Patent Publication Number: US-7587613-B2

Title: Method and apparatus for selectively enabling a microprocessor-based system

Description:
TECHNICAL FIELD 
     The invention relates to microprocessor-based systems. More particularly, the invention relates to enablement of microprocessor-based systems under conditions and circumstances determined by the designer or distributor of the system. 
     DESCRIPTION OF THE PRIOR ART 
     In many instances, the designer or distributor of a microprocessor-based system or device may wish to restrict the conditions or circumstances under which the system or device may be operated. For example, a government may provide a microprocessor-based weapons system to a foreign state and wish to control the duration for, or the locations in which, the system may be operated. 
     One approach to addressing this problem is to restrict the operation of the microprocessor controlling the system or device. Several prior art systems have been suggested to provide such restrictions. For example, U.S. Pat. No. 5,388,156 discloses a system that includes a “a normally closed enclosure, at least one erasable memory element . . . for receiving and storing a privileged access password, . . . a tamper detection switch operatively connected with the erasable memory element, for detecting opening of the enclosure and for invalidating any privileged access password stored in the erasable memory element, . . . and a system processor . . . for controlling access to . . . data stored within the system.” The system is operable only when the privileged password is provided. The tamper detection system ensures that attempts to access the privileged password results in destruction of the password. 
     A different approach is presented in U.S. Pat. No. 5,406,261, where “unauthorized access to a computer system is prevented by controlling power distribution to components within the computer system by a remotely controllable switch. An authorized computer user utilizes a radio frequency, infrared, ultrasonic or other type of wireless coded signal transmitter to send coded signals to a matching wireless receiver within the computer system that controls the power distribution switch.” The system also provides a mechanism to thwart attempts to disable or circumvent the activation process. 
     However, in prior art systems such as these, operation of the system is contingent only upon the presentation of proper authorization information, such as a suitable password or correctly coded signal. What is needed is a method for selectively enabling a microprocessor-based system under conditions or circumstances determined by the designer or distributor of the system at the time of design or distribution. 
     SUMMARY 
     The invention selectively enables a microprocessor-based system. 
     State information describing the operating conditions or circumstances under which a user intends to operate the system is obtained. In the preferred embodiment of the invention, a valid hash value is determined, preferably based on the state information and preferably by locating the valid hash value within a table of valid hash values indexed by the state information. Candidate authorization information is obtained from the user, and a candidate hash value is generated by applying a hashing algorithm to the candidate authorization information, the state information, or a combination of the candidate authorization information and state information. The candidate hash value and the valid hash value are then compared, and the microprocessor-based system is enabled if the candidate hash value matches the valid hash value. In this manner, the designer or distributor of the system can determine, at the time of manufacture or distribution, the conditions and circumstances under which the system may be operated. 
     The preferred embodiment of the invention further incorporates a power-up sequence that is executed before obtaining the state information and candidate authorization information. In addition, if the candidate authorization information is successfully verified and the system is enabled, operation continues in an uninterrupted manner, with the system periodically checking for a reset condition. If a reset condition is detected, a power-down sequence is executed, and the system awaits a power-up condition. Additional state information may be obtained and stored before the power-down sequence is executed. 
     The hashing algorithm is preferably a one-way hashing algorithm, and is applied to only the candidate authentication information. In an alternative embodiment, the hashing algorithm is applied to a catenation of the candidate authorization information and the state information. In another alternative embodiment of the invention, several valid hash values are determined, independent of the state information, by referencing a list of valid hash values. 
     The state information obtained may describe any number of operating conditions or circumstances, such as geographic location, geographic region, date, time, and a prior usage history of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart that shows a method of selectively enabling a microprocessor-based system according to the invention; 
         FIG. 2  is a flow chart that shows a verification operation according to the invention; 
         FIG. 3  is a flow chart that shows a method of determining a valid hash value according to the invention; 
         FIG. 4  is a flow chart that shows a verification operation according to an alternative embodiment of the invention; 
         FIG. 5  is a flow chart that shows a verification operation according to another alternative embodiment of the invention. 
     
    
    
     DESCRIPTION 
     The invention selectively enables the use of a microprocessor-based system by matching candidate authorization information provided by a user to valid authorization information specific to a set of operating conditions or circumstances. 
       FIG. 1  is a flow chart that shows a method of selectively enabling a microprocessor-based system according to the invention. Operation begins when the microprocessor-based system executes a power-up sequence  100 . The power-up sequence may be, for example, an initiation of power provided to the system, such as effected by a user toggling a power switch, or may correspond to the system waking up from a lower activity sleep state to a higher activity state. 
     The microprocessor-based system obtains state information  200 . The state information reflects the operating conditions or circumstances under which the user intends to operate the system. The state information may reflect the geographic location, date, or time of intended operation. The state information may also include a history of previously stored state information retrieved from a memory. For example, the state information may indicate the usage history of the system prior to the time of attempted authorization. 
     To prevent circumvention of the authorization mechanism, the state information is obtained in a manner not subject to tampering by, or interference from, the user. In the case of state information that describes environmental information, such as temperature or location, the state information may be obtained by sensors physically inaccessible to the user. Date and time information may be obtained from a remote time server controlled by the designer or distributor of the system, as is well known in the art. Information detailing the usage history of the system may be retrieved from a tamper resistant, non-volatile memory. Several such memories are described in the prior art and include, for example, EEPROM or battery backed CMOS RAM devices. 
     The microprocessor-based system obtains candidate authorization information  300  from the user wishing to operate the system. In the preferred embodiment of the invention, the candidate authorization information is a password or passphrase. Other embodiments of the invention may incorporate electronic identification cards or biometric information, for example. The candidate authorization information obtained from the user is specific to the conditions or circumstances under which the user wishes to operate the system. For example, the user may be prompted for a password or passphrase specific to operation of the system within a particular geographic region or within a particular range of dates. 
     Once the state information and candidate authorization information have been obtained, the system verifies the candidate authorization information  400 . The verification operation determines if the candidate authorization information matches valid authorization information that is specific to the obtained state information. 
     In the preferred embodiment of the invention, if the verification operation is unsuccessful, the system waits  462  for a predetermined period of time and increments a counter  464  indicating the number of attempted authorizations. The system then checks the counter  466  to determine if the incremented counter value is equal to or less than a predetermined number of maximum allowable attempted authorizations. If the check is successful, the system again obtains candidate authorization information  300  from the user. If the check of the counter fails because the incremented counter value exceeds the maximum allowable number of attempted authorizations, the system enters a terminal shut down state  468 . The system remains in the terminal shut down state until it is serviced by the designer or distributor of the system. 
     If the verification operation is successful, the system allows user operation  500 . Continued operation of the system is allowed while the system periodically checks for a reset condition  600 . Generally, the reset condition corresponds to an expiration of or change in the state for which operation was selectively enabled. Checking for a reset condition may therefore require that the system obtain state information similar to that obtained following execution of the power-up sequence. For example, the reset condition may correspond to the system being transported outside the geographic region for which operation was enabled, for example as detected by an interval GPS receiver. Alternatively, the reset condition may correspond to the operator exceeding a maximum allowable single-session or cumulative operating time. The reset condition may also be triggered by a power-down of the system, effected either by the user or an unexpected loss of power. The reset condition may also be triggered, for example, if the system detects efforts to circumvent or disable the verification mechanism. 
     If no reset condition is detected, user operation of the system continues in an uninterrupted manner. If a reset condition is detected, if necessary, the system obtains additional state information  700 . Obtaining state information at this point in the operation of the invention provides an accurate record of information, such as usage statistics. The system then stores the additional state information  800  in a tamper resistant, non-volatile memory. Storage of the state information allows retrieval of the information when state information is obtained following the execution of the power-up sequence. 
     Once any necessary additional state information has been recorded, the system executes a power-down sequence  900 . The power-down sequence may result in a stoppage of power provided to the system, or may correspond to the system entering into a lower activity sleep state. 
       FIG. 2  is a flow chart that shows a verification operation according to the invention. The state information  250  obtained following execution of the power-up sequence is used to determine  420  a valid hash value  425 . The candidate authorization information obtained from the user  350  is used to generate  430  a candidate hash value  435  using a hashing algorithm. The hashing algorithm is preferably a one-way hashing algorithm, such as the MD5 algorithm or other similar algorithm, as is well known in the art. The candidate hash value and the valid hash value are then compared  450 . If the candidate hash value and valid hash value match one another, the verification operation is successful. If the candidate hash value and the valid hash value do not match one another, the verification operation is unsuccessful. 
     It is important to note that while in the preferred embodiment the state information is obtained before the candidate authorization information, in alternative embodiments of the invention the candidate authorization information is obtained before the state information, or the state information and candidate authorization information are obtained simultaneously. It is only essential that both are obtained before verifying the candidate authorization information. 
       FIG. 3  is a flow chart that shows a method of determining a valid hash value according to the invention. The state information  250  is used to locate  422  the valid hash value  425  within a table of valid hash values  423  that is indexed by the state information. For example, the table may provide a particular valid hash value for a specific date of operation or range of dates of operation. Alternatively, the table may provide a certain valid hash value for a geographic location or geographic region of operation. The table may specify valid hash values with any desired degree of granularity. For example, the table may specify few valid hash values, each corresponding to an expansive geographic region, or the table may specify many valid hash values, each corresponding to a more limited geographic region. 
     In this manner, the designer or distributor of the system determines, at the time of manufacture or distribution, the conditions or circumstances under which the system may be operated. Moreover, the designer or distributor determines the granularity with which potential conditions or circumstances of operation are distinguished from one another. 
     In an alternative embodiment of the invention, the table of hash values is unique to a single unit within a production run of similar systems, and effectively incorporates the identity of the unit within the state information. In this embodiment, the valid authorization information allows a particular unit to be run under particular conditions or circumstances. 
     The table of hash values is preferably stored in a tamper resistant, non-volatile memory. A further measure of security is offered by the one-way nature of the hashing algorithm, which ensures that the valid candidate authorization information that yields the valid hash value cannot be determined from entries in the table of valid hash values. Thus, were a user to gain access to the table of valid hash values, he could not circumvent the verification process. 
       FIG. 4  is a flow chart that shows a verification operation according to an alternative embodiment of the invention. The state information  250  is used to determine  420  a valid hash value  425 . The candidate authorization information  350  and the state information  250  are used to generate  430  a candidate hash value  435  by means of a one-way hashing algorithm. For example, the candidate authorization information may be catenated with the state information prior to being operated on by the hashing algorithm. As in the preferred embodiment of the invention, the candidate hash value and the valid hash value are then compared  450 . If the candidate hash value and valid hash value match one another, the verification operation is successful. If the candidate hash value and the valid hash value do not match one another, the verification operation is unsuccessful. 
     In this embodiment of the invention, because the state information is operated on by the hashing algorithm, the state information is discretized so that a successful comparison of the candidate hash value with the valid hash value is possible. For example, the state information may be thresholded, truncated, or rounded to a desired degree of precision before being operated on by the hashing algorithm. Alternatively, the state information passed to the hashing algorithm may be replaced by a state information entry located within a table of state information. 
       FIG. 5  is a flow chart that shows a verification operation according to another alternative embodiment of the invention. The candidate authorization information  350  and the state information  250  are used to generate  430  a candidate hash value  435  using a one-way hashing algorithm. However, in this embodiment, the state information is not used to determine a single valid hash value. Rather, to determine valid hash values  420 , a list of valid hash values is referenced. The list of valid hash values contains hash values generated by applying the hashing algorithm to state information and candidate authorization information pairings determined by the designer or distributor of the device. As in the preferred embodiment of the invention, the candidate hash value and the valid hash values are then compared  450 . If the candidate hash value matches one of the valid hash values in the list of valid hash values, the verification operation is successful. Otherwise, the verification operation is unsuccessful. 
     Although the invention is described herein with reference to several embodiments, including the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the invention. 
     Accordingly, the invention should only be limited by the following claims.