Patent Publication Number: US-6711683-B1

Title: Compresses video decompression system with encryption of compressed data stored in video buffer

Description:
This application claims priority under 35 USC §119(e)(1) of Provisional Application No. 60/087,262, filed May 29, 1998. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is secure computing systems, especially computer systems that may execute after manufacture field provided programs secured to prevent the user from unauthorized use of selected computer services. The computer system may also be functionally reprogrammable in a secure manner. 
     BACKGROUND OF THE INVENTION 
     There are currently many methods to deliver video programming to a users television besides over the air broadcast. Numerous service providers are available to supply this programming to television viewers. Most of these service providers vend a hierarchy of services. Typically there is a basic service for a basic fee and additional services available for an additional fee. The basic services typically include the broadcast network programming, cable superstations, music and sports programming. These basic services are typically supported by advertizing. These basic programming services thus operate on the same economics as over the air broadcast television. The additional services typically include the so called “premium” programming such as sports and movies. These premium programming services are typically not advertizer supported. These are perceived by the television user as higher value services and television users are willing to pay their service providers additional fees for these services. The service provider passes much of this additional fee to the content providers as their compensation for supplying the programming. There may be one or several tiers of these premium services made available by the service providers. At the top of this programming hierarchy is pay per view programming. Pay per view programming typically includes music concerts and sporting events perceived as time sensitive and highly valuable by the television users. Pay per view may also include video on demand, where the television user requests a particular movie be supplied. This hierarchy of service exists for all current alternative methods of program delivery including television cable, over the air microwave broadcast and direct satellite television. 
     Reception of such alternative programming services has required an additional hardware appliance beyond the user provided television receiver since the beginning of cable television. Initially this additional hardware appliance merely translated the frequency of the signal from the transmission frequency to a standard frequency used in broadcast television. Such a standard frequency is receivable by the user provided television receiver. This additional hardware appliance is commonly know as a “set top box” in reference to its typical deployment on top of the television receiver. Current set top boxes handle the hierarchy of security previously described. 
     In the past these set top boxes have been fixed function machines. This means that the operational capabilities of the set top boxes were fixed upon manufacture and not subject to change once installed. A person intending to compromise the security of such a set top box would need substantial resources to reverse engineer the security protocol. Accordingly, these such fixed function set top boxes are considered secure. The future proposals for set top boxes places the security assumption in jeopardy. The set top box currently envisioned for the future would be a more capable machine. These set top boxes are expected to enable plural home entertainment options such as the prior known video programming options, viewing video programming stored on fixed media such as DVD disks, Internet browsing via a telephone or cable modem and playing video games downloaded via the modem or via a video data stream. Enabling the set top box to be programmed after installation greatly complicates security. It would be useful in the art to have a secure way to enable field reprogramming of set top boxes without compromising the hierarchy of video programming security. 
     SUMMARY OF THE INVENTION 
     This invention is a secure computing system that prevents unauthorized use of compressed video data stored in a first-in-first-out memory buffer in a set top box. In a typical system, the set top box receives an encrypted video data stream, such as representing a premium channel or pay-per-view event. If the use is authorized, a data processor decrypts this data for display. The video data stream is typically transmitted in a compressed form to reduce the necessary channel bandwidth. Current video compression techniques do not compress data uniformly. It is known in the art to include fully transmitted video frames interleaved with differentially encoded frames and predictively encoded frames. For this reason a uniform compressed video data rate does not translate into a uniform decompressed video data rate. Typical set top boxes employ off chip DRAM as a first-in-first-out (FIFO) buffer to prevent the decompression process from overflowing or underflowing. The memory bus traffic between the data processor and the portion of memory used as the FIFO buffer is subject to interception and unauthorized use. 
     The data processor used in this invention is disposed on a single integrated circuit This data processor includes a chip identity read only register storing a unique chip identity number. This unique chip identity number is fixed during manufacture by, for example, laser probing or selective activation of fuse or antifuse links in the chip identity register. The data processor encrypts the compressed video data stream using at least a part of the chip identity number as an encryption key. This encrypted data is stored in the memory area serving as the FIFO buffer. The data is recalled from memory as needed for video decompression. The data processor then decrypts the recalled data employing at least a part of the chip identity number as the decryption key. 
     Using technique the compressed video data stream temporarily stored in compressed form in the FIFO buffer can only be read by the particular data processor having the unique chip identity number. Since the chip identity number is unique to that particular data processor, the video data cannot be processed by another data processor, even another identical set top box system without breaking the code. The encryption and decryption is transparent to the user requiring only a small additional processing capacity within the data processor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of this invention are illustrated in the drawings, in which: 
     FIG. 1 is a block diagram of one embodiment of the secure computing system of this invention; 
     FIG. 2 is an example memory map of the boot read only memory of the digital media processor illustrated in FIG. 1; 
     FIG. 3 is an example memory map of the nonvolatile memory of the set top box illustrated in FIG. 1; 
     FIG. 4 is an example memory map of the read write memory illustrated in FIG. 1; 
     FIG. 5 is a flow chart of the initial operation including the operating system verification of the digital media processor illustrated in FIG. 1; 
     FIG. 6 is a flow chart of the process for verification of an application to the set top box illustrated in FIG. 1; 
     FIG. 7 is a flow chart of the process of verification of a downloaded application program; 
     FIG. 8 is a schematic diagram of a translation look aside buffer preventing virtual memory relocation of a certain page of memory of the digital media processor of FIG. 1; 
     FIG. 9 is a flow chart of the process of encrypting and decrypting compressed video data temporarily stored in random access memory; and 
     FIG. 10 is a flow chart of the process of mode election in a hardware debugger/emulator. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The set top box of the future will enable home entertainment options such as the known video programming options, viewing video programming stored on fixed media such as digital video disks (DVD), Internet browsing via a telephone or cable modem and playing video games downloaded via the modem or via a video data stream. Such a variety of capability can only be provided by a fully programmable data processor which can receive and run downloaded programs. This opens up a host of security issues. Since much of the utility of the system depends on being able to download various applications, the possibility also exists for an unauthorized application being downloaded. Such an unauthorized application may be deliberately written to compromise the hierarchy of security. 
     Fully programmable set top boxes are vulnerable to three main types of attacks. An unauthorized application may interact with the operating system, possibly bypassing security. The set top box nonvolatile memory may be replaced with modified resident applications, but with the original operating system. The nonvolatile memory may be replaced with a new operating system. The most important item to protect is the operating system. If the operating system is compromised, an unauthorized person can do almost anything, including disguising the fact that the operating system is compromised. 
     FIG. 1 illustrates in schematic form the parts of a versatile, programmable set top box system  100 . Set top box system  100  is responsive to inputs from: television cable  101 ; direct satellite receiver front end  103 , digital video disk (DVD)  105 ; an ordinary telephone line  107 ; and infrared remote control  109 . These inputs are conventional and need not be more fully described here. Any interaction of these conventional inputs with the parts of this invention will be more fully described below. 
     The central part of set top box system  100  is the set top box  110 . Set top box  110  includes various interfaces for the inputs including: video analog-to-digital converter  111  connected to the television cable  101 , which may optionally include a cable modem; video analog-to-digital converter  113  connected to direct satellite receiver front end  103 ; a DVD player capable of receiving and reading DVD  105 ; voice band modem  117  connected to telephone line  107 ; and infrared receiver  119  capable of receiving the infrared signals from infrared remote control  109 . 
     Set top box  110  includes several output devices coupled to digital media processor  130 . Video digital-to-analog converter  121  receives a video data stream from digital media processor  130  and supplies an corresponding video signal to television receiver  151 . Typically the desired video data stream is modulated upon a carrier having a frequency which television receiver  151  can normally receive. It is contemplated that video media processor  130  in cooperation with video digital-to-analog converter  121  will be capable of producing a video signal in a plurality of formats. Upon set up of set top box system  100  the particular format will be selected to correspond to the capability of the particular television receiver  151  employed. Audio digital-to-analog converter  123  receives an audio data stream from digital media processor  130  and supplies a base band audio signal to audio system  153 . It is contemplated that this audio signal may encompass plural audio channels (i.e. left and right channels for stereo). It is also contemplated that any particular video source may include plural encoded audio data streams such as alternative languages, descriptive video or other separate audio programs (SAP). Note also that the audio data stream will typically also be modulated on the same carrier as the video signal for reception and demodulation by television receiver  151 . 
     The intelligent part of set top box  110  is digital media processor  130 . Digital media processor  130  is preferable embodied in a single integrated circuit. Note that in order for set top box  110  to be fully secure as intended in this invention, central processing unit  131  and boot ROM  135  must be located on the same integrated circuit. Digital media processor  130  includes central processing unit  131 . Central processing unit  131  is illustrated generically and is not intended to limit the structure employed. Central processing unit  131  preferably includes data processing capability for control functions required for selection of operating mode, channel tuning, security functions and the like. Central processing unit  131  preferably also includes digital signal processing capability for decompressing compressed video and audio signals, decrypting encrypted video signals, converting the received video to the format of the user&#39;s television receiver, operating as a “software” cable modem and voice band modem and demodulating the signal from infrared remote control  109 . Central processing unit  131  may include a microprocessor and a digital signal processor, a single data processor capable of all the necessary functions or a multiprocessor. The exact nature of central processing unit, except for details noted below, is not relevant to this invention. 
     Digital media processor  130  further includes chip identity register  133 . Chip identity register  133  is a programmable readable register holding an identity number unique to the integrated circuit embodying digital media processor  130 . This identity number is preferably implemented as taught in U.S. patent application Ser. No. 08/813,887 entitled “CIRCUITS, SYSTEMS, AND METHODS FOR UNIQUELY IDENTIFYING A MICROPROCESSOR AT THE INSTRUCTION SET LEVEL” and filed Mar. 7, 1997, now U.S. Pat. No. 6,056,133 issued May 16, 2000. As described in this patent application, the unique identification code is formed in a read-only data register by laser probing following integrated circuit test. The unique chip identity number may be specified via selective blowing of fuse or antifuse links or other techniques. This identity number permits a program to verify the exact identity of the particular digital media processor  130  used in the set top box  110 . 
     Digital media processor  130  includes boot read only memory (ROM)  135 . Digital media processor  130  is constructed so that central processing unit  131  begins executing program instructions stored within boot ROM upon each initial application of electric power. An exemplary memory map of boot ROM  135  is illustrated in FIG.  2 . Those skilled in the art will realize that the exact order of storage of the various parts is not as important as the existence of the detailed data types. Boot ROM  135  includes self boot code  201 . Self boot code  201  is the program instructions initially executed by central processing unit  131  upon each initial application of electric power to digital media processor  130 . In addition the known processes for initializing computer systems, self boot code  201  also includes verification program code  202 . Verification program code  202  will be further described below in conjunction with FIG.  5 . Boot ROM  135  also includes a public signature keys. These public signature keys include real time operating system (RTOS) public signature key  203 , first application public signature key  205 , second application public signature key  206  to the Nth application public signature key  207 . These public signature keys are employed in verification of the authorization of programs in a manner that will be further described below. 
     Set top box  110  includes flash (electrically programmable read only memory) EPROM  141  bidirectionally coupled to digital media processor  130 . Flash EPROM  141  serves as the nonvolatile memory for set top box system  100 . This is known as a nonvolatile memory because it retains its contents when electric power is turned off. Nonvolatile memory is needed for the real time operating system (RTOS) and for resident applications. FIG. 3 illustrates an exemplary memory map of flash EPROM  141 . Flash EPROM  141  includes the real time operating system (RTOS)  210 . RTOS  210  includes program code enabling digital media processor  130  to receive and process various data streams as they are received, i.e. in “real” time. RTOS  210  also enables digital media processor  130  to respond to operator control via infrared remote control  109  and infrared receiver  119 . RTOS  210  includes a signature portion  211  whose use will be further described below. Flash EPROM  141  includes program code for the first resident application  220  with its corresponding signature portion  221 . Likewise, flash EPROM  141  included program code for the second resident application  230  and its corresponding signature portion  231  and program code for other resident applications to the Mth resident application  240  and its corresponding signature portion  241 . Flash EPROM  141  optionally includes additional public keys including N+1st public key  251 , N+2nd public key  253  to N+Pth public key  255 . These additional public signature keys are similar to the N public signature keys stored in boot ROM  135 . Their use will be detailed below. 
     Set top box  110  further includes dynamic random access memory (DRAM)  143  bidirectionally coupled to digital media processor  130 . DRAM  143  is a volatile memory that serves as read/write memory to temporarily store transient data during normal operations. DRAM  143  is preferably embodied by synchronous memory employing a RAMBUS interface. FIG. 4 illustrates an exemplary memory map of DRAM  143 . DRAM  143  stores the memory resident part  261  of the real time operating system. Depending upon the particular status of set top box system  100  this memory resident part  261  of the RTOS may differ as known in the art. DRAM  143  stores the memory resident parts  263  of the currently running application or applications. These applications may be resident applications stored in flash EPROM  141  or transient applications stored in other parts of DRAM  143 . Depending upon the status of set top box system  100 , there may be various applications running and their immediately accessible parts will be stored in DRAM  143  for faster access than from flash EPROM  141 . DRAM  143  also stores transient data  265 . This transient data  265  includes temporary data used by the various applications as well as the current control status as controlled by the user via infrared remote control  109  and infrared receiver  119 . DRAM  143  stores the program code of various transient applications such as first transient application  271 , second transient application  273  to Qth transient application  275 . Transient applications are those loaded via cable modem  111 , voice band modem  117  or DVD drive  115  that are intended for use only during the current session of set top box system  100 . These may include video games, Internet browsing and the like. These transient applications are loaded into DRAM  143  each time they are used and then discarded. DRAM  143  also stores compressed video in a first-in-first-out (FIFO) buffer  280 . Video data from television cable  101 , direct satellite receiver front end  103  and DVD  105  will generally be transmitted in compressed form. This saves transmission bandwidth and storage space. One of the tasks of digital media processor  130  is to decompress the video data. Current video compression formats (such as MPEG2) and all contemplated future video compression formats are nonlinear. That is, the different portions of the video data stream are compressed to differing degrees. Thus a constant rate of received video data represents varying amounts of video. Following decompression, digital media processor must supply video data in a constant rate to be viewed. Compressed video FIFO buffer  270  is necessary to smooth out the variations in the rate of receipt. This permits the decompression process to neither overflow with too much compressed data nor underflow with no compressed data ready for decompression. This is possible because the compressed video data stream represents a constant rate video data stream that is to be viewed. Thus the overall average compressed video data rate corresponds to the constant real time viewing rate. 
     FIG. 5 is a flow chart  300  of an example of digital media processor  130  operations controlled by boot ROM  135 . Upon each initial application of electric power to set top box system  100 , digital media processor begins executing the program stored in a predetermined location within boot ROM  135 . Those portions of this program within boot ROM  135  relevant to this invention are illustrated in FIG.  5 . Program  300  first initializes digital media processor  130  (processing block  301 ). This process would include clearing registers and caches, setting the initial operating mode and the like, in a manner known in the art. Following initialization of the processor, program  300  reads the signature portion  211  of RTOS  210  stored in flash EPROM  141  (processing block  302 ). Program  300  next reads the RTOS public key  203  from boot ROM  135  (processing block  303 ). Next program  300  verifies the signature portion  211  of RTOS  210  (processing block  304 ). In accordance with the known art of public key encryption such as the RSA algorithm, signature portion  211  is produced by operating upon all of RTOS  210  with a secret private signature key. The original data of signature portion  211  is recovered by a reverse process employing RTOS public signature key  203  stored in boot ROM  135 . This signature verification process takes into account what is know as a “trap door” function. It is a very difficult process to produce a particular signature portion knowing only the public key. A change of any portion of RTOS  210  is very likely to result in a change in the signature portion  211  in a manner that cannot be predicted from only the RTOS public signature key  203 . Thus it is possible to detect any change in RTOS  210  employing the signature portion  211 . 
     Following the verification, program  300  tests the verified signature portion to determine if RTOS  210  supports secure applications (decision block  305 ). This invention contemplates that digital media processor  130  could be embodied in applications not requiring the security of set top boxes. In such applications, the verified signature portion  211  indicates that the RTOS need not be secured. Note that even a non-secure RTOS must have its stub verified. Failure of the signature verification is fatal whether the RTOS is secure or non-secure. Program  300  bypasses other steps and starts RTOS  210  (processing block  310 ) if this signature portion  211  indicates a non-secure use. This will typically involve loading at least a portion of RTOS  210  into DRAM  143 . It is anticipated that DRAM  143  will allow much faster memory access than flash EPROM  141 . Thus loading portions of RTOS  210  into DRAM  143  will enable faster operation. 
     If the verified signature portion indicates that RTOS  210  is to support secure applications (decision block  305 ), then program  300  tests to determine if RTOS  210  can be verified as correct (decision block  306 ). As described above, the trap door function of the private key signature with public key signature makes it a very difficult task to modify. RTOS  210  without producing an unpredictable modification of signature portion  211 . Thus the initial program stored in boot ROM  135  will almost certainly be able to detect unauthorized modification of RTOS  210 . This verification of RTOS  210  permits the vendor of set top box system  100  to be confident of the security of the system. 
     If the verified signature portion is not verified as secure, then program  300  indicates that RTOS  210  is non-secure (processing block  307 ). Thereafter program  300  takes remedial action (processing block  308 ). This remedial action can take many forms. At the most severe, this remedial action could be complete disablement of set top box  110 . Shutting down media processor  130  will disable set top box  110  since it is the intelligence of set top box  110 . In most secure applications running a non-verified RTOS would be considered very dangerous and the only reasonable remedial action is disabling set top box  110 . In a few cases a less severe remedial action may be appropriate. As a less severe remedial measure, digital media processor  130  could be programmed to no longer interact with video data streams from television cable  101 , direct satellite receiver front end  103  and/or DVD  105 . This mode may permit running local only transient applications. The remedial action could include signaling the set top box vendor or service provider of the security violation via cable modem  111  or voice band modem  117 . The recipient of this notification could then determine either automatically or manually how to deal with the security violation. One method of responding to such a notification of a security violation is to download via cable mode  111  or voice band modem  117  an authorized copy of the RTOS for storage in flash EPROM  143 , overwriting the unauthorized copy. Another method is to download a diagnostic program which will verify and determine the extent of the security violation. At the least severe level most suitable for service providers who supply only advertiser supported program material, is to ignore the security violation and permit operation of the non-secure RTOS. 
     If the verified signature portion is verified as secure, then program  300  indicates that RTOS  210  as verified (processing block  309 ). Thereafter program  300  starts operation of RTOS  210  (processing block  310 ). As described above this would typically involve copying at least portions of RTOS  210  from flash EPROM  141  to DRAM  143 . Following such a copying, program control would be transferred to the RTOS copy in DRAM  143  via a jump instruction. RTOS  210  then enables all the authorized functions of set top box system  100 . 
     The entire RTOS could be encrypted using the private key as an alternative to employing merely a signature verification process. The steps illustrated in FIG. 5 would be similar except that the entire RTOS must be decrypted using the public key rather than just the signature portion. In this event, the decrypted RTOS would be copied to a operating portion of DRAM  143  upon verification. Thereafter program control would be passed to this copy of the RTOS from the boot ROM program via a jump instruction. In this case a non-verified RTOS even if copied into the same part of DRAM  143  will not operate. An incorrect decryption of an unauthorized RTOS  210  would likely result in an inoperable operating system. Thus the remedial action is this case disables set top box  110 . Note the use of a private key to encrypt and a public key to decrypt is the reverse of the usual private key/public key system. Currently, only the RSA system is known to permit this reverse use. 
     FIG. 6 is a flow chart  400  of an example of digital media processor  130  operations when called to load and run a resident application. Following the command to start the resident application program (processing block  401 ), program  400  reads the corresponding signature portion of the resident application stored in flash EPROM  141  (processing block  402 ). 
     Program  400  next reads the corresponding public key from boot ROM  135  or flash EPROM  141  (processing block  403 ). As noted above in the memory maps of boot ROM  135  and flash EPROM  141 , the public keys for resident application programs may be stored in boot ROM  135  or in flash EPROM  141 . Alternatively, set top box  100  may be constructed so that the public keys for some resident applications are stored in boot ROM  135  and the public keys for the remaining resident applications are stored in flash EPROM  141 . Next program  400  verifies the signature portion of the resident application (processing block  404 ). This signature verification process is the same as previously described in conjunction with verification of RTOS  210 . 
     Following the verification, program  400  tests the verified signature portion to determine if the resident application supports security (decision block  405 ). It is contemplated that any resident application that interacts with program content received from television cable  101 , direct satellite receiver front end  103  or DVD  150  will require security. Other resident applications may require security at the option of the application program vendor. Program  400  bypasses other steps, loads the resident application into DRAM  143  and starts the application program (processing block  410 ) if this signature portion indicates a non-secure use. 
     If the verified signature portion indicates that the resident application is to support secure applications (decision block  405 ), then program  400  tests to determine if the resident application can be verified as correct (decision block  406 ). The trap door function of the private key encryption with public key decryption makes it a very difficult -task to modify the resident application program without producing an unpredictable modification of signature portion, thus enabling verification of the authorization of the resident application. 
     If the signature portion is not verified as secure, then program  400  indicates that the resident application is non-secure (processing block  407 ). Thereafter program  400  takes remedial action (processing block  408 ). This remedial action could be any of the many forms described above. 
     If the signature portion is verified as secure, then program  400  indicates that the resident application as verified (processing block  409 ). Thereafter program  400  starts the resident application by transferring at least part of its program code to DRAM  143  and transferring control via a jump instruction. It is contemplated that resident application programs will have access to less than all of the digital media processor functions accessible via RTOS  210 . 
     The entire resident application could be encrypted using the private key as described above. The steps illustrated in FIG. 6 would be similar except that the entire resident application must be decrypted using the public key rather than just the signature portion. As previously described, using this technique means that an unauthorized program will probably crash and disable set top box  110 . 
     FIG. 7 is a flow chart  500  of an example of verification of a downloaded program. Following the command to start downloading an application program (processing block  501 ), program  500  downloads the application as stores it in DRAM  143  (processing block  502 ). Then program  500  reads the corresponding signature portion of the downloaded application stored in DRAM  143  (processing block  503 ). Program  500  next reads the corresponding public key from boot ROM  135  or flash EPROM  141  (processing block  504 ). As noted above in the memory maps of boot ROM  135  and flash EPROM  141 , the public keys for resident application programs may be stored in either boot ROM  135  or in flash EPROM  141 . Next program  500  runs signature verification on the downloaded application program (processing block  505 ). This signature verification process is the same as previously described in conjunction with verification of RTOS  210 . A secure application program will have a signature portion that permits verification of the entire downloaded application program. A non-secure application program will have a verifiable signature stub. 
     Program  500  next tests to determine if the signature or Ha signature stub has been verified (decision block  506 ). If the signature or signature stub has not been verified as proper, then program  500  would indicate a security violation (processing block  507 ) and take remedial action (processing block  508 ). This remedial action could be any of the many forms described above. In addition, another possible remedial action in this instance is to make an a further attempt to download this application. Thus program  500  could loop back to processing block  502  to repeat the download. This remedial action would permit recovery if an authorized application was corrupted, such as by noise or the like, during download. If this option is used, it is preferable to abort this loop after a predetermined number of signature verification failures. 
     Following successful verification of the signature or signature stub, program  500  tests the verified signature portion to determine if the downloaded application supports security (decision block  509 ). Program  500  bypasses other steps, stores and runs the downloaded application program (processing block  512 ), if this signature portion indicates a non-secure use. Note that the downloaded application program may be loaded into flash EPROM  141  if it is intended to be another resident application or into DRAM  143  if it is intended to be a transient application. 
     If the verified signature portion indicates that the downloaded application program supports secure applications (decision block  509 ), then program  500  tests to determine if the downloaded application can be verified as correct (decision block  511 ). The trap door function makes it a very difficult task to modify the downloaded application program without producing an unpredictable modification of signature portion, thus enabling verification of the authorization of the downloaded application program. 
     If the downloaded application program is not verified as correct (decision block  510 ), then program  500  indicates that the downloaded application is non-secure (processing block  507 ). Thereafter program  500  takes remedial action (processing block  508 ). This remedial action could be any of the many forms described above and may include making a further attempt to download this application program. 
     If the downloaded application is verified as correct (decision block  510 ), then program  500  indicates the downloaded application is secure (processing block  511 ). Thereafter program  500  stores and runs the downloaded application program (processing block  512 ). As described above, this storage will be in flash EPROM  141  if the application is a resident application or in DRAM  143  if the application is a transient application. Program  500  starts the downloaded application program by transferring at least part of its program code to DRAM  143  and transferring control via a jump instruction. 
     The entire downloaded application program could be encrypted using the private key as described above. The steps illustrated in FIG. 7 would be similar except that the entire downloaded application must be decrypted using the public key rather than just verifying the signature portion. As previously described, using this technique means that an unauthorized program will probably crash and disable set top box  110 . 
     This security technique relies upon the security of boot ROM  135 . Since boot ROM  135  is located on the same integrated circuit as the other parts of digital media processor  130  and it is a read-only, it is not subject to unauthorized change. Therefore the verification function cannot be changed to verify a unauthorized RTOS. Many of the security functions will be available only to the RTOS based upon program privilege levels. Thus most security functions cannot be easily compromised. The private key used for encryption will only be known to the RTOS supplier, or only to the manufacturer of digital media processor  130 . In addition the public key needed to verify the signature or to decrypt the RTOS is also in the boot ROM. This prevents substitution of another public key in an attempt to cause digital media processor  130  to verify an unauthorized RTOS. Additionally, the resident applications are also secure. The private keys for resident applications can be known only by the application owner, or by the service provider who authorizes the application. 
     The above private key/public key signature verification system will protect against most security attacks. However, if the private key used to authenticate the RTOS is compromised, the security may be defeated by replacing the RTOS with an unauthorized RTOS which will still look authentic. 
     The simplest way to detect a modified RTOS would be to check the resident RTOS against the authorized program. An application program, such as a diagnostic program, could read certain memory locations in the RTOS to see if they contain the expected values. This may not always reveal unauthorized substitution of another RTOS. Many complex data processors such as would be used to embody digital media processor  130  support virtual memory. In a virtual memory environment, the RTOS is quite capable of virtualising itself. Thus the unauthorized RTOS would intercept the confirming read attempts and return the results that the diagnostic application expects from a copy of the authorized RTOS. However, this unauthorized RTOS would run instead of the original RTOS consequently compromising security. The present inventors propose a technique which assures that an application can access a portion of memory directly without being intercepted and translated to a virtual address by the RTOS. 
     FIG. 8 illustrates in block diagram form a translation look-aside buffer (TLB)  137  having a locked page in accordance with this invention. Virtual memory applications translate a virtual address into a physical address. As is known in the art, TLB  137  receives a virtual address on bus  601  and supplies a corresponding physical address on bus  602 . A predetermined number of most significant address bits of the virtual address are supplied to a plurality of comparators  621 ,  623 ,  625  and  627 . The remaining least significant address bits of the virtual address on bus  601  are passed unchanged to the corresponding bits of physical address on bus  602 . Each comparator  621 ,  623 ,  625  and  627  has a corresponding virtual address register  611 ,  613 ,  615  and  617 , respectively. The comparators  621 ,  623 ,  625  and  627  determine if the predetermined number of most significant bits of the virtual address on bus  601  matches the contents of the respective registers  611 ,  613 ,  615  and  617 . Each comparator  621 ,  623 ,  625  and  627  supplied match indication to multiplexer  650 . Multiplexer  650  supplies the predetermined number of most significant bits from one of the physical address registers  641 ,  643 ,  645  and  647 . The physical address register selected by multiplexer  650  corresponds to the comparator  621 ,  623 ,  625  or  627  detecting a match. These most significant physical address bits selected by multiplexer  650  are supplied to the most significant bits of the physical address on bus  602 . Thus TLB  137  substitutes a predetermined number of bits of a physical address for the same number of bits of the virtual address. The number of possible substitutions enabled by the virtual address register and its corresponding comparator and physical address register is limited only by considerations of operation code space to access the registers and the amount of space occupied by the TLB  137 . In the prior art, virtual address registers  611 ,  613 ,  615  and  617  and physical address registers  641 ,  643 ,  645  and  647  are alterable via software. Thus the real time operating system has control of the mapping of virtual addresses to physical addresses. 
     In this invention one of the virtual address registers and the corresponding physical address register are fixed upon manufacture. In the preferred embodiment this pair of registers are mask programmable at metal layers, permitting the locked page to be selected upon manufacture of the integrated circuit including TLB  137  but unalterable following manufacture. FIG. 8 illustrates a fixed virtual address register  611  and its corresponding fixed physical address register  641 . In the preferred embodiment the virtual address stored in fixed virtual address register  621  equals the physical address stored in fixed physical address register  641 . In the preferred embodiment, the critical code to be protected from relocation will be stored in flash EPROM  141  within the boundary of physical addresses covered by this virtual address register. Attempts to write to either fixed virtual address register  611  or fixed physical address register  641  will fail because these registers are fixed in hardware. Preferably there will be no faults or errors generated by an attempt to modify these registers. Alternatively, neither the fixed virtual address register  611  nor the fixed physical address register  641  are accessible via the instruction set architecture. Since the reason that fixed virtual address register  611  or fixed physical address register  641  are fixed is to prevent alteration, no access via the instruction set architecture would ever be required. 
     A further feature of this invention is illustrated in FIG.  8 . Note that the match indication from comparator  621  is supplied directly to multiplexer  650 . The match indication from other comparators form the noninverting input to respective AND gates  633 ,  63 and  637 . Each of these AND gates  633 ,  635  and  637  receives the match indication from comparator  621  on an inverting input. Thus a match indication from comparator  621  prevents supply of a match indication to multiplexer  650  from any other comparator. This prevents an unauthorized person from leaving the original RTOS in place to respond to security queries while attempting to run an unauthorized RTOS from a relocated part of memory. Any memory accesses to the physical memory addresses of virtual address register  611  and physical address register  641  cannot be relocated but are directed to the physical address of the original RTOS. 
     With this invention an unauthorized attempt to relocate the RTOS may occur, but no actual address translation will take place. Thus if the original RTOS is always located in this memory area, a diagnostic program can read signature locations with assurance that the original physical locations are being accessed. Thus the diagnostic program can determine if the RTOS is compromised, and take appropriate remedial action. This remedial action may include any of the remedial actions previously described. 
     The set top box  100  illustrated in FIG. 1 includes an additional potential security problem. DRAM  143  stores a video data stream that has been decrypted but not decompressed. This video data is stored in compressed video FIFO buffer  280 . It is possible for an unauthorized person to intercept this data as it is being transferred from digital media processor  130  to DRAM  143  or as it is being transferred from DRAM  143  and digital media processor  130 . These data transfers will be interleaved with other data traffic between digital media processor  130  and DRAM  143 , but it is feasible to separate the compressed video data. Because the video is compressed, a minimal amount of memory would be required to store this data. Some content providers would like to prevent their video programming from such interception. Note that interception of the video data stream at this point would permit generation of plural, identical and immediately viewable copies of the video. 
     FIG. 9 illustrates in flow chart form a process preventing such unauthorized interception. Following reception of the video data stream (processing block  701 ) digital media processor  130  decrypts the video data stream (processing block  702 ). This decryption is subject to security procedures to ensure that the user is authorized to view this video data stream. Following this decryption of the source program, digital media processor encrypts the video data stream again (processing block  703 ). In this instance a relatively simple encryption is used, such as a simplified DES algorithm. The encryption key is preferably derived from the chip identity number stored in chip identity register  133 . This encrypted data is stored in compressed video FIFO buffer  280  (processing block  704 ). At the appropriate time, the video data is recalled from compressed video FIFO buffer  280  (processing block  705 ). The recalled data is decrypted using the encryption key derived from the chip identity number ( 706 ). This data is then ready for further processing (processing block  707 ). 
     This technique has the advantage that the compressed video data stream temporarily stored in compressed video FIFO buffer  280  can only be read by the particular digital media processor  130 . The chip identity number is unique to that particular digital media processor. The video data cannot be viewed by any other means, even another identical set top box system  100  without breaking the code. This is believed adequate security by most content providers. Additionally, the encryption and decryption is transparent to the user. There only needs to be a small additional processing capacity available within digital media processor  130  beyond the minimal requirement of the particular application. 
     Another potential security problem is created by the hardware debugger/emulator. The semiconductor manufacturer of digital media processor  130  will generally also sell hardware debugger/emulator systems to application program developers, including operating system developers. Generally such hardware debugger/emulator systems by design have unlimited access to all of memory, including “private” areas. Thus a hardware debugger/emulator system of the type known in the art would permit unauthorized breach of the security of set top box system  100 . 
     The following modification to the hardware debugger/emulator system will guard against this potential security problem. The hardware debugger/emulator will operate in two modes, a process mode and a raw mode. In the process mode, the hardware debugger/emulator may only access a particular process or application program. All system access is permitted in the raw mode. 
     FIG. 10 is a flow chart illustrating the process of selecting the mode at the hardware debugger/emulator. Upon start of the hardware debugger/emulator (processing block  801 ), process  800  reads the signature portion  211  of RTOS  210  stored in flash EPROM  141  (processing block  802 ). Process  800  next reads the RTOS public key  203  from boot ROM  135  (processing block  803 ). Next process  800  verifies the signature portion  211  of RTOS  210  (processing block  804 ). Following the verification, process  800  tests the verified signature portion to determine if RTOS  210  supports secure applications (decision block  805 ). As previously described, digital media processor  130  could be embodied in applications not requiring the security of set top boxes. In such applications, the verified signature portion  211  indicates that the RTOS need not be secured. If this is the case, then process  800  bypasses other steps activates the hardware debugger/emulator in raw mode (processing block  806 ). 
     If the RTOS supports secure applications (decision block  805 ), then process  800  checks to determine if the chip identity number stored in chip identity register  133  is of the subset of possible chip identity numbers that permit the raw mode for secure applications (decision block  807 ). Some program developers, particularly RTOS developers, will need access to the raw mode of the hardware debugger/emulator. This invention contemplates that a bit or bits or some subset of the possible coding of the chip identity number will be reserved for hardware debugger/emulators supporting this use. Thus only a certain limited number of the digital media processors  130  will permit raw mode operation of the hardware debugger/emulator in environments supporting the security described above. The manufacturer of digital media processor  130  will supply these particularly identified chips only to trusted program developers. 
     If the chip identity number does not permit raw mode operation (decision block  807 ), process  800  reads a token from the particular process or application program under development in the hardware debugger/emulator. Process  800  then determines if this token is verified as proper (decision block  809 ). This process could take place using the private key encryption and public key decryption described above, or another verification procedure could be employed. If the token is not verified (decision block  809 ), then process  800  take appropriate remedial action (processing block  810 ). The various types of remedial action that could be taken have already been described. If the token is verified (decision block  809 ), then process  800  activates the hardware debugger/emulator in process mode (processing block  811 ). In the process mode, the hardware debugger/emulator may only access a particular process or application program corresponding to the verified token. 
     This process satisfies all the requirements of the users. Program developers who use digital media processor  130  in a nonsecure application will have complete access to the functions of the hardware debugger/emulator. Program developers who use digital media processor  130  in secure applications will have access limited. Most of those program developers will use the secure RTOS and have access only to their own programs as identified by the token encrypted with their corresponding private key. RTOS developers will have complete system access but only to particular digital media processors having the proper chip identity numbers. Thus the manufacturer of digital media processor  130  can have the proper level of control in order protect the security of set top box systems  100 . 
     The security inventions of this patent application have been described in conjunction with a particular type system requiring computer security, i.e. the set top box. Those skilled in the art would realize that the use of these security techniques are not limited to this example. Particularly, almost any computer system requiring that some functions have a degree of security may employ these techniques.