Patent ID: 8307408

Claim:
A file processing system for obtaining a file from a folder storing both a ciphertext file and a plaintext file and for delivering the file to an upper-level application program by using a computer executing the upper-level application program, where said ciphertext file is attached with identification information indicating that the relevant file is a ciphertext file, said file processing system comprising: file read/write means for executing read or write of a file from/to said folder if said upper-level application program requests to read said file stored in said folder or requests to write said file; file distinguishing means for distinguishing whether said read file is a plaintext file or a ciphertext file based on whether or not said read file contains said identification information; and file processing means for deciphering said ciphertext file and delivering a deciphered file produced by excluding said identification information from said read file to said upper-level application program if said read file is a ciphertext file, wherein said identification information is footer information attached after body data in said ciphertext file, wherein if the request to read said file is a request to read data of a pre-determined size from an intermediate position in said ciphertext file to said footer information, said file processing means embeds insignificant information in a part of said identification information, deciphers said read file, removes said insignificant information and delivers the file to said upper-level application program, wherein said file processing means further encrypts data after file edit by said upper-level application program, and modifies an attaching position of said footer information to cipher data, according to an increase/decrease in a file size after the edit and attachment of empty data area, by a size of the increase/decrease in the file size and the attachment of the empty data to generate an updated ciphertext file and disguises a result of the update by attaching the same footer information as the footer information which was attached to the file before the file edit, and wherein said file read/write means writes said updated ciphertext file in said folder.