Abstract:
System for providing a secure file service includes an MLS file service module ( 300 ) comprised of a cryptographic processor ( 302 ). The MLS file service module also includes an MLS file system ( 301 ) hosted by the cryptographic processor. A secure user processor ( 402 ) includes programming and communications hardware for requesting at least one classified file from the MLS file service module. The cryptographic processor includes cryptographic hardware and software to decrypt the classified file. The cryptographic processor is also performs an integrity check on the classified file. Once the file is decrypted and its integrity checked by the cryptographic processor, the MLS file service module serves the classified file to the secure user processor in decrypted form. If the classified file is an executable file, the method also includes selectively enabling a write function for program memory of the secure user processor. This write function is disabled immediately after the classified executable file has been loaded into the program memory to guard against self modifying programs.

Description:
BACKGROUND OF THE INVENTION 
     1. Statement of the Technical Field 
     The inventive arrangements relate to electronic devices for storing and accessing sensitive/classified data. 
     2. Description of the Related Art 
     Electronic computers have the ability to store and process data. Computers typically include some kind of microprocessor with a commercially available operating system such as Linux, Unix, or Microsoft Windows. Many computers also have displays and keyboards for the human/machine interface. The foregoing capabilities make these devices highly useful for a various business and personal applications. 
     Currently, there exist a wide variety of computing devices with conventional operating systems and architectures. These commercially available computers with commercial-off-the-shelf (COTS) operating systems and COTS application programs generally satisfy the processing and data storage requirements of most users. For example, they include applications for word processing, data storage, spreadsheets, time management, and contact management. These applications generally function quite well and have interfaces that are familiar to many users. 
     Some commercially available computing devices and/or software applications incorporate various security measures in an effort to protect data which is stored or processed using the device. For example, encryption technology and password protection features are known in the art. Still, this level of security can be inadequate for managing information that is of a Confidential, Secret, or Top Secret nature, particularly when such information relates to matters of national security. For example, COTS operating systems and applications may not be sufficiently trustworthy for handling this type of information. Such programs can be susceptible to being compromised by various means including hacker attacks, viruses, worms, Trojan horses, and a wide variety of other means that are known to those skilled in the art. 
     Finally, notwithstanding the security limitations of COTS operating systems and applications, the basic architecture and interface systems of many commercial computing devices may leave these devices vulnerable to intrusion. For example, COTS devices do not employ trusted microprocessors, do not employ physical separation of classified and unclassified data processing, nor do they employ physical tamper detection and subsequent memory zeroization. Consequently, transport or processing of classified data using a commercial computer is not generally permitted. 
     Trusted operating systems and applications are generally designed to more rigorously address the problem of computer security. Trusted operating systems undergo evaluation of their overall design, verification of the integrity and reliability of their source code, and systematic, independent penetration evaluation. In contrast, non-trusted operating systems are generally not designed to an equally high level with regard to security precautions. 
     Single-level secure (SLS) is a class of systems that contain information with a single sensitivity (classification). SLS systems permit access by a user to data at a single sensitivity level without compromising data. Thus, SLS data file systems allow information at a single classification to be stored in an information system. The level of access can be limited by the current user security classification sign-on level and a security classification assigned to the secure user processor. 
     Multi-level secure (MLS) is a class of systems that contain information with different sensitivities (classifications). MLS systems permit simultaneous access by a user to data at multiple classification levels without compromising security. Thus, MLS data file systems allow information with different classifications to be stored in an information system. These systems are also designed to provide a user with the ability to process information in the same system. Significantly, however, these systems prevent a user from accessing information for which he is not cleared, does not have proper authorization, or does not have a need-to-know. 
     Users of non-trusted COTS operating systems, as may be found in commercial computers, are not generally allowed access to classified data found in secure file systems. Computers that utilize a trusted operating system (OS) which includes support for an SLS or MLS file system have been developed that are specifically designed to allow for storage of classified data. However, these devices are not generally designed to physically secure the data and zeroize the data upon tamper detection. Nor are they designed to be embedded as a secure component of a host computer system. 
     SUMMARY OF THE INVENTION 
     The invention concerns a method for providing a single-level secure user processor with multi-level secure (MLS) file system services. The method begins by authenticating a user to a cryptographic processor by communicating one or more types of user authentication information to the cryptographic processor. Based on such authentication, the MLS file system services are provided such that the secure user processor has access to files at only one defined security classification level at a time. 
     According to one aspect of the invention, a user can initiate a request for a classified file at a secure human machine interface (HMI). The secure HMI can communicate the request to a secure user processor, which forwards the request to an MLS file service module. In response to the request, a cryptographic processor associated with the MLS file service module accesses an MLS file system containing the classified file. Thereafter the cryptographic processor decrypts the classified file and performs an integrity check on the contents. The classified file is then served to the secure user processor in decrypted form, but only if its integrity has been verified. If the classified file is an executable file, the method also includes selectively enabling a write function for program memory of the secure user processor responsive to the integrity checking step. This write function is disabled immediately after the classified executable file has been loaded into the program memory to guard against self modifying programs. 
     The term “cryptographic processor” as used herein generally refers to a computer processing device that is specifically designed to facilitate cryptographic processing. Such processors generally include one or more hardware based encryption services that facilitate the encryption and decryption of classified files. For example, the hardware encryption services can include a hardware implemented cryptographic algorithm, a random number generator, and/or an exponentiator. 
     It should be understood that the method disclosed herein includes exclusively limiting concurrent access of the secure user processor to files defined within a single security classification level within the MLS file system. This process is accomplished by utilizing a client zeroizer that is responsive to the cryptographic processor to automatically zeroize at least one data store used by the secure user processor. According to one aspect of the invention, this zeroizing step is performed when the secure user processor transitions between a first state in which it has access to the multi-level secure file system at a first security classification level, and a second state in which it has access to the multi-level secure file system at a second security classification level. The method also includes communicating the classified information from the MLS file service module to the secure user processor over a secure path. 
     The method can also include receiving at an MLS file service module a request from the secure user processor for a non-encrypted unclassified file. In response to such request, the MLS file service module accesses the MLS file system containing the unclassified file. Thereafter, the cryptographic processor can verify the integrity of the unclassified file before the file is served to the secure user processor. If the unclassified file is an executable file, the method also includes selectively enabling a write function for program memory of the secure user processor responsive to the integrity checking step. This write function is disabled immediately after the unclassified executable file has been loaded into the program memory to guard against self modifying programs 
     The method can further include selecting the cryptographic processor to include a trusted microprocessor and a trusted operating system executing on the trusted cryptographic processor. The secure user processor can also be chosen so as to include trusted microprocessor hardware. According to one aspect of the invention, the secure user processor is selected so as to include a single level trusted operating system. Alternatively, the secure user processor can be chosen so as to include an untrusted operating system. 
     The invention disclosed herein also includes a system for providing a secure file service. The basic building block of the system is an MLS file service module, which includes a cryptographic processor comprising suitable hardware and software for encrypting and decrypting a classified file. The MLS file service module also includes an MLS file system hosted by the cryptographic processor. The MLS file system contains classified files and is accessible exclusively to the cryptographic processor. The cryptographic processor is programmed so that it is responsive to a secure user processor distinct from the cryptographic processor. The cryptographic processor is provided with programming and data files as necessary for authenticating a user responsive to at least one user authentication information. 
     The secure user processor includes suitable programming and communications hardware for requesting at least one classified file from the MLS file service module that hosts the MLS file system. The cryptographic processor includes suitable cryptographic hardware and software to decrypt the classified file. The cryptographic processor is also programmed to perform an integrity check on the classified file. Once the file is decrypted and its integrity checked by the cryptographic processor, the MLS file service module serves the classified file to the secure user processor in decrypted form. 
     The cryptographic processor is programmed to serve the classified file to the secure user processor in decrypted form, but only if its integrity has been verified. The secure user processor has a program memory into which executable files are loaded. A write enable gate can be provided to limit write access to the program memory of the secure user processor. The cryptographic processor is programmed to selectively enable the write gate responsive to the integrity checking process of an executable file. The cryptographic processor is also programmed to disable this write function immediately after the classified executable file has been loaded into the program memory to guard against self modifying programs. 
     The cryptographic processor used in the system can exclusively limit the extent of MLS file access permitted to the secure user processor. In particular, the cryptographic processor has programming to limit the secure user processor so that it can concurrently access files that are only within a single security classification level within the multi-level secure file system. The system can accomplish this by means of a client zeroizer that is responsive to the cryptographic processor. The client zeroizer is configured for automatically zeroizing at least one data store used by the secure user processor. 
     According to one aspect of the invention, the cryptographic processor is programmed to cause the client zeroizer to automatically perform the zeroizing described herein under certain conditions. For example, the zeroizing can be performed when the secure user processor transitions between a first state in which the secure user processor has access to the multi-level secure file system at a first security classification level, and a second state in which the secure user processor has access to the multi-level secure file system at a second security classification level. 
     The system provides a secure path defining a data communication link between the secure user processor and the cryptographic processor. The system can also include a secure human/machine interface operatively connected to the secure user processor. The secure human/machine interface is configured for communicating user commands to the secure user processor. 
     The cryptographic processor can also include suitable programming and hardware so that it accesses the MLS file system responsive to a request from the secure user processor for an unclassified file. The MLS file service module can thereafter serve the unclassified file to the secure user processor. 
     According to one aspect of the invention, the cryptographic processor is comprised of a trusted microprocessor and a trusted operating system executing on the trusted cryptographic processor. The secure user processor can also be comprised of trusted microprocessor hardware. According to one embodiment, the secure user processor is comprised of a single level trusted operating system. Advantageously, however, the secure user processor can instead be comprised of an untrusted operating system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a single-level secure computing device of the prior art. 
         FIG. 2  is a block diagram of a multi-level secure computing device of the prior art. 
         FIG. 3  is a detailed block diagram of a multi-level secure file service module configured for single-level secure file access and trusted software load support. 
         FIG. 4  is a block diagram of a secure user processor configured for trusted software load support. 
         FIG. 5  is a block diagram of a multi-level secure computing architecture that utilizes the multi-level secure file service module of  FIG. 3  and the secure user processor of  FIG. 4 . 
         FIG. 6  is an alternative embodiment of a multi-level secure computing architecture that utilizes the multi-level secure file service module of  FIG. 3  and the secure user processor of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A block diagram of a single-level secure (SLS) computing device  100  is shown in  FIG. 1 . The SLS computing device  100  can include a secure user processor  102  that includes trusted hardware and single-level trusted software (operating system and application software). As used herein, the term “trusted” is used with reference to computer hardware, operating systems, and/or software applications that have been designed to ensure secure storage, processing and communication of data. Trusted hardware and trusted software can be combined to provide secure data processing. Trusted hardware and software are generally designed and tested to ensure the integrity and reliability of their source code, and their resistance to penetration. In contrast, non-trusted hardware and non-trusted software are generally not designed to an equally high level with regard to security precautions. Accordingly, when integrated into a computer system, those systems are often referred to as non-secure. Commercial-off-the-shelf (COTS) hardware and software is generally not “trusted.” 
     The computing device  100  also includes a user SLS file system  108  in a data store that is used for storing user executable programs and data. Classified data stored in the SLS file system  108  is stored in an encrypted format. A cryptographic engine  104  is provided with trusted hardware and trusted software for providing encryption and decryption services. A crypto file system  110  is also maintained in a data store. The crypto file system  110  is used to store classified data and files used by the cryptographic engine  104 . In contrast to the user SLS file system  108 , user data and applications are not generally stored in the crypto file system  110 . Instead, the crypto file system  110  generally contains cryptographic algorithms, security keys and certificates, audit data, policy profiles, and application data specific to the processing performed by the cryptographic engine  104 . 
     A secure human/machine interface (HMI)  106  is also provided for the SLS computing device  100 . The secure HMI  106  can be comprised of trusted hardware and can provide a trusted path to applications executing on secure user processor  102 . Consequently, secure HMI  106  can prevent invasive or unauthorized applications from monitoring user inputs and system outputs. Secure HMI devices are known in the art and typically can include one or more features to ensure trusted communications between the user and the secure user processor. For example, the secure HMI  106  can provide a suitable interface by which a user can enter data and commands to the computing device  100 . Secure HMI  106  can also include a user display for showing data and information processed by the computing device  100 . 
     A user can request access to a classified data file using the secure HMI  106 . Encrypted files in the user SLS files system  108  are accessed by the secure user processor  102  and provided to the cryptographic engine  104  for decryption. After the file has been decrypted, the cryptographic engine passes the decrypted file back to the secure user processor  102 . Upon completion of any necessary user processing associated with the decrypted classified date file, the secure user processor  102  passes the file back to the cryptographic engine  104  for re-encryption. Thereafter, the encrypted file is returned to the secure user processor  102 , which stores the file in the user SLS file system  108 . 
     Notably, the secure user processor  102  can generally satisfy the security requirements for accessing the single-level secure file system  108 . However, the operating system and applications can be expensive as compared to COTS systems. In particular, the secure user processor must be developed specifically to include trusted software for managing classified files, and especially for managing encryption and decryption services provided by the cryptographic processor. Another disadvantage of this arrangement is that the user single-level secure file system is not generally designed to physically secure the data and zeroize the data upon tamper detection. 
     Referring now to  FIG. 2 , there is shown a multi-level secure (MLS) computing device  200 . MLS computing device  200  can include a secure user processor  202  comprised of trusted hardware and multi-level trusted software (operating system and application software). A secure human/machine interface (HMI)  206  is also provided for the MLS computing device  200 . The secure human/machine interface can be similar to the secure HMI described above relative to  FIG. 1   
     The MLS computing device  200  also includes a user MLS file system  208  in a data store that is used for storing user executable programs and data. Classified data stored in the MLS file system  208  is stored in an encrypted format. A cryptographic engine  204  is provided with trusted hardware and multi-level trusted software for providing encryption and decryption services. A crypto MLS file system  210  is used to store classified data and files used by the cryptographic engine  104 . For example, the MLS file system can separately store and control access to data that is designated as Classified, Secret, or Top Secret. In contrast to the user MLS file system  208 , user data and applications are not generally stored in the crypto MLS file system  210 . Instead, the crypto MLS file system  210  generally contains cryptographic algorithms, security keys, and application data that is specific to the processing performed by the cryptographic engine  204 . 
     Encrypted files in the user MLS files system  208  are accessed by the secure user processor  202  and provided to the cryptographic engine  204  for decryption. After the file has been decrypted, the cryptographic engine passes the decrypted file back to the secure user processor  202 . Upon completion of any necessary user processing associated with the decrypted classified date file, the secure user processor  202  passes the file back to the cryptographic engine  204  for re-encryption. Thereafter, the encrypted file is returned to the secure user processor  202 , which stores the file in the user MLS file system  208 . 
     The secure user processor  202  can generally satisfy the security requirements for accessing the multi-level secure user file system  208 . However, the operating system and applications can be expensive as compared to COTS systems. In particular, the secure user processor must be developed specifically to include trusted software for managing multiple levels of classified files, and especially for managing encryption and decryption services provided by the cryptographic processor. Another disadvantage of this arrangement is that the user multi-level secure user file system  208  is not generally designed to physically secure the data and zeroize the data upon tamper detection. 
     Referring now to  FIG. 3 , there is shown a detailed block diagram of an MLS file service module  300 . The MLS file service module  300  is configured for providing SLS file access to a user MLS file system. As shown in  FIG. 3  a cryptographic processor  302  can host a crypto processor file system  304 . The crypto processor file system  304  can provide storage for various file used by the cryptographic processor  304 . For example, these files can include cryptographic algorithms, keys and certificates, audit data, and policy profiles. The cryptographic processor  302  can also host a user MLS file system  301  comprised of classified information at multiple classification levels. More particularly, the cryptographic processor  302  can provide SLS file access to the MLS file system. Thus, the cryptographic processor  302  can serve files at a single defined security level to a client/user that has signed on at that particular security level after appropriate authentication. Additionally, the cryptographic processor  302  can be programmed to ensure that information loaded into the MLS file system has been provided by a trusted source and that the integrity of the information has been checked. For example, this can be accomplished using checksum/hashing technology. 
     A client SLS access interface  316  can provide communications support for a communication path between the MLS file service module  300  and a client processor. Any suitable physically-secure data communication path can be used for this purpose. Requests from a client processor for access to files and the decrypted data files can be communicated over this interface. 
     According to one embodiment of the invention, the user MLS file system  301  can include files comprising Top Secret information  306 , Secret information  308 , and Confidential information  310 . The files comprising Top Secret information  306 , Secret information  308 , and Confidential information  310  are stored in an encrypted form. These files can include classified data and classified applications software. The classified information files stored in the user MLS file system  301  can be decrypted and integrity checked by the secure cryptographic processor  302  and then served to a client processor using client SLS access interface  316 . In the opposite direction, classified information processed by the client processor is presented by means of client SLS access interface  316  to the cryptographic processor  302 . The cryptographic processor  302  adds an integrity checksum, encrypts the classified data file and stores it in the classified section of the user MLS file system  301  as Top Secret information  306 , Secret information  308 , or Confidential information  310 . In this way, the MLS file service module with SLS access  300  can provide a client processor with integrity-checked unencrypted read/write access to such files at a single security classification level after user authentication. 
     The user MLS file system  301  can also be comprised of files that are unclassified applications  312 . Such applications can be stored in a non-encrypted format. Since a user will not normally need to modify applications software, the cryptographic processor  302  can limit access by a client processor so that the client processor is permitted read only access to the files comprising unclassified applications  312 . The files included in the unclassified applications  312  can be read by the secure cryptographic processor  302 , integrity-checked and then served to the client processor through client SLS access interface  316 . 
     The user MLS file system  301  can also contain files comprising unclassified information  314 . The files comprising unclassified information  314  stored in the user MLS file system  301  can be read by the secure cryptographic processor  302 , integrity checked and then served to the client processor by means of client SLS access interface  316 . In the opposite direction, unclassified information processed by the client processor is presented through client MLS access interface  316  to the cryptographic processor  302  for the addition of an integrity checksum and finally for storage in the unclassified section  316  of the user MLS file system. The MLS file service module with SLS access  300  can provide integrity-checked read/write access to files comprising unclassified information  314 . 
     The MLS file service module includes a file system control interface  322 . The file system control interface can provide a path for trusted user sign-on and authentication for user access to the SLS file access provided by MLS file service module  300 . The file system control interface can be implemented in hardware, in software, or as a combination of hardware and software. Trusted paths for user sign-on and authentication as referenced herein are known in the art. 
     The MLS file service module  300  also includes client zeroize/reset manager  318 . The client zeroize/reset manager  318  can be implemented in hardware, in software, or as a combination of hardware and software. The client zeroize/reset manager  318  can be controlled by cryptographic processor  302 . The cryptographic processor can be programmed to cause the client zeroize/reset manager  318  to automatically zeroize and/or reset any data stores associated with the client processor served by the MLS file service module  300 . The client zeroize/reset manager can zeroize or reset any memory devices or data stores used by the client processor to temporarily store application code, user data, or other file data served to the client processor by the MLS file service module  300 . As will be appreciated by those skilled in the art, such memory devices can include RAM, DRAM, flash memory, video display buffers and any other memory devices used by the client processor for temporarily storing data from files served by the MLS file service module  300 . This automatic process can occur each time that a client processor is transitioned from accessing files at one security classification level to another security classification level. 
     For example, an SLS client processor served by the MLS file service module  300  can be provided with SLS file access to Top Secret Information  306 . Such access can occur after user sign-on and authentication as appropriate for that security level. Thereafter, a user wishing to use the same SLS client processor to access files at a different defined security classification level can log off the MLS file service module  300 . Once logged off, the user can log back on to the MLS file service module  300  with the client processor at a different single defined security classification level. For example, after logging off from the Top Secret level, the client processor can subsequently be permitted access to files comprising a single security classification level such as Secret Information  308 , Confidential Information  310 , or Unclassified Information  314 . 
     Those skilled in the art will appreciate that a client SLS processor of the prior art is normally only able to access classified information at one level. Such SLS processors and their associated software are not designed to accommodate the security demands for handling files at multiple security classification levels. In contrast, the present invention permits an SLS client processor served by the MLS file service module  300  to access information at multiple levels of security classification without the possibility of access violation. An example of such an access violations might include a user attempting to downgrade the security level of information contained in files. In particular, a user who accessed files using the client processor and defined as Top Secret Information  306  could maliciously or unintentionally subsequently try to store such data as Classified Information  310 . Alternatively, such a situation could arise if information from files defined as Classified Information  310  were written unencrypted to the Unclassified Information  314  storage area. SLS processors are not generally designed to address these issues. However, this problem is solved by using the MLS file service module  300  described herein to provide SLS file access. 
     MLS file service module  300  also includes a client trusted load support manager  320 . The client trusted load support manager  320  can be implemented in hardware, in software or as a combination of hardware and software. The client trusted load support manager  320  is controlled by cryptographic processor  302  to authenticate and then selectively control loading of application software into program memory of a secure user processor. Application software for the secure user processor can be stored in the user MLS file system  301 . For example, such application software can comprise Top Secret Information  306 , Secret Information  308 , Confidential Information  310 , or unclassified applications  312 . 
     When a software application is first stored in the user MLS file system  301 , the file can be source authenticated by the cryptographic processor. A variety of well known techniques can be used for this purpose. For example, conventional public key infrastructure (PKI) technology can be used for this purpose. With PKI based techniques, a software source may digitally sign a software application using its private key. The cryptographic processor  302  can verify that signature using a public key issued by a certificate authority within the PKI. This enables the cryptographic processor to verify that the software is authentic. 
     Subsequently, the MLS file service module  300  can receive from a secure user processor a request for a classified file. In response, the cryptographic processor  302  can access the file, and perform any decryption functions that may be required. Then, before serving the file to the secure user processor, the cryptographic processor can perform an integrity check on the file. According to an embodiment of the invention, this integrity checking can be accomplished using checksum/hashing technology. 
     The checksum process can protect the integrity of application software by detecting changes relative to an authenticated version of the software. The process involves adding up some value derived from the basic components of the file. The cryptographic processor  302  can be provided with this information in advance from a trusted source. Subsequently, the cryptographic processor  302  can perform the same operation on the file which is to be loaded into the secure user processor. In this way, the cryptographic processor can compare the software to be loaded to an authenticated version of the original software. If the checksum values match, the cryptographic processor can conclude that the software has not been corrupted or otherwise modified in some malicious way. 
     This integrity check can ensure that the file has not been corrupted in any way. If the file is determined to not to be corrupted, then the file will be served to the secure user processor and loaded into either data or program memory. Alternatively, if the integrity check reveals that the file has been corrupted, then the file will not be served by the MLS file service module to the secure user processor. 
     In  FIG. 3 , the cryptographic processor  302  can be one of several commercially available cryptographic engines. According to one embodiment, the cryptographic processor can be a Sierra II Crypto processor available from Harris Corporation of Melbourne, Fla. The cryptographic processor  302  can include configurable key lengths and can be programmed with one or more encryption algorithms. As illustrated in  FIG. 3 , the MLS file service module  300  can include several control and data ports that are useful for controlling the operation of the cryptographic processor  302 . For example, these can include a crypto ignition key port, a key and certificate fill port, a zeroize switch, and a software load port. The software load port can be used for loading software from a trusted source for executing on the cryptographic processor  302  or a client processor. The zeroize switch can be used to clear the encryption keys and/or the classified information contained in the user MLS file system  301  and the crypto MLS file system  304 . The various control and data ports can be controlled by the client processor or by any other suitable means. 
     The cryptographic processor  302  can include one or more security features. For example, in addition to controlling SLS access to an MLS file system, the cryptographic engine  302  can provide security auditing, security policy enforcement, file integrity checking and/or trusted boot software loading. 
     Referring now to  FIG. 4 , there is shown a block diagram of a secure user processor module  400  that can be used in conjunction with the MLS file service module  300  in  FIG. 3 . Secure user processor module  400  includes a secure user processor  402  comprised of trusted processing hardware. According to one embodiment, the secure user processor  402  can include single level (SL) trusted software. Such SL trusted software can include a SL trusted operating system and SL trusted application software. According to another embodiment, the secure user processor  402  can include non-trusted software. Such non-trusted software can include a non-trusted operating system and one or more non-trusted application programs. The significance of these two different embodiments will be discussed in greater detail in relation to  FIGS. 5 and 6 . 
     As shown in  FIG. 4 , the secure user processor  402  includes a data communication link with the MLS file service module  300 . This data communication link provides the secure user processor with SLS access to the user MLS file system  301  through the client SLS access interface  316 , as described above. The secure user processor module  400  also includes suitable data store facilities. These data store facilities can be used for temporarily storing any necessary data or software applications used by the secure user processor  402 . Such data store facilities can include data memory and program memory. The data memory can include RAM  404  and non-volatile RAM (NVRAM)  406 . The program memory can include RAM  408  and Flash/EEPROM  410 . 
       FIG. 4  also shows a control signal path from client zeroize/reset manager  318  to the secure user processor  402 . The control signal path can be implemented in hardware, in software, or in a combination of hardware and software. One or more data stores associated with the secure user processor can zeroized in response to a control signal received by means of this control signal path. For example, such data stores can include data memory comprising RAM  404  and NVRAM  406 . Such data stores can also include program memory comprised of RAM  408  and flash/EEPROM  410 . Other data stores can also be zeroized in response to the control signal from the client zeroize/reset manager  318 . For example, such data stores can include a video buffer memory (not shown). 
     The secure user processor module  400  also includes write enable gates  412  and  414  that selectively control write access to program memory including RAM  408  and Flash/EEPROM  410 , respectively. A control signal from the client trusted load manager  320  can be used to selectively control the write enable gates  412 ,  414 . The write enable gates  412 ,  414  can be used in conjunction with the client trusted load manager  320 . This arrangement can allow the MLS file service module to ensure that write access to program memory (RAM  408 , Flash/EEPROM  410 ) is provided only after the software has been authenticated and integrity checked by the cryptographic processor  302  utilizing appropriate means. Moreover, client trusted load manager  320  can thereafter disable write access to the program memory of secure user processor  402 . By disabling the write access immediately after the application program has been loaded into memory, the client trusted load manager can protect the secure user processor against viruses and other types of self-modifying application software. 
     Referring now to  FIG. 5 , there is shown a block diagram of a first embodiment of a multi-level secure (MLS) computing architecture  500  that is comprised of MLS file service module  300  and secure user processor module  400 . In  FIG. 5 , the secure user processor module  400  includes a secure user processor  402   a  according to a first embodiment as described above. In particular, the secure user processor  402   a  utilizes trusted hardware and SL-trusted software (operating system and application software). 
     The MLS computing architecture  500  also includes a secure HMI  504 . Secure HMI devices are known in the art and typically can include one or more features to ensure trusted communications between the user and the secure user processor. The secure HMI  504  is comprised of trusted hardware. Secure HMI  504  interfaces with the secure user processor  402   a  by means of a trusted communication link. Any suitable physically-secure data communication path can be used for this purpose, provided that it offers trusted communications between the secure user processor  402   a  and the secure HMI  504 . This trusted communication link can be used for communicating user commands, data, and any information to be displayed on the secure HMI. It can also be used to facilitate user sign-on as hereinafter described. Trusted communication links as described herein are known in the art. 
     The MLS computing architecture in  FIG. 5  provides SLS file service to an MLS file system, and trusted software loading. The architecture in  FIG. 5  provides additional capabilities as compared to the prior art SLS computing device  100  shown in  FIG. 1 , thereby overcoming several of its limitations. The single-level trusted software running on secure user processor  402   a  is much simpler and thus less expensive to design, develop, and test/certify as compared to the SL-trusted software required for the secure user processor  102  in computing device  100 . The SL-trusted operating system utilized on secure user processor  402   a  does not need to implement a trusted file system which is normally a significant portion of the SL-trusted OS development effort. The SL-trusted software applications utilized on secure user processor  402   a  do not need to invoke decryption services upon file read from the file system and do not need to invoke encryption services upon file write to the file system. The absence of these requirements significantly reduces the design, development and testing/certification effort for those software applications. 
     In addition to the foregoing advantages, the secure user processor  402   a  can now be used to access files at multiple security classification levels. Such SLS access to the user MLS file system  301  is advantageously achieved by placing responsibility for MLS security with the MLS file service module  300 , and more particularly, with the cryptographic processor  302 . Cryptographic processor  302  selectively controls the client zeroize/reset manager  318  to ensure that MLS file system data is zeroized in the data memory and program memory contained in secure user processor module  400 . The cryptographic processor  302  is programmed so that this zeroizing process occurs whenever the secure user processor  402   a  transitions between accessing files at one security classification level versus files at another security classification level. This is a significant improvement over the capabilities and design efficiencies offered by existing architectures. 
     Yet another advantage of the MLS computing architecture in  FIG. 5  is its ability to ensure trusted software loading for the secure user processor module  400 . The MLS file service module  300 , and more particularly, the cryptographic processor  302  can ensure trusted file loading by integrity checking any software applications before they are loaded into the program memory of the secure user processor  402   a . Such integrity checking can be accomplished by using suitable means such as checksum/hashing techniques as previously described. 
     It is noted that although the software executing on secure user processor  402   a  is simpler and potentially less expensive than the software utilized by the secure user processor  102  in the prior art, the software executing on secure user processor  402   a  still needs to be designed, developed, and tested/certified to single-level secure standards. The software on secure user processor  402   a  still needs to be SL-trusted so that it can provide the trusted path to the file system control interface  322  to support trusted user sign-on services. 
     Referring now to  FIG. 6 , there is shown a block diagram for another embodiment of a computer architecture for an MLS computing device  600 . The MLS computing device  600  is similar to the MLS computing device  500  to the extent that it is also comprised of MLS file service module  300  and secure user processor module  400 . However, in  FIG. 6  the secure user processor module  400  includes a secure user processor  402   b  according to a second embodiment as described above. The secure user processor  402   b  utilizes trusted hardware similar to the secure user processor  402   a . However, rather than using SL-trusted software, the secure user processor  402  uses non-trusted software (operating system and application software). For example, COTS software, which is inexpensive and readily available, can be used for this purpose. 
     A secure HMI  604  is also provided. The secure HMI  604  is comprised of trusted hardware. Secure HMI  604  interfaces with the secure user processor module  400  by means of a physically-secure communication link. Any suitable physically-secure data communication path can be used for this purpose. This physically-secure data communication link can be used for communicating user commands, data, and any information to be displayed on the secure HMI. Notably, in the architecture shown in  FIG. 6 , this physically-secure communication link is not used to facilitate user sign-on because the software utilized by secure user processor module  400  is not trusted. Instead, a separate trusted communication link is provided directly between the secure HMI  604  and the file system control interface  322 . 
     The secure user processor  402   b  also communicates with the MLS file service module  300 . In particular, the secure user processor  402   b  can communicate with the client SLS access interface  316  (but not the file system control interface  322 ). The client SLS access interface  316  provides services as described above. 
     The architecture in  FIG. 6  provides the same capabilities as the SLS computing device  500  shown in  FIG. 5 , while overcoming one of its major limitations. In contrast to the computing device  500 , the software running on secure user processor  402   b  is COTS software that is highly familiar to the user and does not require expensive custom development. The tradeoff to this approach is that secure user processor  402   b  cannot provide the trusted path to the file system control interface  322  to support trusted user sign-on services. Referring to  FIG. 6 , it is seen that trusted human/machine interface  604  must now support two separate interfaces, one trusted file system control interface  322  to the file service module  300  to handle user authentication and a second physically-secure interface to secure user processor module  400  for all normal user input/output such as running software applications. This advantageous arrangement can permit a user to use familiar COTS operating systems and applications installed on the secure user processor  402   b , while still having the benefit of access to classified files at multiple defined security levels in the MLS file service module  300 . 
     In  FIGS. 5 and 6  various communication links are provided respectively between the secure user processor module  400  and the MLS file service module  300 . It should be understood that the foregoing communication links can be implemented by any suitable means and in different physical configurations, provided it is physically secure. For example, the data communication link can be through a direct connection (e.g. USB, PCMCIA) interface. Such a direct connection can create the appearance that the MLS file service module  300  is a local disk drive. However, in order to establish a trusted path for user sign-on/sign-off, suitable trusted path methods can be used to provide the communication link. Trusted path methods of this type are well known to those skilled in the art. 
     As an alternative to the direct connection approach described above, the MLS file service module  300  can be embedded in the computer on an I/O bus (e.g. PCI) to provide the appearance of a local disk drive, but within the same physically secure enclosure. In this way, a secure path can be provided between the secure user processor and the file service module. Yet another alternative can include embedding the MLS file service module  300  on a host computer motherboard. Consequently, the data communication can occur over a data communication link within the same physically secure enclosure to establish a secure path. 
     The invention described and claimed herein is not to be limited in scope by the preferred embodiments herein disclosed, since these embodiments are intended as illustrations of several aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.