Abstract:
The security of Near Field Communications (NFC) against eavesdroppers is improved by encoding a hidden key, private to an initiating device, onto the carrier wave when a target device communicates its data via the carrier. The initiating device, in possession of the hidden key, is enabled to undo the scrambling effects of the hidden key to thereby privately interpret the data encoded onto the carrier wave by the target device. In various aspects, the hidden key is a one-time-use key that is randomly generated, used, and then discarded, preventing malicious parties from gaining the hidden key to later interpret intercepted data signals.

Description:
BACKGROUND 
     Near Field Communications (NFC) include several standards by which two devices in close proximity to one another may exchange information wirelessly. These standards, which include ISO/IEC 18092, specify several aspects of establishing a connection between the devices and how data are to be exchanged. NFC has historically relied on proximity as a security feature, but as malicious parties and eavesdroppers gain greater sophistication in their receiving hardware and in disguising devices to implement man-in-the-middle attacks at the point of data exchange, additional security features are needed to maintain confidence in the privacy of NFC transactions. One such security feature is to encrypt the signals passed between the devices via secret keys; however, encryption adds processing overhead (slowing communications) and is vulnerable to the secret keys being exposed. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. This summary is not intended to identify all key or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter. 
     Systems, methods, and computer readable storage devices including instructions for providing secure Near Field Communications (NFC) are discussed herein. One of the benefits of NFC is that its handshake process between two communicating devices is fast, so that two devices may exchange data quickly. Unfortunately, part of that speed is realized by foregoing an authentication (e.g., a username/password pair) of the communicating devices and instead relying on the short range of the communications to exclude malicious parties or eavesdroppers. 
     To provide greater security, while retaining the speed benefits inherent to NFC, the present disclosure provides for the actively communicating device in a pair of communicating devices to randomly modulate the magnetic carrier by which the devices communicate. The passively communicating device encodes its data onto the carrier oblivious to the modulation, and the actively communicating device maintains a cache of the modulations so that their effect on the communications from the passively communicating device can be removed by the actively communicating device. The actively communicating device receives data from the passively communicating device that is interpretable by the actively communicating device, but eavesdropping devices receive a signal that is scrambled and uninterpretable. The actively communicating device privately maintains the hidden key used to modulate the carrier, and may discard the key as the signal is interpreted to forgo the possibility that an eavesdropped may recover the hidden key and thereby interpret intercepted messages. 
     Examples are implemented as a computer process, a computing system, or as an article of manufacture such as a device, computer program product, or computer readable medium. According to an aspect, the computer program product is a computer storage medium readable by a computer system and encoding a computer program comprising instructions for executing a computer process. 
     The details of one or more aspects are set forth in the accompanying drawings and description below. Other features and advantages will be apparent from a reading of the following detailed description and a review of the associated drawings. It is to be understood that the following detailed description is explanatory only and is not restrictive of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate various aspects. In the drawings: 
         FIG. 1  illustrates two devices communicating via Near Field Communications; 
         FIGS. 2A-2G  are examples of spectrograms interpreting a Near Field Communication signal; 
         FIG. 3  is a flow chart showing general stages involved in an example method for securing Near Field Communications; 
         FIG. 4  is a block diagram illustrating example physical components of a computing device; and 
         FIGS. 5A and 5B  are block diagrams of a mobile computing device. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description refers to the same or similar elements. While examples may be described, modifications, adaptations, and other implementations are possible. For example, substitutions, additions, or modifications may be made to the elements illustrated in the drawings, and the methods described herein may be modified by substituting, reordering, or adding stages to the disclosed methods. Accordingly, the following detailed description is not limiting, but instead, the proper scope is defined by the appended claims. Examples may take the form of a hardware implementation, or an entirely software implementation, or an implementation combining software and hardware aspects. The following detailed description is, therefore, not to be taken in a limiting sense. 
       FIG. 1  illustrates an actively communicating device  100   a  (i.e., an initiator) in communication with a passively communicating device  100   p  (i.e., a target) via a magnetic carrier  110  according to an NFC standard, such as, for example, ISO/IEC 18092. In various aspects, the passively communicating device  100   p  may be a passive device (with no power source of its own) or an active device communicating as a passive device and therefore may include any of the components illustrated for the actively communicating device  100   a . For example, the passively communicating device  100   p  may be a credit card and the actively communicating device  100   a  a credit card reader, or the passively communicating device  100   p  may be a first mobile telephone and the actively communicating device  100   a  a second mobile telephone. 
     In various examples, the actively communicating device  100   a  and the passively communicating device  100   p  are illustrative of a multitude of computing systems including, without limitation, desktop computer systems, wired and wireless computing systems, mobile computing systems (e.g., mobile telephones, netbooks, tablet or slate type computers, notebook computers, and laptop computers), hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, printers, gaming devices, key fobs and cards, and mainframe computers. The hardware of these computing systems is discussed in greater detail in regard to  FIGS. 4, 5A, and 56 . 
     The actively communicating device  100   a  includes a power source  120   a , a key buffer  130   a , a modulator  140   a , an antenna  150   a , and a demodulator  160   a . The power source  120   a  in various aspects may be a battery or an electrical connection to an external power source, such as, for example, a wall socket or another device (e.g., when the actively communicating device  100   a  is an attachment or dongle for a computer to enable that computer to communicate via NFC). The power source  120   a  provides power to the other components of the actively communicating device  100   a  to generate, modulate, and interpret NFC signals, and in some aspects provides power, carried by the magnetic carrier  110 , to the passively communicating device  100   p  to induce it to generate and modulate NFC signals. 
     The modulator  140   a  is configured to generate the magnetic carrier  110  via the antenna  150   a  and encode data onto it. In various aspects, the data encoded into the magnetic carrier  110  include signals used for initiating communications with other communicating devices  100  as well as the hidden key. For example, a handshake signal or response request signal may be encoded onto the magnetic carrier  110  by the modulator  140   a  to induce the passively communicating device  100   p  to provide the information it stores to the actively communicating device  100   a . In another example, while the passively communicating device  100   p  encodes its information onto the magnetic carrier  110 , the actively communicating device  100   a  will encode the hidden key onto the magnetic carrier  110  to secure the information from eavesdroppers. In some aspects, the modulator  140   a  begins encoding the hidden key onto the magnetic carrier  110  in response to a timed delay from the end of a handshake procedure or a start-of-signal message from the passively communicating device  100   p . Similarly, in different aspects, the modulator  140   a  ceases encoding the hidden key onto the magnetic carrier  110  in response to reaching a message length previously encoded onto the magnetic carrier  110 , reaching an end of the hidden key, or receiving an end-of-signal message from the passively communicating device  100   p.    
     In some aspects, the hidden key is modulated onto the magnetic carrier for the duration that the passively communicating device  100   p  encodes information onto the magnetic carrier  110 . In other aspects, the modulator  140   a  encodes the hidden key onto the magnetic carrier  110  for a longer period of time than the passively communicating device  100   p  transmits its information (e.g., starting in anticipation of message encoding, continuing after message encoding) to further disguise the message encoded by the passively communicating device  100   p . In yet other aspects, the modulator  140   a  encodes the hidden key onto the magnetic carrier  110  for a shorter period of time (e.g., starting after a first bit is received, ending after a payload is completely received but not an end-of-signal-message). 
     The demodulator  160   a  is configured to observe the magnetic carrier  110  as received by the antenna  150   a  to interpret information encoded onto the magnetic carrier  110 . In various aspects, the demodulator  160   a  is communicated to the modulator  140   a  to alert the modulator  140   a  of received messages (e.g., start-of-signal, end-of-signal, etc.). In other aspects, the demodulator  160   a  is communicated to a memory storage device and a processor so that information received from the passively communicating device  100   p  may be acted upon by the actively communicating device  100   a . For example, the demodulator  160   a  may pass a credit card number received from a passively communicating device  100   p  of a smart card to a memory storage device and processor for use to determine whether to authorized a transaction using that smart card. 
     The demodulator  160   a  is configured to unscramble the information received from the passively communicating device  100   p  from the effects of the hidden key also modulated onto the magnetic carrier  110 . In various aspects, the demodulator  160   a  uses the hidden key to produce a version of the magnetic carrier  110  without the effects of the hidden key, which is compared against a static threshold to determine a binary value encoded by the passively communicating device  100   p  in a given time frame on the magnetic carrier  110 . In other aspects, the demodulator  160   a  determines a binary value encoded by the passively communicating device  100   p  onto the magnetic carrier  110  in light of the hidden key by modulating a threshold according to the hidden key to provide a threshold that changes to match the effects of the hidden key during a given time frame of transmission. 
     In some aspects, the demodulator  160   a  may also perform error correction on the received information (e.g., using a check sum or error correction code) and frame synchronization procedures on the magnetic carrier  110  (adjusting when the modulator  140   a  sends a next bit or a phase of the magnetic carrier  110 ), or may signal the modulator  140   a  that an error has occurred in the reception of information from the passively communicating device  100   p . For example, when random errors (e.g., bit flips) are intentionally introduced to further obfuscate the message, the demodulator  160   a  may apply a Reed Solomon code to correct these intentional errors. 
     In various aspects, the modulator  140   a  and the demodulator  160   a  are part of a single device or may be two separate devices. Similarly, in various aspects, the modulator  140   a  and the demodulator  160   a  may be configured for various modulation schemes, including Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM), which encode and decode data onto and from a carrier by changing an amplitude of the carrier, a frequency of a carrier, and a phase of a carrier respectively according to the information to transmit. One of ordinary skill in the art will be familiar with various modulation schemes and the interpretation thereof. 
     The key buffer  130   a  is in communication with the modulator  140   a  and the demodulator  160   a , and is configured to store a hidden key for protecting the information exchanged from the passively communicating device  100   p  to the actively communicating device  100   a  from eavesdroppers and other malicious parties. In various aspects, the key buffer  130   a  includes a random number generator that is used to produce a series of bits with random binary values to comprise the hidden key. In some aspects, the hidden key is produced on demand—where a bit is produced to encode the magnetic carrier  110  for a given time frame, to interpret the magnetic carrier  110  at the given time frame, and then discarded once the given time frame has been interpreted. A given bit may be discarded by overwriting it with a next bit in the hidden key, shifting a register to queue the next bit in the series comprising the hidden key for use by the actively communicating device  100   a , or by erasing it from the key buffer  130   a . In other aspects, the key buffer  130   a  is provided a hidden key for repeated use, which may be hardcoded to the actively communicating device  100   a  or provided for a given communications session. The hidden key is kept privately by the actively communicating device  100   a  and is not shared to the passively communicating device  100   p , which may encode its information to the magnetic carrier  110  oblivious to the modulation effects imparted by the actively communicating device  100   a  according to the hidden key. 
     The passively communicating device  100   p  includes a data store  170   p , a modulator  140   a , and an antenna  150   p . The modulator  140   p  and antenna  150   p  of the passively communicating device  100   p  operate similarly to the modulator  140   a  and antenna  150   a  of the actively communicating device  100   a , except that they do no generate the magnetic carrier  110 , but piggyback the passively communicating device&#39;s information onto the magnetic carrier  110  generated by the actively communicating device  100   a ; modulating it to carry the information for reception by the actively communicating device  100   a . The antennas of the devices  100  effectively form a transformer when the devices  100  are in proximity and magnetically coupled to one another. In various aspects, the data store  170   p  of the passively communicating device  100   p  is a passive data store (e.g., an identifier number on a key fob, and account number of a credit card) hard coded onto a chip, or an active data store (e.g., a register in computer memory that is provided for communication between the devices. The passively communicating device  100   p  receives the magnetic carrier  110  generated by the actively communicating device  100   a  and modulates that magnetic carrier  110  via the modulator  140   p  according to the information stored in the data store  170   p  to affect the magnetic carrier  110 . 
     Each communicating device  100  alternates when it encodes information for interpretation by the other communicating device  100 . In various aspects, a guard time between encoded signals ensures that the communicating devices  100  do not “talk over” one another, which may be signaled by an end of frame signal and/or a message length indicator encoded onto the magnetic carrier  110  when a given communicating device  100  encodes information for the other communicating device  100 . The hidden key, however, is not meant for interpretation by the passively communicating device  100   p  or any other device—it is private to the actively communicating device  100   a —and is encoded onto the magnetic carrier  110  by the actively communicating device  100   a  during the time period that the passively communicating device  100   p  encodes its information onto the magnetic carrier  110 ; safeguarding that information from eavesdroppers. 
     In various aspects, the magnetic carrier  110  transfers power from the actively communicating device  100   a  to the passively communicating device  100   p  to induce the modulator  150   p  to encode the information in the data store  170   p  onto the magnetic carrier  110 . 
       FIGS. 2A-2G  are example spectrograms  200  interpreting an NFC signal. Although examples are given herein primarily in terms of Amplitude Modulation (AM), the present disclosure is not limited to application in AM devices. The present disclosure is envisioned as being equally applicable to other modulation schemes, including, but not limited to: Frequency Modulation (FM) and Phase Modulation (PM). 
     As will also be appreciated, although the example spectrograms  200  are shown as idealized sine and square waves for purposes of illustration, several wave types that are non-idealized are possible and envisioned for use with the present disclosure including, but not limited to: sine, saw-tooth (forward or reverse biased), square, and triangular. Additionally, although interpretation of the signals is shown via on-off keying, other interpretation schemes (e.g., differential encoding) are also envisioned for use with the present disclosure. Further, it will be appreciated that the constant amplitudes shown are idealized, and a dip in amplitude is expected when communicating devices  100  come into proximity to one another due to the increased load on the actively communicating device  100   a  to induce passive mode communications from the passively communicating device  100   p , especially when the passively communicating device  100   p  moves relative to the actively communicating device  100   a.    
       FIG. 2A  illustrates an example carrier wave  210  as a sine wave. In the ISO/IEC 18092 standard for NFC, the carrier wave  210  will have a frequency of 13.56 MHz and an operating volume (i.e., signal amplitudes) for its field strength between 1.5 and 7.5 A/m in un-modulated conditions, although other communication standards that employ the present disclosure may use different frequencies and operating volumes for the carrier wave  210 . The actively communicating device  100   a  generates the carrier wave  210  for use as the magnetic carrier  110 , which is induced on the passively communicating device  100   p  to encode data onto, which is then read by the actively communicating device  100   a.    
       FIG. 2B  illustrates an example data signal  220  from a passively communicating device  100   p . The data signal  220  encodes information stored in the data store  170   p  as a time series comprising several time frames in which individual bits are encoded. Each of the spectrograms  200  in  FIGS. 2A-G  are shown over a time period from an initial time (t 0 ) to a final time (t 8 ) to illustrate examples via the transmissions and interpretations of an eight-bit byte. During each time frame (e.g., between t 0  and t 1 , t 1  and t 2 , t 2  and t 3 , etc.), the passively communicating device  100   p  may encode a bit onto the carrier wave  210 . Depending on the bit-rate set for transmission, the actual duration of a time frame may vary in different aspects. 
     Eight time frames are illustrated in the spectrograms  200  to illustrate the encoding and transmission of a byte. In the present disclosure, to differentiate decimal and binary representations of numbers, binary numbers are presented in eight-bit bytes with a space between each group of four bits and a subscript two following the second group (e.g., 0000 0000 2  is the binary representation of zero for purposes of the present disclosure). The values of individual bits are discussed as being ONE/TRUE, ZERO/FALSE, or UNKNOWN. As illustrated in  FIG. 2B , the data signal  220  represents 1100 1100 2  (decimal  204 ), where the bits for the first, second, fifth, and sixth time frames are ONE/TRUE and the bits for the third, fourth, seventh, and eighth time frames are ZERO/FALSE. 
       FIG. 2C  illustrates the carrier wave  210  of  FIG. 2A  as modulated by the example data signal  220  of  FIG. 2B . In the current example, the amplitude of the carrier wave  210  has been modulated to encode 1100 1100 2  from the data signal  220  of  FIG. 2B . The actively communicating device  100   a  constructs an observed message  230  from the modulated carrier wave  210  according to one or more sampling methods and encoding methods. As illustrated, the observed message  230  reconstructs the data signal  220  of  FIG. 2B , which is interpreted against a decoding threshold  240  to extract the data encoded onto the carrier wave  210 . As will be understood in an AM implementation, values above the decoding threshold  240  are interpreted to be ONE/TRUE and values below the decoding threshold  240  are interpreted to be ZERO/FALSE. Similar thresholds for frequency or phase changes are set for interpreting signals sent via FM and PM schemes. 
     In various aspects, the decoding threshold  240  may include an uncertainty range, such that any value within the uncertainty range from the decoding threshold  240 , despite falling on one side or the other of the decoding threshold, is determined to be UNKNOWN as its position relative to the decoding threshold  240  is too close to accurately determine the true value of a corresponding bit. 
     The decoding threshold  240  is set, as illustrated in  FIG. 2C , based on the modulated operating volume to determine whether the observed message  230  in a given time frame represents a ONE/TRUE or a ZERO/FALSE. Unfortunately, any device within range to receive the modulated carrier wave  210  of  FIG. 2C  may reconstruct the data encoded by the passively communicating device  100   p  onto the carrier wave  210 ; not just the actively communicating device  100   a.    
       FIG. 2D  illustrates an example hidden key signal  250 . Although shown with a similar amplitude to the data signal  220  of  FIG. 2B  and with similar amplitudes internally between each time frame, the strength of the hidden key signal  250  may vary from the strength of the data signal  220  and the amplitude of each time frame may also vary. For example, although only two amplitudes are illustrated in  FIG. 2D , multiple different amplitudes that affect the carrier wave  210  to different extents when encoded thereon may be applied. Additionally, the maximum amplitude of the hidden key signal  250  is set such that the modulated signal will conform to the upper and lower power bands of the applicable standard of communication, and in some aspects is set as close as possible to the modulation depth (differentiating bits of different values in an AM scheme) to provide greater confusion between message values intercepted by an eavesdropper. 
     The hidden key signal  250 , as illustrated, represents 1010 1010 2  (decimal  170 ). The values of at least a portion of the bits of the hidden key signal  250  are stored in the key buffer  140   a  of the actively communicating device  100   a  for use in modulating the carrier wave  210  to secretly secure the transmission of data from the passively communicating device  100   p  and to demodulate the observed message  230  when it has been secretly secured.  FIG. 2E  illustrates the carrier wave  210  as modulated by the hidden key signal  250 , which if interpreted by another device would produce an observed message matching the hidden key signal  250 , which if interpreted against the appropriate decoding threshold  240  would yield the hidden key value of 1010 1010 2 . Because the carrier wave  210  as modulated by the hidden key signal  250  should not be transmitted without the passively communicating device  100   p  also modulating the carrier wave  210  according to the data signal  220 , the observed signal  230  should not match the hidden signal  250 ; the spectrogram  200  of  FIG. 2E  is provided as an illustrative example to discuss the internal operation of the actively communicating device  100   a.    
       FIGS. 2F and 2G  illustrate the carrier wave  210  of  FIG. 2A  as modulated by the example data signal  220  of  FIG. 2B  and the hidden key signal  250  of  FIG. 2D . As will be apparent, the observed message  230  in  FIGS. 2F and 2G  shows more than two amplitude levels to which the carrier wave  210  has been modulated. In the illustrated examples, when the corresponding bits from data signal  220  and the hidden key signal  250  are both ONE/TRUE or ZERO/FALSE, the resulting amplitude in the observed message  230  may be resolved to properly to return the corresponding bit from the data signal  220 . When the corresponding bits from data signal  220  and the hidden key signal  250  have different values, however, the resulting amplitude in the observed message  230  cannot be reliably resolved to the value of the data signal  220  without knowledge of the values of the hidden key signal  250 . An eavesdropper who observes the carrier wave  210  and attempts to interpret the observed message  230  to extract the transmitted data therefore would misinterpret the transmitted data or determine the value of such bits to be UNKNOWN. For example, if the eavesdropper used the prior decoding threshold  240 , as is shown in  FIG. 2F , the observed message  230  would be incorrectly interpreted to be 1110 1110 2  (decimal  238 ). In another example, if the eavesdropper were to set the decoding threshold  240  evenly between the high value and the low value of the observed message  230 , half of the bits would be interpreted as UNKNOWN (i.e., bits two, three, six, and seven in the illustrated example), leaving the transmitted value ambiguous to the eavesdropper. 
     In various aspects, to interpret the observed message  230  in light of the hidden key, the actively communicating device  100   a  may either modulate the decoding threshold  240  by the hidden key signal  250  to produce the modulated threshold  260  shown in  FIG. 2G  or demodulate the observed message  230  by the hidden key signal  250  to revert the observed message of  FIGS. 2F and 2G  to that shown in  FIG. 2C  (and then use the static decoding threshold  240  to interpret the carrier wave  210  and produce the data signal  230 ). As illustrated in  FIG. 2G , the modulated threshold  260  changes as the carrier wave  210  was changed by its modulation by the hidden key signal  250 , thus allowing the threshold by which the observed message  230  is decoded to shift, privately, on the actively communicating device  100   a  to correctly interpret the data signal  220  as 1100 1100 2  in  FIG. 2G  as opposed to 1110 1110 2  in  FIG. 2F  without having to expose or leave the hidden key open to exposure to any potential eavesdroppers. 
       FIG. 3  is a flow chart showing general stages involved in an example method  300  for securing NFC signals from eavesdroppers. Method  300  begins at OPERATION  310 , where two devices come within proximity for NFC signals to be exchanged. In various aspects, the actively communicating device  100   a  generates an NFC signal in response to a user initiating NFC communications, or may periodically or constantly generate an initiate command signal to detect passively communicating devices  100   p  that enter within communication proximity to the actively communicating device  100   a . In various aspects, proximity may be set as a distance between devices (e.g., 5 cm), a minimum signal strength of the carrier wave  210  from the initiating device (e.g., at least 0.175 A/m) that can induce the target device to respond. 
     Once two devices are within communicative proximity, the actively communicating device  100   a  will initiate a handshake procedure with the passively communicating device  100   p  at OPERATION  320 . During a handshake procedure for passive mode communications between devices, the initiating device (the actively communicating device  100   a ) may use a known series of data (e.g., a series of n bits encoding a known sequence via differential encoding) to set timing parameters on the target device (the passively communicating device  100   p ) and/or to provide initial power to the target device if it lacks an internal power source. 
     Once the handshake procedure is complete, method  300  proceeds to DECISION  330  to determine whether a response is received from the passively communicating device  100   p . When it is determined that no response has been received within a response window, method  300  may conclude or return to OPERATION  320  to attempt the handshake procedure again to initiate communications with the passively communicating device  100   p . When it is determined that a response has been received, method  300  continues to OPERATION  340 . 
     At OPERATION  340  the hidden key is obtained. In various aspects, the hidden key is generated on demand bit-by-bit, where a bit with a random binary value is generated and provided to secure—and then interpret—a time frame of the data signal  220  before being discarded. In other aspects, the hidden key is generated as a series of randomly valued bits in response to a length of a message from the target device encoded in the data signal  220 . In yet other aspects, a hidden key is obtained by requesting it from a memory storage device, on which the hidden key is hardcoded or produced by an external process to secure NFC signals for a given communications session. 
     Proceeding to OPERATION  350 , the initiator device modulates the magnetic carrier  110  according to the hidden key to secure the transmission of information from the target device. In various aspects, the magnetic carrier  110  is modulated according to the hidden key in anticipation of the target device transmitting information and/or is modulated after the target device has ceased transmitting information to ensure that all of the transmission from the target device is secured by the hidden key. In other aspects, the magnetic carrier  110  is modulated according to the hidden key only for a portion of the information from the target device, such as, for example, to secure the transmission of a payload portion of a transmission frame, but not header/footer information of the transmission frame. The initiator device, in various aspects, starts encoding the hidden key onto the carrier wave  210  in response to a timed delay (such as from the end of a handshake signal), reception of a message start from the target device, or n time frames from the start of a message from the target device. In aspects using differential encoding, such as Manchester encoding, the hidden key is encoded onto the carrier wave  210  several times per each bit in the message (e.g., twice per bit) and synchronized with bit transmission to hide changes in value. 
     At OPERATION  360  the response from the target device is interpreted to retrieve the data signal  220  from the carrier wave  210 . Depending on the modulation scheme, the initiator device may compare changes in the carrier wave&#39;s amplitude, frequency, or phase against a corresponding decoding threshold  240  that may be static or dynamic in light of the hidden key. When using a static threshold, the initiator device demodulates the carrier wave  210  to remove the effect of the hidden key, which only it knows, to properly interpret the data signal  220  encoded onto the carrier wave  210  by the target device. When using a dynamic threshold, the threshold is modulated according to the hidden key to account for its effect on the modulated carrier wave  210 . 
     It is determined at DECISION  370  whether the response from the passively communicating device  100   p  has ended. In various aspects, an end-of-signal message, a span of length n time frames where the carrier wave  210  encodes only the hidden key, known message frame lengths of a given communication standard, or a message length encoded earlier onto the carrier wave  210  by the target device are used to determine when the response has ended (or to predict when it will end). When it is determined that the response has ended, method  300  proceeds to OPERATION  380 . When it is determined that the response has not ended (i.e., is still ongoing), method  300  returns to OPERATION  350  for the initiator device to continue protecting the information encoded onto the carrier wave  210  by the hidden key. 
     At OPERATION  380  the initiator device ceases modulating the carrier wave  210 . The initiator device may, in some aspects, transmit or initiate a new handshake or other request for additional information from the target device or a different target device, or method  300  may conclude. 
     While implementations have been described in the general context of program modules that execute in conjunction with an application program that runs on an operating system on a computer, those skilled in the art will recognize that aspects may also be implemented in combination with other program modules. Generally, program modules include routines, programs, components, data structures, and other types of structures that perform particular tasks or implement particular abstract data types. 
     The aspects and functionalities described herein may operate via a multitude of computing systems including, without limitation, desktop computer systems, wired and wireless computing systems, mobile computing systems (e.g., mobile telephones, netbooks, tablet or slate type computers, notebook computers, and laptop computers), hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, and mainframe computers. 
     In addition, according to an aspect, the aspects and functionalities described herein operate over distributed systems (e.g., cloud-based computing systems), where application functionality, memory, data storage and retrieval and various processing functions are operated remotely from each other over a distributed computing network, such as the Internet or an intranet. According to an aspect, user interfaces and information of various types are displayed via on-board computing device displays or via remote display units associated with one or more computing devices. For example, user interfaces and information of various types are displayed and interacted with on a wall surface onto which user interfaces and information of various types are projected. Interaction with the multitude of computing systems with which implementations are practiced include, keystroke entry, touch screen entry, voice or other audio entry, gesture entry where an associated computing device is equipped with detection (e.g., camera) functionality for capturing and interpreting user gestures for controlling the functionality of the computing device, and the like. 
       FIGS. 4, 5A, and 5B  and the associated descriptions provide a discussion of a variety of operating environments in which examples are practiced. However, the devices and systems illustrated and discussed with respect to  FIGS. 4, 5A, and 5B  are for purposes of example and illustration and are not limiting of a vast number of computing device configurations that are utilized for practicing aspects, described herein. 
       FIG. 4  is a block diagram illustrating physical components (i.e., hardware) of a computing device  400  with which examples of the present disclosure may be practiced. In a basic configuration, the computing device  400  includes at least one processing unit  402  and a system memory  404 . According to an aspect, depending on the configuration and type of computing device, the system memory  404  comprises, but is not limited to, volatile storage (e.g., random access memory), non-volatile storage (e.g., read-only memory), flash memory, or any combination of such memories. According to an aspect, the system memory  404  includes an operating system  405  and one or more program modules  406  suitable for running software applications  450 . According to an aspect, the system memory  404  includes an initiator controller  490  application—configured to enable the computing device  400  to act as the actively communicating device  100   a . The operating system  405 , for example, is suitable for controlling the operation of the computing device  400 . Furthermore, aspects are practiced in conjunction with a graphics library, other operating systems, or any other application program, and are not limited to any particular application or system. This basic configuration is illustrated in  FIG. 4  by those components within a dashed line  408 . According to an aspect, the computing device  400  has additional features or functionality. For example, according to an aspect, the computing device  400  includes additional data storage devices (removable and/or non-removable) such as, for example, magnetic disks, optical disks, or tape. Such additional storage is illustrated in  FIG. 4  by a removable storage device  409  and a non-removable storage device  410 . 
     As stated above, according to an aspect, a number of program modules and data files are stored in the system memory  404 . While executing on the processing unit  402 , the program modules  406  (e.g., initiator controller  490 ) perform processes including, but not limited to, one or more of the stages of the method  300  illustrated in  FIG. 3 . According to an aspect, other program modules are used in accordance with examples and include applications such as electronic mail and contacts applications, word processing applications, spreadsheet applications, database applications, slide presentation applications, drawing or computer-aided application programs, etc. 
     According to an aspect, aspects are practiced in an electrical circuit comprising discrete electronic elements, packaged or integrated electronic chips containing logic gates, a circuit utilizing a microprocessor, or on a single chip containing electronic elements or microprocessors. For example, aspects are practiced via a system-on-a-chip (SOC) where each or many of the components illustrated in  FIG. 4  are integrated onto a single integrated circuit. According to an aspect, such an SOC device includes one or more processing units, graphics units, communications units, system virtualization units and various application functionality all of which are integrated (or “burned”) onto the chip substrate as a single integrated circuit. When operating via an SOC, the functionality, described herein, is operated via application-specific logic integrated with other components of the computing device  400  on the single integrated circuit (chip). According to an aspect, aspects of the present disclosure are practiced using other technologies capable of performing logical operations such as, for example, AND, OR, and NOT, including but not limited to mechanical, optical, fluidic, and quantum technologies. In addition, aspects are practiced within a general purpose computer or in any other circuits or systems. 
     According to an aspect, the computing device  400  has one or more input device(s)  412  such as a keyboard, a mouse, a pen, a sound input device, a touch input device, etc. The output device(s)  414  such as a display, speakers, a printer, etc. are also included according to an aspect. The aforementioned devices are examples and others may be used. According to an aspect, the computing device  400  includes one or more communication connections  416  allowing communications with other computing devices  418 . Examples of suitable communication connections  416  include, but are not limited to, radio frequency (RF) transmitter, receiver, and/or transceiver circuitry; universal serial bus (USB), parallel, and/or serial ports. 
     The term computer readable media, as used herein, includes computer storage media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, or program modules. The system memory  404 , the removable storage device  409 , and the non-removable storage device  410  are all computer storage media examples (i.e., memory storage.) According to an aspect, computer storage media include RAM, ROM, electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other article of manufacture which can be used to store information and which can be accessed by the computing device  400 . According to an aspect, any such computer storage media is part of the computing device  400 . Computer storage media do not include a carrier wave or other propagated data signal. 
     According to an aspect, communication media are embodied by computer readable instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transport mechanism, and include any information delivery media. According to an aspect, the term “modulated data signal” describes a signal that has one or more characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media include wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency (RF), infrared, and other wireless media. 
       FIGS. 5A and 5B  illustrate a mobile computing device  500 , for example, a mobile telephone, a smart phone, a tablet personal computer, a laptop computer, and the like, with which aspects may be practiced. With reference to  FIG. 5A , an example of a mobile computing device  500  for implementing the aspects is illustrated. In a basic configuration, the mobile computing device  500  is a handheld computer having both input elements and output elements. The mobile computing device  500  typically includes a display  505  and one or more input buttons  510  that allow the user to enter information into the mobile computing device  500 . According to an aspect, the display  505  of the mobile computing device  500  functions as an input device (e.g., a touch screen display). If included, an optional side input element  515  allows further user input. According to an aspect, the side input element  515  is a rotary switch, a button, or any other type of manual input element. In alternative examples, mobile computing device  500  incorporates more or fewer input elements. For example, the display  505  may not be a touch screen in some examples. In alternative examples, the mobile computing device  500  is a portable phone system, such as a cellular phone. According to an aspect, the mobile computing device  500  includes an optional keypad  535 . According to an aspect, the optional keypad  535  is a physical keypad. According to another aspect, the optional keypad  535  is a “soft” keypad generated on the touch screen display. In various aspects, the output elements include the display  505  for showing a graphical user interface (GUI), a visual indicator  520  (e.g., a light emitting diode), and/or an audio transducer  525  (e.g., a speaker). In some examples, the mobile computing device  500  incorporates a vibration transducer for providing the user with tactile feedback. In yet another example, the mobile computing device  500  incorporates input and/or output ports, such as an audio input (e.g., a microphone jack), an audio output (e.g., a headphone jack), and a video output (e.g., a HDMI port) for sending signals to or receiving signals from an external device. In yet another example, the mobile computing device  500  incorporates peripheral device port  540 , such as an audio input (e.g., a microphone jack), an audio output (e.g., a headphone jack), and a video output (e.g., a HDMI port) for sending signals to or receiving signals from an external device. 
       FIG. 5B  is a block diagram illustrating the architecture of one example of a mobile computing device. That is, the mobile computing device  500  incorporates a system (i.e., an architecture)  502  to implement some examples. In one example, the system  502  is implemented as a “smart phone” capable of running one or more applications (e.g., browser, e-mail, calendaring, contact managers, messaging clients, games, and media clients/players). In some examples, the system  502  is integrated as a computing device, such as an integrated personal digital assistant (PDA) and wireless phone. 
     According to an aspect, one or more application programs  550  are loaded into the memory  562  and run on or in association with the operating system  564 . Examples of the application programs include phone dialer programs, e-mail programs, personal information management (PIM) programs, word processing programs, spreadsheet programs, Internet browser programs, messaging programs, and so forth. According to an aspect, initiator controller  490  application is loaded into memory  562 . The system  502  also includes a non-volatile storage area  568  within the memory  562 . The non-volatile storage area  568  is used to store persistent information that should not be lost if the system  502  is powered down. The application programs  550  may use and store information in the non-volatile storage area  568 , such as e-mail or other messages used by an e-mail application, and the like. A synchronization application (not shown) also resides on the system  502  and is programmed to interact with a corresponding synchronization application resident on a host computer to keep the information stored in the non-volatile storage area  568  synchronized with corresponding information stored at the host computer. As should be appreciated, other applications may be loaded into the memory  562  and run on the mobile computing device  500 . 
     According to an aspect, the system  502  has a power supply  570 , which is implemented as one or more batteries. According to an aspect, the power supply  570  further includes an external power source, such as an AC adapter or a powered docking cradle that supplements or recharges the batteries. 
     According to an aspect, the system  502  includes a radio  572  that performs the function of transmitting and receiving radio frequency communications. The radio  572  facilitates wireless connectivity between the system  502  and the “outside world,” via a communications carrier or service provider. Transmissions to and from the radio  572  are conducted under control of the operating system  564 . In other words, communications received by the radio  572  may be disseminated to the application programs  550  via the operating system  564 , and vice versa. 
     According to an aspect, the visual indicator  520  is used to provide visual notifications and/or an audio interface  574  is used for producing audible notifications via the audio transducer  525 . In the illustrated example, the visual indicator  520  is a light emitting diode (LED) and the audio transducer  525  is a speaker. These devices may be directly coupled to the power supply  570  so that when activated, they remain on for a duration dictated by the notification mechanism even though the processor  560  and other components might shut down for conserving battery power. The LED may be programmed to remain on indefinitely until the user takes action to indicate the powered-on status of the device. The audio interface  574  is used to provide audible signals to and receive audible signals from the user. For example, in addition to being coupled to the audio transducer  525 , the audio interface  574  may also be coupled to a microphone to receive audible input, such as to facilitate a telephone conversation. According to an aspect, the system  502  further includes a video interface  576  that enables an operation of an on-board camera  530  to record still images, video stream, and the like. 
     According to an aspect, a mobile computing device  500  implementing the system  502  has additional features or functionality. For example, the mobile computing device  500  includes additional data storage devices (removable and/or non-removable) such as, magnetic disks, optical disks, or tape. Such additional storage is illustrated in  FIG. 5B  by the non-volatile storage area  568 . 
     According to an aspect, data/information generated or captured by the mobile computing device  500  and stored via the system  502  are stored locally on the mobile computing device  500 , as described above. According to another aspect, the data are stored on any number of storage media that are accessible by the device via the radio  572  or via a wired connection between the mobile computing device  500  and a separate computing device associated with the mobile computing device  500 , for example, a server computer in a distributed computing network, such as the Internet. As should be appreciated such data/information are accessible via the mobile computing device  500  via the radio  572  or via a distributed computing network. Similarly, according to an aspect, such data/information are readily transferred between computing devices for storage and use according to well-known data/information transfer and storage means, including electronic mail and collaborative data/information sharing systems. 
     Implementations, for example, are described above with reference to block diagrams and/or operational illustrations of methods, systems, and computer program products according to aspects. The functions/acts noted in the blocks may occur out of the order as shown in any flowchart. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     The description and illustration of one or more examples provided in this application are not intended to limit or restrict the scope as claimed in any way. The aspects, examples, and details provided in this application are considered sufficient to convey possession and enable others to make and use the best mode. Implementations should not be construed as being limited to any aspect, example, or detail provided in this application. Regardless of whether shown and described in combination or separately, the various features (both structural and methodological) are intended to be selectively included or omitted to produce an example with a particular set of features. Having been provided with the description and illustration of the present application, one skilled in the art may envision variations, modifications, and alternate examples falling within the spirit of the broader aspects of the general inventive concept embodied in this application that do not depart from the broader scope.