Abstract:
A computer may be secured from attack by including a trusted environment used to verify a known monitor. The monitor may be used to determine a state of the computer for compliance to a set of conditions. The conditions may relate to terms of use, such as credits available for pay-per-use, or that the computer is running certain software, such as virus protection, or that unauthorized peripherals are not attached, or that a required token is present. The monitor may send a signal directly or through the trusted environment to a watchdog circuit. The watchdog circuit disrupts the use of the computer when the signal is not received in a given timeout period.

Description:
BACKGROUND 
   A trusted platform module (TPM) for use in computing devices such as personal computers is known. The purpose of a TPM is to provide computer identity and secure services related to transactions, licensing of application and media, protecting user data, and special functions. 
   Trusted platform modules are commercially available, for example, a TPM is available from STM Microelectronics, the ST19WP18 module. The TPM stores keys and subsequently uses those keys to authenticate application programs, Basic Input/Output System (BIOS) information, or identities. However, use of the TPM is voluntary and according to current and anticipated standards and implementations cannot be used to mandate a condition on the computing device. Some business models assume the computer is out of the direct control of the computer owner/supplier, for example, a pay-per-use business model. In such an instance, circumvention of TPM services may be possible, and if circumvention occurs, may have an undesirable negative impact on the business. 
   SUMMARY 
   A trusted platform module (TPM) may be used to authenticate a monitor program that enforces conditions on a computing device. Owner keys injected or written to the TPM may be used to require that a monitor approved by the owner is operational. In turn, the approved monitor has access to resources of the TPM by way of monitor&#39;s authenticated status. Such a secure resource of the TPM may be, for example, a general purpose input/output (GPIO) port. A simple watchdog timer may be configured to reset the computer on a timed interval unless the watchdog timer is restarted within the interval period by a signal received using the GPIO. 
   By configuring the computer in this manner, the TPM may be used to help ensure a known monitor is running, and the watchdog timer may be used to help ensure that neither the monitor nor the TPM are disabled or tampered. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a network interconnecting a plurality of computing resources; 
       FIG. 2  is a simplified and representative block diagram representative of a computer in accordance with an embodiment of the current disclosure; 
       FIG. 3  is a simplified and representative block diagram showing a hierarchical representation of functional layers within the computer of  FIG. 2 ; 
       FIG. 4  is a simplified and representative block diagram of a computer architecture of the computer of  FIG. 2 ; 
       FIG. 5  is a simplified and representative block diagram of an alternate computer architecture of the computer of  FIG. 2 ; 
       FIG. 6  is simplified and representative block diagram of the TPM; and 
       FIG. 7  is a flow chart depicting a method of locking-on a TPM using a monitor. 
   

   DETAILED DESCRIPTION 
   Although the following text sets forth a detailed description of numerous different embodiments, it should be understood that the legal scope of the description is defined by the words of the claims set forth at the end of this disclosure. The detailed description is to be construed as exemplary only and does not describe every possible embodiment since describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims. 
   It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘ —————’  is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term by limited, by implication or otherwise, to that single meaning. Finally, unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. § 112, sixth paragraph. 
   Much of the inventive functionality and many of the inventive principles are best implemented with or in software programs or instructions and integrated circuits (ICs) such as application specific ICs. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts in accordance to the present invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the preferred embodiments. 
     FIG. 1  illustrates a network  10  that may be used to implement a dynamic software provisioning system. The network  10  may be the Internet, a virtual private network (VPN), or any other network that allows one or more computers, communication devices, databases, etc., to be communicatively connected to each other. The network  10  may be connected to a personal computer  12  and a computer terminal  14  via an Ethernet connection  16 , a router  18 , and a landline  20 . On the other hand, the network  10  may be wirelessly connected to a laptop computer  22  and a personal digital assistant  24  via a wireless communication station  26  and a wireless link  28 . Similarly, a server  30  may be connected to the network  10  using a communication link  32  and a mainframe  34  may be connected to the network  10  using another communication link  36 . 
     FIG. 2  illustrates a computing device in the form of a computer  110 . Components of the computer  110  may include, but are not limited to a processing unit  120 , a system memory  130 , and a system bus  121  that couples various system components including the system memory to the processing unit  120 . The system bus  121  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus. 
   Computer  110  typically includes a variety of computer readable media. Computer readable media can be any available media that can be accessed by computer  110  and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by computer  110 . Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Combinations of the any of the above should also be included within the scope of computer readable media. 
   The system memory  130  includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM)  131  and random access memory (RAM)  132 . A basic input/output system  133  (BIOS), containing the basic routines that help to transfer information between elements within computer  110 , such as during start-up, is typically stored in ROM  131 . RAM  132  typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  120 . By way of example, and not limitation,  FIG. 2  illustrates operating system  134 , application programs  135 , other program modules  136 , and program data  137 . 
   The computer  110  may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only,  FIG. 2  illustrates a hard disk drive  141  that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive  151  that reads from or writes to a removable, nonvolatile magnetic disk  152  and an optical disk drive  155  that reads from or writes to a removable, nonvolatile optical disk  156  such as a CD ROM or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not united to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive  141  is typically connected to the system bus  121  through a non-removable memory interface such as interface  140 , and magnetic disk drive  151  and optical disk drive  155  are typically connected to the system bus  121  by a removable memory interface, such as interface  150 . 
   The drives and their associated computer storage media discussed above and illustrated in  FIG. 2 , provide storage of computer readable instructions, data structures, program modules and other data for the computer  110 . In  FIG. 2 , for example, hard disk drive  141  is illustrated as storing operating system  144 , application programs  145 , other program modules  146 , and program data  147 . Note that these components can either be the same as or different from operating system  134 , application programs  135 , other program modules  136 , and program data  137 . Operating system  144 , application programs  145 , other program modules  146 , and program data  147  are given different numbers here to illustrate that, at a minimum, they are different copies. A user may enter commands and information into the computer  20  through input devices such as a keyboard  162  and pointing device  161 , commonly referred to as a mouse, trackball or touch pad. Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  120  through a user input interface  160  that is coupled to the system bus, but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB). A cathode ray tube  191  or other type of display device s also connected to the system bus  121  via an interface, such as a video interface  190 . In addition to the monitor, computers may also include other peripheral output devices such as speakers  197  and printer  196 , which may be connected through an output peripheral interface  190 . 
   The computer  110  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  180 . The remote computer  180  may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  110 , although only a memory storage device  181  has been illustrated in  FIG. 1 . The logical connections depicted in  FIG. 1  include a local area network (LAN)  171  and a wide area network (WAN)  173 , but may also include other networks. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
   When used in a LAN networking environment, the computer  110  is connected to the LAN  171  through a network interface or adapter  170 . When used in a WAN networking environment, the computer  110  typically includes a modem  172  or other means for establishing communications over the WAN  173 , such as the Internet. The modem  172 , which may be internal or external, may be connected to the system bus  121  via the user input interface  160 , or other appropriate mechanism. In a networked environment, program modules depicted relative to the computer  110 , or portions thereof, may be stored in the remote memory storage device. By way of example, and not limitation,  FIG. 1  illustrates remote application programs  185  as residing on memory device  181 . 
   The communications connections  170   172  allow the device to communicate with other devices. The communications connection  170   172  are an example of communication media. The communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. A “modulated data signal” may be a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Computer readable media may include both storage media and communication media. 
   The trusted platform module  125  or other trusted environment, discussed in more detail below, may store data, and keys and verify executable code and data. The trusted platform module specification states in section 4.5.2.1, “As part of system initialization, measurements of platform components and configurations will be taken. Taking measurements will not detect unsafe configurations nor will it take action to prevent continuation of the initialization process. This responsibility rests with a suitable reference monitor such as an operating system.” Because the TPM is not defined as an enforcement tool the further enhancements described below supplement the common TPM. 
   A watchdog circuit  126  may be configured to measure a period of time and when the time expires trigger a signal  127  that disrupts the operation of the computer  110 . The disruption may be a system reset that causes the computer  110  to reboot. The disruption may interrupt data on the system bus  121  or a peripheral bus. To prevent the watchdog  126  from disrupting the operation of the computer  110 , a signal over communication connection  128  may be required to reset the period of time and start the timing process again. As shown in  FIG. 2 , the watchdog timer reset signal may be carried over communication connection  128 . As discussed more below, the TPM  125  may initiate the watchdog timer reset responsive to a signal from a monitor program. The steps described in the following may be used to help ensure that a specific, desired, monitor is present and operating by using the combination of the TPM  125  and watchdog circuit  126 . 
     FIG. 3 , a simplified block diagram showing a hierarchical representation of functional layers within a representative computer such as that of  FIG. 2 , is discussed and described. A trusted platform module  202  may be hardware that resides below the basic input/output structure (BIOS)  204 . The TPM  202  may act as a resource to the computer and higher level operations, such as the BIOS  204 . The BIOS may activate a monitor  206 . The monitor  206  resides below the operating system  208  at the monitor level  210 . The monitor  206  may access and use resources of the TPM  202  to carry out policies associated with the operation of higher level entities. The operating system  208  supports the major functions of the computer  110  and may be responsible (after initial bootstrap processes hand over control) for communication, user input/output, disk and other memory access, application launch, etc. The operating system may also directly access and use the TPM  202 . As shown, first and second applications  212   214  may run on the operating system  208 . In some cases, the monitor may enforce policies related to both the operating system  208  and the applications  212   214 . For example, before application  214  may be launched from disk  216 , the operating system may check licensing status, depicted by line  218 , to determine if the application  214  meets a given criteria for launching. The criteria for launch and subsequent metering of applications using a monitor function are discussed in more detail in US patent application “Method for Pay-As-You-Go Computer and Dynamic Differential Pricing” filed on Dec. 8, 2004 as Ser. No. 11/006,837. Briefly, the monitor  206  may be used to measure and meter application programs, utilities and computer resources, for example, in a pay-per-use or pre-paid scenario. 
   Referring briefly to  FIG. 6 , the TPM  202  is discussed in more detail. The TPM  202  may have an internal memory  502  comprising both volatile and non-volatile memory, at least part of which may be secure from tampering or unauthorized write operations. The memory may store an owner key  504  for use in validating entities that claim affiliation with the owner for the purpose of configuring the TPM  202  and for establishing trust with an outside entity. The memory may also include, among other things, a platform configuration register (PCR)  506 . The PCR  506  may be used to store a hash or other strong identifier associated with the monitor  206 . The TPM  202  may also include a clock  508  and cryptographic services  510 . Both may be used in the authentication and authorization processes as will be discussed below in more detail. The TPM  202  may also include a bus  512 , sometimes referred to as a Single-pin Bus or general purpose input/output (GPIO). In one embodiment, the GPIO  512  may be coupled to the watchdog circuit, as described elsewhere. 
   The TPM  202  may also be coupled to a general purpose bus  514  for data communication within the computer, for example, a process running the monitor  206 . Using the bus  514 , or in some cases another mechanism  516 , the TPM  202  may be able to measure the monitor. The measurement of the monitor may include checking a cryptographic hash of the monitor, that is, checking a hash of the memory range occupied by the monitor. The PCR may be used to store the measurement data  506 . The owner key  504  may be affiliated with the hash of the monitor  506 , for example, by a digitally signed hash of the monitor that requires the owner key  504  for confirmation. The owner key  504  may be written or injected into the TPM  202  at the time of manufacture, or later, for example, at the time of delivery to a customer. The owner key  504 , then, is used to authenticate the monitor  206 . 
   In an exemplary embodiment, the monitor  206  is measured by a trusted module preceding it in the boot sequence, for example, by the BIOS  204 . The monitor measurement, such as a hash computed by the BIOS  204 , may be stored in the TPM PCR  506  via the bus  514 . When the TPM  202  validates the measurement (hash), the TPM  202  may then allow access to the monitor  206  unique keys and/or other secrets allocated to the monitor  206  and stored in the TPM  202 . The TPM  202  will allocate to any monitor corresponding keys and secrets to whatever measurement the monitor&#39;s measurement matches. 
   The TPM may be programmed with an owner key  504  and a corresponding monitor metric  506 , i.e. a hash of a known monitor  206 . The owner key is used to program or update the monitor metric  506 , such that only the entity in possession of the owner key  504  may set the PCR register  506  for the known monitor  206 . The standard TPM  202  has a characteristic that only a monitor  206  verified against a given measurement  506  may have control of the GPIO  512 . When the GPIO  512  is connected in a tamper-resistant manner to the watchdog circuit  126 , a chain of trust may be completed. That is, only a verified monitor  206  may control the GPIO  512  and only the GPIO  512  may be used to restart the watchdog circuit  126 . Therefore, while the monitor  206  may be replaced or altered, only the monitor  206  verified by PCR  506  set by the owner key  506  may be used to restart the timer of the watchdog circuit  126 . Thus only the authorized monitor may be used to prevent the watchdog from disrupting the computer  110  by, for example, resetting, the computer  110 . The timer of the watchdog circuit  126  may be set to a period selected to allow restoration of a corrupted or tampered computer  110 , but short enough to prevent significant useful work to be done on the computer  110 . For example, the watchdog may be set to disrupt the computer  110  every 10-20 minutes, unless restarted by the validated monitor  206 . 
   The owner secret  504  and the monitor measurement  506  may be programmed in a secure manufacturing environment, or may be field programmed using transport keys known to the entity programming the owner key  504 . Once the owner key  504  is known, the programming entity, for example, a service provider, may set the measurement of the monitor that will determine what monitor is given access to the GPIO bus. The owner key  504  may be required to re-program the owner key. The use of derived keys may facilitate key distribution, scaling and protection from widespread loss should a local owner key  504  be compromised. Key management techniques are known in the data security arts. 
     FIG. 4  is a block diagram of a representative architecture of a computer  300 , the same or similar to computer  110 . The computer may have a first and second interface bridges  302   304 . The interface bridges  302   304  may be connected by a high speed bus  306 . The first interface bridge  302  may be connected to a processor  308 , graphics controller  310  and memory  312 . The memory  312  may host a monitor program  314 , as well as other general purpose memory uses. 
   The second interface bridge  304  may be connected to peripheral buses and components, for example, universal serial bus (USB)  316 , Integrated Drive Electronics (IDE)  318 , or Peripheral Component Interconnect (PCI)  320 , used to connect disk drives, printers, scanners, etc. The second interface bridge may also be connected to a TPM  322 . As discussed above, the TPM  322  may have secure memory  324  for key and hash data, and a general purpose input/output (GPIO)  326 . The TPM  322  may be physically or logically coupled to the monitor by connection  328 . As discussed, the BIOS  204  may measure the monitor  206  and store the measurement in the TPM  322 , which allocates to the monitor  314  keys and secrets corresponding to the provided measurement. The monitor  314  is therefore given access to the resources and data locked with these keys and secrets. The connection  328  may also be used by the monitor to control the GPIO  326  for the purpose of sending a signal to the watchdog circuit  330 . The signal may cause the watchdog to reset. When the signal is not received by the watchdog circuit  330  in a time period proscribed by a setting in the watchdog circuit  330 , a reset, or other disruptive signal may be sent over connection  332 . To discourage tampering, the connection between the GPIO  326  and the watchdog circuit  330  may be protected, for example, by potting or routing between circuit board layers to prevent manual restarting of the watchdog circuit  330 . The computer reset signal connection  332  may be similarly protected from tampering, or at least a portion of the reset signal connection  332  between the watchdog circuit  330  and the main processor computer reset point (not depicted). 
     FIG. 5  is a representative block diagram of an alternate architecture of the computer of  FIG. 2 . Comparing to the description of  FIG. 4 , like numbered components are the same. The watchdog circuit  330  has been moved into the second interface bridge  304  showing a representative illustration of how the watchdog circuit  330  may be combined into another circuit to improve tamper resistance. The integration of the watchdog circuit  330  to the second interface bridge chip  304 , while itself appropriate, is only illustrative. Since the second interface bridge  304  is a major component of the computer architecture, the desired level of disruption may be carried forth from within the second interface bridge  304 . Therefore, a connection from a watchdog circuit external to the second interface bridge  304 , such as connection  332 , may not be required. 
   In this alternate architecture, the GPIO  326  may not be used to signal the reset to the watchdog circuit  330 . Instead, a message may be sent over logical connection  334  directly from the monitor  314  to the watchdog circuit  330 . 
   Because a sufficient level of trust may not exist between the two entities ( 314   330 ) the message may be signed using keys held in the TPM  322 . For example, these keys may be associated with the monitor  314  during first boot (e.g. on the manufacturing line—for the sake of trustworthiness). Keys may be assigned arbitrarily, or, as mentioned above, keys may be hierarchically derived from a master key and known data such as a root certificate, serial number or manufacturing sequence number, etc. The watchdog timer  330  may be configured to respect only messages signed using these keys, for example, during the first boot of the computer  110  on the assembly line. In addition, the monitor locks these keys into the TPM  322 , such that only a monitor  314  identically measured has access to these keys. A variant of this architecture is that the monitor relies on the TPM  322  to allocate it these keys uniquely and respectively to its measurement. 
   During normal operation the monitor  314  may request the TPM  322  to sign on its behalf the message to be sent to the watchdog timer  330 . The TPM  322  signs the message with the keys that correspond to the monitor  314  (per its measurement that was stored into the TPM  322  by the BIOS during each boot). The monitor  314  may receive the signed message from the TPM  322  over logical connection, for example, connection  328  and then provide it to the watchdog circuit  330  over logical connection  334 . 
   When the watchdog circuit  330  receives the message, the watchdog circuit  330  may use the keys (set during manufacturing) to authenticate the message. Alternately, it may request verification using key or secret in the TPM  322  using logical connection  336 . If another monitor is running, it will measure differently, resulting in different keys &amp; secrets being allocated by the TPM. Therefore, the alternate monitor will not be able to sign the message properly such that it will be authenticated by the watchdog circuit  330 . Consequently, the watchdog circuit  330  will initiate a sanction, such as firing a reset of the computer  110  after the expiration of its timing interval. The use of signed or encrypted messages may reduce the opportunity for attacks on the logical connections  328  and  334 . 
     FIG. 7 , a flowchart illustrating a method to lock a trusted platform module (TPM) always “on” using monitor, is discussed and described. A typical TPM, for example, TPM  125  may be optionally enabled by the user. As described below, the method will help ensure that both the TPM  125  remains enabled, and that a monitor  206  selected by the owner of the business will be executed, at the risk of sanctions such as disabling the computer  110 . 
   Starting with application of power at the start  402 , the computer  110  may initiate the various hardware components through normal boot mechanisms. This applies to the TPM  322  as well. The boot sequence may follow a Trusted Computing Platform Alliance (TCPA) methodology. The Core Root of Trust for Measurements(CRTM) (not depicted) measures the BIOS  133  and stores  403  its measurement into the TPM  322 . Then the CRTM loads and executes the BIOS  133 . (The CRTM may ideally be stored in a trustworthy location in the computer  110  which is very difficult to attack). 
   The BIOS  133  may execute in a conventional fashion, initiating and enumerating various computer components, with one exception—it may measure each software module before loading and executing it. Also, it may store these measurements into the TPM  322 . Particularly, it may measure the monitor  314  and store  405  the monitor measurement into the TPM  322 . 
   The TPM  322  allocates 408 keys and secrets uniquely and respectively to the monitor measurement. The essence is that the TPM  322  consistently allocates 408 unique keys &amp; secrets that correspond to a given measurement. Consequently, the secrets available to a monitor  314  are unique, consistent and respective. As a result any monitor may lock resources such that will be exclusively available only to that particular monitor. For example, this enables the linking of the genuine monitor  314  to the watchdog circuit  330  by programming the GPIO  326  connected to the watchdog circuit  330  to respect only the measurement associated with the genuine monitor  314 . The GPIO  326  is then available only to a monitor that measures identically to the genuine monitor  314 . 
   Regardless of whether the loaded monitor is genuine or not, the boot sequence loads and executes  410  the monitor. The normal boot process may continue  411  and assuming a successful boot, normal operation  412  of the computer  110  follows. 
   As soon as the monitor  314  is loaded and executed at  410  it starts its loop ( 413 - 419 ). First, the monitor  314  sends  413  a message to the watchdog circuit  330  via the TPM GPIO  326 . The message may signal the TPM  322  to use the GPIO  326  to signal the watchdog circuit  330  to restart its timer (not depicted). 
   After sending the message to the TPM  322 , the monitor returns to the testing state  414 . The monitor may test  414  that the state of the computer  110  complies with a current policy. The current policy may involve the specific presence or absence of known programs, utilities or peripherals. The test may also be related to metering or other pay-per-use metrics. For example, the test may check for available provisioning packets for consumption vs. specific application program operation. In another embodiment, the test may be related to operation during a specific time period, such as calendar month. 
   When the test  414  fails, the No branch may be followed  416 , where the monitor acts in accordance with the policy. The action may be just a warning code sent to the operating system or a warning message presented to user. The action may be some sanction imposed on the operating system and user, e.g. limiting or eliminating a certain function of the computer. This may apply to hardware and/or software functions. For instance, the computer may be slowed down, certain software may be disabled, or certain devices may be disabled, e.g. a webcam. More severe sanctions may be to limit the amount of RAM available to the OS, or to reduce the Instruction-Set-Architecture available to the operating system. In an exemplary embodiment, one course of action available to the monitor  314  when a non-compliant condition is found may be to not take action to restart the timer of the watchdog circuit  330  and let the watchdog circuit  330  impose a sanction. 
   When the test succeeds, the Yes branch from  414  may be followed. In either case, execution waits  419  for an interval before returning to step  413 . The wait interval avoids exhausting the computer&#39;s resources by repeatedly running the monitor  314 . Obviously, this wait interval  419  should be some fraction of the watchdog timer counting period. The determination of a usable fraction may be the likelihood that normal operation of the computer would delay execution completion of the loop. Then the loop returns to step  413  discussed above. The period for repeating the loop may be set to any time less than the watchdog circuit timeout period, otherwise an unwarranted disruption may take place. 
   When the TPM  322  receives  420  the message, the TPM  322  acts according to the monitor measurement. If the measurement is deemed non-genuine  420  fails, the No branch may be taken to box  422 , which takes no action, i.e. the signal to the watchdog circuit  330  is not sent. No further action may be needed by the TPM  322  because the watchdog circuit  330  will disrupt the computer  110  unless steps are taken to stop it. Optionally, the TPM  322  may, at  422 , generate an error for logging generate a warning/error code, notify the operating system and may display a message to the user. 
   When the TPM  322  verifies that the monitor measurement is genuine, the GPIO  326  may be activated to signal  424  the watchdog circuit  330  to restart its timer. As discussed above, restarting the watchdog circuit timer prevents the watchdog circuit  330  from initiating a disruptive action, such as a reset of the computer  110 . The watchdog circuit  330  may then restart  426  the timer at its initial value. The timer will then count  428  and test  430  for expiration of a pre-determined time. The timer period may be settable. Timer implementation is known and whether the timer counts up to a given number, down to zero, counts to a set clock time, or other mechanism, is a design choice. 
   If the timer has not expired, the no branch from  430  may be taken back to  428 , which will take another count from the timer. When time has expired, the yes branch from  430  may be taken and the watchdog may enforce a sanction by disrupting  432  the computer. The disruption may be a system reset, causing a re-boot, disabling of peripherals, etc. The period for the watchdog circuit timer to count down to a disruption  432  may be enough to allow a user to correct a non-compliant condition on the computer  110 , but should be frequent enough to restrict reliable or useful activity on the computer  110 . 
   The link from  432  to  426  may be conceptual. If the disruption is implemented by a reset of the whole computer, this link is moot. In the event of a more subtle disruption, e.g. slowing the computer down, this link is used to restart the count down and may result in a more disabling disruption, for example, cause a reset. 
   It can be seen that two purposes of the owner of a business associated with supplying computers on a pay-per-use or other underwriter may be accomplished by the above method. First, if the TPM  322  is disabled because the user opted out of using the TPM  322  or hacked the computer to disable the TPM  322 , messages to the watchdog circuit  330  will not be generated and the computer  110  will be disrupted. 
   Similarly, if the TPM  322  is enabled and operational, but the monitor is altered or replaced, possibly to alter or ignore the policies in effect (e.g. usage policies), the TPM will not honor the monitor requests. Practically, an altered monitor measurement is different than the measurement of the genuine monitor. Consequently, when the monitor measurement is stored into the TPM  322 , it will allocate a set of keys and secrets respective and unique to the altered monitor, and different from those needed for operation of the GPIO  326 . As a result any message from the altered monitor to the TPM to signal the GPIO  326  will not be honored. Therefore, the watchdog circuit  330  will not receive restart signals and the computer  110  will be disrupted. 
   In both cases, the TPM  322  must be enabled and the genuine monitor  314  must be in place and operational for correct operation of the computer  110 . 
   Other uses for the above method and apparatus may be envisioned. For example, part of the boot process may require presentation of credentials by an authorized user. If correct credentials are not presented, the boot process may not load the genuine monitor, which will ultimately result in the disabling of the computer  110 .