Abstract:
Methods, systems, apparatuses and products are disclosed for providing security circuits. Exemplary embodiments including semiconductor chips on circuit boards are shown, together with application in a movie stick/movie player pair. 
     Such systems provide for and improve on the means for clocked logic security support beyond what is available in current security products while being capable of embodiment in low cost technologies such as programmable gate arrays.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application claims priority to U.S. provisional patent application No. 61/189,642 which is incorporated in its entirety by this reference. This patent application also claims priority to United States provisional patent application entitled METHODS, APPARATUSES, AND PRODUCTS FOR A SECURE CIRCUIT by inventor Philip Sydney Langton filed 6 Aug. 2009 and which is also incorporated in its entirety by this reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to devices for security which may be incorporated into personal computers, and devices sharing similar architectures, especially low-cost embedded systems, including but not limited to consumer and/or widely-provided customer service devices. More particularly, the invention relates to an architecture, a design and implementation techniques for a circuit for providing security features. 
       BACKGROUND OF THE INVENTION 
       [0003]    SOTOP (Single Secure On-time Only Programmable) and SCSM (Single Circuit Secure Memory) are closely related circuits. Either or both may be used to create security devices with uncommon architectures that can provide for consumable keys (one-time keys as a limiting case, perpetual keys as the opposite) and high security. Such security devices are based on simplicity favored over ever-growing complexity. A hardware/microcode/firmware approach is used. 
         [0004]    Extremely high security is possible at great (often unaffordable) cost using conventional software-based implementations as an alternative to the invention. Secure microprocessors have been abandoned in many applications such as cellphones—not for reason that their security is unneeded, but rather because their cost is prohibitive. 
         [0005]    In any case, whether using previously developed solutions or the invention, adding a secure module to an insecure assembly will not automatically make the assembly secure. Rather a secure module merely provides the opportunity to design and build a secure assembly. Because the invention focuses on lowest cost rather than highest possible security it opens the door to low cost assemblies since, not only is the incorporated security module itself cheap but the cost of assembly features to accommodate it are low cost also. 
         [0006]    One problem of using highly innovative solutions to meet commercial needs for superior security/cost tradeoffs is that because of the out-of-the-mainstream approach used unusual components have been needed and this augurs against very low cost until and unless extremely high volumes justify an ASIC (application-specific integrated circuit). Embodiments of the invention address that problem, and more. 
       SUMMARY OF THE INVENTION 
       [0007]    The disclosed invention includes, among other things, a manufacture comprising two clocked logic subsystems each including a PGA (programmable gate array) circuit, a configurator circuit, a configuration memory and an input circuit coupled to write to the configuration memory. The input circuit of each subsystem is operable to write to the configuration memory of the other when it is unclocked. 
         [0008]    According to other aspects of the present invention, a SOTOP and/or a SCSM may be included and functional. 
         [0009]    According to another aspect of the present invention, two closely coupled FPGAs may be bonded to a single substrate. 
         [0010]    In an embodiment of still another aspect of the present invention, a secure movie stick and player may be provided. 
         [0011]    An advantage provided by the present invention is that it provides for a low cost and complexity with an excellent level of cost-effectiveness concurrent with data that is retained in a non-volatile memory in which the data may be created and stored during operation, while providing that in the event of an attack by eavesdropping then any cracking can be made short-lived and unprofitable. 
         [0012]    Other solutions may provide just some of the above, or all of the above but only when connected to a trusted server. Any requirement for the use of a trusted server may be a disadvantage or even infeasible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The aforementioned and related advantages and features of the present invention will become better understood and appreciated upon review of the following detailed description of the invention, taken in conjunction with the following drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and in which: 
           [0014]      FIG. 1  shows an exemplary assembly configured according to an embodiment of the invention; 
           [0015]      FIG. 2  is a block diagram that shows an architectural structure an approach to PGA component initialization according to an embodiment of the invention; 
           [0016]      FIGS. 3A and 3B  show plan and side views of a PGA package as used in forming part of an assembly configured according to an embodiment of the invention; 
           [0017]      FIG. 4  shows an assembly  400  in side view according to an embodiment of the invention; 
           [0018]      FIG. 5A ,  5 B,  5 C respectively show side, bottom and top views of an exemplary embodiment of the present invention; 
           [0019]      FIG. 6A ,  6 B,  6 C show side, bottom and top views of a Flash Movie Stick mass storage device according to an exemplary embodiment of the present invention; 
           [0020]      FIG. 7A ,  7 B,  7 C show three views (side, bottom and top) of a Flash Movie Stick mass storage device according to an embodiment of the invention; 
           [0021]      FIGS. 8A ,  8 B,  8 C and  FIG. 9  show four views of a token device assembly according to an embodiment of the invention; 
           [0022]      FIG. 10  shows a movie stick and player connected to a television according to an embodiment of the invention; 
           [0023]      FIG. 11  shows a movie stick and player connected to a computer according to another embodiment of the invention; 
           [0024]      FIG. 12  shows sets of keys held in devices according to an embodiment of the invention; 
           [0025]      FIG. 13  shows simplest case of movie validation and playing according to an embodiment of the invention; 
           [0026]      FIG. 14  shows process of key revocation according to an embodiment of the invention; 
           [0027]      FIG. 15  shows an example process of movie validation after key revocation according to an embodiment of the invention; 
           [0028]      FIG. 16  shows a case of movie validation and playing using hashes according to an embodiment of the invention; 
           [0029]      FIG. 17  shows a case of movie validation and playing using encryption according to an embodiment of the invention; 
       
    
    
       [0030]    For convenience in description, identical components may have been given the same reference numbers in the various drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The description of well-known components is not included within this description so as not to obscure the disclosure or take away or otherwise reduce the novelty of the present invention and the main benefits provided thereby. An exemplary embodiment of the present invention will now be described with reference to the Figures. 
         [0032]    Refer to  FIG. 1  which shows an exemplary assembly configured according to an embodiment of the invention. A PGA (programmable gate array)  150  is present. Commonly termed an FPGA (field programmable gate array), such devices are well-known in the art, however commercial FPGA products tend to incorporate further subsystems, including some of those described herein (below), and other subsystems. 
         [0033]    PGA may be endowed with a RAM  160  (Random Access Memory), usually on the same substrate. In some implementations RAM  160  may be an integral part of PGA  150  implemented typically in a bistable circuit, for example a D-type flip-flop. 
         [0034]    PGA is also coupled to an I/O (Input/Output) block  170  which can provide connection off-chip (present but not shown in  FIG. 1 ). 
         [0035]    In embodiments, Configurator  140  may read from NVCM  130  (Non-volatile configuration memory) at chip power-on initialization so as to responsively configure PGA  150  steered by the contents of the NVCM  130  and, optionally, initialize the data represented in some or all of RAM  160 . 
         [0036]    In some embodiments PGA  150 , RAM  160  and I/O block  170  may substantially occupy a single chip substrate  180 . In this case Configurator  140 , Configuration memory (for example NVCM  130 ) and other parts are external—this is sometimes termed a “two-chip solution” even though more than two chips may be used. 
         [0037]    In more modern embodiments more subsystems may be included in a single chip. In a preferred embodiment of the invention the entire set of subsystems shown in  FIG. 1  (and optionally more) is included as a single chip  198 . 
         [0038]    Still referring to  FIG. 1 , Program Processor  120  is typically a state machine that uses clocked logic to write to the NVCM at a time that the PGA logic is not running. That is to say when the main system clock (not shown in  FIG. 1 ) is stopped. Program Processor  120  may have some form of internal clocking but more typically recovers a clock signal from SPI (Serial Programming Interface)  110 . Program Processor  120  does much more than recover the clock however, it is the means by which information enters NVCM  130  to crucially control PGA  150  and initialize RAM  160  from power-on reset. The use of a serial interface for programming is a convenience but is not essential to the invention; any of many various types of input may be used to good effect. 
         [0039]    In an embodiment, the set of components may be formed onto a single “FPGA known good die”  190 , thus providing a part that may be, for example, wire-bonded or solder beaded and encapsulated as is, or formed into a larger assembly. 
         [0040]    Information may enter Program Processor  120  in at least two ways. Firstly, the logic controls that are later used to initialize the PGA  150  may be formed when the chip is one-time configured, this may considered a late part of the manufacturing process. PGA initialization takes place when the system clock is unfrozen, or upon power-up if the clock is never frozen at any time that the power is turned on. 
         [0041]    PGA  150  may be set up with the logic for embodying a CPU (Central Processor Unit, especially a RISC (Reduced Instruction Set Computer) processor) in the Programmable Gate Array  150  and RAM  160  may also be set up with firmware to then control the RISC CPU formed in the PGA  150 . Implementing a RISC CPU in this way allows a higher level of program abstraction as contrasted with the flip-flops, latches, LUT (lookup table) level and similar features inherent in typical PGA capabilities. This early operation of Program Processor  120  is described further below in connection with  FIG. 2 . 
         [0042]    A second type of operation of SPI  110  with Program Processor  120  is that it may be driven from I/O block  175  from the lower half of the system depicted in  FIG. 1 . As may be apparent SPI  115 , Program Processor  125 , NVCM  135 , Configurator  145 , PGA  155 , RAM  165  and I/O block  175  form an entire, substantially symmetrical sub-system to that described above as ref. 190 . It is a security feature of commercial FPGA parts that reading from the NVCM is not allowed except by the Configurator, which, by design, does not allow readout such as for design cloning which may be illegitimate if unauthorized. 
         [0043]    The clock may thus be stopped on one PGA ( 150  or  155 ) while an NVCM ( 135  or  130  respectively) is being written to. For example, PGA  150  may cause RISC code to be executed that performs I/O to stop the clock on PGA  155  and also writes into SPI  115 . Then, later it may be the turn of PGA  150  to stop the clock on PGA  150  and write into SPI  110 . 
         [0044]    Moreover, it will be apparent to a person of ordinary skill in the art who is familiar with SOTOP circuits described in the prior art that a SCSM can be built with such an arrangement. 
         [0045]      FIG. 2  is a block diagram that shows an architectural structure an approach to PGA (Programmable Gate Array) component initialization according to an embodiment of the invention. 
         [0046]    There are a number of ways for a client to introduce their own initialization data into an FPGA (Field Programmable Gate Array) production run, all of which are known industry standards. In the first instance of an industrially known standard, the clients may build their own proprietary client-side firmware directly into the FPGA Chip Development Programmer  200  fabric. This allows the clients to store and retrieved their own dynamic IP (intellectual property) keys, and the client later may do a final lockdown into NVCM (non-volatile configuration memory). 
         [0047]    In the second instance of an industrially known standard, the client and/or clients may provide their own proprietary software&#39;s Firmware, or a Firmware Client  201 , and/or hardware security to firewall off critical information within the manufacturing environment. 
         [0048]    In the third instance of an industrially known standard, the client may embed FPGA Gateway System Firmware  202  obtained from an outside vendor into a UPGP (Universal Production Gang Programmer)  203 , or directly into the FPGA. 
         [0049]    In the fourth instance of an industrially known standard, a Client may Virtually  204  input their own data into a production run of manufactured FPGAs by using a UPGP  203  to store and retrieved their own file system via a Master FPGA  205  key embedded directly into the UPGP  203 . 
         [0050]    In the fifth instance of an industrially known standard, the client may use Initialize Lock Module Keys  206  to establish a Secure Tunnel  207  into the UPGP  203  to pre-authenticate end users. 
         [0051]    In the sixth instance of an industrially known standard, the client may insert a Novel Protection Switching circuit  208  into the FPGA design itself. In the final instance of an industrially known standard, the client may use a Secure Remote Network  209  to transmit their firmware to the manufacturer of the FPGA. 
         [0052]      FIGS. 3A and 3B  show plan and side views of a PGA package as used in forming part of an assembly configured according to an embodiment of the invention.  FIG. 3A  shows a plan view of the solder-ball side of an FPGA that may be used. In this example the chip is 12 mm by 12 mm and has a total of 284 solder balls each at 0.5 mm ball pitch. This is a chip-scale ball grid array.  FIG. 3B  shows the same chip in profile or side view, a single row of 22 balls is visible close to one edge. FPGA components are available in many types of packages, the use of a BGA (ball grid array or ball grid package) is merely exemplary and enabling, but not critical. Other types of packages, variously mounted, may be used within the general scope of the invention. 
         [0053]      FIG. 4  shows an assembly  400  in side view according to an embodiment of the invention. FPGAs  410  and  415  may be of the same type as the FPGA described earlier (ref.  190  in  FIG. 1 , also  FIG. 3A  and  FIG. 3B ). 
         [0054]    Referring again to  FIG. 4 , FPGAs  410  and  415  may be bonded by solder balls  430  to a double-sided substrate  420  (typically of fiber glass resin or similar insulating material with conductors buried therein). Copper fingers  425 ,  426  may be provided for off-assembly connection. 
         [0055]    In such an assembly much of the interconnection is hidden within substrate  420  and between the two FPGAs  410 ,  415 . Moreover ball-grid chips are difficult to disassemble without destroying the assembly, the components parts, or at the very least making the intrusion tamper-evident. Whilst every system is ultimately hackable the construction of an embodiment such as this is well beyond the capabilities of a home handyman to breach. This fits well with the philosophy of OFID products generally—that highly cost effective security is provided cheaply, but the most extreme security at almost any price is something outside the scope of OFID products. 
         [0056]    The effect of combining two commonly available two circuit memory chips and tightly coupling them where each can, in turn, take full control of the other (but only one at a time) allows a specialized SCSM device or a SOTOP device to be built by closing off general purpose access from the outside and yet allowing data to, in effect, circulate in a loop between the two subsystems. 
         [0057]    Where still more security is required it is feasible, where economic concerns permit, to implement using known good die FPGAs in place of ball grid packaged FPGAs. Two adjacent known good dice can be interconnected on a single chip (in effect a double-sized die). However, unless volumes are high this additional security is bought at increased cost, which is against the most optimal use of the invention. 
         [0058]    Off-assembly communication with the outside world and the application of consumable memory security devices is well-known though novel and valuable techniques can be further refined based on the hardware/microcode/firmware based assemblies described above. 
         [0059]    Using manufactures based on the circuits described above, a large variety of products are enabled. Optimally, but not essentially, such products would have a number of requirements. A first and primary requirement is low cost and hence—complexity, especially as may be reflected in the cost and number of components called for. The invention may find application not where the highest possible security is required but where only a low cost is afforded but nonetheless an excellent level of cost-effectiveness is sought in the security that is, in fact, provided. A second advantage of the inventive circuit is energy performance, notably that data is retained in a non-volatile way even when no measurable amount of power is used. Portable consumer-grade primary cell powered devices come to mind. A third advantage is that data (in the broadest sense) may be created and stored during operation; some alternative solutions are based on a once-and-for-all-time locking up of information at the manufacturing or distribution stage. 
         [0060]    A fifth advantage of the security aspects circuit is that if it is attacked by eavesdropping (or a substantially similar man-in-the-middle type of attack) then any cracking can be made short-lived and unprofitable. This arises out of an ability for the supplier/distributor to control the revoking or expiry of security keys. The user cannot stimulate the device to revoke some of its keys on the road to cracking the remainder of them. 
         [0061]    Finally, other solutions may provide all of the above—but only when connected to a trusted server. In effect the use of a trusted server extends the reach of the total security system. While that approach can be wholly satisfactory in some implementations it is vulnerable to attack of the central area of control, which is likely to result in a high cost for protecting it from attack. Moreover sometimes adequate (as contrasted with excellent) security is desired even when connection (via Internet or otherwise) to a trusted system such as a remote server is infeasible. 
         [0062]      FIG. 5A ,  5 B,  5 C respectively show side, bottom and top views of an exemplary embodiment of the present invention. Into plastic housing  901  the PCBA  603  (printed circuit board assembly) may be placed. Plastic housing  901  may act to protect the PCBA and the USB connector  602 . The sealing of plastic housings are known industry standards. 
         [0063]      FIG. 6A ,  6 B,  6 C show side, bottom and top views of a Flash Movie Stick mass storage device with a PCBA 603  with SMT (Surface-mount technology) components according to an embodiment of the invention. SMT parts include BGA (Ball-Grid Array) package  601 , this may include SMDs (Surface-mount devices), FPGAs (field-programmable gate arrays), and ASICs (application-specific integrated circuits) all of which are known industry standards. USB (Universal Serial Bus) type A Connector  602 , and/or other SMD connectors that are known industry standards including USB-A USB-B, Mini-B, Micro-AB and Micro-B. NS: non-standard connectors also exist for specific proprietary purposes, and not are interoperable with USB-IF (USB Implementers Forum) compliant equipment. Printed circuit board, or PCBA  603  is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. Capacitors  604  may be a SMD or two-terminal through-hole passive electronic component consisting of a pair of conductors separated by a dielectric. Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials that are known industry standards. Resistor  605  may be a two-terminal through-hole, SMD electronic or one or more Array SMD components. Tantalum capacitors  606  may be SMD, and/or through-hole tantalum forms of electrolytic capacitor. Diodes  607  may be SMD or two-terminal device. 
         [0064]      FIG. 6A  shows a side view of a TOSP (Thin small-outline package) Flash memory  608  which is a non-volatile computer memory that can be electrically erased and reprogrammed. There are a variety of small form-factor IC (integrated circuit) carriers other than TSOP, for example BGA (Ball-Grid Arrays) package, SOIC (Small-outline integrated circuit), PSOP (Plastic small-outline package), SSOP (Shrink small-outline package), and TSSOP (Thin-shrink small outline package). 
         [0065]      FIGS. 7A ,  7 B,  7 C show three views of a Flash Movie Stick mass storage device views assembly PCBA with SMT configuration according to an embodiment of the invention. Flash memories  608  in the  FIG. 7B  top view shows four non-volatile computer memory chips  608  that can be electrically erased and reprogrammed. TOSP is shown but not critical, other formats may be used. In  FIG. 7C , the Flash memory bottom view, there is a non-volatile computer memory  608   
         [0066]      FIGS. 8A ,  8 B,  8 C show three views of a token device assembly and there are at least two classes of meaning of token in computing: The views show PCBA  603  and an SMT configuration.  FIG. 8B  top view shows FPGA  601 . 
         [0067]      FIG. 9  shows a bottom view of the token device of  FIGS. 8A ,  8 B,  8 C with a small memory secondary storage  608  which may be a secure or insecure Flash memory. 
         [0068]      FIG. 10  show an exemplary system that exploits the advantages described above. It particularly illustrates a technique for key revocation. It should be realized however that this is but an example to illustrate capabilities and is not a limiting application of the invention. 
         [0069]    Referring to  FIG. 10 , more specifically it shows a so-called movie stick and player connected to a television according to an embodiment of the invention. A STICK  1010  contains content such (for example) as a movie recorded in a Flash memory technology. The STICK  1010  contains a microprocessor-based, single-secure memory circuit with SOTOP. It is powered and communicates only via the USB slave port. Use of USB slave  1004  is exemplary, not limiting. The STICK  1010  also encodes in SOTOP multiple arrays of keys, described with reference to other figures, below. 
         [0070]    Keys may typically be 32 bit random numbers used as one-time secret keys. 32 bits is sufficient to ensure that a brute force attack will be ineffective if each wrong guess is penalized with a significant delay. The type of key used is not critical. 
         [0071]    Still referring to  FIG. 10 , a PLAYER  1000  is externally powered and provides a USB Master port  1005  onto which the STICK  1010  is attached. Again USB is exemplary only. In this case the Player incorporates a modulator  1025  to generate a suitable signal for a TV (television)  1020 . A stream of data encoding the movie is received from the STICK  1000  by the PLAYER  1010  and modulated for the TV  1020 . 
         [0072]    The movie may be stored in the clear or encrypted. If encrypted, then the stream must be decrypted, either in the PLAYER  1000 , in the TV  1020  or in another circuit interposed between the two (not shown). This exemplary embodiment illustrated one possible application—control of the authority and authentication to stream data such as a movie, not the format of the movie (irrespective of how encrypted or otherwise encoded). 
         [0073]    Referring to  FIG. 11  an alternative embodiment has the movie playing on a computer  1110  rather than a TV. As contrasted with  FIG. 11 , a USB Slave port has replaced the modulator. The Computer  1110  is presumed to include necessary equipment such as a USB Master port and a display (not shown in  FIG. 11 ). 
         [0074]    Refer to  FIG. 12 . This shows some of the information  1210  encoded into a typical STICK and information  1220  encoded into a typical PLAYER. The STICK represents a movie stick for a particular title, in this case designated “Title  57 ” in any suitable digital encoding. Also digitally recorded into the STICK are the Movie&#39;s Images and Sound, which may or may not be encrypted. 
         [0075]    Still referring to  FIG. 12 , in the STICK a 3 by 9 array of keys is encoded. These are a FK (first key), SK (second key) and TK (third key) for each of 9 sets. Fewer or greater than 9 sets may be provided. Keys may for example be 32 bit numbers chosen randomly prior to the manufacturing process. 
         [0076]    Still referring to  FIG. 12 , in the PLAYER the same keys for Title  57  are recorded, but the keys for many other titles are also recorded, a little of Title  58  is shown by way of example. Since the PLAYER does not record movie images it may use comparable storage technology to store many keys, tens of millions perhaps in some applications. In all cases the keys may be secured using SOTOP circuits to make cracking and hacking difficult as described elsewhere. 
         [0077]    Now follows an exemplary description of how the devices may be used to authenticate movie playing. A strength of the product that embodies the invention is based on the single-secure circuit concepts which effectively limit that the device can be cracked only by an eavesdropping approach (or an extremely expensive approach using sophisticated equipment). It is, by design, resistant to malware since there is no means to enter code or data into either device. Data may be erased only; code may neither be erased nor changed. 
         [0078]      FIG. 13  shows a simple authentication and play process assuming that no keys have been revoked. The STICK “hello&#39;s” the PLAYER and then announces is has Title  57 . The PLAYER confirms it “knows” of Title  57 . The STICK sends FK 1 , which is the first key of set one. The PLAYER validates FK 1  against its own key memory and imposes a delay if there is an error to hamper would-be hackers/crackers. 
         [0079]    Assuming no error, the PLAYER having validated FK 1  then sends the STICK SK 1 , which the stick checks against the STICK&#39;s memory. All being in order authentication is complete and playing may begin. 
         [0080]      FIG. 16  shows HOW an alternative embodiment could exchange a cryptographically hashed message consisting of FK 1  and a cryptographic salt. The salt could be a semi-random selection of bytes from TK 1  or derived from data that is unique to the STICK or Title. In this case the player could run the same hashing algorithm on SK 1  and a match of the result would authenticate the STICK. This would further frustrate eavesdropping attacks against the exchange. 
         [0081]      FIG. 17  shows HOW an alternative secure embodiment could exchange authentication messages encrypted with the keys from the STICK and the PLAYER. In this case after the STICK identifies itself as containing Title  57  the STICK sends the authentication request message combined with a salt encrypted with key FK 1 . The PLAYER decrypts the message with FK 1  and if it gets a coherent authentication request the source is authenticated. The PLAYER then responds with an authentication request message combined with a salt that is encrypted with SK 1  which the stick decrypts with SK 1  and mutual authentication is complete. 
         [0082]      FIG. 14  shows how a key may be revoked. In this example a new movie STICK (for title  58 ) is used to revoke FK 1  of Title  57 . The practical effect of doing this is to make any and all cracked or hacked titles unreliable and short lived. 
         [0083]    Still referring to  FIG. 14 , after the initial hellos and announcement of Title  57 , the STICK sends a message (command) to revoke FK 1 . The PLAYER confirms that it knows FK 1 , but it does not yet revoke FK 1 . First the STICK must prove its authority to revoke FK 1 , which it does by sending SK 2  (not SK 1 ). Note: this is the first time it is possible to eavesdrop on SK 2 . Thus, FK 1  of Title  57  cannot be revoked until Title  58  is published. This puts control of the timing of “second generation” cracking of Title  57  in the hands of the distributor not the cracker. Thus cracking is made unprofitable. 
         [0084]    After completion of the revocation of FK 1  of Title  57 , the movie STICK for Title  58  restarts the Hello sequence so it can stream Title  58  such as in the manner already described in connection with  FIG. 4  for Title  57 . Many minor (or major) variations to the protocol will be apparent to persons of ordinary skill in the art within the context of the invention. 
         [0085]      FIG. 14  describes how the STICK for Title  57  may be authenticated and plays even after FK 1  has been revoked. First the STICK offers FK 1 . But FK 1  has been revoked in the PLAYER, so the PLAYER sends TK 1  to the STICK. This code TK 1  is checked by the STICK and authenticates that the PLAYER is genuine and therefore entitled (in effect) to demand the first key from the second set (FK 2 ). FK 2  is the authenticated in the PLAYER and SK 2  is authenticated in the STICK and movie streaming can begin. 
         [0086]    Thus, if fake devices are created by eavesdropping, several things make life hard for the malfeasants. First, it is not possible for a fake STICK to revoke FK 1  until FK 1  has been revoked legitimately because a prematurely created fake command does not know SK 2  and the cracker has no means to find it. 
         [0087]    Also, it is not possible to a fake PLAYER to tease FK 2  out of the stick prematurely (before FK 1  is legitimately revoked) because a hacker who creates a fake PLAYER cannot discover TK 1  by eavesdropping until after FK 1  is legitimately revoked. 
         [0088]    Applying the hashing and encryption techniques from  FIG. 16  and  FIG. 17  to the revocation protocol will be apparent to persons of ordinary skill in the art within the context of the invention. 
         [0089]    A business case could be constructed for example in which keys are revoked monthly so that cracked machines become somewhat incompatible and/or unreliable each month. Of course this does nothing to prevent eavesdropping of decrypted movies and the creation of entirely fake distribution systems since that involves the huge disadvantages of complexity embraced by other systems. But this embodiment of the invention does ensure that fake movie sticks will not play reliably in any genuine movie players that even occasionally play genuine movie sticks. And it further ensures that fake movie players will not reliably play genuine movie sticks. 
         [0090]    The embodiments described above are exemplary rather than limiting and the scope and bounds of the present invention should be determined from the claims. Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.