Using CFS, the Cryptographic Filesystem

If you want to keep private your personal
files, such as those containing phone numbers, correspondence or
journals, you could keep them in a hidden directory named
~/.private with mode 0700, so only you could read the files. Are
you chuckling yet? Then let's consider employing a stronger privacy
technique: cryptography. Specifically, let's look at Matt Blaze's
open-source Cryptographic Filesystem (CFS) for UNIX and
Linux.Briefly, CFS allows you to safeguard your files in encrypted
form in a normal directory. By using a key (or password, if you
will), you temporarily decrypt your files to clear-text form for
the window of time in which you need to work with them.CFS makes your clear-text files available to you via a local
loopback NFS mount; the CFS documentation refers to this as an
"attach". Modifications you make to your clear-text files then are
reflected automatically in the encrypted versions. You end your CFS
session with a "detach", which makes your clear-text files
disappear until the next time that you attach them.This article reports some of the benefits and methods of
using CFS as of version 1.4.0beta2. Some handy tools for use with
CFS also accompany this article; see the Resources section.CFS vs. Other ToolsOther ways to improve your privacy with open source tools are
available; there's TCFS, the Transparent Cryptographic Filesystem,
and OpenSSL, among other tools. Here's a brief summary of the
relative merits of some of them, including TCFS, CFS and
OpenSSL:

• CFS: runs in user space, and no kernel patches are
  required. CFS uses an ordinary NFS loopback (a local NFS export
  with a local mount) that may create some security worries. Use
  caution in exporting directories. CFS was developed on SunOS and
  BSDI, then ported to Linux and other OSes, which bodes well for its
  ongoing utility. CFS supports several choices of encryption
  algorithms.
• TCFS: requires a Linux-specific NFS module or
  kernel configuration. The tighter kernel bindings and extended
  filesystem attribute requirements yield better security but,
  potentially, less portability.
• OpenSSL: runs in user space, and no kernel patches
  are required. OpenSSL supports a wide variety of encryption
  methods, as well as support for hardware tokens. OpenSSL is
  available for Linux, MS Windows and other environments. OpenSSL
  handles encryption or decryption of only one stream or file at a
  time, as of version 3.4.
• OpenSSH: apples and oranges. You might use OpenSSH
  in conjunction with the other tools, but OpenSSH is mainly for
  interactive session privacy, not stored data privacy.
• Linux loop device mount: comes with Red Hat Linux.
  At this time, DES appears to be the only serious encryption method
  available for loop device mounts. It requires preparation of a
  fixed-size container file and either root privileges or user
  permissions on loop device files. See mount(8) and
  losetup(8).

Installing CFSA source RPM, cfs-1.4.0.beta2j-6.2a.src.rpm, is available
with the other tools accompanying this article on the
LJ FTP site; see the Resources section. The
beta2j version of the RPM includes, in addition to the components
of the base beta2: one more security patch for Linux; two Red Hat
Linux-friendly setup scripts, cfs.init and cfs-setup; and two handy
tools, decrypt and dpw.py. All of these are broken out separately
for those of you disinclined to use RPMs. This RPM was tested on
Red Hat Linux 6.2, 7.1 and 7.2.Always consider searching for later versions of CFS in either
RPM or tarball form, and check for security patches. CFS version
1.4.1 exists as of this writing (see the Resources section); it
adds support for NetBSD but no new features or bug fixes.NFS is a prerequisite for using CFS. Be very selective with
whom you share your filesystem resources--don't export your root
directory and everything below it to the whole world. Consider
using a personal firewall to forbid external access to most service
ports, especially the ports the NFS and RPC port mapper
dæmons use, 2049 and 111 (TCP and UDP), respectively.In the following examples of commands, prompts are shown in
bold type. The # shell prompt indicates root privileges; $ is the
prompt for ordinary (non-root) users of bash and Bourne shells.
Make any appropriate adjustments for your choice of shell.Install the CFS source RPM package with the usual RPM command
as root:# rpm -iv
cfs-1.4.0.beta2j-6.2a.src.rpmAfterward, build and install the CFS package as follows,
again as root:

        # cd /usr/src/redhat/SPECS
        # rpm -bb cfs.spec
        # cd ../RPMS/i386
        # rpm -ivv cfs-1.4.0.beta2j.i386.rpm

If you have difficulties installing this particular RPM, by
all means seek out and install a more suitable RPM or tarball of
the CFS distribution. Adapt the value-added files accompanying this
article (on the FTP site) to your own needs and tastes. In
particular, note that some NFS set up is required. See the
cfs-setup script accompanying this article or read Matt Blaze's
document "CFS Installation and Operation" (see Resources).Getting Started with CFSThe following instructions are suitable for use with Red Hat
Linux 6.2, 7.1, 7.2, and 7.3; you may need to make some adjustments
for your variant of Linux.Make certain that NFS is running:# ps auxww | grep
rpc.mountdIf rpc.mountd isn't running, then crank up NFS:# /etc/rc.d/init.d/nfs
startThen start the CFS dæmon, cfsd, by running its
boot-time startup file as root:# /etc/rc.d/init.d/cfsd
startAs yourself, create a private notes directory and attach it.
We'll demonstrate two ways of doing this:

1. The easy way uses the decrypt tool that accompanies
   this article (see the Resources section):

        $ decrypt  -init
        Key: (type your key here to create the private directory, and
                remember the key)
        Again: (type your key again here)
        Key: (retype your key to proceed with the attachment)

        $ mkdir ~/cdata
        $ cd ~/cdata
        $ cmkdir notes
        Key: (type your key here and remember it for future use)
        Again: (type your key again here)
        $ cattach notes $LOGNAME-notes
        Key: (re-type your key)

(In the example above, the predefined environment variable
$LOGNAME contains your login name. It's used in order to avoid name
collisions, but feel free to substitute a simpler clear-text
directory name.)In both cases it may take a minute or two before the CFS
dæmon (cfsd) makes available the clear-text directory,
$LOGNAME-notes.Next, create a test file in the attached clear-text
directory, as follows:

        $ pushd /mnt/crypt/$LOGNAME-notes
        $ echo "Test." > test.txt
        $ popd

End your CFS session, and see what transpired in the relevant
directories:

        $ cdetach $LOGNAME-notes
        $ ls /mnt/crypt
        $ ls -R ~/cdata

The listing of ~/cdata should show an obscured name for your
test.txt file, such as 03fa2aa5242d5a741866a6605de1ae3b.Re-attach the directory in order to verify that your test
file is still there. Again, there are two ways to do that:

1. Here's the easy way, using
   decrypt:

        $ decrypt
        Key: (retype your key)

        $ cd ~/cdata
        $ cattach notes $LOGNAME-notes
        Key: (retype your key)

Next, verify that your test file is still there:$ cat
/mnt/crypt/$LOGNAME-notes/test.txtNow go in search of the documentation for CFS, which includes
on-line man pages for the commands cmkdir, cattach, cdetach,
cpasswd and others. The underpinnings of CFS are described well in
Matt Blaze's papers, "CFS Installation and Operation" and "A
Cryptographic Filesystem for Unix". Read these using nroff
-ms /usr/doc/cfs*/notes.ms. Among other useful tidbits in
these papers is a suggestion for speeding up CFS performance by
modifying the NFS rsize and wsize mount options.After that, look for the README file that accompanies this
article (see the Resources section), and check out the decrypt and
dpw.py tools. The decrypt script simplifies the management of your
private directories with CFS. Try this command:$ decrypt
-helpThe dpw.py tool provides a graphical user interface for
searching a private file of passwords you maintain with decrypt.
Run dpw.py and click the help button. The dpw.py tool requires the
standard Python module Tkinter, among others.Strengths and Vulnerabilities of CFSCFS's strengths include certain kinds of error reduction or
error prevention:

• After working with your clear-text files, CFS
  doesn't require a separate re-encryption step, thus avoiding the
  problem of re-encrypting with the wrong key.
• Revision control, at least with RCS, is less
  error-prone. In contrast, where files are individually decrypted
  with OpenSSL, accidentally checking-in the clear-text file would
  leave it exposed.
• CFS supports an inactivity timeout so the
  clear-text file isn't accidentally left available for long periods
  of time. Be sure to use the timeout option (-i) with the cattach
  CFS command.

Vulnerabilities to consider when using CFS and some other
privacy tools:

• Keyboard snooping can expose your secret key when
  you type the key for encryption or decryption.
• Privileged users (intruders or not) can snoop out
  your attached clear-text files through various means.
• Your clear-text files may be exposed on the network
  in various ways. OpenSSH can help to some extent, but it's best to
  confine your use of CFS and OpenSSL to your unshared local host's
  directly connected console and keyboard and to confine your private
  data and clear-text attach points to your local host's
  filesystems.
• Consider keeping some private files in separate
  private directories; that way, not everything can be compromised
  simultaneously.
• When applying revision control tools to your
  private files, think carefully about how to keep your clear-text
  files exposed only temporarily, for example, in the face of CVS's
  directory copying approach. With CFS, consider using RCS in the
  clear-text attach directory, as in cd
  /mnt/crypt/mycleartext && co -l myjournal.

Matt Blaze's CFS documents more thoroughly examine CFS's
security issues and design considerations.ConclusionWe shouldn't delude ourselves that CFS alone is going to
protect us if we attract the interest of tenacious snoopers or if
we're careless with our network security. We should use CFS for the
same reason that we lock our doors and secure our windows at home.
It's not necessarily going to prevent the worst, but offering some
obstacles may help to keep things safer, longer.A mobile laptop computer running Linux likely would be a
fabulous place to employ CFS. A laptop, being largely
self-contained and unshared, offers fewer vulnerabilities when
other practical security precautions are employed, such as erecting
a personal firewall and disabling unnecessary network services. And
in the event that your laptop is stolen, your CFS-encrypted private
files most likely will remain unseen. Don't forget to back
up.On my wish list of desired improvements to the open-source
version of CFS would be hardware security token support, perhaps
borrowed from OpenSSL. Requiring a hardware security token
ameliorates the problem of password exposure from keyboard
sniffing, although not necessarily keystroke capture over time.
Also desirable would be a port of CFS to MS Windows for use with
multiboot hosts.AcknowledgmentsMy thanks to Greg
Hao and Claude
Johnson for test-driving the CFS installation instructions
and accompanying scripts on Red Hat Linux 7.x. Thanks also to
Enrico
Scholz for the base SRPM.ResourcesThe
files accompanying this article, including the
cfs-1.4.0.beta2j CFS source RPM, decrypt, dpw.py, cfs-setup and
README files.The
base SRPM from which the cfs-1.4.0.beta2j source RPM was
created for this article (see previous item).Blaze, Matt. "CFS Installation and Operation". 1997. This
paper is available in the CFS documentation directory,
nroff -ms /usr/doc/cfs*/notes.ms. Or look for
notes.ms among the C source files, e.g. in
/usr/src/redhat/BUILD/cfs*/.Blaze, Matt. "A Cryptographic Filesystem for Unix". 1993.
This paper also is available in notes.ms, along with the preceding
paper.Mauriello, Ermelindo. "TCFS: Transparent Cryptographic
Filesystem". 1997. Available on
Linux Journal's web
site.Matt Blaze's own
CFS software distribution page. The latest release as of
this writing is cfs-1.4.1, while the version described in this
article is cfs-1.4.0beta2. Version 1.4.1 adds support for NetBSD
but offers no new features or bug fixes. For Linux you may continue
to use cfs-1.4.0beta2j.www.openssl.org:
home of OpenSSLwww.openssh.com:
home of OpenSSHJerry Sweet is a system
administrator and programmer in the USA.

email: cfs-tools@akamail.com