Encrypting Remote Files with EFS

[Zack]

We are in the midst of deploying EFS to protect specific folders on laptop
hard drives. We want EFS used only for that purpose—locally; as such, we
do not want users to have the ability to encrypt files that are residing
on file servers. According to my understanding of EFS, which seems to be
confirmed by the quote below from Windows help, users shouldn’t be able to
do so unless we specifically enable file server(s) to be trusted for
delegation in AD.

“In a domain environment, remote encryption is not enabled by default. To
enable encryption for a specific computer, your network administrator can
make that computer trusted for delegation. For more information, consult
your network administrator.â€

However, some of our servers are allowing files to be encrypted and
decrypted remotely—and these servers are *not* marked as trusted for
delegation in AD. Further, the user that encrypted the file can scoot
over to another PC, log in as themselves, and access the file—and we have
no CA infrastructure in place; these are locally-generated EFS
certificates that do not chain back past the local client machine. The
certificate thumbprints in the personal store for the user account on the
two PCs do not match, yet they can access the file just the same, while
other user accounts cannot.

I’m thoroughly confused by this behavior, and would appreciate any experts
chiming in and cluing me in as to why 1) some servers are allowing remote
encryption, while others are not, and 2) why locally-generated EFS certs
are behaving this way.

Our environment:

-Windows 2000 native-mode domain

-All DCs are Win2k, file servers are a 2k/2003 mix

-Clients are 2000/XP; the OS of the client/server doesn’t seem to matter—
some 2k3 servers allow remote encryption, some don’t, and some 2000
servers allow, while others don’t.

Thanks,

-Zack-

 

[Star Fleet Admiral Q]

The user encrypting on one, logging on to another and being able to decrypt
sounds about right, as these are "domain" profiles, so as they move from
machine to machine, the same domain profile would be loaded to the
PC/laptop - I'll assume these are roaming profiles? Maybe not?

As far as the cert footprint, which one are you comparing, the "public" or
"private" cert - the "public" one should be different, as the are different
machines, but the "private" cert should be identical.

For the rest, some of the other experts will need assist, as why some do and
some don't, I agree with you, if none are trusted, then it shouldn't allow
none.

--

Star Fleet Admiral Q @ your Service!

http://www.google.com
Google is your "Friend"

 

[Sam Jost]

Hi Zack,

Ah well, I played around with EFS a bit and got some small input for
you:

if you encrypt a file on your local client hdd windows will create a
local zertificate (A) you need to access this file.

if the same domain user encrypts a file on some other local comps hdd
it will create a new local zertifacte (B) there, too, to encrypt the
file.

if you encrypt a file on a domain server hdd, the server will create a
domain zertifacte (C) to encrypt the file.

if you now attach the hdd of the first local to the second comp, you
will not be able to decrypt the file (unless you export the (A) first
and import it to the second comp) since the local zertificate (B) will
be used.

but if you try to access the file on the server hdd windows will use
the server based zertificate (C) and can decrypt it just fine.


I dont like this behaviour as well. I in my case do not want the server
to store data and zertificates in one place, I'd rather like the zert
to reside on the client only, so when someone steals my server they got
no chance of accessing the data on the hdd. Well, but thats just me...

cu,
Sam

 

[Guest]

Zack, were the servers at one time trusted for delegation? If so, you will
need to reboot the machines to clear EFS cache. EFS will cache up to 100
users' certificates (for performance). Users who had previously encrypted on
the servers would still be able to do so until the cache is cleared.

If the servers were never trusted for delegation, please give a few more
details. Exactly which client OSes (XPsp1, XPsp2, W2Ksp4, etc.) are able to
access which non-TFD servers (W2Ksp4, WS2003, WS2003sp1, etc.).

Thanks.
Pat

 

[Guest]

Just to clarify, only the servers would need to be rebooted.

Thanks.
Pat