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               Cryptographic File System under Linux HOW-TO
                            LINUX SECURITY FAQ
                              March 14, 1996
     Copyright (C) 1996 Alexander O. Yuriev ([email protected])
                             CIS Laboratories
                             TEMPLE UNIVERSITY
                                    USA

       This document describes how to compile, install and setup CFS
       that was written by Matt Blaze of AT&T, under Linux.  The following
       copyright statement copied directly from CFS 1.12 describes
       the restrictions on the CFS usage:

* The author of this software is Matt Blaze.
*              Copyright (c) 1992, 1993, 1994 by AT&T.
* Permission to use, copy, and modify this software without fee
* is hereby granted, provided that this entire notice is included in
* all copies of any software which is or includes a copy or
* modification of this software and in all copies of the supporting
* documentation for such software.
*
* This software is subject to United States export controls.  You may
* not export it, in whole or in part, or cause or allow such export,
* through act or omission, without prior authorization from the United
* States government and written permission from AT&T.  In particular,
* you may not make any part of this software available for general or
* unrestricted distribution to others, nor may you disclose this software
* to persons other than citizens and permanent residents of the United
* States and Canada.
*
* THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED
* WARRANTY.  IN PARTICULAR, NEITHER THE AUTHORS NOR AT&T MAKE ANY
* REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE MERCHANTABILITY
* OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR PURPOSE.

       Although the information in this document is believed to be
       correct, neither the Author nor CIS Laboratories, nor Temple University
       provides any kind of WARRANTIES and is not/are not responsible for
       what happens if you follow these guidelines. The information in this
       document is provided AS IS!

ABOUT CFS

       CFS provides application-independent encryption/decryption of the
       filesystem layer that does not require modification of the
       underlying filesystem code nor any kind of modification of the
       kernel source.  The symmetric cipher implemented in the mainstream
       version of CFS is based on the modified DES cipher running in CBC
       mode making the brute-force attack against the usual 56-bit DES
       key-space unrealistic.  The structure of CFS makes replacement of
       the mainstream DES cipher with a Fast-DES or any other symmetric
       cipher an extremely straightforward process.  Please refer to the
       "White" paper about CFS for more information
       (ftp://bach.cis.temple.edu/pub/Papers/cfs.ps)

COMPILING AND INSTALLING CFS

       CFS does not compile "out of the box" under Linux. Follow these
       instructions to get CFS running or your Linux system. There are
       several methods to make CFS work under Linux, the cleanest one of
       which is based on the modifications performed by Olaf Kirch. His
       version of CFS is available from:

       ftp://ftp.mathematik.th-darmstadt.de/pub/linux/okir/cfs-1.1.2.tar.gz

       Olaf signed the modified archive. The PGP signature for the modified
       version of the cfs-1.1.2 can be obtained from

       ftp://ftp.mathematik.th-darmstadt.de/pub/linux/okir/cfs-1.1.2.pgp

       In single-user mode, compile CFS by using the "make" command.

       After compilation is completed, install "cfsd", "cdetach", "ccat",
       "cmkdir", "cname" and "cattach" to the /usr/local/sbin directory
       with the ownership "root:wheel" and the access mode "551".
       Generate a list of MD5 hashes of the clean binaries. Now copy these
       files together with the "md5sum" to a media such as an image of a CD
       or a floppy and make the media write protected.

       Create the directory /.cfsfs which will be used as a hook for the
       CFS server.  Make that directory owned by root:root and protected
       with access mode "000".  Create the directory /securefs which will
       become a root of the CFS tree.

       Add the following lines into your /etc/rc.d/rc.local:

       echo -n "Initializing secure filesystem: "
       if [ -x /usr/local/sbin/cfsd ]; then
               /usr/local/sbin/cfsd > /dev/null
               echo -n "cfsd "
               /bin/mount -o port=3049,intr localhost:/.cfsfs /securefs
               echo -n "loopback "
               echo "done"
       else
          echo "Cryptographic Filesystem is not installed"
       fi

       Users of the Caldera Network Desktop and Red Hat Commercial Linux
       distributions should add the file "cfsfs" that is attached at the end
       of this document to their /etc/rc.d/init.d directory. Then symlink
       the file "S65cfsfs" to it in the appropriate run-level directories
       using the command:

               ln -s ../init.d/cfsfs S65cfsfs

       in /etc/rc.d/rcX.d, where X is a run-level number, add the line:

               /.cfsfs         localhost

       to /etc/exports. Finally, add the line:

               portmap: 127.0.0.1

       to the /etc/hosts.allow file.

       You should now restart your computer.  When it comes back into a
       multiuser mode, issue a mount command to verify that CFS is running.
       If everything was successful, you should see a new line in a list of
       filesystems:

localhost:/.cfsfs on /securefs type nfs (rw,port=3049,intr,addr=127.0.0.1)


CREATING CFS DIRECTORY

       To create a CFS protected directory called "secret" use the command

               cmkdir secret

       You will be requested to supply and verify the passphrase.  If you
       succeed, a new directory named "secret" will appear in the current
       directory. This directory will contain encrypted information which
       will be accessible only in the encrypted form unless it is attached
       to the CFS tree.

       In order to add the "secret" directory to a list of directories
       managed by CFS, it has to be attached to the CFS tree using the
       command:

               cattach secret Big-Secret

       CFS will request you to type the access passphrase. If it matches
       the passphrase supplied to the "cmkdir" command that created the
       directory originally, then the information in the secret directory
       will be accessible in a non-encrypted form under /securefs/Big-Secret
       to the user who supplied the correct passphrase.  Please note that
       usually it takes about a minute to attach a protected directory to
       the CFS tree. When the user is finished manipulating the information
       they should issue the command:

               cdetach Big-Secret

       to destroy the access key. This command removes the directory
       "secret" from the list of directories managed by CFS making it
       impossible to access cleartext information in that directory until
       it is again attached using the "cattach" command.

PROTECTION OF CFS

       In order to grant a user access to encrypted parts of the directory
       tree, CFS requires the user to supply a passphrase that is used to
       generate a set of access keys. A compromise of a passphrase allows
       an intruder to access the encrypted information through the Unix
       security model. Therefore it is extremely important to protect
       access passphrases. There are two basic ways that can be used by
       intruders to gain access to your passphrase. They are (1) Sniffer
       attacks (2) Attack against the protocol.  The following simple
       guidelines can be used to minimize the possibility of a successful
       attack against CFS:

               1. Make sure that the CFS binaries are not compromised in
                  any form.

                       * Ensure that "cattach", "ccat", "cmkdir", "cname",
                         the CFS server "cfsd" and finally, "cdattach"
                         are not replaced with Trojan versions that record
                         access passphrases or, in a case of "cfsd",
                         access keys.

                       * Ensure that the CFS server is not compromised in a
                         way that it does not perform the encryption
                         procedure correctly.

                       * An attack against "cdeattach" usually involves a
                         small modification that prevents correct
                         destruction of access keys allowing an intruder
                         to gain access to a supposedly detached part of
                         the directory tree.

                  The simplest way to verify that binaries are not
                  compromised is to statically link them and place them on
                  a CD.  Another way is to again statically link the
                  binaries, use "md5sum" message-digest calculator and
                  write their MD5 hashes onto a write-protected media.
                  Prior to using any CFS programs on a system, mount
                  the floppy disk and compare MD5 hashes of binaries on the
                  system with the hashes of the clean statically linked
                  copies located on the floppy disk, replacing the
                  compromised versions.

               2. Keyboard grabbers used to grab passphrases as they are
                  being typed rely on the fact that most users are careless
                  enough to ignore the following simple guidelines:

                       1. When typing a passphrase in an xterm, make sure
                          that the xterm program is not compromised and use
                          the "Secure Keyboard" option while typing the
                          passphrase. This prevents keystrokes from being
                          intercepted by X grabbers.

                       2. Type passphrases from a terminal attached directly
                          to a serial port of the system when such terminal
                          is available.

                       3. Make sure that your pty and ttys permissions
                          disallow others from reading your keystrokes
                          directly from the device node.

               3. Never type your passphrase across the network, even if
                  the network is located behind a firewall and you trust
                  everybody who is connected to your network not to use
                  sniffers. This also applies to networks that use
                  scrambling routers, because there is absolutely no
                  guarantee that routers use a strong encryption or do not
                  have a back door or a loophole that potentially can allow
                  an intruder to defeat encryption used by a router.  If
                  you have to type your password across the network, do it
                  only if you are using an encrypted tunnel between systems
                  such as the one created by the deslogin(8) protocol.

               4. Always de-attach CFS protected trees from the  filesystem when
                  not using them, even when you are leaving your system for
                  "only" a couple of minutes.

KNOWN PROBLEMS WITH CFS

       At this moment there is only one problem that can be reproduced.
       "Permission denied" error is generated when a user attempts to
       access the files located on a compact disc.


CREDITS

       The following people helped in the preporation process of this
       document: Topher Hughes of the Dickinson College, Elie Rosenblum of
       the Montgomery Blair High School, Mario D. Santana of the Florida
       State University, Daniel P Zepeda and Olaf Kirch.


====================[cfsfs]======================
#!/bin/sh
#
# $Header: /Secure/secure-doc/linux/CFS/RCS/CFS-Doc,v 1.4 1996/03/15 04:49:37 alex Exp alex $
#
# cfsfs         Crypto filesystem
#
# Author:       Alexander O. Yuriev <[email protected]>
#               Derived from cron

# Source function library.
/etc/rc.d/init.d/functions

# See how we were called.
case "$1" in
 start)
       echo -n "Starting Crypto Filesystem: "
       if [ -x /usr/local/sbin/cfsd ]; then
               /usr/local/sbin/cfsd > /dev/null
               /bin/mount -o port=3049,intr localhost:/.cfsfs /securefs
               echo "done"
       else
               echo -n "Crypto Filesystem is not installed"
       fi
       touch /var/lock/subsys/cfsfs
       ;;
 stop)
       echo -n "Stopping Crypto filesystem: "
       umount /securefs
       killproc cfsd
       echo
       rm -f /var/lock/subsys/cfsfs
       ;;
 *)
       echo "Usage: cfsfs {start|stop}"
       exit 1
esac

exit 0
====================[end of cfsfs]======================

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