Home

nShield: A Noninvasive NFC Security System for Mobile Devices

1. Figure 12 nShield achieves an atten Figure 13 Delay caused by deter Figure 14 Accuracy of attenuation uation range of about 10dB oscilloscope A Nexus 7 serves as the NFC initiator in this experiment We vary the field strength near the tag by changing the distance between the initiator and the tag The PRR associated with each field strength value is computed from 100 transmissions The field strength measurements are normalized Fig 11 a and b show the PRR FS curves of Mifare Classic tag and Mifare Ultrlight tag respectively We can see that all the curves have narrow transition regions lt 0 2 dB in which the PRR values quickly increase from 0 to 1 We further observe that Mifare Ultralight tag has a narrower transition region than the Mifare Classic tag 0 05 dB vs 0 2 dB This is because the Mifare Ultralight tag has a much smaller antenna size making it more sensitive to the field strength For each tag we can see that the field strength required for a successful first polling round is lower than that for the second polling round As mentioned in Section 6 2 this is due to the fact that the tag has more time to harvest energy before the first round of polling 8 4 Attenuation Range and Granularity nShield provides a wide attenuation range and fine atten uation granularity which allows it to precisely control the strength of the NFC RF field to the optimal level This sub section evaluates the attenuation range
2. Given the Q factors of transmitter antenna Q and the harvesting antenna Qn the maximum power transfer efficiency of the NFC antenna pairs can be expressed as 25 0 1 2 a 2 1 V1 U0 U kVQ Qn 3 where k is the coupling coefficient with 0 being completely uncoupled and 1 being perfectly coupled k depends on many factors such as the distance between the two anten nas antenna alignment and etc For NFC since the com munication pairs are always placed in proximity k is usually above 0 1 8 For each nShield installation k is largely a constant value as nShield is fixed on the back of the mobile device Due to the NFC communication bandwidth require ment about 1 8 MHz 8 the Q factor of the transmitting antennas Qz is about 15 for most NFC devices 17 As a result the maximum power transfer efficiency of nShield is largely determined by the Q factor of the harvesting an tenna Qn A high power transfer efficiency can thus be achieved by using harvesting antennas with high Q factors above 50 For example if k Q and Qn of an NFC energy harvesting system are 0 2 15 and 100 respectively a maxi mum power transfer efficiency of 77 could be achieved A key insight of this analysis is that the harvesting antenna cannot be reused by the NFC transceiver due to the con flicting requirements of the Q factors Therefore to support efficient energy harvesting and reliable NFC communication at the same
3. nShield can harvest a power of about 55 mW com pared to mere 10 2 mW and 17 7 mW of NFC Discover kit and NFC WISP respectively The significant improvement Tire Software define radio Dedicated ASIC NFC radio NFC X capability upports NFC A implemented amming only radio NFC B NFC F HW accelerated SW encoding No TX support RX capability upports NFC A implemented NFC A NFC B and NFC NFC B NFC F HW accelerated SW decoding HW decoding Optimization High Q antenna N A Harvestable power 55 mW maximum 30 mW transferred to antenna Max initiator 100 tag emulation 66 subcarrier faut System pwr Table 3 Comparison of hardware of nShield and EnGarde Energy harvesting on the energy harvesting efficiency enables nShield to power additional components and perform sophisticated operations to ensure system security To date the most relevant work to ours is EnGarde 21 EnGarde is a hardware NFC security device that jams on going malicious NFC transactions Different from RFID guardian and Proxmark III EnGarde is optimized for mobile devices and harvests energy from NFC transmissions How ever EnGarde protects NFC by censoring the content of NFC transactions and hence cannot defend against eaves dropping attacks We provide a comparison between the hardware of the two systems which is summarized in Table 3 nShield is built based on a software define radio SDR which is capable of transmitting to and receiving fr
4. 9 b shows the power transferred to the harvesting antenna at different output voltages We can see that the transferred power decreases linearly when the output volt age increases When the output voltage of the harvesting antenna is zero the antenna receives the maximum power However it also delivers virtually no power to the system resulting in an extremely low power harvesting efficiency as observed from both Fig 9 a and b When the out put voltage is about half of the antenna open circuit volt age the maximum power is harvested although the power transferred to the antenna is significantly lower These re sults show that in order to deliver the maximum power to the system the battery and the harvesting antenna must achieve a voltage matching We next evaluate the power harvesting and transfer effi ciencies of nShield We only use Adafruit PN532 break board as initiator in this experiment because the transmis sion power of Nexus 7 cannot be accurately measured due to its packaging The transmission power of the PN532 board can be obtained by measuring the current draw on the TVD D pin of PN532 chip which supplies power to its internal coil exciting circuits The harvesting antenna is connected with a potentiometer which serves as a variable load Fig 10 a shows the amount of power transmitted trans ferred and harvested under different loads to the harvesting antenna We can see that the transmission power increase
5. effi Figure 11 PRR FS curves of two N harvested from Nexus 7 and PN532 ciency and power transfer efficiency FC tags breakboard A heavier lighter load to the harvesting antenna creates a slightly stronger weaker mutual coupling which in turn leads to a stronger weaker detuning effect The detuning effect changes the impedance of the antenna resulting in less power transferred The highest transmission power is about 440 mW Fig 10 b shows the computed energy transfer and har vesting efficiencies We can observe that the energy transfer efficiency increases linearly with the load to the harvesting antenna while the energy harvesting efficiency is a parabo la curve which peaks at the voltage matching point 11 V When the output voltage of the harvesting antenna is be low 4 V the energy transfer efficiency is close to 1 At this point most of the transmitted energy is absorbed by the harvesting antenna and the strength of the RF field created by the initiator is significantly attenuated The energy har vesting efficiency peaks at 24 4 when the output voltage of the harvesting antenna is 11 V We discuss the energy harvesting efficiency in Section 10 8 2 System Power Consumption and Lifetime We use an Agilent 34410A benchtop multimeter to mea sure the power consumption of nShield The results are sum marized in Tab 1 The most power consuming states are data reception and transmission This is because the MCU ha
6. strength of the attenuated signal over tion distances of two tags with differ over distance distance sniffing distance the eavesdropping attack becomes signifi cantly more difficult However the optimal attenuation level varies significantly for different initiators Specifically Fig 2 shows that to reduce the signal power to an undecodable level for sniffers the NFC signal needs to be attenuated by 9 8 dB NFC Breakboard 5 9 dB Neuxs 7 4 2 dB Note 2 and 2 2 dB Galaxy Nexus respectively Such significant diversity is caused by the differences in initiator implementations such as the size of antenna We now show that for a given initiator the maximum allowed attenuation level also varies significantly across tar gets We measure the maximum communication distances between the NFC breakboard and two passive tags Mifare Classic and Mifare Ultralight with different attenuation lev els applied to the RF field Fig 3 shows that the communi cation distances decrease when the attenuation level increas es However the Mifare Classic can tolerate a maximum attenuation level of about 9 dB while Mifare Ultralight can only tolerate about 3 dB This huge difference is the result of the diverse receiving sensitivities of tags 3 3 Discussion We now summarize the results of our experimental study First current NFC implementations emit significantly more RF power than intended As a result the passive eaves dropping dist
7. successful initiator tag communications for 10 seconds We manually determine the ground truth optimal attenuation level for each tag initiator distance by examining all attenuation levels from a high to low order We use an Agilent probe to form a small loop an tenna and connect the probe to an Agilent DSOX2024A os cilloscope to measure the attenuated RF field strength We then run the adaptive attenuation algorithm for ten times and measure the resulted RF field strength of each run Fig 14 shows that 90 of the estimation errors of the Mifare Classic tag at distances of 0 cm 2 cm and 4 cm fall below 0 3 dB 0 34 dB and 0 52 dB respectively For the Mifare Ultralight tag at distances of 0 cm 1 cm and 2 cm 90 the errors fall below 0 12 dB 0 16 dB and 0 35 dB re spectively The mean errors of the two tags are only 0 29 dB and 0 1 dB respectively We can observe that Mifare Ultra light tag generally incurs smaller error than Mifare Classic tag This may be because the Mifare Ultralight tag has a much smaller antenna size which makes it more sensitive to the field strength As a result it has a narrower transi tion region which conforms the finding in Section 8 3 This makes Mifare Ultralight tag more responsive to our adap tive attenuation algorithm resulting in a smaller estimation error Next we evaluate the eavesdropping distances achieved with our sniffer at different initiator tag distances We record the eavesdropping d
8. the PRR AL curve is very narrow see Section 8 3 this ensures that the optimal level would not be accidentally discarded Algo rithm 6 1 shows the pseudo code of the adaptive RF field attenuation algorithm nShield exploits the target discovery process which is always performed by the initiator in the initial phase of the communication to perform adaptive RF field attenu ation The NFC initiator periodically performs this process by broadcasting NFC discovering probes at a rate about 3Hz on Android smartphones If a target NFC device which can be a tag or another NFC initiator working in active mode hears this probe it will send an acknowledgement message back to the initiator The initiator will then con firm the discovery of the target device by broadcasting a response The two devices will then exchange a few mes sages back and forth to learn a few parameters such as IDs and capabilities There are several advantages of exploit ing this process for adaptive RF field attenuation First the NFC target discovery process is mandatory in all NFC communication modes and NFC standards NFC A NFC B and NFC F 9 Second this process does not involve the data payload The communication conducted during adaptive RF field attenuation might be eavesdropped due to the possibly high initiator transmission power However this does not lead to security breach since there is no data payload exchange Once adaptive RF field attenuation i
9. time a dual antenna configuration one high Q factor antenna and one low Q factor antenna must be adopted According to 1 to improve Q factor of an NFC antenna we can either increase its inductance or decrease its series resistance In our harvesting antenna design shown in Fig 6 we use wide antenna tracks to decrease the series resis tance and closely couple the antenna tracks to increase the inductance The parasitic capacitance also contributes to the series resistance of the antenna We adopt a single layer antenna to decease the parasitic capacitance The resulted high Q factor ensures that when the transmitter antenna and the harvesting antenna are closely coupled the harvest ing antenna can receive most of the radiated energy The implementation details of the harvesting antenna are given in Section 7 5 2 Energy Management Circuit Another major factor that affects the amount of power harvested to the system is the design of the energy manage ment circuit The energy received by the harvesting antenna has to be transferred to the energy storage components in the system e g batteries or super capacitors A common practice for maximizing power transfer is to match the out put impedance of the antenna with the input impedance of the load 24 The maximum power that can be transferred Proad can be expressed as Proad eee Ricoa Vane 0 25Pmaz 4 Rant Rioad 4Rant where Uant open is the open circuit root mean
10. transmitted The TX control cir cuit can generate different clock frequencies according to the data rates of the modulation schemes nShield reduces the risk of eavesdropping by absorbing the excessive RF pow er radiated by the initiator with an adjustable attenuator which is multiplexed with the load modulator The receiving antenna is connected to a peak detector which removes the AM carrier from the RF signal The hardware based demodulator on the MCU demodulates the baseband signal from which the raw data is retrieved A key novelty in the design of nShield is to exploit the hand shake mechanism in the target discovery process to determine the optimal transmission power of the initiator Specifically n Shield infers whether the previous messages are successfully received by examining the logical relationship between con secutive initiator messages To reduce the delay of deter mining the optimal attenuation level nShield adopts a bi nary search algorithm to accelerate the search nShield falls asleep to conserve energy when no NFC signal is present A low power wakeup circuit connected with the peak detector generates an interrupt signal to wake up the system once NFC RF field is present Fig 6 shows a prototype system of nShield The size of the circuit board and the antenna is 5 5 cm by 5 3 cm and 9 6 cm by 9 6 cm respectively We note that this antenna is spe cially designed for Nexus 7 tablet The size of antenna can be reduc
11. voltage which is dynamically set by the adaptive RF field attenuation algorithm described in Section 6 2 us ing an onboard DAC A novel design of nShield is that the attenuator is multiplexed with the load modulator of the NFC transmitter This design reduces the cost and size of nShield Our experiment in Section 8 4 shows that nShield can achieve an attenuation range of 10 86 dB which is suf ficient for the purpose of regulating NFC RF field strength 6 2 Adaptive RF Field Attenuation Algorithm nShield adapts the signal attenuation level dynamically to ensure reliable communication between the initiator and the target device nShield equally divides the whole atten uation range into N discrete levels The goal of adaptive RF field attenuation is to find the optimal attenuation lev el in the N levels with which the attenuated field strength is just enough to support reliable bi directional communica tions between the initiator and the target Fig 8 illustrates the relationship between Packet Reception Ratio PRR and the attenuation levels AL nShield tries to use an attenu ation level as high as possible while ensuring the resulted PRR to be close to 1 i e high communication reliability Aopt Shown on Fig 8 is the optimal attenuation level 1 Packet Reception Ratio 0 5 Atrans 123456789 p Attenuation Level Figure 8 An illustration of the attenuation level vs Packet Reception Ratio relationship N However a k
12. well as transmitting data to the initiator The smaller antenna is responsible for overhearing data from the initiator We show in Section 5 that this dual antenna configuration is essential for maximizing the energy harvesting efficiency without sac rificing the receiving performance as the receiving antenna and the harvesting antenna require fundamentally different design methods Battery and Management Circuit Control Load Modulator 8 Attenuator Gate Q voltage a TX D amp Control z Demodulat Z Peak Baseband teruh emodulator x Detector signa Wakeup a Circuit A mer Figure 5 Block Diagram of nShield The harvesting antenna is connected with an RF bridge rectifier which rectifies the RF signal to a DC voltage The DC voltage is then regulated to provide power to the sys tem and charge a 20 mAh on board battery In Section 5 we show that the voltage matching between the harvesting antenna and the battery plays a critical role in maximizing the amount of power harvested by the system The load modulator is connected with the rectifier which alters the load of the harvesting antenna to transmit data to the NFC initiator Since the load modulation based communication scheme adopted by NFC standard requires strict timing n Shield employs a hardware TX control circuit to accurately generate the clock used by the load modulation and precisely synchronize the data to be
13. which allows it to provide security protection for a diverse set of NFC platforms Last ly nShield only introduces insignificant delay up to 2 2 s to NFC data communications 12 REFERENCES Adafruit PN532 breakboard http www adafruit com products 364 2 Americans spend 58 mins a day on smartphones http www experian com blogs marketing forward 2013 05 28 americans spend 58 minutes a day on their smartphones 3 DES wikipedia site http en wikipedia org wiki Data_Encryption_Standard 4 How soon is now NFC smartphones and physical access control systems http blogs gartner com mark diodati 2011 10 31 how soon is now nfc smartphones and physical access control systems 5 Impedance matching wikipedia site 6 Mobile payments today http www mobilepaymentstoday com research 400 Contactless NFC 7 Near field communication NFC 2014 2024 http www prnewswire com news releases near field communication nfc 2014 2024 227654461 html 8 Near field communication wikipedia http en wikipedia org wiki Near_field_communication 9 NFC forum technical specifications http www nfc forum org specs spec_list 10 NFC WISP project site http www alansonsample com research NFC WISP html 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 NXP PN532 user manual http www nxp com documents user_manual 1415
14. 20 pdf Proxmark 3 project site http www proxmark org RSA wikipedia site http en wikipedia org wiki RSA_ 28cryptosystem 29 ST discovery kit http www st com web en catalog tools FM116 SC1444 PF253360 Still not a wallet NFC has a second life as a safe simple pairing tool http gigaom com 2013 08 08 still not a wallet nfc has a second life as a safe simple pairing tool Strasbourg NFC ticketing moves to commercial launch http wow nf cworld com 2013 07 05 324901 strasbourg nfc ticketing moves to commercial launch Ti HF antenna design notes http www ti com rfid docs manuals appNotes HFAntennaDesignNotes pdf WEP wikipedia site http en wikipedia org wiki Wired_Equivalent_Privacy A Dementyev J Gummeson D Thrasher A Parks D Ganesan J R Smith and A P Sample Wirelessly powered bistable display tags In UbiComp 2013 H Falaki R Mahajan S Kandula D Lymberopoulos R Govindan and D Estrin Diversity in smartphone usage In Mobisys 2010 J J Gummeson B Priyantha D Ganesan D Thrasher and P Zhang Engarde protecting the mobile phone from malicious nfc interactions In MobiSys 2013 G Hancke Practical attacks on proximity identification systems In Security and Privacy 2006 IEEE Symposium on pages 6 pp 333 2006 E Haselsteiner and K Breitfu Security in near field communication nfc In Printed handout of Workshop on RFID Security July 2006 J J Karak
15. FC signal reception antenna is prototyped using the same materials and techniques except that it has much thinner tracks We use an impedance analyzer to tune the Q factor of the antenna to the optimal value of 15 17 The harvest ing and receiving antennas are then glued together The two prototype antennas can be easily mass manufactured using flexible thin film circuits We implement an NFC transceiver on nShield The re ception path is composed of a peak detector a comparator and a software decoder The RF signal from the antenna is first converted to baseband signal by the peak detector and then converted to clean logic levels by the comparator The decoder is implemented in software on the MCU To de crease the computational overhead hardware components on the MCU are adopted to assist the decoding Specifi cally a hardware timer is adopted to timestamp the transi tions of the logic levels and a DMA controller is employed to automatically transfer the timestamps to the RAM This design automatically collects samples without software in tervention enabling low power asynchronous decoding The data is then verified using CRC and reported to upper layer protocols For transmission nShield adopts the load modu lation communication techniques 9 in which the load of the antenna is modulated according to the data to be transmit ted We adopt a high speed MOSFET Fairchild FDV301N as the load modulator multiplexed with attenuat
16. NFC sniffers NFC signals can be eaves dropped from as far as 2 4 m away which is much further than the intended NFC working distance This poses a seri ous concern for security privacy sensitive NFC applications such as contactless payment 3 A MEASUREMENT STUDY In this section we experimentally study the passive eaves dropping distance of NFC transmissions Specifically we measure the physical distance at which the signals from ini tiators and targets can be successfully decoded i e eaves dropped Moreover we study the impact of transmission power attenuation on the passive eavesdropping distance of different NFC devices The results provide important moti vation for the design of nShield We note that the actual eavesdropping distance depends on many factors such as initiator implementation initiator position NFC working mode active or passive and envi ronmental factors e g background noise Our measure ments are conducted in typical settings and an exhaustive evaluation of all these factors is beyond the scope of this pa per Nevertheless our results raise serious concerns about the physical security of NFC due to the significant discrep ancies between the actual and intended working distances and shed lights on possible defense mechanisms 3 1 Experimental Setup Our experiment is conducted using NFC initiators tags and a sniffer Commercial off the shelf NFC transceivers do not make good sniffers for tw
17. ance is at least an order of magnitude of the intended NFC communication range This issue greatly in creases NFC users risk of being eavesdropped Second the NFC RF field strength can be effectively attenuated exter nally to enhance the security of NFC without sacrificing the communication reliability However the desired attenuation level varies significantly with the specific working conditions including initiator transmission power target reception sen sitivity initiator target distance and etc Therefore simple solutions such as an external signal attenuator with fixed amount of power reduction would not work for all scenarios These results have several important implications for the security of NFC systems Properly implemented cryp tosystems can offer strong security assurance even when the communication could be eavesdropped However as NFC is usually considered physically secure many upper layer protocols of NFC applications do not implement encryption or only adopt short keys in encryption algorithms such as DES 3 With an passive eavesdropping distance up to 244 cm as shown in our study these systems hence are exposed to malicious attacks For instance the leakage of pairing code during NFC based Bluetooth paring could lead to possible passive eavesdropping or even man in the middle attack on the following data communications This issue is aggra vated in active NFC communication scenarios where both NFC devi
18. and step that can be achieved by nShield We manually tune the DAC connected with the attenuator to sweep through its entire voltage out put range with a step of 0 05 V To measure the attenuated signal strength we use an Agilent probe to form a small loop antenna and connect the probe to an Agilent DSOX2024A oscilloscope We record the measured peak to peak ampli tude Vpp of the NFC signal Fig 12 a depicts the signals that are maximally at tenuated and unattenuated We can see that nShield can significantly decrease the strength of NFC signals as the Vpp of the signal decreases from 2 14 V to only 0 216 V af ter the maximum attenuation level is applied Fig 12 b shows the computed attenuation levels with different DAC output We can observe that the effective attenuation re gion roughly takes about a quarter of the full output scale of the DAC ranging from 0 8 V to 1 4 V This is due to the characteristic of the attenuator on nShield which is a high speed switching MOSFET The MOSFET is complete ly shut down when the gate voltage is below 0 8 V and is saturated when the gate voltage is above 1 4 V Therefore it operates as a variable attenuator only when the gate volt mining attenuation level level determined by nShield age is between 0 8 V and 1 4 V The maximum attenuation 10 86 dB is achieved when the MOSFET is saturated We can also observe that the attenuation is nonlinear with the DAC output resulting in a n
19. ash Transmission lines and filter networks Macmillan New York 1950 M Kesler Highly resonant wireless power transfer Safe efficient and over distance 2013 H S Kortvedt and S F Mj lsnes Eavesdropping near field communication In The Norwegian Information Security Conference NISK 2009 A Kurs A Karalis R Moffatt J D Joannopoulos P Fisher and M Solja i Wireless power transfer via strongly coupled magnetic resonances science 317 5834 83 86 2007 Z N Low R Chinga R Tseng and J Lin Design and test of a high power high efficiency loosely coupled planar wireless power transfer system Industrial Electronics IEEE Transactions on 56 5 1801 1812 2009 C Miller Exploring the nfc attack surface Proceedings of Blackhat 2012 M R Rieback G N Gaydadjiev B Crispo R F H Hofman and A S Tanenbaum A platform for rfid security and privacy administration In LISA 2006 D Welch and S Lathrop Wireless security threat taxonomy In Information Assurance Workshop 2003 IEEE Systems Man and Cybernetics Society pages 76 83 2003
20. bution of this work is the analysis of the factors affecting the energy harvesting efficiency and the design of a highly effective energy har vesting system nSheild is capable of harvesting significant amount of power 55 mW from commodity mobile devices which is at least a 1 8X improvement over the state of the art NFC based energy harvesting systems Together with the extremely lo power design it enables nShield to provide the host uninterrupted protection against malicious eaves dropping Lastly the small form factor self sustainability and transparency to OS makes nShield an attractive solu tion to retrofit existing mobile devices with protection a gainst passive eavesdropping In summary we make the following key contributions in this paper 1 We conduct an experimental study on the feasibility of passive NFC eavesdropping with a specially designed inexpensive NFC sniffer We show that commodity NFC enabled devices can be eavesdropped from up to 240 cm away which is at least an order of magni tude further than the intended NFC communication distance Moreover although external signal attenua tion is effective in reducing NFC transmission power the desired attenuation level that can still sustain data communication is highly dependent on the NFC hard ware tags sensitivity and the physical distance To our best knowledge this is the first empirical study on passive NFC eavesdropping in practical settings 2 We des
21. ces actively transmits using high transmission pow ent attenuation levels er and eavesdropping attacks on both of the devices could be launched over distance Moreover the feasibility of N FC eavesdropping attack renders encryption the last line of defense against attacks Unfortunately with the rapid ad vance of decryption techniques many once considered safe encryption protocols including WEP 18 DES 3 and R SA 13 have been demonstrated vulnerable when sufficient encrypted data is observed through eavesdropping 4 OVERVIEW OF nShield 4 1 Design Objectives and Challenges It is shown in Section 3 that current NFC initiator imple mentations emit significantly more RF power than intended which greatly increases the user s risk of being eavesdropped This result motivates us to develop an NFC security protec tion device called nShield that dynamically regulates the strength of the RF field radiated by NFC initiators nShield regulates the RF strength by absorbing the excessive RF power with its own antenna nShield can be easily stuck on the back of mobile devices and is solely powered by the ab sorbed RF energy thus eliminating offline charging Specif ically we have the following design objectives Adaptive RF field strength regulation Today s N FC devices exhibit significant diversity in terms of initia tor transmission power and the receiver sensitivity nShield must be able to dynamically adjust t
22. d the tar get then exchange a few parameters back and forth to learn the capabilities of each other before the start of the real data communication On an NFC enabled Android phone when the screen of the phone is unlocked the NFC radio is ac tivated and the discovery process starts automatically and continues until a target device is discovered During this process the discovery probes are broadcast at a frequency of about 1 4 Hz Using NFC antennas a device can harvest energy from the RF field generated by NFC initiators within close proximity a few centimeters However the amount of energy that can be harvested during the probing is usual ly very limited as NFC radios have a low duty cycle 10 during the probing phase Passive eavesdropping attacks are harmful to wireless com munications in several ways They could not only compro mise the privacy security of the system but also serve as the early steps of other more damaging attacks 31 e g the man in the middle attacks 31 Another reason that makes passive eavesdropping attacks especially harmful is that they are hard to detect as they do not actively transmit any sig nal and are usually launched from distance NFC is gen erally considered to be a secure wireless technology against eavesdropping due to its short communication range How ever current NFC implementations often emit significantly more RF power than intended Our study shows that with specially designed
23. djusted to mitigate the potential risks of eavesdropping Existing efforts on NFC security can be classified into two basic categories Several solutions improve the security of NFC by adding more security elements such as additional secret keys to the native OS of mobile devices 23 How ever the mobile device would become vulnerable if the in tegrity of the OS is compromised e g after being rooted The second category employs additional hardware devices to secure NFC 30 12 However these hardware systems are bulky and power hungry which are ill suited for mobile devices In a recent work 21 a hardware security device is developed to harvest energy from NFC transmissions and jam malicious interactions However due to the low energy harvesting efficiency the system may not provide uninter rupted protection The above approaches are designed to prevent content based malicious attacks and none of them can protect NFC from eavesdropping attacks In this paper we propose a novel noninvasive NFC se curity system called nShield to protect NFC against passive eavesdropping nShield is a credit card sized thin pad that can be easily stuck on the back of mobile devices see Fig 6 nShield implements a novel adaptive RF attenuation scheme in which the extra RF energy of NFC transmission s is determined and absorbed by nShield At the same time nShield scavenges the extra RF energy to sustain the perpetual operation A key contri
24. e been conducted on the dis tance of eavesdropping RFID proximity cards In 22 the authors measure the passive eavesdropping distance of the communication between a commercial reader and a Philips Mifare card using a wide band sniffer The results show that the possible eavesdropping distance is more than 4 m 22 In 23 the authors analyze the security of NFC and esti mate the passive eavesdropping distance of NFC to be about 10m However this result is not experimentally validated In 26 the maximum passive eavesdropping distance of N FC is empirically measured to be 30 cm using Mifare tags and an oscilloscope However the antennas of Mifare tags used in their experiments are not optimized for eavesdrop ping To our best knowledge our work is the first empirical study on the practical passive NFC eavesdropping distance under realistic experimental settings We have designed and implemented a prototype NFC sniffer Its small form factor and high sensitivity demonstrated the feasibility of launch ing passive eavesdropping attack from distance In particu lar we are able to achieve a 2 4 m eavesdropping distance with our portable NFC sniffer see Section 3 Several approaches have been proposed to protect NFC from malicious attacks A common solution is to modify the OS of mobile devices 23 to enhance the security of NFC How ever the mobile device would become vulnerable if the in tegrity of the OS is compromised e g b
25. ed for smartphones without sacrificing the energy harvesting efficiency and attenuation performance The size of the prototype circuit board can be shrunk significantly by removing unnecessary components like debug port buttons and LEDs As a result nShield can be easily fit on diminu tive thin film circuit boards which could be stuck to the back of small size mobile devices The total component cost of our prototype implementation is under 20 and could be further reduced when nShield is mass manufactured Figure 6 Antenna and circuit of nShield mounted on the back of a Google Nexus 7 tablet 5 MAXIMIZING HARVESTED ENERGY nShield is powered solely by the energy harvested from NFC transmissions The capability of harvesting a large amount of power not only enables the uninterrupted pro tection of nShield but also helps increase the attenuation range of the host s NFC transmission power Fig 7 shows the block diagram of the energy harvesting subsystem of nShield which comprises a harvesting antenna and an en ergy management circuit These two components work together to determine the amount of power that could be harvested We show that they must be carefully designed to maximize the harvested power We define the following two terms to characterize the performance of energy harvesting Energy power transfer efficiency is defined as the ratio of the amount of energy power transferred to the harvest ing antenna to the am
26. enuate the emitted RF field The thickness and the area of the aluminum foil are adjusted to create different RF field strength while the maximum passive eavesdropping distances are measured with our sniffer We use a loop an tenna connecting with an Agilent oscilloscope to measure the RF field strength after attenuation Fig 2 shows that as expected for all the 4 tested initia tors the passive eavesdropping distances decrease when the attenuation level increases When the strength of the NFC RF field is just enough to support reliable communication our sniffer can achieve a maximum passive eavesdropping distance of around 80 cm which is 67 NFC Breakboard 48 Neuxs 7 39 Note 2 and 31 Galaxy Nexus shorter than those without attenuation With such a short 250 2500 z ee T 10 NFC Breakboard 5 Poa NFC Breakboard Mifare Classic Song E oo ER Nexus 7 a 200 ooo Nexus 7 E a Mifare Ultralight Z 000 Galaxy Nexus z Galaxy Nexus 58 E S zan pa Note 2 B 150 Note 2 g 1500 a S z z 8 1000 g 100 ti gt 500 3 50 Reha ua E o RF strength threshold for Mifare Classic a W 0 0 0 0 100 200 300 0 2 4 8 10 12 14 0 2 4 _ 6 8 10 Distance cm Attenuation dB 3 Attenuation dB Figure 1 The received signal Figure 2 The received signal Figure 3 The maximum communica strength of the unattenuated signal
27. es and the open circuit output voltages of the harvesting antenna We then compute the power harvest ed by the system and the power transferred to harvesting antenna under different loads Fig 9 a depicts the harvested power under different antenna output voltages We can see that the curves are parabolas with the maximum power of 55 mW at 5 V and 90 mW at 12 V respectively when Google Nexus 7 and P N532 breakboard are used The amount of power that can be harvested from PN532 breakboard nearly doubles that from Google Nexus 7 This is because PN532 breakboard has a much higher transmission power than Nexus 7 ac cording to our measurement However as the antenna is optimized for working with Nexus 7 nShield cannot harvest the maximum amount of power from PN532 breakboard In particular the maximum power is harvested at 12 V out put and the battery voltage on nShield is only 4 8 V This voltage mismatch limits the maximum harvested power to be only 57 mW An impedance matching block is required to shift the open circuit voltage to around 10 V for PN532 breakboard as discussed in Section 5 2 On the other hand nShield can receive the maximum power when working with Nexus 7 due to the tight voltage matching These result s also confirm that a super capacitor is a poor choice for energy storage on nShield since the voltage of super capac itors varies significantly with its discharging level resulting a poor voltage matching Fig
28. ey challenge in the design of nShield is that without prior knowledge about the target device such as reception sensitivity and initiator target distance nShield cannot know what RF field strength would support reli able communications NFC work in a poll response fash ion in which the target only transmits after it was polled by a message from initiator We refer to the process of a polling and its subsequent response as a polling round To find out wether an attenuated field strength can support bi directional communication the initiator has to attempt a polling round with the attenuation level in question nShield learns if a polling round is successfully complete by exam ining the logic of the polling messages of consecutive polling rounds In particular some polling messages such as the Single Device Detection Request and the Select Request de fined in the NFC A standard can only be transmitted if the previous polling round succeeds When overhearing such polling messages nShield infers that the previous polling round ends successfully As shown in Section 8 3 for the passive communication mode the field strength required for completing the first polling round is lower than that for completing later polling rounds This phenomenon is caused by insufficient energy left on the tag after the first polling round Passive tags rely on the energy from the NFC RF field to operate After activating the RF field the initiator pauses fo
29. fall below 2 2 s while the mean delay is 2 1 s An interesting phenomenon is that the delay of Mifare Classic incurred at a distance of 4 cm is smaller than those incurred at 2 cm and 0 cm This is because the delay is largely proportional to the number of steps that the adaptive attenuation algorithm has to take to find the optimal attenuation level which varies between 6 and 12 in nShield Thus a longer communication distance could possibly incur a shorter delay We also notice that the adaptive attenuation algorithm is resilient to minor tag initiator distance variation as nShield can almost always find the optimal attenuation level within 2 2 seconds We did observe some long delays 3s to 4s although they are rare lt 5 Our further investigation indicates that they are caused by occasional initiator halts in which the initiator pauses its transmission for 1 to 2 seconds while the RF field remaining active Finding the exact reason of this long initiator halt is left for future work 8 6 Accuracy and Effectiveness of Adaptive At tenuation We evaluate the accuracy of adaptive attenuation algorith m in estimating the optimal attenuation level in this subsec tion The initiator we use in this experiment is the PN532 breakboard For each tag under test we evaluate the opti mal attenuation level with different tag initiator distances We define the optimal attenuation level as the highest at tenuation setting that can support
30. he amount of absorbed power to ensure that the remaining RF power is just enough to sustain successful NFC communications As nShield has no prior knowledge about the receiving sensitivity of the target a trial and error approach is needed to determine whether NFC communications can be sustained at a partic ular power level However trying all possible attenuation levels incurs high delay due to the wide attenuation range and the low frequency of NFC transmissions Noninvasive operation The operation of nShield should not rely on either initiator nor target In other words it should work in a standalone manner with no physical con nections to neither initiator nor target This requires nShield to be a self sustained self powered device which has its own CPU and power source Moreover it should be transparent to the host without the need to communicate with the host or modify the NFC protocols The noninvasive and trans parent nature of nShield enables it to easily retrofit the ex isting NFC devices with security protection However a key challenge presented by this design is that as nShield cannot interact with either initiator or target it has to determine the right transmission power solely based on the overheard transmissions Unintermittent protection nShield should provide the host devices unintermittent protection against passive eaves dropping In particular the down time of protection caused by battery depletion sho
31. icient for keeping the battery charged In the rare case when the mobile de vice is only infrequently unlocked for a long period nShield may deplete its battery To address this issue we adopt a technique called tag emulation to have the initiator signifi cantly increase its duty cycle Specifically nShield emulates itself as a passive IS014443A tag and responds to the prob ing messages sent by the initiator As a result it triggers the initiator to stay active This leads to a 10X increase of the initiator output energy allowing nShield to be rapid ly charged However this process may interfere with NFC transactions as the initiator cannot communicate with other target devices when the tag emulation is active We adop t the following adaptive mechanism to address this issue First nShield pauses the tag emulation for 1 second every 2 seconds allowing the initiator to discover other target de vices during the pause Second nShield only activates tag emulation when the discharging level of the onboard battery is lower than 30 6 ADAPTIVE RF FIELD ATTENUATION 6 1 Attenuator nShield reduces the risk of being eavesdropped by atten uating the NFC RF field strength using the harvesting an tenna The level of attenuation to the RF field is adjusted by the load of the harvesting antenna nShield adopts a MOSFET as the variable load i e attenuator to the anten na The resistance of the MOSFET is controlled by its gate terminal
32. ign an NFC security system called nShield to protect NFC from passive eavesdropping attacks As a key novelty nShield absorbs the excessive RF energy of NFC to attenuate the signal strength against pas sive eavesdropping while the absorbed RF energy is scavenged for its perpetual operation By exploiting the NFC target discovery process nShield intelligent ly determines the right attenuation level that is just enough to sustain reliable data communication As a result it can promptly and precisely control the signal strength of NFC transmissions mitigating the risk of passive eavesdropping 3 We carefully analyze the factors that affect the NFC energy harvesting efficiency and apply several design techniques to the antenna and hardware of nShield to maximize the amount of harvested energy which in clude quality factor optimization voltage matching and tag emulation As a result nShield can harvest significantly more power 1 8X and 3 1X than the two state of the art NFC energy harvesting systems This capability enables nShield to provide the host uninter rupted protections against passive eavesdropping at tacks 4 We implement a prototype of nShield and evaluate it s performance via extensive experiments Our results show that nShield has extremely low power consump tion high energy harvesting efficiency and can adap tively attenuate the signal strength of NFC transmis sions in a variety of realistic settings while
33. initiator transmissions the eavesdropping distance of tag transmissions is significantly shorter due to the much weaker signal strength of the tag response We acknowl edge that better hardware design and more advanced signal processing techniques could achieve even longer eavesdrop ping distances Nevertheless our results are already suffi cient to demonstrate that the current NFC implementations on smartphone and tablet platforms are subject to passive eavesdropping from a distance at least an order of magnitude longer than the intended NFC communication range Pre Amplifier To PC ADC gt Antenna AM Demodulator Figure 4 Block diagram of the NFC sniffer used in the measurement study A promising approach to defending against passive eaves dropping is to reduce the transmission power of the initiator However the current NFC chipsets adopt fixed transmission power which leaves attenuating the signal externally the on ly choice We need to answer the following two questions in order to design an external signal attenuator 1 what is the maximum attenuation level that could be applied with out sacrificing the reliability of data communication and 2 what is the resulted passive eavesdropping distance We investigate these questions in the second experiment We adopt the same experimental setting as in the first experi ment except that we cover the initiators with thin aluminum foils to att
34. istances at which the received signal strength of the initiator falls below 100 mV by following the same procedure of the measurement study in Section 3 The results are summarized in Table 2 It can be seen that for each tag the eavesdropping distance decreases with the initiator tag distance This is because a longer initiator tag distance requires a stronger signal strength to ensure reliable communication which increases the eavesdropping distance We also notice that the Mifare Ultralight tag always incurs longer eavesdropping distances than Mifare Classic tag This is because the low sensitivity receiver of the Mifare Ultra light tag requires higher transmission power to maintain reli able communication The shortest eavesdropping distances for the two tags are 48 cm and 70 cm respectively It is worth noting that even after significant reduction the re sulted eavesdropping distance may still be further than the expected NFC working distance This is largely due to the fundamental design trade off of NFC nShield could apply higher attenuation to decrease the eavesdropping distance to only a few centimeters but this would significantly re duce the reliability of the NFC communication eee Initiator tag Distance Ultralight Table 2 Eavesdropping distances after attenuation 9 RELATED WORK Near Field Communication NFC is a new short range wireless communication standard evolved from HF RFID technology Several studies hav
35. ment is to show two key observations based on which the adaptive RF field attenuation algorithm is designed 1 the transition regions on the PRR FS curves are very nar row and 2 the field strength required for completing the first polling round is higher than the subsequent rounds We attach a thin aluminum antenna to the back of each tag to measure the field strength using an Agilent DSOX2024A The exact amount of harvested power is not given in 21 However it is expected to be much lower than 30 mW due to the load source mismatch and the loss on rectifying and regulating components a a Tag Mifare Classic a Tag Mifare Classic Unattenuated Signal 1 1 s Vpp 2 14V ba 2 S oap eee Maximally 5 0 6 8 05 Cau attenuated signal S T T1 Vpp 216mV a7 4em 0 2 2 f f f f f f of 1 f f 1 1 f 0 0 05 0 1 0 15 0 2 0 25 0 3 0 35 0 4 9 22 24 28 3 32 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 Time s ei k Delay s z 2 Error dB b j b Tag Mifare Ultralight b Tag Mifare Ultralight 0 1 i step Th panatag emman aaa g2 0 002948 ce g j i 08 Fg Sai 1 1 i 0 6 0cm z6 1 i g 3 2 1cm S 8 i STS G04 a Zz 1 10 68 dB attenuation 120m 10 i H 4 0 2 12 1 ra f o 0 0 2 0 4 0 6 0 8 1 2 2 5 3 3 5 4 0 0 05 0 1 0 15 02 025 03 0 35 0 4 0 45 DAC Output Scale Delay s Error dB
36. n NFC enabled smartphones will be shipped in 2013 7 And over 50 of the smart devices to be shipped in 2015 will have NFC support 4 The growing popularity of NFC has enabled a range of applications from contactless payment 6 and ticketing 16 to device pairing 15 for ad hoc data exchange A major trait of NFC is its short communication range usually within 10 cm which is the result of the fast de caying magnetic induction between the antennas of NFC transmitter and receiver The short communication range is favored by many security sensitive applications such as con tactless payment since it provides a natural physical pro tection against various attacks particularly malicious eaves dropping Unfortunately as NFC is still a relatively new and developing technology its implementation on mobile devices often have design flaws which may be exploited to compromise application security 29 In particular our ex perimental study described in this work shows that current NFC radios emit significantly more RF energy than intend ed With a specially designed portable NFC sniffer we are able to eavesdrop NFC transmissions from up to 240 cm away which is at least an order of magnitude further than the intended NFC communication distance These findings raise major concerns on the physical security of NFC More over this issue is aggravated by the fact that current NFC chipsets adopt fixed transmission power which cannot be a
37. n harvest significant amount of power 55 mW and adap tively attenuates the signal strength of NFC in a variety of realistic settings while only introducing insignificant delay up to 2 2 s Categories and Subject Descriptors C 2 0 Computer communication Networks General Security and protection B 0 Hardware General Keywords NFC Eavesdropping Smartphone Energy Harvesting Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full cita tion on the first page Copyrights for components of this work owned by others than ACM must be honored Abstracting with credit is permitted To copy otherwise or re publish to post on servers or to redistribute to lists requires prior specific permission and or a fee Request permissions from permissions acm org MobiSys 14 June 16 19 2014 Bretton Woods New Hampshire USA Copyright 2014 ACM 978 1 4503 2793 0 14 06 15 00 http dx doi org 10 1145 2594368 2594376 Guoliang Xing Michigan State University 3115 Engineering Bld East Lansing MI 48824 glxing msu edu 1 INTRODUCTION In recent years the Near Field Communication NFC technology is increasingly available on the new generation of smartphones tablets and smart accessories It is estimated that more than 200 millio
38. nShield A Noninvasive NFC Security System for Mobile Devices Ruogu Zhou Michigan State University 3115 Engineering Bld East Lansing MI 48824 zhouruog msu edu ABSTRACT The Near Field Communication NFC technology is gain ing increasing popularity among mobile users However as a relatively new and developing technology NFC may also introduce security threats that make mobile devices vulnera ble to various malicious attacks This work presents the first system study on the feasibility of and defense again passive NFC eavesdropping Our experiments show that commod ity NFC enabled mobile devices can be eavesdropped from up to 240 cm away which is at least an order of magnitude of the intended NFC communication distance This finding challenges the general perception that NFC is largely im mune to eavesdropping because of its short working range We then present the design of a hardware security system called nShield With a small form factor nShield can be at tached to the back of mobile devices to attenuate the signal strength against passive eavesdropping At the same time the absorbed RF energy is scavenged by nShield for its per petual operation nShield intelligently determines the right attenuation level that is just enough to sustain reliable data communication We implement a prototype of nShield and evaluate its performance via extensive experiments Our re sults show that nShield has low power consumption 23 uW ca
39. ntrol capabilities which allow mobile devices to configure their NFC transmission power from software This eliminates the need of accessory se curity hardware like nShield In such a case the adaptive attenuation algorithm of nShield can be integrated by the NFC driver to attenuate the transmission power Thanks to the high energy harvesting efficiency the n Shield platform is capable of powering additional hardware components like sensors Moreover it can be used as a software defined radio platform for studying NFC protocols 11 CONCLUSION This paper presents a novel noninvasive security system called nShield to protect NFC against passive eavesdrop ping nShield dynamically attenuates the signal strength of NFC transmissions by absorbing the excessive RF energy nShield intelligently determines the amount of absorbed en ergy so that the attenuated signal strength is just enough to sustain successful NFC communications As a result in or der to launch an attack the eavesdroppers must be in close proximity of the mobile device making possible security breach significantly more challenging We have implement ed a prototype of nShield and evaluated its performance via extensive experiments We show that nShield can harvest up to 55 mW power which outperforms two state of the art NFC energy harvesting systems by 1 7X and 3 1X respec tively Moreover nShield can accurately attenuate the NFC signal strength in fine granularity
40. o reasons First they typically have a small antenna size due to the form factor constraints of mobile devices which greatly limits the receiving sensitiv ity Second the commercial NFC transceivers are specially optimized for working in close distance with the target We have designed an NFC sniffer for our experiments Fig 4 shows the block diagram of the sniffer which consists of a 30 cm by 23 cm antenna a pre amplifier and an ADC that is connected to a PC via USB to upload the collected samples The NFC signal overheard by the antenna is am plified and demodulated by the pre amplifier and the AM demodulator respectively The signal is then digitalized by ADC and transmitted to PC for decoding Our sniffer has a size of a tablet and average power consumption of 120 mW Therefore it can be easily connected to a mobile device via the micro USB interface to form a mobile sniffer The NFC initiator devices used in this study include a Google Nexus 7 tablet two smartphones Google Galaxy Nexus and Sam sung Galaxy Note 2 and an Adafruit PN532 NFC break board 1 The NXP PN532 NFC chipset is adopted by the NFC breakboard while all the other devices employ the NX P PN544 NFC chipset These two chipsets are currently the most popular NFC chipsets used on commercial off the shelf mobile devices Both chipsets use fixed transmission pow er which cannot be configured by software 11 We use an NXP Mifare Classic tag as target 3 2 Resul
41. om NFC initiators The SDR can be programmed to support stan dard and custom protocols However as SDR relies on soft ware radio stack to decode and encode messages it tends to incur longer delays In the case of nShield hardware compo nents demodulator modulator etc are utilized to acceler ate the encoding decoding which significantly reduces the delay EnGarde on the other hand employs a hardware based NFC transceiver TI TRF7970A that incurs shorter delay than SDR based transceiver However EnGarde only employs the receiving chain of the hardware transceiver due to its dual antenna configuration Although EnGarde imple ments a simple transmitter that can generate jamming sig nals it does not support data transmissions The capability of transmission is critical for tag emulation which increases the amount of energy harvested from initiator significant ly Another disadvantage of this configuration is the result ed high power consumption since the hardware transceiver employed by EnGarde is mainly designed for power hungry NFC initiators Moreover the hardware based transceiver does not support the development of new physical and link level protocols The energy harvesting system of nShield also differs sig nificantly from that of EnGarde Although a dual antenna configuration is employed by both systems it is used to meet fundamentally different requirements Specifically EnGarde employs the dual antenna configurati
42. on for tag proximity de tection while nShield adopts it for improving power harvest ing efficiency The harvesting antenna of nShield is specially designed to achieve high Q factor nShield also employs a technique called voltage matching which carefully matches the output voltage of the antenna to that of the battery to maximize the amount of power harvested On the another hand EnGardes does not perform any load source match ing which significantly limits the power harvesting efficien cy Moreover EnGarde does not support tag emulation due to the lack of transmission capability and can only trigger the initiator to raise its duty cycle to 66 by using jam ming This further lowers the amount of energy harvested Lastly the active power consumption of EnGarde is much higher than nShield 32 7 mW vs 8 7 mW due to the use of hardware based transceiver 10 DISCUSSION Although NFC does not support single initiator multiple target communication the presence of multiple target de vices may lead to collisions in the discovery process NFC standards require the initiator to resolve collisions observed in discovery process using anti collision techniques similar to RFID standards and interact with resolved targets one by one after the discovery process nShield currently does not consider the multiple tag case However nShield can learn if a collision has occurred by overhearing the traffic from the initiator and act accordingly H
43. onconstant attenuation steps The maximum step occurs when the MOSFET operates n ear the middle of the effective attenuation region For a 16 bit DAC with 2 3 V reference the maximum step is 0 0029 dB The wide attenuation range and fine attenuation step allows nShield to precisely attenuate the RF field with wide strength range to the optimal level This ensures nShield to best protect the security of NFC while maintaining reliable communication 8 5 Delay of Adaptive Attenuation The delay caused by the adaptive attenuation algorithm is a critical performance metric for nShield since a long delay would have significant impact on the user s experience In this section we measure the delay introduced by the adap tive attenuation algorithm using a Mifare Classic tag and a Mifare Ultralight tag We define the delay as the inter val from the time instant when the initiator sends the first probe to the tag to the time instant when the optimal at tenuation level is determined We use the hardware timer on nShield to timestamp these events and measure the de lay For each tag we measure the delay associated with 3 different optimal attenuation levels by varying the tag initiator distances To illustrate the delay in practical set tings we hold the tags with hands which introduces small tag initiator distance variations during communications We repeat the experiment at each distance for 20 times Fig 13 shows that most of the delays
44. only intro ducing insignificant delay 2 BACKGROUND NFC employs the fast decaying magnetic induction be tween the antennas of transmitter and receiver for commu nication in close distance The typical working distance of NFC using compact antenna coils with the size of a credit card is a few centimeters An NFC communication process involves an initiator and a target Initiator devices are usu ally smartphones tablets and POS terminals which initiate the NFC communication with the target The target devices can either be those devices or proximity cards NFC has t wo working modes i e passive mode and active mode The passive mode employs the same communication techniques as those used by the proximity card in which the target device is powered by the RF field emitted by the initiator and transmits by modulating the RF field In the active mode both initiator and target are powered by their own energy sources The ASK and PSK modulation schemes are employed by NFC to support a number of data rates 106 kpbs 212 kbps and 424 kbps An NFC communication process always begins with target discovery in which the NFC initiator discovers the nearby NFC targets and learns the capability of the discovered tar gets The initial phase of discovery process is probing in which the initiator broadcasts discovery messages periodi cally to find nearby target devices An NFC target device responds after it hears the probe The initiator an
45. or which can be easily driven by the onboard DAC due to its very low gate driving voltage less than 1 V The bridge rec tifier is implemented by four NXP PMEG600 low forward drop Schottky diodes to minimize the energy loss on rectify ing To generate accurate baud rates and subcarrier frequen cies a 13 56 MHz crystal oscillator and a hardware clock di vider are employed We implemented the 15014443A NFC A protocol on nShield which supports a data rate of 106 kbps Since the modulation demodulation tasks are mainly handled by hardware higher data rates can also be easily supported by nShield Moreover since many protocols are implemented in software nShield can be easily customized to meet the requirements of different applications As a software defined radio platform nShield can also be config ured to provide malicious content protection functions 21 nShield employs several techniques to optimize its power consumption For example at runtime unused components are shut down The clock rate of the MCU is also dynami cally adjusted according to the workload To further reduce power consumption nShield enters sleep state when no NFC RF field is detected During sleep all onboard components except the low power time keeping timer are shut down 8 EXPERIMENTATION In this section we study the performance of nShield using a set of experiments We adopt two initiators Google Nexus 7 tablet and Adafruit PN532 breakboard and t
46. ount of energy power transmitted by the NFC initiator Energy power harvesting efficien cy is defined as the ratio of the amount of energy power transferred to the receiving system after rectifying and regu lation to the amount of energy power transmitted by the NFC initiator Obviously for any wireless power transfer system energy power harvesting efficiency is always lower than energy power transfer efficiency Energy Management Circuit Impedance Trans Block Antenna To System Battery amp Switching heni Regulator System Figure 7 Block Diagram of energy management circuit on nShield 5 1 Harvesting Antenna When the communication between an NFC initiator and a target device commences energy transfers from the trans mitting antenna to the harvesting antenna via resonant in ductive coupling 25 through air The NFC antennas are essentially inductors which have inductance as well as se ries resistance The radiation efficiency of NFC antennas can be quantified using quality factor or Q factor which is the ratio of the inductive reactance to the series resistance of the antenna at 13 56 MHz wL 27 12rL Q 1 R R where w is the working frequency of the antenna and L and R are the inductance and the series resistance of the anten na respectively The Q factors of the transmitter antenna and the harvesting antenna largely determine the power har vesting efficiency between antennas
47. owever this extension is left for future work nShield significantly improves the amount of harvested energy over existing NFC based energy harvesting systems 21 14 19 10 However compared to specialized wireless power transfer systems 28 that often achieve power har vesting efficiencies of at least 70 nShield s efficiency is much lower 24 4 This is mainly because the current NFC initiator is not optimized for high efficiency wireless power transfer The antenna on NFC transmitter has low Q factor which significantly limits the power transfer effi ciency Moreover achieving high efficiency also requires that the transmitter and receiver must be precisely tuned to the same resonant frequency which varies with the transmitter receiver distance High efficiency inductive power trans fer systems adopt several techniques including resonant fre quency auto tuning and antenna impedance auto tuning to deal with the detuning effects Unfortunately these mecha nisms are not implemented on NFC initiators We acknowledge that a complete redesign of the NFC ini tiator would be a more effective way to improve physical security However such a clean slate approach may prove challenging in practice due to the need of involving many players from IC to device manufacturers Moreover this would leave the legacy devices already shipped exposed to malicious attacks The next generation NFC chipsets may have native transmission co
48. r certain time to charge the tag before starting the first polling round The length of this charging period is usually much longer than the interval between consecutive polling rounds Even if the RF field strength was not sufficient to sustain the succes sive polling the first polling round may still succeed due to the energy harvested from the initial charging period As a result for passive communication mode the success of the first polling round after the activation of the RF field is not a good indicator if the field strength is strong enough for sustaining bi directional communication In our design we deem a field strength sufficient only if it can support the first three consecutive polling rounds Algorithm 6 1 Adaptive RF Field Attenuation Input N number of attenuation levels Output nopt optimal attenuation level Used sub function Comm n attempt communication with attenuation level n This sub function returns success only if the first three polling rounds are completed successfully with the attenuation level n Nopt round Nupper Niower 2 end while gt return Nopt 1 Nupper N 2 Niower 1 3 Nopt N 2 4 while Nupper Niower gt 2 do Dd if Comm nopt success then 6 Nupper round Nupper Nopt 2 7 else 8 Niower Nopt 9 end if 10 11 12 An interesting question is that with N different attenu ation levels in what order should nShield attempt commu nication
49. s A naive solution is to attempt with all N levels from a high to low or low to high order until an attenuation level for supporting reliable bidirectional communication is found However this approach incurs high delay at least several seconds We adopt the Binary Search Algorithm B SA to accelerate the search process With BSA the search starts from the middle of all attenuation levels Depend ing on whether the following polling rounds are successful or not BSA discards the lower or higher half of the level s that unlikely contain the optimal level For example if any of the three following polling round fails BSA discards all the levels that are higher than the currently attempted level BSA repeats this process with the remaining levels until there is only one level left However due to the tran sition region on the PRR AL curves see Section 8 3 BSA may fail to locate the optimal attenuation level This is be cause whether an attenuation level in the transition region such as Atrans on Fig 8 can support a successful polling round is probabilistic When the polling rounds attempted with Atrans succeed all the attenuation levels higher than Atrans including the optimal level Aopt would be discarded To address this issue we adopt a modified BSA in nShield It works in the same way as the original BSA except that it only discards half of the higher levels after three success ful polling rounds As the transition region of
50. s done the following data communication is protected from passive eavesdropping If the adaptive RF field attenuation is not finished yet in the last phase of the target discovery process nShield will jam the communication to force the initiator to restart the process 7 IMPLEMENTATION We implemented a prototype nShield which is shown in Fig 6 We use a TI MSP430F2618 as the MCU on nShield It integrates many low power components used by nShield such as comparator ADC DAC and DMA controller A 4 8 V 20 mAH NiMh battery is adopted to store the harvested energy We implement the harvesting antenna using layered tapes and aluminum foil To maximize the attenuation range the size of the harvesting antenna should be slightly larger than the antenna on the NFC initiator so that all magnetic flux generated by the initiator would undergo the attenuation before reaching the target For example our prototype an tenna attached to Nexus 7 has a dimension of 9 6 cm by 9 6 cm slightly larger than the NFC antenna in Nexus 7 We build the base of the antenna using 2mm thick layered tapes We apply the aluminum foil to one side of the base and cut the foils into 7 mm wide tracks to reduce the series resis tance The tightly coupled tracks increase the inductance of the antenna The combination of high inductance and low series resistance leads to a high Q factor gt 100 which is essential for achieving high energy transfer efficiency The N
51. s when the load becomes lighter The change of the transmis sion power is due to the detuning effect in which the tuning of the initiator s antenna is varied by the mutual coupling between the harvesting antenna and the initiator antenna a Power Harvested a Input power power transferred and harvested 1 a Tag Mifare Classic x y MAX 90 mW 12V t v o a gt S SS e o amp 3S S Power mW Power mW Google Nexus 7 PN532 Breakboard 20 25 gles m 1st polling round 2nd polling round larvested power Transferred power 10 15 Antenna Output Voltage V 0 b Power Transferred to Antenna 10 15 Antenna Ouput Voltage V b Efficiencies of energy transfer and harvesting 05 0 05 Normalized Field Strength dB b Tag Mifare Ultralight Google Nexus 7 PN532 Breakboard 1 Power harvesting efficiency 7 0 8 Power transfer effici a L z 0 8 ower transfer efficiency g 06 J 5 2007 NMAX 110 mW 5V 7 3 0 6 a 04 m 1st polling round 7 H hE i 04 2nd polling round 400 N 180 mW 12V 02 x P a eee StS E S 0 se j 1 1 1 1 L 5 o 15 20 25 4 6 10 12 14 16 18 20 45 A 0 5 0 0 5 1 15 Antenna Output Voltage V Antenna Ouput Voltage V Normalized Field Strength dB Figure 9 Power transferred and Figure 10 Power harvesting
52. s to work at a higher system clock rate to meet the strict timing requirements of the NFC data reception and trans mission and several system components e g TX control circuit need to be powered on Although the idle RX TX power consumption are high their impact on system life time is actually insignificant since nShield spends most of the time in the sleep state with a power consumption of on ly 23 uW This is due to the fact that the NFC initiator is usually inactive most of the time e g when the mobile device is locked during which nShield is asleep Thanks to the large amount of power harvested from N FC transmissions and low power design nShield can sustain its operation solely on the harvested energy NFC standard requires initiators to insert long guard time between consec utive polling rounds 9 As a result NFC initiators are in idle listening most of the time when activated This causes nShield to be idle during most of its active period leading to an average active power consumption of 8 7 mW As nShield can harvest 55 mW power from an active NFC initiator it maintains a net power gain of 46 3 mW during its active state For typical Android devices the integrated NFC ini Tale listening 9 8 mW 23 uW 8 7 mW 13 1 18 1 mW mw Table 1 System power consumption under different s tates tiators are duty cycled at 10 21 during probing With its low sleep power consumption the battery on nShield can s ta
53. square volt age inducted on the harvesting antenna Rant and Rioaa are the impedances of the antenna and the load respectively and Pmaz is the maximum power that the harvesting anten na can receive We can see that Proaad equals a quarter of Pmaz When and only when Ricad Rant However the perfect impedance matching is impossible for energy harvesting systems since the input impedance of the energy management circuit Rioaa varies significant ly with the system load To solve this problem instead of matching impedance nShield employs voltage matching Since Rant and Rioaa are in series when Rant Rioad the voltage across Rane and Ricaa denoted as Uant and Uioad respectively are also identical i e Uant Uloaa 0 5U ant open Therefore an alternative way to achieve the maximum power transfer is to match Ujoad to 0 5Uant open Since Uant open is a constant value when the harvesting an tenna is attached to the initiator the maximum power trans fer can be achieved by letting Uioaa 0 5Uant open A key question is how to stabilize Uioaa when system load varies nShield connects the battery directly to the output of the rectifier which makes Ujoaa stay equal to the voltage of the battery Ubat Since most batteries have stable output volt age regardless the discharging level and the output current system load the optimal energy transfer rate can be al ways maintained However 0 5Uant open could be difficul
54. t to match with Ubat in practice as the harvesting antenna and the ener gy management circuit are usually separately designed to meet different requirements e g Q factor system power consumption system voltage etc An impedance transfor mation block such as L section circuit or RF transformer 5 can be employed to shift Uant open to a given voltage Al though an impedance transformation block is not required by our current implementation of nShield it would be re quired if nShield employs a Lithium battery 3 6 V It is also worth noting that super capacitors are ill suited for n Shield as their output voltages vary significantly with the discharging levels To protect the batteries we use a linear regulator and MOSFET switches to manage the charging We do not use a switching regulator since it tends to alter the voltage matching point thus reduces the energy harvest ing efficiency Fig 7 shows the design of energy management circuit of nShield Our experiment in Section 8 1 shows that nShield can harvest 55 mW power constantly from the NFC initiators on typical smartphones 5 3 Tag Emulation As discussed in Section 2 the initiator adopts a low prob ing rate 21 when no target device is nearby which only allows limited amount of energy to be harvested Never theless we show in Section 8 2 that as long as the host device is active for more than 429 seconds day the energy harvested during the probing phase is suff
55. ts In the first experiment we measure the passive eavesdrop ping distances of both initiator and tag without attenuating the RF field radiated by the initiator We place the initiators on a desk with the antennas of the devices facing forward We activate one initiator at a time The Mifare tag is placed in parallel and 1 cm from the antenna of the activated ini tiator We place the sniffer near the initiator and gradually move it away from the initiator Fig 1 shows the signal strength of the initiators that is measured by the sniffer at different distances As expect ed the received signal strength decreases over distance We can see that the signal is capped when the initiator sniffer distance is short as the output voltage of the sniffer cannot exceed the voltage of its battery We implemented a Miller decoder in Matlab to decode these samples We find that the signal can be decoded if its strength is above 100 mV When the strength is lower the signal to noise ratio SNR is too low for successful decoding As shown in Fig 1 the 100 mV signal strength corresponds to physical distances of 152 cm 131 cm 116 cm and 244 cm respectively when Nexus 7 Note 2 Galaxy Nexus and Adafruit NFC breakboard are used as initiators We are also able to decode the signal transmitted by the tag at maximum distances of 91 cm with Nexus 7 85 cm with Note 2 67 cm with Galaxy Nexus and 121 cm with Adafruit NFC breakboard Compared to the
56. uld be minimized As discussed in Section 2 nShield scavenges energy from the NFC RF field which is available only when the host device is active e g when the screen of a smartphone is unlocked When ener gy harvesting is not possible nShield has to survive using the energy scavenged previously Moreover to keep the s mall form factor nShield cannot adopt bulky high capacity batteries Due to these challenges nShield must minimize its power consumption as well as maximize the amount of power harvested from the host device However wireless charging is inherently inefficient 27 especially for periph erals like nShield that has tight cost budget and form factor constraints 4 2 System Overview nShield is composed of two major components a software defined passive NFC radio platform and an adaptive RF field attenuation algorithm The software defined platfor m is capable of receiving data from and transmitting data to NFC initiators attenuating the NFC RF field using it s antenna and harvesting energy from the RF field The adaptive attenuation algorithm dynamically determines the highest attenuation level that can still ensure communica tion reliability according to the overheard NFC traffic Fig 5 shows the system architecture of nShield An on board M CU runs signal processing tasks such as encoding decoding nShield has two tuned loop antennas The larger antenna is used for harvesting energy from the NFC initiator as
57. wo tags Mi fare Classic and Mifare Ultralight We choose these devices not only because they are representative NFC devices on the market but also due to their diverse characteristics For example the Adafruit PN532 breakboard has a large anten na and can transmit a large amount of power about 450 mW while Google Nexus 7 has a much smaller antenna and much lower transmission power about 200 mW The Mifare Classic tag has an antenna size of a credit card which is very common among passive tags while Mifare Ultralight only has an antenna size of a coin which is considered to be a weak tag The testing equipments we use include an Ag ilent DSOX2024 oscilloscope an Agilent 34410A benchtop multimeter an Extech handheld multimeter and an SDR Kits VNWA3 Vector Network Analyzer 8 1 Amount of Harvested Power We measure the amount of power that can be harvested by nShield and the power transfer and harvesting efficiency with two experiments in this subsection In the first experiment we employ both of the initiators for testing The harvesting antenna shown in Fig 6 is attached to the back of Google Nexus 7 and to the surface of the PCB antenna on PN532 breakboard We connect a potentiometer to the antenna as the load The output voltage and current of the antenna under different loads are measured with an Agilent 34410A benchtop multimeter A linear regression is applied to the results to compute the in ternal resistanc
58. y fully charged if the mobile device is unlocked for average 429 seconds per day which can be met by smartphones and tablets in most circumstances 20 2 When the discharging level of the onboard battery is low nShield automatically activates tag emulation which increases the charging rate by 10X to rapidly charge the battery Moreover even when energy harvesting is not possible e g NFC is disabled the lifetime of a fully charged nShield still exceeds one month thanks to its low sleep power consumption The above results show that nShield s capability of har vesting high amount of power plays a significant role in achieving the perpetual operation As nShield can be only charged when the screen of the device is unlocked the min imum harvested power for sustaining nShield depends on how the users interact with mobile devices A recent survey 28 shows that on average U S users spend 58 minutes on smartphones per day which is more than enough for nShield to stay fully charged However for light smartphone users the harvesting power should be sufficiently high Compared to EnGarde whose harvested power is only about 30 mW nShield decreases the minimum active time of the phone by more than 50 7 15 min vs 15 5 min 8 3 Receiver Characteristics In this subsection we study the receiving characteristics of passive NFC tags by measuring the PRR FS Packet Re ception Ratio vs Field Strength curves The purpose of this experi
59. y rooting the de vice 21 To address this issue several systems adopt addi tional hardware security devices RFID guardian 30 pro vides protection by actively jamming suspicious NFC trans actions However active jamming consumes considerable power and requires bulky hardware e g RF amplifier and large battery which significantly limits RFID guardian s applications Proxmark III 12 is a widely used RFID NFC software defined radio that is capable of detecting an attack and generating jam signals However it must be plugged in as its FPGA based design consumes significant power about several hundred milliwatts Furthermore none of these ap proaches can provide anti eavesdropping protection NFC is ideal for energy harvesting due to the condensed RF field strength generated by its high transmission pow er and short communication range Energy harvesting en ables a mobile device to replenish its energy in the presence of NFC RF field The NFC Discover kit 14 from ST in clude a sensor board can be wirelessly powered by nearby NFC initiators NFC WISP 19 10 is a software defined passive tag platform which is capable of harvesting energy from NFC transmissions and conducting simple sensing and computational tasks A key difference between the energy harvesting component of nShield and the above two system s is the amount of power harvested With extensive op timizations to harvesting antenna and energy management circuit

nShield: A Noninvasive NFC Security System for Mobile Devices

Contents

Download Pdf Manuals

Related Search

Related Contents