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The Encrypting File System (EFS) on Microsoft Windows is a feature introduced in version 3.0 of NTFS〔(【引用サイトリンク】title=File Encryption (Windows) )〕 that provides filesystem-level encryption. The technology enables files to be transparently encrypted to protect confidential data from attackers with physical access to the computer. EFS is available in all versions of Windows developed for business environments (see Supported operating systems below) from Windows 2000 onwards.〔EFS is available on Windows 2000 Server and Workstation, on Windows XP Professional, on Windows Server 2003 and 2008, and on Windows Vista and Windows 7 Business, Enterprise and Ultimate. EFS is ''not'' available on Windows XP Home Edition, nor on the Starter, Basic, and Home Premium editions of Windows Vista and Windows 7. It could not be implemented in the Windows 9x series of operating systems, since they did not natively support NTFS, which is the foundation for EFS.〕 By default, no files are encrypted, but encryption can be enabled by users on a per-file, per-directory, or per-drive basis. Some EFS settings can also be mandated via Group Policy in Windows domain environments. Cryptographic file system implementations for other operating systems are available, but the Microsoft EFS is not compatible with any of them.〔(【引用サイトリンク】title=Cryptographic Filesystems, Part One: Design and Implementation )〕 See also the list of cryptographic file systems. ==Basic ideas== As long as the operating system is running on a system without file encryption, access to the files will have to go through OS-controlled user authentication and access control lists. If an attacker gains physical access to the computer, however, this barrier can be easily circumvented. One way would be to remove the disk and put it in another computer with an OS installed that can read the filesystem, or simply reboot the computer from a boot CD containing an OS that is suitable to access the local filesystem. The most widely accepted solution is to store the files encrypted on the physical media (disks, USB pen drives, tapes, CDs and so on). In the Microsoft Windows family of operating systems EFS enables this measure, although on NTFS drives only, and does so using a combination of public key cryptography and symmetric key cryptography to make decrypting the files extremely difficult without the correct key. However, the cryptography keys for EFS are in practice protected by the user account password, and are therefore susceptible to most password attacks. In other words, encryption of files is only as strong as the password to unlock the decryption key. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Encrypting File System」の詳細全文を読む スポンサード リンク
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