Encrypted File System. EFS is a subsystem of the NTFS file system available in windows 2000 and later winodws operating systems. EFS can be used to encrypt any file or folder in the NTFS file system. A symmetric key is used to encrypt the data. A symmetric is one which can be used to encrypt as well as to decrypt data. This method is used because it makes the encryption/decryption process much faster. This key is then encrypted using a private key associated with the user account of the user that is encoding the data. This is stored in an alternate data stream of the encrypted file. In this way, the data is kept private from other users that have physical access to the computer.