Hands-on Project 117 Encryption and decryption

Encryption Process Summary

The following list summarizes the steps EFS performs to encrypt a file:

1. The user profile is loaded if necessary.

2. A log file is created in the System Volume Information directory with the name Efsx.log, where x is a unique number (for example, Efs0.log). As subsequent steps are performed, records are written to the log so that the file can be recovered in case the system fails during the encryption process.

3. Base Cryptographic Provider 1.0 generates a random 128-bit FEK for the file.

4. A user EFS private/public key pair is generated or obtained. HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\EFS\CurrentKeys\CertificateHash identifies the user's key pair.

5. A DDF key ring is created for the file that has an entry for the user. The entry contains a copy of the FEK that has been encrypted with the user's EFS public key.

6. A DRF key ring is created for the file. It has an entry for each Recovery Agent on the system, with each entry containing a copy of the FEK encrypted with the agent's EFS public key.

7. A backup file with a name in the form Efs0.tmp is created in the same directory as the file to be encrypted

8. The DDF and DRF key rings are added to a header and augment the file as its EFS attribute.

9. The backup file is marked as encrypted, and the original file is copied to the backup.

10. The original file's contents are destroyed, and the backup is copied to the original. This copy operation results in the data in the original file being encrypted because the file is now marked as encrypted.

11. The backup file is deleted.

12. The log file is deleted.

13. The user profile is unloaded (if it was loaded in step 1).

If the system crashes during the encryption process, either the original file remains intact or the backup file contains a consistent copy. When Lsasrv initializes after a system crash, it looks for log files under the System Volume Information subdirectory on each NTFS volume on the system. If Lsasrv finds one or more log files, it examines their contents and determines how recovery should take place. Lsasrv deletes the log file and the corresponding backup file if the original file wasn't modified at the time of the crash; otherwise, Lsasrv copies the backup file over the original, partially encrypted file and then deletes the log and backup. After Lsasrv processes log files, the file system will be in a consistent state with respect to encryption, with no loss of user data.

The Decryption Process

The decryption process begins when a user opens an encrypted file. NTFS examines the file's attributes when opening the file and then executes a callback function in the EFS driver. The EFS driver reads the $EFS attribute associated with the encrypted file. To read the attribute, the driver calls EFS support functions that NTFS exports for EFS's use. NTFS completes the necessary steps to open the file. The EFS driver ensures that the user opening the file has access privileges to the file's encrypted data (that is, that an encrypted FEK in either the DDF or DRF key rings corresponds to a private/public key pair associated with the user). As EFS performs this validation, EFS obtains the file's decrypted FEK to use in subsequent data operations the user might perform on the file.

EFS can't decrypt an FEK and relies on Lsasrv (which can use CryptoAPI) to perform FEK decryption. EFS sends an LPC message by way of the Ksecdd.sys driver to Lsasrv that asks Lsasrv to obtain the decrypted form of the encrypted FEK in the $EFS attribute data (the EFS data) that corresponds to the user who is opening the file.

When Lsasrv receives the LPC message, Lsasrv executes the Userenv.dll (User Environment DLL) LoadUserProfile API function to bring the user's profile into the registry, if the profile isn't already loaded. Lsasrv proceeds through each key field in the EFS data, using the user's private key to try to decrypt each FEK. For each key, Lsasrv attempts to decrypt a DDF or DRF key entry's FEK. If the certificate hash in a key field doesn't refer to a key the user owns, Lsasrv moves on to the next key field. If Lsasrv can't decrypt any DDF or DRF key field's FEK, the user can't obtain the file's FEK. Consequently, EFS denies access to the application opening the file. However, if Lsasrv identifies a hash as corresponding to a key the user owns, it decrypts the FEK with the user's private key using CryptoAPI.

Because Lsasrv processes both DDF and DRF key rings when decrypting an FEK, it automatically performs file recovery operations. If a Recovery Agent that isn't registered to access an encrypted file (that is, it doesn't have a corresponding field in the DDF key ring) tries to access a file, EFS will let the Recovery Agent gain access because the agent has access to a key pair for a key field in the DRF key ring.

Decrypted FEK Caching

Traveling the path from the EFS driver to Lsasrv and back can take a relatively long time—in the process of decrypting an FEK, CryptoAPI uses results in more than 2000 registry API calls and 400 file system accesses on a typical system. The EFS driver, with the aid of NTFS, uses a cache to try to avoid this expense.

Decrypting File Data

After an application opens an encrypted file, the application can read from and write to the file. NTFS calls the EFS driver to decrypt file data as NTFS reads the data from the disk and before NTFS places the data in the file system cache. Similarly, when an application writes data to a file, the data remains in unencrypted form in the file system cache until the application or the cache manager uses NTFS to flush the data back to disk. When an encrypted file's data writes back from the cache to the disk, NTFS calls the EFS driver to encrypt the data.

As stated earlier, the EFS driver performs encryption and decryption in 512-byte units. The 512-byte size is the most convenient for the driver because disk reads and writes occur in multiples of the 512-byte sector.

Hands-on-project 48 -- Encrypting a File

How to Encrypt a File

1. From Windows Explorer or My Computer, navigate and select the folder you want encrypted. In this example, a folder "Encrypted Folder" will be encrypted so that any file placed inside this folder will be encrypted.
   
   
   
   
2. Right click the folder and choose properties and then click the Advanced... button.
   
   
   
   
3. Check the Encrypt contents to secure data attribute. Note that a folder may be compressed or encrypted, but not both. Click OK again when done.
   
   
   
   If there are already files and or subfolders inside this folder, an additional pop up dialog will appear, otherwise you are done (go to step 4). This additional dialog (Figure 4) will present two options for you:
   
   • Apply changes to this folder, subfolders and files - Choose this to encrypt the folder so that everything inside this folder is encrypted, and this includes files and folders that are later moved to or created inside this folder.
   • Apply changes to this folder only - Choose this to encrypt only the folder so that all files/folders subsequently moved or created in this folder will be encrypted. Existing files and folders are not encrypted.
     
   Choose one radio button and then click OK.
   
   
   
   
4. Notice the folder name turns green when the folder has an encrypt attribute set. New files and folder placed inside will also have green names (encrypted). When you copy or move a file out of an encryption folder, the green color of a file/folder name means encryption has been preserved. If the color is black, it means the file has been decrypted.
   
   

EFS File Sharing - Sharing Your Encrypted Files

You may share an encrypted file with additional users after you have encrypted the file. You can only do this on a per file basis. EFS file sharing allows other users you designate with the ability to decrypt and encrypt your original encrypted file. These users may also move, copy, or delete the encrypted file if they have such file permissions.

Once a file has been initially encrypted, file sharing is enabled through a new button. After right clicking and selecting the Advanced Properties of an encrypted file, a user may be added by selecting the new Details... Button.

You will be presented with a window showing who has EFS access to this file.

Click the Add... button to add more users.

You may add other users (not groups) from the local machine or from the Active Directory, provided the user has a valid EFS certificate. Users without a valid EFS certificate will not be shown. A valid EFS certificate is automatically created whenever a user encrypts a file, and the user can simply encrypt a file to have one created automatically. Select a user you want to add. If the user is in active directory, you can find the user via the Find User... button. Click OK to return and view the user has been added to the EFS file share list. Click OK again (3 times) and you are done.

Backing Up Your EFS Certificate and Keys

It is important to back up your EFS certificate and keys in the event your user account profile may become corrupted or deleted, in which case you will not have the private key to decrypt your files. If did not backup your keys, but have permitted other users to EFS share your encrypted files, those users can recover your data. You should back up your certificate and keys to an external storage media (floppy, USB mini drive) and have it locked away.

To export your EFS keys and certificate from your computer for backup purposes, do the following:

• Launch Microsoft Internet Explorer web browser.
• From the Tools menu, click Internet Options.
• On the Content tab, in the Certificates section, click Certificates.
• Click the Personal tab.
• Notice there may be several certificates present, depending on whether you have installed these certificates for other purposes.
  
  
  
  
• Select one certificate at a time until the Certificate Intended Purposes field shows Encrypting File System (Red highlighted area on image). This is the certificate that was generated when you encrypted your first file/folder.
• Choose the Export button to start the Certificate Export Wizard, and click Next.
• Choose Yes to export the private key, and then click Next.
• Choose Enable Strong protection, and then click Next.
• You will be prompted to type a password to protect your private key. Do not forget this password. Reconfirm the password and click Next.
• Specify where you want to save the certificate and key into a single file with a .pfx extension. You can specify this path to a storage media such as a floppy disk or USB mini drive and click Next.
• Choose Finish and you are done. Move the media to safe storage.