Abstract:
A method and apparatus for forming a security enclosure having improved fold retention. In particular, the enclosure is formed by folding a flexible tamper respondent cloth around an electronic assembly. An adhesive on the inner folded surfaces of the cloth temporarily retains the folds. The enclosure is then exposed to heat and pressure to promote improved adhesion strength of the adhesive, thereby improving fold retention.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates generally to the detection of intrusions into electronic assemblies, and more particularly, to the retention of a security enclosure capable of detecting such intrusions. 
   2. Related Art 
   In electronic network commerce applications, it is a requirement to protect the contents of the associated computer systems from being unlawfully read or modified. It is conventional practice to provide certain encryption schemes in which data is transmitted and received in an encrypted form and only authorized people who have the encryption key codes may read or modify the data. However, an unauthorized person with sufficient skills and knowledge may attempt to bypass software encryption controls by making a physical attack against the computer hardware to attempt a direct interrogation of the memory components and other devices. Defense from these types of attacks requires that tamper resistant physical packaging be provided for critical encryption components, in such a way that unauthorized attempts to gain entry are detected and encryption key codes are immediately erased. 
   One means of providing physical protection against intrusion is the use of an electrical grid surrounding the encryption module, which when broken triggers the requisite signal to disable the unit. It is known in the art to surround and protect an encryption module with a membrane consisting of one or more flexible dielectric layers having electrically conductive traces thereon. The membrane is electrically connected to the module, then wrapped, folded and bonded around the module, wherein a sticky pressure sensitive bonding adhesive adheres the membrane around the module. The traces are intentionally made fragile so that they are damaged if an attempt is made to remove the membrane. Further the membrane may be potted in a molding material, which offers further protection as its removal would also damaged the traces. 
   While the membrane must meet the physical security requirements, it must not be so sensitive that it falsely triggers the erasure of the key codes as a result of handling during the manufacturing assembly process, or subsequently due to environmental conditions associated with changes in temperature, humidity or atmospheric pressure. Accordingly, one of the drawbacks in the current art is that the security membranes intended for wrapping, folding, and bonding to an enclosure may be too stiff to readily fold as a result of the thickness and other properties of the various layers. As a result, during the assembly folding process, a fold may be completed but the stiffness of the membrane may result in poor retention of the fold, as the elastic strain energy associated with bending the membrane overwhelms the adhesive strength of the bonding adhesive. This can result in two conditions. First, the unfolding can damage the fragile circuit traces as the adhesive pulls against them during unfolding of the membrane. Second, unfolding can produce the formation of openings or tunnels through which the subsequently applied molding materials may leak into the interior of the enclosure, resulting in the possibility of an immediate failure or potentially a reduction in reliability of the internal components. 
   Thus, there is a need for better means for performing the assembly wrapping, folding, and bonding operation in such a way that the membrane is not damaged, and that molding material can not subsequently leak into the interior of the enclosure. Contrary to meeting this requirement stands the fact that the available membrane materials have certain physical properties associated with their materials selection and cross-sections that can not be readily altered, and further the fact the conductive traces are intentionally fragile so as to detect any security attack. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, the first embodiment provides a method of forming a security enclosure, comprising: providing an electronic assembly; enclosing the assembly in a tamper respondent wrap, such that the wrap forms fold lines at a first and second end of the assembly; placing the enclosed assembly in a fixture, wherein the fixture comprises a base upon which the assembly rests, a first stationary arm mounted on the base holding the fold lines at the first end of the assembly, a second arm slidably mounted on the base, and a traversing mechanism to bias the second arm toward the fold lines at the second end of the assembly; and heating the enclosed assembly. 
   The second embodiment of the present invention provides a method of producing a tamper respondent enclosure, comprising: enclosing a cryptographic processor in a tamper respondent sheet, wherein an adhesive material secures the enclosure; holding the enclosed cryptographic processor such that the adhesive material remains intact; and applying heat to the enclosed cryptographic processor to strengthen the adhesive material. 
   The third embodiment of the present invention provides a method of forming a security enclosure, comprising: providing a circuit card; enclosing the card in a tamper respondent cloth, wherein an adhesive secures fold lines of the cloth; holding the fold lines of the cloth to maintain adhesive contact; and heating the enclosed card. 
   The fourth embodiment of the present invention provides a method of assembling a security enclosure comprising: providing a fixture; providing an enclosure having a cloth member thereon; placing the enclosure in the fixture; heating the enclosure; and removing the enclosure from the fixture. 
   The fifth embodiment of the present invention provides an apparatus for securing a security enclosure, comprising; a base upon which a security enclosure rests; a first stationary arm mounted on the base, which holds a first end of the security enclosure; a second arm slidably mounted on the base; and a traversing mechanism to bias the second arm toward a second end of the security enclosure. 
   The foregoing and other features and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
       FIG. 1  depicts a top view of a tamper respondent wrap in accordance with the present invention; 
       FIG. 2A  depicts an electronic assembly in accordance with the present invention; 
       FIG. 2B  depicts the electronic assembly of  FIG. 2A  having an enclosure therearound in accordance with the present invention; 
       FIG. 3  depicts a first step in enclosing the assembly of  FIG. 2A  within the wrap of  FIG. 1  in accordance with the present invention; 
       FIG. 4  depicts a second step in enclosing the assembly of  FIG. 2A  within the wrap of  FIG. 1  in accordance with the present invention; 
       FIG. 5  depicts a third step in enclosing the assembly of  FIG. 2A  within the wrap of  FIG. 1  in accordance with the present invention; 
       FIG. 6  depicts a fourth step in enclosing the assembly of  FIG. 2A  within the wrap of  FIG. 1  in accordance with the present invention; 
       FIG. 7  depicts a fifth step in enclosing the assembly of  FIG. 2A  within the wrap of  FIG. 1  in accordance with the present invention; 
       FIG. 8  depicts a graph illustrating the bending modulus of the wrap of  FIG. 1  in accordance with the present invention; and 
       FIG. 9  depicts a clamping device used in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Although certain embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the embodiment. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale. 
   Referring to the drawings,  FIG. 1  shows a tamper respondent wrap or cloth  10  as known and used in the art. The tamper respondent cloth  10  may be a sheet of composite material similar to one made by a division of W. L. Gore (Dundee, Scotland), as described in a patent to MacPherson (U.S. Pat. No. 5,858,500), which is herein incorporated by reference. In particular, the tamper respondent cloth  10  comprises a laminate formed of a number of separate layers, including a delamination respondent layer, and a pierce and laser respondent layer. 
   The respondent layers of the tamper respondent cloth  10  comprise electrically responsive line elements that are disposed on a film material. The pierce and laser respondent layer is intended to detect efforts to penetrate the security enclosure by means of forming a small hole through the tamper respondent cloth  10 . The delamination respondent layer is intended to detect efforts to peel the tamper respondent cloth  10  away from the outer surface of the inner enclosure. The delamination respondent layer is provided with an adhesive characteristic such that peeling it away from a surface will damage the electrically responsive materials therein. These respondent layers are adhered together by means of an adhesive. Thus the tamper respondent cloth  10  comprises respondent film layers that are bonded together with an adhesive. Moreover, an adhesive is provided to retain the folded and overlapping portions upon folding. 
   A top view of one such respondent layer is shown in  FIG. 1 . The layer includes an electrically insulating film  12 , made of polyester film or other similar material, having a plurality of diagonally extending ink traces or lines  14  formed on a first and second side of the film  12  (one side shown). The lines  14  are formed by printing carbon loaded polyester ink on the surface of the film  12 . The lines  14  printed on each side of the film  12  are then selectively connected at the edges  13  of the film  12 . The lines  14  form a plurality of continuous conductors which break easily if attempts are made to penetrate the delamination respondent layer or the pierce and laser respondent layer. 
   Connection between the lines  14  and an enclosure monitor of the electronic assembly (described infra) is provided by an integrated ribbon cable  18 . Like the lines  14 , the ribbon cable  18  is formed by printing carbon loaded polyester ink onto a first surface of the layer  12 . Connectors  16 , also formed on the first surface of the layer  12 , make the connection between the ribbon cable  18  and the lines  14 . 
     FIG. 2A  shows an electronic assembly such as a cryptographic processor card  20  in accordance with the present invention. The cryptographic processor card  20  contains an encryption module  22  which carries the secured sensitive information, a memory  24  which stores a key or code necessary to access the stored information in the encryption module  22 , an erase circuit  26  which erases the stored information in the encryption module  22  in the event the tamper respondent cloth  10  around the cryptographic processor card  20  is breached, an enclosure monitor  28  which monitors the resistence of the lines  14  of the cloth  10  and activates the erase circuit  26  in the event a breach is detected, and a battery  30 , all of which are mounted on a printed circuit board  32 . The cryptographic processor card  20  further includes a plurality of connecting or ribbon cables  34  (one of which is shown), which are used to connect multiple enclosures to one another on a board (not shown), as known in the art. 
   The cryptographic processor card  20  may then be positioned inside a housing  100  comprising for example a top half  102  and a bottom half  104  of a sheet metal box, as illustrated in  FIG. 2B . The two halves  102 ,  104  may be joined together, and include openings  106  as need to insert electrical cables into the processor card  20 . The housing  100  may be designed to provide a suitable surface for wrapping the tamper respondent cloth  10 . 
   The tamper respondent cloth  10  is then wrapped around the housing  100  containing the cryptographic processor card  20  in a manner similar to that of a gift-wrapped present, as illustrated in  FIG. 3 . The cloth  10  overlaps near the middle of the cryptographic processor card  20 , on the underside of the card  20 . The connectors  16  and ribbon cable  18  ( FIG. 1 ) are wrapped within the cloth  10  such that the ribbon cable  18  interconnects with the enclosure monitor  28  within the enclosure. 
   A top flap  36  at each end of the cloth  10  is folded down over the ends  38  of the housing  100  containing the card  20  (one end  38  is shown in  FIG. 4  for ease of illustration). An adhesive on the inner surface of each top flap  36  adheres each top flap  36  to the respective ends  38  for ease of assembly. As illustrated in  FIG. 5 , the connection cable  34  at the one end are folded upward to abut the folded top flap  36  (noting that only one end of the cryptographic processor card  20 , the end nearest the encryption module  22 , has the connection cable  34  extending therefrom in the current illustration, however, the present invention is not intended to be limited to the quantity nor location of the connection cable shown). 
   As illustrated in  FIG. 6 , the side flaps  40  at each end  38  of the housing  100  containing the card  20  are folded inward to overlap the top flap  36  at each end  38 , and the connection cable  34  at the one end. As with the top flap  36 , each side flap  40  includes an adhesive on the inner surface of each flap  40  such that each side flap  40  adheres to each top flap  36  at each end  38  of the housing  100  containing the card  20 , and the connection cable  34  at one end  38  of the card  20 , during assembly. Each bottom flap  42  is then folded upward to overlap the top  36  and side flaps  40 , as well as the connection cable  34  (at the one end  38  of the card  20 ), as illustrated in  FIG. 7 . As with the top and side flaps  36 ,  40 , each bottom flap  42  includes an adhesive on the inner surface of the flap  42  to adhere each bottom flap  42  to the connection cable  34  and/or the side and top flaps  36 ,  40 . 
   It should be noted that the present invention is not intended to be limited to the order of folding the tamper respondent cloth  10  around the card  20  described above. In contrast, the side flaps  40  may be folded inward first, followed by either the top  36  or bottom flaps  42 . Alternatively, the bottom flap  42  may be folded upward first, followed by either the side flaps  40  or the top flap  36 , and so on. 
   The enclosure  44 , having fold lines  45  at each end  38  thereof, is then placed in a clamping device or fixture  46  similar to the one shown in  FIG. 9 . The clamping device  46  includes a base  48  and a plurality of legs  50  thereunder. A stationary clamping arm  52  is securely mounted to a first end of the base  48 . A traversing clamping arm  54  is slidably mounted to the base  48 . A biasing or traversing mechanism  56 , located at a second end of the base  48 , is connected to the traversing clamping arm  54 . The traversing mechanism  56 , comprising a biasing screw, a hydraulic mechanism, an electromechanical sensor motor, etc., functions to bias the traversing clamping arm  54  toward and/or away from the stationary clamping arm  52 . 
   In practice, the enclosure  44  is placed on the base  48  of the clamping device  46 , such that one end  56  of the enclosure  44  is positioned against the stationary clamping arm  52 . The traversing clamping arm  54  is then biased toward the other end  56  of the enclosure  44  via the biasing mechanism  56 . 
   Once the traversing clamping arm  54  of the clamping device  46  is adjusted such that the enclosure  44  fits snugly between the traversing clamping arm  54  and the stationary clamping arm  52 , the enclosure  44  and clamping device  46  are exposed to a temperature of approximately 40–90° C., and preferably between 50–70° C. (because the ink lines  14  may begin to soften and reflow at temperatures above approximately 80° C.), 60° C. being the optimal temperature, for approximately 1 hour (refer to the temperature chart of  FIG. 8 ). The enclosure  44  is then removed from the clamping device  46 . Thereafter, the enclosure  44  may undergo additional processing as known in the art, i.e., applying a polyurethane coating, etc. 
   Heating the tamper respondent cloth  10  initially causes the layers of adhesive to soften, thereby allowing the pierce and laser respondent layer to slide past the delamination respondent layer in the fold areas such that the cloth  10  bends more easily. Upon continued heating the adhesive cross-links or cures due to thermal aging, thereby making the adhesive become more solidified. After removing the heat, the adhesive continues to harden in the folded position during cooling. As a result, the folded cloth  10  forming the enclosure exhibits improved fold retention and reduced stress. And unlike the previous methods of forming enclosures, the cloth  10  does not come un-wrapped during processing. Accordingly, the subsequent encapsulant material, e.g., a polyurethane coating, will not flow past the folds into the inner enclosure to damage the cryptographic processor card  20 , as often happens with conventional methods. 
   It should be noted that the enclosure  44  described and illustrated herein is only one example of the type of enclosure that may be use in combination with the present invention. The present invention is in no way intended to be limited to use in conjunction with electronic assemblies of this size, shape and form. Rather, the enclosure may take the form of a wedge-shaped enclosure, a cuboid, a cube, etc. 
   While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.