Secret Agent Plan
A reader's encrypted files are safe and recoverable if he turned on the Data Recovery Agent.
- By Bill Boswell
I have a laptop which has broken down—doesn't
want to switch on any longer! Now my problem is this:
I had several files encrypted on this laptop—very important data
to say the least! I used to back up my data onto a CD-RW using Windows
XP's Backup Tool and I always left the data encrypted while backing up.
Unfortunately I never backed up the private key and encryption certificate.
When I tried to restore my data onto another PC at work, the restore
was successful, but I couldn't open the encrypted files. I have no access
at all to my laptop, as it won't switch on. Is there any way that I could
open the encrypted files?
Anthony: You might be in the clear. If the laptop was a
member of an Active Directory domain, the encrypted files can be recovered.
First, some background: When you encrypt a file using the Encrypting
File System, the EFS driver talks to the Microsoft Base Cryptographic
Provider to get a random number from the Random Number Generator (RNG).
This random number becomes the cipher key that EFS uses to encrypt the
Windows XP SP1 uses the Advanced Encryption Standard (AES) Rjindahl algorithm
to encrypt the file. So does Windows Server 2003. Windows 2000 uses DESX
or Triple-DES, where DES stands for the old Data Encryption Standard (now
defunct). This may sound like geek trivia, but it could become important
To protect the portability of the files, EFS encrypts the cipher key
used to encrypt the file and stores the key along with the file. To do
this encryption, EFS uses a public key issued to the user by the Base
Crypto Provider on the local machine. The private key, as you discovered,
resides in the local profile of the user who encrypts the file.
Help from Bill
Got a Windows or Exchange question or need troubleshooting
help? Or maybe you want a better explanation than provided
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the best questions get answered in this column.
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Here's where things get interesting when it comes to solving this problem.
EFS also encrypts a second copy of the cipher key using the public key
issued to the domain's Administrator account. This account is called the
Data Recovery Agent, or DRA.
The DRA private key resides in the Administrator profile of the first
domain controller in the domain. (There's a wrinkle to this that I'll
get to in a minute.) So, knowing that you need access to the private key
corresponding to the public key used to encryption the cipher, here's
what you do.
- Take the backup file (bkf) and restore it at the first domain controller
in the domain.
- Log on using the Administrator for the domain. Don't use an account
with Administrator privileges. It must be the actual account called
- Open one of the encrypted files. This should succeed because the
Administrator account's private key will decrypt the cipher key for
Okay, that sounds pretty simple. Here's some reasons why it might not
work. When I said that the domain Administrator account was the DRA, that's
only correct in a brand new installation of Active Directory or if you
promoted a Windows NT 4.0 PDC then logged on as Administrator.
But, if you promoted an NT4 PDC then logged on using any other administrative
account, then that account becomes the DRA. So, after the PDC upgrade,
if you logged on using your Anthony account, then you became the DRA for
the entire domain. The public key corresponding to the private key for
your account on the newly upgraded PDC is used to encrypt cipher keys
on every member computer.
So, if logging on as Administrator doesn't get access to the files, and
this server is an upgraded PDC, go through the list of profiles under
Documents and Settings and see if you can figure out which of the accounts
was the first administrator to log onto the machine following the upgrade.
This account will have a set of hidden cryptographic files in the profile.
You can also determine the name of the DRA account used by EFS when it
encrypted the files via the Efsinfo utility in the Windows Server 2003
support tools. You can run that version of Efsinfo on Windows 2000. Open
a command prompt and go to the folder where the recovered encrypted files
reside. Run efsinfo /r to list the recovery
If you're able to open the files but you only see gibberish inside, then
you have a different sort of problem. A Windows 2000 domain controller
uses DESX or Triple DES for file encryption, so you won't be able to decrypt
files encrypted on Windows XP SP1, which uses AES for file encryption.
In this case, you'll need to transport a copy of the EFS private key to
an XP SP1 desktop or a Windows Server 2003 server then recover the backup
To transport the key, while logged on as the DRA at the first domain
controller in the domain, launch the Certmgr.msc console from %windir%\System32
and drill down to the Personal certificates. Right-click the File Recovery
certificate and select Export from the menu. This opens a Certificate
Export wizard. Just follow the wizard to save the private key to a transportable
file. Give the file a strong password.
Then put a copy of the file on a Windows XP SP1 desktop and log on as
the DRA and double-click the file. This launches the Certificate Import
wizard. Walk through the wizard to put the certificate in the default
At that point, you should be able to open the encrypted files.
Whew! Hopefully one of those possibilities worked for you and you're
now viewing the encrypted files. You can clear the Encryption flag then
put the files on a different laptop and encrypt them again and don't forget
to get a backup of your local profile.
But... There's another possibility and it's not a pretty one.
Unfortunately, Windows XP does not require a DRA to encrypt a file. (Windows
2000 Professional does require a DRA.) So, if the laptop was not
able to locate the public key of the DRA in Active Directory, it would
have encrypted the files without any DRA. Here's how you'll know if this
If you run Efsinfo /r and it says that it
can't find a recovery agent, then that's doom. The only possibility that
might save your files is if you ever used a roaming profile for the account
you used to log onto the laptop. If so, a copy of the private EFS key
resides in that roaming profile. Configure your account to use the roaming
profile again and log on using your domain account and see if you can
access the files.
If none of that works, then at least you'll have peace of mind knowing
that bad guys can't open the files, either. Hope this helps.