Securing Windows Server 2008 R2 : Encrypting File System

Often, the data content is not only important, but may also be of a confidential nature. One way to enable users to protect data that they know is confidential is to train them on the usage of EFS. EFS is a file encryption technology that allows end users to store data content on NTFS volumes in encrypted format. EFS is a user-specific technology, meaning that once a file has been encrypted while a user is logged in with a particular username, only that logged-on username can be used to read or decrypt the file.

Enabling EFS is as easy as checking a box. Each file and folder on an NTFS volume has an attribute that is used to enable encryption. See Figure 1 for an example of this attribute at the folder level. Once you have enabled a folder for EFS, all files placed into that folder will automatically be encrypted. EFS is a not a new technology, but Microsoft has enhanced some aspects of it with the release of Windows Server 2008 R2. One of the key areas of change for EFS revolves around the introduction of support for new algorithms, as we will discuss in the next section.

EFS keys and algorithms

EFS utilizes both symmetric and asymmetric key technology to encrypt and secure data on NTFS volumes. A symmetric key is a single key which can quickly be used to encrypt or decrypt larger amounts of data. Symmetric keys are often used to encrypt content because of the speed advantage they have over key pairs. EFS utilizes symmetric keys to secure data content.

Asymmetric key pairs are a complimentary pair of keys. One of the keys is used to encrypt while the other to decrypt. Asymmetric keys are slower when dealing with large amounts of data, and so, are not used in EFS to secure data, but are instead used to secure the symmetric key. So, ultimately, it is a combination of keys that are used by EFS to secure a user’s data in the file system; a single key to encrypt the data content and a key pair to secure the single key.

In earlier iterations of EFS, Microsoft has employed industry standard encryption algorithms such as Triple DES (3DES) and Data Encryption Standard X (DESX). As encryption standards have developed and improved, Microsoft has continued to update EFS to support the newer protocols, as was evident with the release of Windows XP SP1. From Windows XP SP1, forward EFS began utilizing Advanced Encryption Standard (AES) as its primary encryption mechanism.

The newest version of EFS, included with Windows Server 2008 R2 and Windows 7, has followed in the same footsteps as the preceding versions and has been improved to reflect the algorithm standards that exist today. The following represent the algorithms supported by the Windows Server 2008 R2 iteration of EFS:

• Advanced Encryption Standard

• Secure Hash Algorithm (SHA)

• Elliptic Curve Cryptography (ECC)

• Smart card-based encryption

A critical addition to the preceding list is the new support for ECC. Many environments today are required to comply with stricter regulatory requirements. The addition of ECC allows for these high-security environments to comply with Suite B encryption requirements as set forth by the National Security Agency. Today, Suite B compliance is utilized by United States government agencies to protect classified information.

EFS and policy enforcement

With additional compliance regulations existing in many environments today, administrators often need a mechanism to control the enforcement of certain security policies. In Windows 7 and Windows Server 2008 R2, you have the capability to control the way EFS behaves in the Local Computer Policy on the machine. Utilizing the Local Computer Policy, you have the ability to enforce ECC as well as configure other settings such as if Smart Cards are required for EFS usage. Since Local Computer Policy settings are administrated individually on each computer, it makes it very difficult to use these settings in a larger environment.

The most common way to enforce policy onto large groups of machines in an AD environment is by utilizing Group Policy. In order to address EFS policy enforcement on a broader scale, Microsoft has incorporated settings into Group Policy to allow the capability to control and enforce settings centrally for new EFS components. You will file EFS settings within a Group Policy under Computer Configuration | Policies | Windows Settings | Security Settings | Public Key Policies | Encrypting File System.

In Suite B compliance environments, the usage of RSA encryption algorithms is not allowed and only ECC may be used for EFS. Group Policy has three ECC pertinent settings, Allow, Require, and Don’t Allow, which are displayed in Figure 2.

The Allow setting simply allows the use of ECC, but does not enforce it. This means that both RSA and ECC are available when this setting has been configured. If you are in an environment that requires Suite B compliance, Allow is not an appropriate setting. Instead, you would want to select the second radio button for Require. Require prevents the use of RSA and enforces that ECC be the only protocol in use with EFS. The final setting of Don’t Allow blocks the usage of ECC, thus all EFS key sets will be generated utilizing RSA.

Cipher.exe

Cipher.exe is a command that has existed for some time, and essentially embodies a command prompt version of the file system GUI exposure of EFS, but with a little more dynamism. Since administrators are often looking for a faster and more manageable way to perform mundane or repetitive tasks, Cipher.exe fits well into an administrator’s toolbox.

Cipher.exe can help to reduce the administrative burden associated with EFS by allowing bulk EFS actions to be performed against the file system. It has the capability to target files and folders and force encryption or decryption throughout directory structures. Instead of having to move files into a designated folder manually for them to be encrypted, Cipher.exe allows targeting for encryption via the command prompt. Additionally, it can also be used to assist with other EFS administrative tasks such as key management tasks. One such task includes the generation of new user keys, another includes updating encrypted files with new user encryption keys.

With Cipher.exe, two switches exist that allow the administrator to request new EFS keys: /K and /R. /K will allow an administrator to create a new certificate and key, while /R allows an administrator to generate a new recovery certificate and key.

Since Windows 2008 R2 has introduced the capability to generate keys based on the ECC algorithm, Microsoft has enabled the specification of the ECC protocol and key length with the new /ECC:length switch. The new ECC switch can be used in conjunction with the /K and /R switches to allow the administrator to impact key generation in two ways: first, ECC keys will be generated, and second, the key length for the ECC keys can be specified. The options for the ECC key length include 256, 384, and 521 key lengths. ECC keys can only be used with self-signed certificates.