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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 onwards. Cryptographic file system implementations for other operating systems are available, but the Microsoft EFS is not compatible with any of them.
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. 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 EFS component driver then uses the symmetric key to decrypt the file. 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 FAT The most significant way of preventing the decryption-on-copy is using backup applications that are aware of the "Raw" APIs.
In other words, the files are "copied" e. Two significant security vulnerabilities existed in Windows EFS, and have been variously targeted since. In Windows , the local administrator is the default Data Recovery Agent, capable of decrypting all files encrypted with EFS by any local user.
EFS in Windows cannot function without a recovery agent, so there is always someone who can decrypt encrypted files of the users.
Any non-domain-joined Windows 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. 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.
In Windows , 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 computer and resets a local user account's password, [7] the attacker can log in as that user or recovery agent and gain access to the RSA private key which can decrypt all files. This is because the backup of the user's RSA private key is encrypted with an LSA secret, which is accessible to any attacker who can elevate their login to LocalSystem again, trivial given numerous tools on the Internet.
In Windows XP and beyond, the user's RSA private key is backed up using an offline public key whose matching private key is stored in one of two places: the password reset disk if Windows XP is not a member of a domain or in the Active Directory if Windows XP is a member of a domain.
This means that an attacker who can authenticate to Windows XP as LocalSystem still does not have access to a decryption key stored on the PC's hard drive. In Windows , XP or later, the user's RSA private key is encrypted using a hash of the user's NTLM password hash plus the user name — use of a salted hash makes it extremely difficult to reverse the process and recover the private key without knowing the user's passphrase.
Also, again, setting Syskey to mode 2 or 3 Syskey typed in during bootup or stored on a floppy disk will mitigate this attack, since the local user's password hash will be stored encrypted in the SAM file. Once a user is logged on successfully, access to his own EFS encrypted data requires no additional authentication, decryption happens transparently.
Thus, any compromise of the user's password automatically leads to access to that data. Windows can store versions of user account passphrases with reversible encryption, though this is no longer default behaviour; it can also be configured to store and will by default on the original version of Windows XP and lower Lan Manager hashes of the local user account passphrases, which can be attacked and broken easily.
It also stores local user account passphrases as NTLM hashes, which can be fairly easily attacked using " rainbow tables " if the passwords are weak Windows Vista and later versions don't allow weak passwords by default.
To mitigate the threat of trivial brute-force attacks on local passphrases, older versions of Windows need to be configured using the Security Settings portion of Group Policy to never store LM hashes, and of course, to not enable Autologon which stores plaintext passphrases in the registry.
Further, using local user account passphrases over 14 characters long prevents Windows from storing an LM hash in the SAM — and has the added benefit of making brute-force attacks against the NTLM hash harder. When encrypting files with EFS — when converting plaintext files to encrypted files — the plaintext files are not wiped, but simply deleted i.
This means that, unless they for example happen to be stored on an SSD with TRIM support, they can be easily recovered unless they are overwritten. To fully mitigate known, non-challenging technical attacks against EFS, encryption should be configured at the folder level so that all temporary files like Word document backups which are created in these directories are also encrypted. When encrypting individual files, they should be copied to an encrypted folder or encrypted "in place", followed by securely wiping the disk volume.
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. 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.
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