Need advice on file encryption

Sin_Lam_Tan-2 · June 21, 2006, 2:39am

Currently I am working on encryption filter driver. When a plain file is
modified (receive app write IRP), I would like to encrypt the whole file
(not just the current buffer in the write IRP), when is the correct way to
do this?

I tried to encrypt the plain file during close/cleanup (after detecting it
has been modified), i.e. delayed encryption, this approach sometimes gets
sharing violation when trying to open file for encryption at that moment.
The scenario of plain file modification using a test program is as below:
a/ Open a plain file
b/ Modified it.
c/ Close it.
d/ Open it again
e/ Read the content

I tried to encrypt the content using a system thread between c/ and d/. If I
get sharing violation status during driver file open, I will try again in a
short while (0.5s). This approach does not work properly for my own test
program as the content is not encrypted when it reach e/. For normal file
operation in windows explorer works fine. So this delay encryption is a bad
approach.

Is it possible to do whole file encryption during the first app write IRP
(not paging IO)? Can I use the File Object in app write IRP?

Thanks for any advice!

Regards,
SL

OSR_Community_User · June 21, 2006, 3:31am

Please, pretty please, before you start doing asking about encryption,
read the older archives. File encryption is in top ten of the topics here,
I am sure you will find tons of threads and tons of informations.

L.

OSR_Community_User · June 21, 2006, 7:48am

The simple answer to your question is in fact covered in the archives
(as Ladislav suggested) because there are plenty of ways to “get around
the sharing violation” issue. In the scenario you’ve described,
however, you shouldn’t be getting a sharing violation since the file
isn’t in use.

However, I suspect there’s a whole class of issues that you aren’t
considering in your analysis:

How are you going to handle failures while you are encrypting this
file? If you do “on the fly conversion” you risk leaving the file in an
intermediate corrupt state unless you have an explicit plan for dealing
with this.
How do you want this to be visible to the user? If you do “convert
the entire file on the first write” that means the first user write is
going to take a LONG time if the file is of any size at all (e.g.,
imagine this is a 1GB file - hardly unusual these days.)
How do you handle multiple concurrent access? You’re looking at ONE
application having the file opened. What will you do when two
applications open the file in succession (assuming sharing allows this?)

In our own work, we found this issue (what we refer to as “policy
change” because we’re moving from one encryption model to a second
encryption model) to be an interesting challenge on all these fronts.
We use sparse streams to perform asynchronous conversions of this type
(deciding to fetch the data from the correct stream based upon whether
it is present in one or the other.) This gives us a model for failure
recovery (we can resume conversion after failure without any data loss
or corruption.) Of course, only some file systems support streams, which
is why we chose to build a layered file system for other FSDs.

I hope this helps. Best of luck!

Regards,

Tony

Tony Mason
Consulting Partner
OSR Open Systems Resources, Inc.
http://www.osr.com

-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com] On Behalf Of Tan Sin Lam
Sent: Wednesday, June 21, 2006 2:39 AM
To: ntfsd redirect
Subject: [ntfsd] Need advice on file encryption

Currently I am working on encryption filter driver. When a plain file is

modified (receive app write IRP), I would like to encrypt the whole file

(not just the current buffer in the write IRP), when is the correct way
to
do this?

I tried to encrypt the plain file during close/cleanup (after detecting
it
has been modified), i.e. delayed encryption, this approach sometimes
gets
sharing violation when trying to open file for encryption at that
moment.
The scenario of plain file modification using a test program is as
below:
a/ Open a plain file
b/ Modified it.
c/ Close it.
d/ Open it again
e/ Read the content

I tried to encrypt the content using a system thread between c/ and d/.
If I
get sharing violation status during driver file open, I will try again
in a
short while (0.5s). This approach does not work properly for my own test

program as the content is not encrypted when it reach e/. For normal
file
operation in windows explorer works fine. So this delay encryption is a
bad
approach.

Is it possible to do whole file encryption during the first app write
IRP
(not paging IO)? Can I use the File Object in app write IRP?

Thanks for any advice!

Regards,
SL

Questions? First check the IFS FAQ at
https://www.osronline.com/article.cfm?id=17

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OSR_Community_User · June 21, 2006, 7:55am

> Please, pretty please, before you start doing asking about encryption,

read the older archives. File encryption is in top ten of the topics
here, I am sure you will find tons of threads and tons of informations.
The problem is often enough not that a poster does not search, instead the search phrases used may have been inappropriate. Thus it would be helpful if you suggest those phrases along with the suggestion to search ...

Just my 2 cents,

Oliver

May the source be with you, stranger

ICQ: #281645
URL: http://assarbad.net

OSR_Community_User · June 21, 2006, 8:15am

> The problem is often enough not that a poster does not search,

instead the search phrases used may have been inappropriate.

Good point. So, just search for “encryption” and “filter driver”,
it should be enough

L.

Gary_Little-2 · June 21, 2006, 10:01am

Need I say that most if not all, of this can be more easily handled by the
drive hardware. If he has not considered it the OP should consider an HDD
solution that utilizes encryption logic on the drive; e.g. Seagate has the
Momentus FDE, and Hitachi has an FDE drive as well.

Gary G. Little
-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com] On Behalf Of Tony Mason
Sent: Wednesday, June 21, 2006 6:47 AM
To: Windows File Systems Devs Interest List
Subject: RE: [ntfsd] Need advice on file encryption

The simple answer to your question is in fact covered in the archives
(as Ladislav suggested) because there are plenty of ways to “get around
the sharing violation” issue. In the scenario you’ve described,
however, you shouldn’t be getting a sharing violation since the file
isn’t in use.

However, I suspect there’s a whole class of issues that you aren’t
considering in your analysis:

How are you going to handle failures while you are encrypting this
file? If you do “on the fly conversion” you risk leaving the file in an
intermediate corrupt state unless you have an explicit plan for dealing
with this.
How do you want this to be visible to the user? If you do “convert
the entire file on the first write” that means the first user write is
going to take a LONG time if the file is of any size at all (e.g.,
imagine this is a 1GB file - hardly unusual these days.)
How do you handle multiple concurrent access? You’re looking at ONE
application having the file opened. What will you do when two
applications open the file in succession (assuming sharing allows this?)