Patent Publication Number: US-2021168933-A1

Title: Tamper detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/263,261, filed Jan. 31, 2019, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This application relates generally to tamper-proofing of electronic systems, and more particularly to tamper-proof housings for sensitive electronic circuitry at the printed circuit board (PCB) level. 
     Many forms of electronic security are made vulnerable to penetration if an adverse party has physical access to the system. Reverse engineering, decapsulation, hardware-based man-in-the-middle attacks, and other methods can enable an attacker with physical access to system busses and connected integrated circuits (ICs) to circumvent system-level and/or device-level security. Physical attack tampering typically takes place in order for certain intellectual property (IP) assets to be uncovered, stolen, altered, manipulated, destroyed, or otherwise compromised. Such IP assets can include software and its related data, including as examples financial information, authentication keys, or firmware images; or hardware, such as sensitive chip-level or PCB designs, or other physical systems such as clock sources, digital sequence sources, or actuator control. Prior art tamper-proof system coverings include, for example, potting, electro-mechanical switches configured to break contact on tampering, PCB tamper mesh enclosures, or a switch or button using inductive sensing coils, Hall effect detection, or ambient light detection. 
     SUMMARY 
     In described examples, an enclosure for circuitry includes a platform, a charge source, a first capacitive plate, a second capacitive plate, and a capacitive sensor. The circuitry is fixedly coupled to the platform. The first capacitive plate is also fixedly coupled to the platform, and either alone, or together with the platform, surrounds a volume containing the circuitry and the charge source, the charge source electrically coupled to and configured to charge the first capacitive plate. The second capacitive plate is fixedly coupled to the platform without touching the first capacitive plate, and either alone, or together with the platform, surrounds the first capacitive plate. The second capacitive plate is configured so that there is an electric potential difference between the first capacitive plate and the second capacitive plate. The capacitive sensor is electrically connected to the first capacitive plate and configured to determine when a capacitance between the first and second capacitive plates is changed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a side view of an electrical block diagram view of a system  100  including a secure enclosure for tamper detection using capacitive sensing. 
         FIG. 2  shows an example plan and schematic view of the system of  FIG. 1 . 
         FIG. 4  shows an example of a process for tamper detection using capacitive sensing. 
         FIG. 3A  shows a three-quarters perspective view of an example implementation of a system including a secure enclosure (visible in  FIG. 3C ) for tamper detection using capacitive sensing. 
         FIG. 3B  shows a three-quarters perspective view of the example system implementation of  FIG. 3A . 
         FIG. 3C  shows a three-quarters perspective view  308  of the example system implementation of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a side view of an electrical block diagram view of a system  100  including a secure enclosure  102  for tamper detection using capacitive sensing, where by way of introduction secure enclosure  102  includes a conductive structural inner volume ( 120 ,  122 ,  126 ,  128 ) and a conductive structural outer volume ( 130 ,  132 ,  136 ). The conductive structural outer volume surrounds the conductive structural inner volume, thereby presenting capacitance between the inner and outer volumes. As shown in  FIG. 1 , a control unit  104 , a battery  106 , and a secure asset  108  (such as one or more ICs) are mounted on a top surface  110  of a top layer  140  of a multi-layer PCB  112  (such as an FR4 PCB). The control unit  104  includes a capacitive sensor  105  and logic for determining whether tampering has occurred based on capacitance measurements of the sensor. (“Capacitive sensor” refers to a sensor which senses capacitance. Changes in capacitance are preferably measured as relative changes in capacitance. In some embodiments, changes in capacitance can be measured as absolute changes in capacitance.) Changes in capacitance between the conductive structural inner volume and the conductive structural outer volume (e.g., changes greater than a threshold) indicate tampering. 
     The PCB  112  has a top layer  140 , a middle layer  142 , and a bottom layer,  144 . The top layer  140  has a top surface  110 . The middle layer  142  has a middle-top surface  116  at an interface between the top layer  140  and the middle layer  142 , and a middle-bottom surface  124  at an interface between the middle layer  142  and the bottom layer  144 . The bottom layer has a bottom surface  134 . 
     The “inside” of the secure enclosure  102  refers herein to the interior of the above-introduced inner volume, which is between (enclosed by) an inner top shield  120 , such as a hollow rectangular parallelepiped open along one larger face and mounted on the top surface  110 , and an inner bottom shield  126 , such as a conductive plate located on the middle-bottom surface  124  (between the middle layer  142  and bottom layer  144  of the PCB  112 ). The “outside” of the secure enclosure  102  refers herein to a volume external to the above-introduced outer volume, which is beyond an outer top shield  130 , such as a hollow rectangular parallelepiped open along one larger face but larger than inner top shield  120  and mounted on the top surface  110 , and an outer bottom shield  136 , such as a conductive plate mounted on the bottom surface  134 . The inner top shield  120 , inner bottom shield  126 , outer top shield  130 , and outer bottom shield  136  are further described below. There is preferably a single electrical connection  114  connecting circuitry inside the secure enclosure to circuitry outside the secure enclosure  102 , to avoid providing routes—physical openings in the conductive structural volumes—by which an attacker might attempt to gain physical access to the inside of the secure enclosure. The connection  114  preferably runs along the middle-top surface  116 , thereby embedding a length of the connection  114  within the PCB  112  to make tampering with the connection  114  more difficult, and connects to the control unit  104 . 
     In some embodiments, electrical activity within the secure enclosure  102 , or between the inside and outside of the secure enclosure  102 , can affect sensed capacitance. This can be mitigated by, for example, PCB design, such as routing (direction and size of connections); bandwidth (or associated data rate) of signals; and timing and/or forced synchronicity capacitance measurements (for example, the system could force local electrical activity to stop when capacitance measurements are taken). 
     The control unit  104  is preferably connected by electrical lines  118  to the battery  106  and the secure asset  108  to mediate input and output (I/O) between the secure asset  106  and the outside of the secure enclosure  102 , preventing I/O if tampering is detected. The control unit  104  is electrically connected through an electrical connection  119  to the inner top shield  120 . The inner top shield  120  surrounds the inside of the secure enclosure  102  without gaps in the inner top shield  120  or between the inner top shield  120  and the PCB  112 . The inner top shield  120  is made of material appropriate as a capacitive plate (e.g., conductor), preferably selected to act as electromagnetic (EM) shielding, both with respect to photons (that is, the capacitive material is opaque), and with respect to electric and magnetic fields. The inner top shield  120  is mounted on the top surface  110  of the PCB  112  and is electrically connected to the control unit  104  and the battery  106 , and to one or more capacitive sense vias  122 . Capacitive sense vias are conductive, and electrically couple the control unit  104  (and its integrated capacitive sensor  105 ) to the top and bottom inner shields  120 ,  130 . One such via  122  is shown in  FIG. 1 , but as shown in  FIG. 2 , multiple adjacent capacitive sense vias  122  are preferably implemented. 
     In the example shown in  FIG. 1 , the battery  106  is connected to the inner top shield  120  via the control unit  104  (using the electrical connection  119 ), so that the control unit  104  can regulate the charge on the inner top shield  120  and the inner bottom shield  126 . That is, the battery  106  is connected to power the control unit  104 , and the control unit  104  applies charge to the inner top shield  120 , the capacitive sense vias  122 , and the inner bottom shield  130 . 
     The capacitive sense vias  122  interpenetrate the PCB  112  from the top surface  110  to the middle-bottom surface  124 . The capacitive sense vias  122  are electrically connected to the inner bottom shield  126 . The inner bottom shield  126  is mounted on the bottom surface  124  of the PCB  112 , covers an area of the middle-bottom surface  124  matching and aligned (e.g., vertically) with an area of the top surface  110  covered by the inner top shield  120 , and is made of a conductive material selected to act as a capacitive plate and as EM shielding. A return line  128  electrically connects the inner bottom shield  126  to a channel of the control unit  104 . As further described below, e.g., with respect to  FIG. 2 , there are preferably multiple capacitive sense vias  122 , located within a perimeter of the top and bottom inner shields  120 ,  130  (viewing the system  100  looking towards and perpendicularly to the top surface  110 , as in  FIG. 2 ). The preferred arrangement of the vias  122 ,  132  can also be described as staggered placement of the capacitive sense vias  122  and ground vias  132 , or analogized to an alternating crenellation (with ground vias  132  in the upper portions of crenellation notches, and capacitive sense vias  122  in lower portions of crenellation notches, or vice versa). From the preceding, therefore, one skilled in the art should now appreciate, as introduced earlier, that the inner top shield  120 , capacitive sense vias  122 , and inner bottom shield  126  (together referred to herein as the inner shield) together surround and enclose the secure enclosure  102 . 
     The outer top shield  130  is mounted on the PCB  112  so that the outer top shield  130  surrounds and is near to, but not in electrical contact with, the inner top shield  120 . That is, the inner top shield  120  is nested within the outer top shield  130 . There are preferably no gaps in the outer top shield  130 , or between the outer top shield  130  and the PCB  112 , to prevent exterior access to the inside of the secure enclosure  102 . The outer top shield  130 , like the inner top shield  120 , is made of a conductive material, preferably selected to act as a capacitive plate and as EM shielding. The inner and outer top shields  120 ,  130  can be mounted on the PCB using, for example, solder or mounting brackets which create a reliable electrical connection between the shields and power or ground (respectively). 
     The outer top shield  130  is connected to multiple ground vias  132 , which are connected to a ground (not shown) located outside of the secure enclosure  102 . The ground vias  132  interpenetrate the PCB  112  from the top surface  110  to the bottom surface  134 . The ground vias  132  are electrically connected to the outer bottom shield  136 . The outer bottom shield  136  is mounted on the bottom surface  134  of the PCB, covers an area of the bottom surface  134  matching and aligned (e.g., vertically) with an area of the top surface  110  covered by the outer top shield  130 , and is made of a material selected to act as a capacitive plate and as EM shielding. The ground vias  132  are preferably disposed in a ring (i.e., surrounding alignment, but not necessarily circular) near the perimeter of the outer bottom shield  136 . The capacitive sense vias  122  and the ground vias  132  are preferably “blind” vias, that is, they are preferably not externally visible or physically accessible when the secure enclosure  102  is fully assembled (for example, a portion of the capacitive sense vias  122  connected to the middle-bottom surface  124  is covered by the inner bottom shield  126 , and a portion of the ground vias  132  connected to the bottom surface  134  is covered by the outer bottom shield  136 ). The outer top shield  130 , the ground vias  132 , and the outer bottom shield  136  together surround and enclose the secure enclosure  102 , the inner top shield  120 , the capacitive sense vias  122 , and the inner bottom shield  130 . 
     Preferably, secure asset  108  devices are placed only on a side of the PCB  112  enclosed by the inner and outer top shields  120 ,  130 . (In some embodiments, volumes which can fit secure assets  108  can be located on more than one side of the PCB  112 , such as on both sides of a planar PCB  112 .) Also, preferably, sensitive power planes and signal traces reside within the secure enclosure  102 . 
     The inner top shield  120  and the outer top shield  130  act as a top plate capacitor, driven by the control unit  104  (powered by the battery  106 ) using the capacitive sense vias  122 , and grounded by the ground vias  132 . Similarly, the inner bottom shield  126  and the outer bottom shield  136  act as a bottom plate capacitor, driven by the battery  106  using the capacitive sense vias  122 , and grounded by the ground vias  132 . EM field lines run between the inner top shield  120  and the outer top shield  130 , and between the inner bottom shield  126  and the outer bottom shield  136 . The control unit  104  preferably controls the inner top shield  120 , capacitive sense vias  122 , and inner bottom shield  130  to charge and (partially) discharge at a high frequency, for example, 1 MHz. 
     The control unit  114  is configured to measure changes in capacitance of the top and bottom plate capacitors. An empty volume  138  between the inner and outer top shields  120 ,  130  or inner and outer bottom shields  126 ,  136  (PCB material, rather than an empty volume, is located between the bottom shields  126 ,  136  in the example shown in  FIG. 1 ) can be filled using a dielectric material comprising air or another dielectric material, such as a dielectric material with increased capacitance and/or reduced production handling requirements and/or cost. 
       FIG. 2  shows an example plan and schematic view  200  of the system  100  of  FIG. 1 . From the above description and the plan view of  FIG. 2 , one skilled in the art will further appreciate that the outer perimeter of inner top shield  120  encloses each of the control unit  104 , the battery  106 , and the secure asset  108 . Further, the plural capacitive sense vias  122  form a generally inner perimeter which, from  FIG. 1 , is understood as into the page from the perspective of  FIG. 2 , and that is located within the outer boundary (perimeter) of inner top shield  120  and within the outer boundary (perimeter) of outer top shield  130 . Similarly, the plural ground vias  132  form a generally outer perimeter which, also from  FIG. 1 , is understood as into the page from the perspective of  FIG. 2 , and that is located outside the outer boundary (perimeter) of outer top shield  130  and/or overlapping the outer boundary (perimeter) of outer top shield  130 . 
     In operation and as further detailed below with respect to  FIG. 4 , the control unit  104  can detect changes in capacitance as between shields  120 ,  126 ,  130 ,  136 , for instance were such a change to occur based on an attempt to tamper with the system  100 . Changes in capacitance measured by the control unit  104  can occur as a result of, for example, one of the shields  120 ,  126 ,  130 ,  136  being moved, removed, distorted, or deflected (changing distances between plates), drilled through or ablated (changing the size, and therefore total conductor area, of a plate; and/or shorting the inner and outer plates if a drill bit is conductive), or charged by an exterior source (changing the charge on a plate). Changes in capacitance measured by the control unit  104  can also occur as a result of, for example, a drill bit (or other device for removing material) being used to access the inside of the secure enclosure  102  and drilling through or contacting a ground via  132  or a capacitive sense via  122  on the way; or an electrically conductive probe contacting one (or more) of the shields  120 ,  126 ,  130 ,  136  or the vias  122 ,  132 . 
     As shown in  FIG. 2 , the capacitive sense vias  122  and the ground vias  132  are preferably arranged in closely spaced concentric geometries around a perimeter of the secure enclosure  102 , with inner ring vias (capacitive sense vias  122 ) having alternating positions (staggered) with outer ring vias (ground vias  132 ), for example to prevent the PCB  112  from being drilled through by an attacker seeking access to the secure enclosure. (A break in the via rings, through which the connection  114  passes, is shown for clarity and simplicity. In preferred embodiments, the connection  114  is routed between the capacitive sense vias  122  and the ground vias  132  without a break in the via rings.) Also, the perimeter of the capacitive sense vias  122  is preferably within the perimeter of the top and bottom inner shields  120 ,  130 , and the perimeter of the ground vias  132  is preferably within or overlaps the perimeter of the top and bottom outer shields  126 ,  136 . Together, the shields  120 ,  126 ,  130 ,  136  and the vias  122 ,  132  thus provide a physical barrier against attacks attempting to physically access the secure enclosure, while allowing sufficient spacing to be arranged in or on the surface of the PCB  112  to pass power and/or signaling traces from the outside to the inside of the secure enclosure  102 . 
     The outer shield is electrically connected and surrounds the secure enclosure  102  in three dimensions, forming a Faraday cage around the secure enclosure  102 . That is, the outer shield will generally block EM fields originating outside the secure enclosure  102  from penetrating to affect the inside of the secure enclosure  102  (isolating the secure enclosure  102  from external galvanic and photonic signals). This means that the outer shield being connected to the system ground shields the secure enclosure  102  from EM-based attacks (such as electrical overvoltage stress), and from unintended EM interference (reducing noise within the secure enclosure  102 ). This also makes the control unit  104  less sensitive to false tamper detection events, such as during system assembly or other intended end-user handling, because the outer shield being connected to system ground means that in ordinary handling, changes in capacitance in the top and bottom plate capacitors will generally be caused only by removal of the top or outer bottom shield  126 ,  136 . 
     In some embodiments, when the control unit  104  detects tampering (a change in capacitance in the top and/or bottom plate capacitors), it can cause the secure asset  108  to be disabled. For example, the secure asset  108  can be caused to delete sensitive data, make sensitive data unchangeable, or trigger a physically self-destructive event (for example, burning an entire array of programmable fuses to make data previously stored in a select portion of the fuses unreadable). 
       FIG. 3A  shows a three-quarters perspective view  300  of an example implementation of a system  100  as shown in and described with respect to  FIG. 1A .  FIG. 3A  shows the system  100  with the inner top shield  120  (not visible) and the outer top shield  130  covering the secure enclosure  102 . Outer brackets  302  (partially visible) hold the outer top shield  130  in place against the PCB  112 . The ground vias  132  are visible outside the perimeter of the outer top shield  130 . 
       FIG. 3B  shows a three-quarters perspective view  304  of the example system  100  implementation of  FIG. 3A . In  FIG. 3B , the outer top shield  130  is exploded away from the surface of the PCB  112  to reveal the inner top shield  120 . Inner brackets  306  hold the inner top shield  120  in place against the PCB  112 . 
       FIG. 3C  shows a three-quarters perspective view  308  of the example system  300  implementation of  FIG. 3A . In  FIG. 3C , the outer top shield  130  and the inner top shield  120  are exploded away from the surface of the PCB  112  to reveal the secure enclosure  102 . The capacitive sense vias  122  are visible within the perimeter of the inner top shield  120 . (For clarity and simplicity, the inner and outer top shields  120 ,  130  are spaced relatively far apart in  FIGS. 3A, 3B, and 3C . The capacitive sense vias  122  and ground vias  132  are preferably closer together than shown—for example, close enough to deter and/or prevent physical attack by drilling into the secure enclosure  102 , as described above.) The control unit  104 , the battery  106 , and the secure asset  108  are also visible within the space comprising the secure enclosure  102  when the outer and inner top shields  120 ,  130  are fixedly attached to the PCB  112 . 
     As shown in  FIGS. 3A, 3B, and 3C , when the outer and inner top shields  120 ,  130  are fixedly attached to the PCB  112  by the outer and inner brackets  302 ,  306 , the conductive structural inner volume ( 120 ,  122 ,  126 ,  128 ) is capacitively coupled to the conductive structural outer volume ( 130 ,  132 ,  136 ). Accordingly, changes in the capacitance between the conductive structural inner volume ( 120 ,  122 ,  126 ,  128 ) and the conductive structural outer volume ( 130 ,  132 ,  136 ) (as measured by the capacitive sensor  105  in the control unit  104 ) indicate tampering—that is, attempts to physically or electrically access the inside of the secure enclosure  102 . 
       FIG. 4  shows an example of a process  400  for tamper detection using capacitive sensing. In step  402 , a volume containing a secure asset (for example, circuitry) is enclosed within an inner shield, the inner shield including a charged inner capacitive plate coupled to a charge source within the volume, and capacitive sense vias coupling the inner capacitive plate to a capacitive sensor  105  located within the volume. In step  404 , the volume and the inner shield are enclosed within an outer shield, the outer shield including a grounded outer capacitive plate, and ground vias coupling the outer capacitive plate to a ground, the outer shield not touching the inner shield. In step  406 , a capacitance between the inner capacitive plate and the outer capacitive plate is measured using the capacitive sensor  105 . In step  408 , the circuitry is operated in dependence on the measuring—for example, if a measured change in capacitance is detected, then tampering is thusly presumed as the cause and a portion of the secure asset is caused to become inoperable. Also, a reporting element can be triggered when tampering is detected. For example, an alarm can be activated, information about the detected change in capacitance can be stored in storage within the secure enclosure, or a signal indicating the detected tampering can be sent to the outside of the secure enclosure. 
     Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. 
     In some embodiments, there is more than one electrical connection between the inside and the outside of the secure enclosure. 
     In some embodiments, there is a direct electrical connection between the exterior of the secure enclosure and the secure asset. 
     In some embodiments, there is no electrical connection between the inside and the outside of the secure enclosure. 
     In some embodiments, power requirements of the control unit and secure asset are small. In some embodiments in which the secure enclosure is isolated from power sources outside of the secure enclosure, a small battery, such as a coin cell battery, can be used. 
     In some embodiments, only one of the inner top and outer shields comprises electromagnetic shielding. In some embodiments, only one of the bottom inner and outer shields comprises electromagnetic shielding. 
     In some embodiments, I/O between the secure enclosure and the exterior is routed through the control unit. In some embodiments, the control unit is not connected to communicate with the secure asset. In some embodiments, the control unit is not electrically connected to the secure asset. 
     In some embodiments, an MSP430FR2633 or other MSP430 CapTIvate enabled device comprises or is used in a control unit. These devices are commercially available from Texas Instruments. 
     In some embodiments, the secure enclosure can contain, for example, one or more of: a processor, a memory, or a communications device. 
     In some embodiments, more than two via rings are used. In some embodiments, vias are arranged other than in a ring. 
     In some embodiments, tamper sensors in addition to the control unit are used, such as photon or pressure sensors. 
     In some embodiments, the battery is selected to be large enough to last for a projected lifetime of the secure asset. In some embodiments, the battery&#39;s lifetime defines the useful lifetime of the secure asset. In some embodiments, power is supplied by wires extending from the outside to the inside of the secure enclosure. That is, the charge source for charging the top and bottom inner shields can be powered by a power source  146  outside the secure enclosure, and connected to the control unit by, for example, the connection. 
     In some embodiments, capacitive sensing and data protection (such as a hardware-level security key) can be implemented on the same device. 
     In some embodiments, one or more of the outer top shield, the inner top shield, the outer bottom shield, and the inner bottom shield, is detachable. In some embodiments, one or more of the outer top shield, the inner top shield, the outer bottom shield, and the inner bottom shield, is not detachable (for example, is soldered onto the PCB or is connected to the PCB using an adhesive). 
     In some embodiments, voltage, temperature, and humidity monitoring subsystems can be included in a tamper detection system, insider or outside of the secure enclosure, to assist in detecting physical tamper attacks, and/or to provide measurements which can be analyzed to compensate for environmental factors which can affect capacitance measurements. In some embodiments, these subsystems are located inside the secure enclosure for use in conjunction with the capacitance measurement system. In some embodiments, these subsystems are integrated into the control unit (for example, integrated into an integrated circuit which includes capacitance measurement circuits). In some embodiments, the timing of capacitance measurements, and/or the timing of activating voltage, temperature, and humidity monitoring subsystems subsystems, can be controlled to appear non-deterministic to an attacker, for example, using random or pseudorandom timing for measurement polling events. 
     In some embodiments, devices within the secure enclosure are powered only by a battery or other energy storage fully contained within the secure enclosure. In some embodiments, devices within the secure enclosure can receive power from outside the secure enclosure. 
     In some embodiments, the secure asset and/or the control unit is programmable from outside of the secure enclosure. In some embodiments, the secure asset can be reprogrammed by the control unit. In some embodiments, the control unit can be reprogrammed by the secure asset. 
     In some embodiments, an inner shield and an outer shield fully surround the PCB (or other platform on which the secure asset is mounted). In some such embodiments, there is only one inner shield and only one outer shield. In some such embodiments, the shields alone (without the PCB) are sufficient to surround and enclose the secure enclosure. 
     In some embodiments, the control unit changes the operation of the secure asset (for example, causing reprogramming or destruction of portions or all of the secure asset) if a change in capacitance between the inner and outer plate capacitors is greater than a threshold. In some embodiments, the threshold is dependent on a state of charging the inner plate capacitor or on environmental factors (such as sensed voltage, temperature, and humidity). 
     In some embodiments, the charge source for the inner capacitive plate is other than a battery. 
     In some embodiments, the inner top shield and the outer top shield are shaped as five-sided boxes, as N-sided polyhedra with N−1 closed sides, or as continuous portions of a sphere. 
     In some embodiments, the electrical connection between the inside and outside of the secure enclosure can be configured to provide into the secure enclosure (from a communications node  148  outside of the secure enclosure to which the electrical connection is connected) one or more of power, control signals for the circuitry and/or the capacitive sensor, or data for use in operation of the circuitry and/or the capacitive sensor. 
     In some embodiments, ground vias and/or capacitive sense vias are larger or smaller. In some embodiments, more or fewer ground vias and/or capacitive sense vias are used. 
     In some embodiments, polyhedral shapes made of conductive material, including polyhedral shapes other than rectangular parallelepipeds, which open on one or more faces and configured to fixedly or removeably attach to the PCB without a gap in the shape or between the shape and the PCB, can be used as shields (capacitive plates). 
     In some embodiments, inner and outer bottom shields are mounted on the bottom surface and enclose an empty volume, similarly to the inner and outer top shields. In some such embodiments, circuitry is located within the empty volume enclosed by the inner and outer bottom shields. 
     In some embodiments, there are gaps in one or more of the inner top shield, the outer top shield, the inner bottom shield, or the outer top shield, or between one or more of the shields and the PCB (or other platform on which devices in the secure enclosure are mounted), such that the gaps are too small for an attacker to use to gain access to the inside of the secure enclosure. 
     In some embodiments, the control unit uses different (or randomized) frequencies to charge and discharge the inner shield (the top and bottom inner plate and the capacitive sense vias). 
     In some embodiments, the outer shield is electrically coupled to a ground, but is not at a voltage of the ground. For example, the outer shield can be electrically coupled to a ground via a resistor and/or one or more other impedance elements. In some embodiments, the outer shield is configured so that there is an electric potential difference between the outer shield and the inner shield (for example, other than a potential difference corresponding to a charged inner shield and a grounded outer shield). 
     In some embodiments, a conductive structural inner volume does not include capacitive sense vias and/or an inner bottom plate. In some embodiments, a conductive structural outer volume does not include ground vias and/or an outer bottom plate. In some embodiments, different capacitive sensors measure capacitance with respect to inner and outer top plates than with respect to inner and outer bottom plates and/or with respect to ground vias and capacitive sense vias. 
     In some embodiments, the outer shield(s) can be coated and/or covered with a non-conductive material. In some embodiments, exposed portions of vias can be coated and/or covered with a non-conductive material. 
     In some embodiments, the secure enclosure is hermetically sealed. 
     In some embodiments, the secure enclosure and/or the empty volume is wholly or partially filled with a non-conductive material, such as a potting material (encapsulation material). 
     In some embodiments, a key can be transmitted into the control unit from outside the secure enclosure which disables at least part of the control unit functionality; for example, to allow for intended access to the interior of the secure enclosure. 
     In some embodiments, capacitive sense vias can be located outside the perimeter of the inner top and/or bottom shields and within the perimeter of the outer top and/or bottom shields. In some embodiments, capacitive sense vias can be located to overlap the perimeter of the inner inner top and/or bottom shields. In some embodiments, ground vias can be located inside the perimeter of the outer top and/or bottom shields and within the perimeter of the inner top and/or bottom shields. 
     In some embodiments, no return line is used. In some embodiments, the battery and the control unit are coupled to the ground via the capacitive coupling between the inner and outer shields. 
     In some embodiments, the inner top plate is driven (charged) separately from the inner bottom plate and/or the capacitive sense vias. That is, the inner top plate, the inner bottom plate, and the capacitive sense vias can be driven using separate channels from the capacitive sensor (or otherwise directly or indirectly from the battery or other charge source). Power can also be distributed on the same or additional separate channel(s) to other devices within the secure enclosure. 
     In some embodiments, the connection uses a medium other than electricity through a wire to transmit signals, e.g., photonic or galvanic signal transmission. 
     In some embodiments, the capacitive sense vias are capacitively coupled to the ground vias.