Шифрующая файловая система в Windows (EFS)

Feature in Microsoft Windows
The Encrypting File System (EFS) on Microsoft Windows is a feature introduced in version 3.0 of NTFS 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 except the home versions (see Supported operating systems below) from Windows 2000 onwards. 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. See also the list of cryptographic file systems.

When an operating system is running on a system without file encryption, access to files normally goes through OS controlled user authentication and access control lists. However, if an attacker gains physical access to the computer, this barrier can be easily circumvented. One way, for example, would be to remove the disk and put it in another computer with an OS installed that can read the filesystem; another, would be to simply reboot the computer from a boot CD containing an OS that is suitable for accessing the local filesystem. The most widely accepted solution to this 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, the encryption of a file is only as strong as the password to unlock the decryption key.

EFS works by encrypting a file with a bulk symmetric key, also known as the File Encryption Key, or FEK. It uses a symmetric encryption algorithm because it takes less time to encrypt and decrypt large amounts of data than if an asymmetric key cipher is used. The symmetric encryption algorithm used will vary depending on the version and configuration of the operating system; see Algorithms used by Windows version below. The FEK (the symmetric key that is used to encrypt the file) is then encrypted with a public key that is associated with the user who encrypted the file, and this encrypted FEK is stored in the $EFS alternative data stream of the encrypted file. To decrypt the file, the EFS component driver uses the private key that matches the EFS digital certificate (used to encrypt the file) to decrypt the symmetric key that is stored in the $EFS stream. The EFS component driver then uses the symmetric key to decrypt the file. Because the encryption & decryption operations are performed at a layer below NTFS, it is transparent to the user and all their applications. Folders whose contents are to be encrypted by the file system are marked with an encryption attribute. The EFS component driver treats this encryption attribute in a way that is analogous to the inheritance of file permissions in NTFS: if a folder is marked for encryption, then by default all files and subfolders that are created under the folder are also encrypted. When encrypted files are moved within an NTFS volume, the files remain encrypted. However, there are a number of occasions in which the file could be decrypted without the user explicitly asking Windows to do so. Files and folders are decrypted before being copied to a volume formatted with another file system, like FAT32. Finally, when encrypted files are copied over the network using the SMB/CIFS protocol, the files are decrypted before they are sent over the network. The most significant way of preventing the decryption on copy is using backup applications that are aware of the "Raw" APIs. Backup applications that have implemented these Raw APIs will simply copy the encrypted file stream and the $EFS alternative data stream as a single file. In other words, the files are "copied" (e. g. into the backup file) in encrypted form, and are not decrypted during backup. Starting with Windows Vista, a user's private key can be stored on a smart card; Data Recovery Agent (DRA) keys can also be stored on a smart card.

Two significant security vulnerabilities existed in Windows 2000 EFS, and have been variously targeted since. In Windows 2000, the local administrator is the default Data Recovery Agent, capable of decrypting all files encrypted with EFS by any local user. EFS in Windows 2000 cannot function without a recovery agent, so there is always someone who can decrypt encrypted files of the users. Any non domain joined Windows 2000 computer will be susceptible to unauthorized EFS decryption by anyone who can take over the local Administrator account, which is trivial given many tools available freely on the Internet. In Windows XP and later, there is no default local Data Recovery Agent and no requirement to have one. Setting SYSKEY to mode 2 or 3 (syskey typed in during bootup or stored on a floppy disk) will mitigate the risk of unauthorized decryption through the local Administrator account. This is because the local user's password hashes, stored in the SAM file, are encrypted with the Syskey, and the Syskey value is not available to an offline attacker who does not possess the Syskey passphrase/floppy.

In Windows 2000, the user's RSA private key is not only stored in a truly encrypted form, but there is also a backup of the user's RSA private key that is more weakly protected. If an attacker gains physical access to the Windows 2000 computer and resets a local user account's password,

Anyone who can gain Administrators access can overwrite, override or change the Data Recovery Agent configuration. This is a very serious issue, since an attacker can for example hack the Administrator account (using third party tools), set whatever DRA certificate they want as the Data Recovery Agent and wait. This is sometimes referred to as a two stage attack, which is a significantly different scenario than the risk due to a lost or stolen PC, but which highlights the risk due to malicious insiders. When the user encrypts files after the first stage of such an attack, the FEKs are automatically encrypted with the designated DRA's public key. The attacker only needs to access the computer once more as Administrator to gain full access to all those subsequently EFS encrypted files. Even using Syskey mode 2 or 3 does not protect against this attack, because the attacker could back up the encrypted files offline, restore them elsewhere and use the DRA's private key to decrypt the files. If such a malicious insider can gain physical access to the computer, all security features are to be considered irrelevant, because they could also install rootkits, software or even hardware keyloggers etc. on the computer – which is potentially much more interesting and effective than overwriting DRA policy.

Files encrypted with EFS can only be decrypted by using the RSA private key(s) matching the previously used public key(s). The stored copy of the user's private key is ultimately protected by the user's logon password. Accessing encrypted files from outside Windows with other operating systems (Linux, for example) is not possible—not least of which because there is currently no third party EFS component driver. Further, using special tools to reset the user's login password will render it impossible to decrypt the user's private key and thus useless for gaining access to the user's encrypted files. The significance of this is occasionally lost on users, resulting in data loss if a user forgets his or her password, or fails to back up the encryption key. This led to coining of the term "delayed recycle bin", to describe the seeming inevitability of data loss if an inexperienced user encrypts his or her files. If EFS is configured to use keys issued by a Public Key Infrastructure and the PKI is configured to enable Key Archival and Recovery, encrypted files can be recovered by recovering the private key first.

No other operating systems or file systems have native support for EFS.