 3, 4) in which RF signals (RF0Ut1, RF0Ut2, RF0Ut3) comprising different device specific fingerprints are emitted. As a result, identification of an RF device is ensured or can be reliably prevented.

Description

RF device emitting an RF signal and method for operating an RF device

FIELD OF THE INVENTION

The invention relates to an RF device emitting an RF signal and to a method for operating an RF device emitting RF signals. More specifically, the invention relates to an RF device comprising device specific fingerprints in emitted RF signals and to a method for operating an RF device emitting RF signals comprising device specific fingerprints.

BACKGROUND OF THE INVENTION

In the context of the present application, the term uncloneable with respect to characteristics or signals means that the characteristics or signals are impossible or at least extraordinary hard to reproduce.

All manufacturing processes for producing electronic devices and components have internal tolerances leading to differences in the devices produced by these manufacturing processes. In practice, large efforts are put into optimizing such processes in order to minimize the manufacturing differences. However, no two devices will be exactly identical.

Although undesirable from a manufacturing point of view, the differences between devices caused by the manufacturing process can be useful. The small differences in construction lead to small differences in the characteristics of the devices. If these differences in the characteristics can be measured, they can be used for identifying the device. Thus, due to the small differences in the characteristics, the devices show device specific fingerprints which are unique.

Currently digital circuits which exploit device specific characteristics are an active topic of research. For device identification using digital signals, two main approaches can be found. According to a first approach, the device is used as an unknown and uncloneable one-way function. Some challenge-response pairs of the device are extracted from the device and stored in a database. Thus, by comparing a response of the device to a challenge with the content of the database, identification of the device is possible. According to a second approach, the device is used as an uncloneable ROM (Read Only Memory) from which a secret key is extracted. These two approaches are in fact similar, since each one-way function can also be looked at as a lookup in a ROM. However, using digital signals has some disadvantages. The biggest disadvantage is that a digital signal can be easily reproduced and, hence, the result should either not be revealed or never be used again. Not revealing the result requires that cryptographic functionality is present on the device. Never using the result again limits the number of identifications (authentications) that can be performed.

In order to overcome these problems concerning digital signals, efforts have been undertaken to identify devices by fingerprints contained in RF signals emitted by these devices. With respect to RF devices, the fingerprints are characteristics present in the emitted RF signal which are device specific and caused by small differences in the construction of the device; e.g. as a result of manufacturing tolerances. In case of such RF devices emitting RF signals, these characteristics are unique and uncloneable. The techniques that have been tried rely on oversampling, filtering and modeling of the analog output signal to obtain a fingerprint of the device. Compared to the digital devices, in RF devices emitting analog RF signals, the fingerprint is harder to detect due to the efforts during manufacturing mentioned above for equalizing the devices. A possible solution for making such RF devices better identifiable would be to enlarge the manufacturing tolerances. However, it is obvious that this would be contrary to the aim of providing specific well-defined properties for a large number of different devices. Thus, there remains the problem that it is much harder to extract sufficient information from such devices because the differences between signals of different devices are too small and hard to distinguish from unavoidable noise. Hence, only a small number of bits can be extracted from the device and used for identification. Thus, the problem remains that the amount of information content (entropy) which can be extracted from the documented behavior of such an RF device can be too limited to enable obtaining a valid unambiguous fingerprint. However, on the other hand, in some cases it cannot be ruled out that identification of such a device is possible even if it is undesired.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the present invention to provide an RF device and a method for operating an RF device with which unambiguous identification of an RF device emitting RF signals can be reliably enabled or prevented, respectively.

This object is solved by an RF device emitting an RF signal according to claim 1. The device has a configurable structure operable in a plurality of different configurations in which different device specific fingerprints are present in emitted RF signals to provide different device specific fingerprints at different points in time. Since an RF signal comprising a device specific fingerprint is provided, this analog signal is much harder to reproduce than a digital signal. As such, the fingerprint is uncloneable and need not be kept secret. Thus, no additional cryptographic functionality is necessary in the device. A plurality of different configurations of the device is provided in which different device specific fingerprints are emitted. The configuration space of the device is augmented.

As a result, if reliable identification is desired, responses of the RF device to different challenges corresponding to different configurations can be used for identification. In this case, since a plurality of fingerprints is provided, the amount of information content (entropy) allowing identification is enlarged. Further, the probability of collisions in fingerprints between different devices, i.e. two or more devices comprising identical fingerprints within the measurement limits, is reduced and can be made arbitrarily small. For r different configurations, e.g. corresponding to r parameters, the probability of r fingerprints corresponding to r sets of parameters decreases as r increases. Thus, a unique identification is allowed, if desired. On the other hand, if identification of the RF device shall be prevented, the RF device can be adapted such that the plurality of configurations is randomly selected. Thus, by randomizing the fingerprint, the achievable entropy can be reduced to a level which prevents identification of the RF device. Preferably, the configurable structure is formed by a plurality of different internal circuits which can be selectively activated and provide different device specific fingerprints in the emitted RF signal. In this case, it can be ensured that the device reliably provides different fingerprints for the plurality of different configurations.

If the device specific fingerprints are unique signal components resulting from manufacturing tolerances, it can be ensured that they are uncloneable.

According to an aspect, the different configurations are activated in response to signals received by the RF device. In this case, the RF device can be externally induced to emit the plurality of different fingerprints such that identification is possible in a fast and reliable way. If the signals received by the RF device are RF signals, this is possible without any physical contact to the RF device.

According to another aspect, the device is adapted to randomly change the configuration between different configurations over time. In this case, identification of the device can be prevented and emitted RF signals can be prevented from being tracked, since the emitted fingerprints change randomly. If the RF device is an RFID tag, reliable identification of objects provided with the RF device is allowed in a convenient way.

The object is further solved by a method for operating an RF device emitting RF signals according to claim 8. The method comprises the step: controlling the RF device to operate, at different points in time, in a plurality of different configurations in which RF signals comprising different device specific fingerprints are emitted. As a result, reliable identification can be ensured, if desired. Further, identification of the RF device can be reliably prevented, if desired.

If the method comprises the step of randomly changing the configuration of the RF device between different configurations of the plurality of configurations over time, signals emitted by the RF device can be prevented from being tracked. If the configuration of the RF device is changed independently from signals received by the RF device, it can be prevented that identification from the external using signals applied to the RF device is possible. In this case, signals received by the RF device may include signals applied to the RF channel as well as e.g. signals applied in another way such as signals from a PCI bus in the case of an Ethernet card or the like.

According to an aspect, the method comprises the steps: for each configuration, analyzing an RF signal emitted by the RF device for the device specific fingerprint; and identifying the specific RF device from the device specific fingerprints obtained for different configurations. Thus, a plurality of fingerprints is used for identifying the device. As a result, an uncloneable identification can be provided within achievable measurement ranges.

If the RF device is controlled to operate in the plurality of different configurations by transmitting signals to the RF device, identification of the RF device from the external is reliably enabled.

Preferably, the RF device is identified by comparing analyzed device specific fingerprints to expected characteristics. In this case, the comparison to expected values allows fast identification and authentication.

According to an aspect, the expected characteristics are signed and stored on the RF device and accessible from outside the RF device. In this case, the RF device, e.g. an RFID tag, can provide the expected characteristics to a reader which can authenticate the device by verifying the signature and comparing the measured fingerprints to those provided by the device as expected characteristics. If changing the configuration of the RF device is realized by subsequently activating different internal circuits, it can be ensured that different fingerprints are reliably provided for the plurality of different configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail hereinafter, by way of non- limiting examples, with reference to the embodiments shown in the drawings.

Fig. Ia) schematically shows the general structure of an RF device. Fig. Ib) schematically shows the general structure of an RF device according to an embodiment.

Figs.2a) to 2c) schematically show different configurations in an embodiment.

Fig. 3a shows a conventional RFID tag.

Fig. 3b shows an RFID tag according to an example.

DESCRIPTION OF EMBODIMENTS

The general structure of an RF device will first be described with reference to Fig. 1. Fig. 1 schematically shows the general structure of a conventional RF device 100. As can be seen in Fig. 1, an antenna 5 receiving an RF input signal RF_1n is provided. The RF input signal RF_1n is passed to a modulator 6 which modulates the RF input signal RF_1n based on modulation data 7 and outputs an RF output signal RF_0Ut. In such a structure as shown in Fig. 1, due to manufacturing tolerances etc. every component of the RF device 100 contributes to the RF fingerprint of the RF device 100. Typically, the following properties of the RF device 100 will be variable, i.e. contributing to the device specific fingerprint: the transmission frequency fr of the RF output signal RF_0Ut and the amount of harmonics produced by the oscillator contained in the RF device 100; resonance frequency, quality factor, etc. of the antenna 5; the timing of the modulation data 7; non-linearities, transients, etc. of the modulator 6, and further properties. As has been discussed above, the variations existing in the components of the RF device 100 can be too small to allow identification with sufficient precision. An embodiment will now be described with reference to Fig. 2. As schematically indicated in Fig. 2, the circuits of the RF device 1 according to the embodiment have a configurable architecture, i.e. the RF device 1 can be operated in a plurality of different configurations. In the example shown in Fig. 2, the modulator 6 is implemented to be configurable. The modulator 6 can be operated in a plurality of different configurations based on a configuration vector 8. For example, the configuration vector 8 may comprise a plurality of parameters (p_n, ..., P3, p₂, Pi) controlling the modulator 6 to be operated in a plurality of different configurations defined by the parameters. However, the embodiment is not limited to this and instead of the modulator 6 or additionally thereto other circuits of the RF device 1 can be configurable.

In each of the configurations, the RF device 1 outputs an RF output signal RF₀Ut comprising a characteristic fingerprint. However, the fingerprints will be different for different configurations and thus the RF device 1 is adapted to output a plurality of different fingerprints corresponding to the plurality of different configurations. The plurality of different configurations of the RF device 1 can for example be realized by a plurality of different (physical) internal circuits 2, 3, 4 which are selectively activated based on the parameters of the configuration vector 8. This is schematically shown in Fig. 2. As can be seen in Fig. 2a), at a first time the RF device 1 is in a first configuration in which a first internal circuit 2 is used in the RF device 1. Thus, in response to an RF input signal RF_1n, the RF device 1 outputs an RF output signal comprising the characteristic fingerprint of the first internal circuit 2, schematically indicated as RF_0Uti- At a second time, see Fig. 2b), instead of the first internal circuit 2 a second internal circuit 3 is used in the RF device 1. As a consequence the RF output signal comprises a characteristic fingerprint caused by the second internal circuit 3, as schematically indicated as RF_0Ut2- Fig. 2c) shows a third configuration at a third time in which a third internal circuit 4 is used and the RF output signal comprises a characteristic fingerprint caused by the third internal circuit (RF_0Ut3). Although three different configurations are illustrated in Fig. 2, the invention is not limited to this number and more or less different configurations are possible.

Now, it will be described how the implementation in an RF device 1 can be practically realized for example. The method can be realized for example by first identifying a component (or group of components) which contributes to the RF signal fingerprint and the contribution of which is observable. It should be noted that in many cases every (or almost every) component contributes to the RF fingerprint but not all components have a contribution large enough to be noticeable in the presence of noise. In the example, next multiple instances of this component (or group of components) are added and means for selecting one or more instances of this component (or group of components) to operate are provided. It should be noted that the component (or group of components) may need to be adapted in order to work correctly, if multiple components can be selected simultaneously. These steps can be repeated for as many components as considered appropriate. A specific realization with the RF device 1 being formed by an RFID tag will be described with reference to Figs. 3a and 3b. Fig. 3a shows a conventional RFID tag comprising an inductor 10, a capacitor 11, a (load) resistor R and a switch S. The system shown in Fig. 3a uses load modulation for sending a message and the resistor R is switched on or off to send a signal using the switch S. The switch S is e.g. formed by a MOSFET or other suitable switching component. According to the specific realization according to an example for application of the present invention shown in Fig. 3b, the resistor R and corresponding switch S are replaced by multiple parallel (load) resistors R₁, R₂, R3, ..., R_n provided with corresponding switches S₁, S₂, S3, ..., S_n. These load resistors could be switched one at a time which results in n possible configurations. If k load resistors are

combinations for the value k = n/2. It should be noted that for this realization a slight modification to the component is needed; the resistance of the multiple resistors R₁, R₂, R3, ..., R_n should be k-times the resistance of resistor R. Thus, according to this realization a plurality of different configurations is provided in the RF device. Since the components of the different configurations differ in their properties due to manufacturing tolerances, a plurality of different fingerprints is provided.

Next, it will be described how the RF device 1 can be operated.

The RF device can be used as a PUF (physically uncloneable function). A reader device communicating with the RF device can provide a challenge to the RF device causing the RF device to switch to a set of parameters, i.e. a specific configuration, for its communication settings. The device specific fingerprinting of the specific configuration of the RF device yields the response to the challenge.

For example, the RF device 1 is adapted such that the parameters for the configuration vector 8 are provided by the RF input signal RF_1n. Thus, depending on the properties of the RF input signal RF_1n, the RF device 1 is controlled to operate in a specific one of the plurality of different configurations. By applying, at different points in time, a plurality of different RF input signals to the RF device 1 as challenges and detecting the response of the RF device 1, i.e. the respective RF output signals RF_0Ut, a plurality of fingerprints of the RF device 1 can be obtained. Since a plurality of fingerprints is obtained, the amount of device specific information (entropy) which can be used to identify the RF device 1 is augmented. From the plurality of fingerprints, the RF device 1 can be reliably identified according to the following way: A database can be provided in which reference characteristics/fingerprints are stored. In this case, the measured fingerprints are compared to the reference fingerprints and the RF device 1 can be reliably identified based on the plurality of measured fingerprints. Thus, the RF device can be used for online device authentication.

Alternatively, the identity of the device and reference fingerprints can be signed and stored on the device. In this case, the RF device (e.g. an RFID tag) can send the signed fingerprints to a reader which can authenticate the RF device by verifying the signature and comparing the measured fingerprints to those sent by the RF device. Thus, the RF device can be used for offline device authentication.

As an alternative to controlling the different configurations by the applied RF input signal, the device could cycle through a pre-determined subset of parameters (different configurations) for its communication settings in a pre-determined sequence. Fingerprinting of the emitted RF output signals can then be used to identify the RF device. This can be done during normal operation of the RF device.

Further, the device could be used for covert channel communication. In this case, a reader communicating with the RF device is provided with k device specific fingerprints of RF signals corresponding to k different configurations (different values of the communication parameters) of the RF device. The RF device can be adapted to encode a message using the k "words" from the set of fingerprints. Since the reader is provided with the plurality of fingerprints, it can reliably decode the message. As an alternative, the RF device 1 can be adapted to reliably prevent unambiguous identification through its device specific fingerprints. This can e.g. be relevant if, for privacy reasons, it is not desired to "leave fingerprints". The movements of the RF device could e.g. be tracked using its device specific fingerprints. However, if this is not desired, the RF device can be adapted to randomly change its configurations such that different fingerprints are contained in the emitted RF signals with these different fingerprints randomly distributed over time. Thus, tracking can be reliably prevented if sufficiently many different configurations (fingerprints) are possible. If the different configurations are independent from applied RF input signals, the device cannot be identified from the external by its device specific fingerprints.

Claims

CLAIMS:

1. RF device (1) emitting an RF signal (RF_0Ut); the device having a configurable structure (2, 3, 4) operable in a plurality of different configurations in which different device specific fingerprints are present in emitted RF signals (RF_0Ut) to provide different device specific fingerprints at different points in time.

2. The RF device according to claim 1, wherein the configurable structure (2, 3, 4) is formed by a plurality of different internal circuits which can be selectively activated and provide different device specific fingerprints in the emitted RF signal (RF_0Ut).

3. The device according to any one of claims 1 or 2, wherein the device specific fingerprints are unique signal components resulting from manufacturing tolerances.

4. The device according to any one of claims 1 to 3, wherein the different configurations (2, 3, 4) are activated in response to signals (RF_1n) received by the RF device.

5. The device according to claim 4, wherein the signals (RF_1n) received by the RF device are RF signals.

6. The device according to any one of claims 1 to 3, wherein the device is adapted to randomly change the configuration between different configurations over time.

7. The device according to any one of claims 1 to 6, wherein the RF device is an RFID tag.

8. A method for operating an RF device (1) emitting RF signals (RF_0Ut), the method comprising the step: controlling the RF device (1) to operate, at different points in time, in a plurality of different configurations (2, 3, 4) in which RF signals (RF₀Ut₁, RF_0Ut2, RF_0Ut3) comprising different device specific fingerprints are emitted.

9. The method according to claim 8, further comprising the step: randomly changing the configuration of the RF device (1) between different configurations of the plurality of configurations (2, 3, 4) over time.

10. The method according to claim 9, wherein the configuration of the RF device (1) is changed independently from signals (RF_1n) received by the RF device.

11. The method according to claim 8, further comprising the steps: - for each configuration (2, 3, 4), analyzing an RF signal (RF_0Utl, RF_0Ut2, RF_0Ut3) emitted by the RF device (1) for the device specific fingerprint; and identifying the specific RF device (1) from the device specific fingerprints obtained for different configurations.

12. The method according to claim 11, wherein the RF device (1) is controlled to operate in the plurality of different configurations by transmitting signals (RF_1n) to the RF device.

13. The method according to any one of claims 11 or 12, wherein the RF device (1) is identified by comparing analyzed device specific fingerprints to expected characteristics.

14. The method according to claim 13, wherein the expected characteristics are signed and stored on the RF device (1) and accessible from outside the RF device.

15. The method according to any one of claims 8 to 14, wherein changing the configuration of the RF device (1) is realized by subsequently activating different internal circuits (2, 3, 4).