Patent ID: 7210043

Claim:
A computer system having an input/output processing unit for executing a file access, an access execution unit for requesting a file access via the input/output processing unit in response to a user instruction, and an access control unit for performing access control when the access is executed, wherein the access control unit comprises: a storage unit protected from the access execution unit; a file list stored in the storage unit describing security levels of files; a user list stored in the storage unit describing clearances of users; an access control processing unit for determing whether the file access is legal in accordance with the file list, the user list, an access type of the file access, information identifying a file, and information identifying a user, wherein if a host OS program of the access control processing unit is tampered with, a guest OS of the access control processing unit is instructed to invalidated one or more functions of the host OS program; an enciphering unit for encrypting a file when storing the file on a storage medium; a deciphering unit for decryping the encrypted file when retrieving the encrypted file from the storage medium; wherein the storage unit stores at least one cipher key commonly used among a plurality of security levels for each file, the cipher key used for both encrypting and decrypting; and an access monitor unit which: when the input/output processing unit executes a file access, sends the access type, the information identifying the file, and the information identifying the user to the access control processing unit; receives a validity determination result of the file access from the access control processing unit; and if the file access is legal, causes the input/output processing unit to execute the file access, and if the file access is illegal, inhibits the file access.