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 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. The symmetric encryption algorithm used will vary depending on the version and configuration of the operating system see Algorithms used by Windows version below. 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. In other words, the encryption of a file is only as strong as the password to unlock the decryption key.ĮFS works by encrypting a file with a bulk symmetric key, also known as the File Encryption Key, or FEK. However, the cryptography keys for EFS are in practice protected by the user account password, and are therefore susceptible to most password attacks.

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. 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). 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. However, if an attacker gains physical access to the computer, this barrier can be easily circumvented. 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. ( February 2010) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this section by adding citations to reliable sources.

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