To: linux-kernel@vger.kernel.org
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Subject: Encrypted Filesystem
Message-ID: <OFA97B290B.67DE842E-ON87256E27.0061728C-86256E27.0061BB0E@us.ibm.com>
From: Michael A Halcrow <mahalcro@us.ibm.com>
Date: Mon, 26 Jan 2004 11:46:29 -0600
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I have some time this year to work on an encrypted filesystem for
Linux.  I have surveyed various LUG's, tested and reviewed code for
currently existing implementations, and have started modifying some
of them.  I would like to settle on a single approach on which to
focus my efforts, and I am interested in getting feedback from the
LKML community as to which approach is the most feasible.

This is the feature wish-list that I have compiled, based on personal
experience and feedback I have received from other individuals and
groups:

 - Seamless application to the root filesystem
  - Layered over the entire root filesystem
  - Unencrypted pass-through mode with minimal overhead
  - Files are marked as  ``encrypted'' or ``unencrypted'' and treated
    accordingly by the encryption layer
 - Key->file association
  - As opposed to key->blkdev or key->directory granularity
  - No encryption metafiles in directories, instead placing that
    information into Extended Attributes
  - May break some backup utilities that are not EA-aware; may require
    another mode where encryption metadata is stored in a header block
    on the encrypted file
  - Directories can be flagged as ``encrypted-only'', where any new
    files created in that directory are, by default, encrypted, with
    the key and permissions defined in the directory's metadata
  - Processes may have encryption contexts, whereby any new files they
    create are encrypted by default according to the process'
    authentication
  - Make as much metadata about the file as confidential as possible
    (filesize, executable bit, etc.)
 - Pluggable encryption (I wouldn't mind using a block cipher in CTR
   mode)
 - Authentication via PAM
  - pam_usb
  - Bluetooth
  - Kerberos
  - PAM checks for group membership before allowing access to certain
    encrypted files
 - Rootplug-based LSM to provide key management (necessary to use
   LSM?)
 - Secret splitting and/or (m,n)-threshold support on the keys
 - Signatures on files flagged for auditing in order to detect
   attempts to circumvent the encryption layer (via direct
   modifications to the files themselves in the underlying filesystem)
 - Ad-hoc groups for access to decrypted versions of files
  - i.e., launch web browser, drop group membership by default (like
    capability inheritance masks) so that the browser does not have
    access to decrypted files by default; PAM module checks for group
    membership before allowing access (explicit user authorization on
    application access requests)
 - Userland utilities to support encrypted file management
 - Extensions to nautilus and konqueror to be able to use these
   utilities from a common interface (think: right-click, encrypted)
 - Distro installation integration
 - Transparent shredding, where the underlying filesystem supports it
 - Versioning and snapshots (CVS-ish behavior)
 - Design to work w/ SE Linux

These are features that have been requested, but are not necessarily
hard requirements for the encrypted filesystem.  They are just
suggestions that I have received, and I am not convinced that they are
all feasible.

There are several potential approaches to an encrypted filesystem with
these features, all with varying degrees of modification to the kernel
itself, each with its own set of advantages and disadvantages.

Options that I am aware of include:

 - NFS-based (CFS, TCFS)
  - CFS is mature
  - Performance issues
  - Violates UNIX semantics w/ hole behavior
  - Single-threaded

 - Userland filesystem-based (EncFS+FUSE, CryptoFS+LUFS)
  - Newer solutions, not as well accepted or tested as CFS
  - KDE team is using SSHFS+FUSE

 - Loopback (cryptoloop) encrypted block device
  - Mature; in the kernel
  - Block device granularity; breaks most incremental backup
    applications

 - LSM-based
  - Is this even possible?  Are the hooks that we need there?

 - Modifications to VFS (stackable filesystem, like NCryptfs)
  - Very low overhead claimed by Erez Zadok
  - Full implementation not released
  - Key->directory granularity
  - Evicts cleartext pages from the cache on process death
  - Uses dcache to store attaches
  - Other niceties, but it's not released...

My goal is to develop an encrypted filesystem ``for the desktop'',
where a user can right-click on a file in konqueror or nautilus and
check the ``encrypted'' box, and all subsequent accesses by any
processes to that file will require authentication in order for the
file to be decrypted.  I have already made some modifications to CFS
to support this functionality, but I am not sure at this moment
whether or not CFS is the best route to go for this.

I have had requests to write a kernel module that, when loaded,
transparently starts acting as the encryption layer on top of whatever
root filesystem is mounted.  For example, an ext3 partition may have
encrypted files strewn about, which are accessible only after loading
the module (and authenticating, etc.).

Any advise or direction that the kernel community could provide would
be very much appreciated.

Thanks,
Mike
.___________________________________________________________________.
                         Michael A. Halcrow 
       Security Software Engineer, IBM Linux Technology Center 
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