  Loopback Encrypted Filesystem HOWTO
  Copyright by Ryan T. Rhea, zzrhear@pobox.winthrop.edu
  v1.0, 22 September 1999

  This document explains how to setup and then use a filesystem that,
  when mounted by a user, dynamically and transparently encrypts its
  contents.  The filesystem is stored in a regular file, which can be
  hidden or named non-conspicuously such that it would most likely be
  overlooked.  This allows for a high level of secure storage of data.
  ______________________________________________________________________

  Table of Contents

  1. Before you begin

  2. Introduction

  3. Summary

  4. Details

  ______________________________________________________________________

  1.  Before you begin

  This process requires the kernel source code, knowledge of compiling
  this code, and a lot of patience.  I highly recommend having a boot
  disk ready.  Also, be sure to have a backup before you permanently
  store your important data on the encrypted filesystem - it can be
  curroupted like any other filesystem.

  As a minimum, you will have to patch to at least version 2.2.9 of the
  linux kernel before continuing.  There are further instructions on
  applying patches in the ``'' section later in this document.

  Kernel source can be found at:

       <ftp://ftp.kerneli.org/>

  There is a HOWTO on the process of recompiling kernels at:

       <http://metalab.unc.edu/LDP/HOWTO/>

  This document may be reproduced and distributed in whole or in part,
  without fee, subject to the following conditions:

    The copyright notice and this permission notice must be preserved
     complete on all complete or partial copies.

    Any translation or derived work must be approved by the author in
     writing before distribution.

    If you distribute this work in part, instructions for obtaining he
     complete version of this manual must be included, and a means for
     obtaining a complete version provided.

    All source code in this document is placed under the GNU General
     Public License, available via anonymous FTP from:
       <ftp://prep.ai.mit.edu/pub/gnu/COPYING/>

  2.  Introduction

  The process uses the device '/dev/loop*' (where * can be 0-7 on most
  installations) to mount a loopback filesystem.  The same process can
  be used without encryption to store a linux filesystem on a non-linux
  partition.  There is a HOWTO on this at the LDP site mentioned
  previously.

  Different types of encryption can be used, including XOR, DES,
  twofish, blowfish, cast128, serpent, MARS, RC6, DFC, and IDEA.  The
  program 'losetup' (loopback setup) is what associates your encrypted
  file with a filesystem and it's cipher type.  According to Alexander
  Kjeldaas, who maintains kerneli.org and the international crypto
  patches, DES and losetup are currently incompatible.  This is due to
  differences in the way the two handle parity bits.  There are no plans
  to support DES as it is much more insecure than the other ciphers.

  Twofish, blowfish, cast128, and serpent are all licensed free for any
  use.  The others may or may not have licensing restrictions.  Several
  of them are candidates for the AES standard.  The finalists will
  provide royalty free use of their ciphers worldwide.

  This document uses the serpent algorithm because it is strong yet
  remarkably fast, and it's freely distributable under the GPL.
  According to it's documentation, serpent uses a 128-bit block cipher
  designed by Ross Anderson, Eli Biham and Lars Knudsen.  It provides
  users with the highest practical level of assurance that no shortcut
  attacks will be found.  The documentation on serpent as well as the
  source code can be found at:

       <http://www.cl.cam.ac.uk/~rja14/serpent.html>

  3.  Summary

  There are many steps involved in the process.  I will provide ``'' for
  these steps in the next section. I thought it would be nice to provide
  a summary first to provide reference (if you are experienced with
  unix/linux you probably don't need the details anyway).  Here they are
  summarized as follows:

  1. Download the newest international crypto patch (I used

       <http://ftp.kerneli.org/pub/kerneli/>

  2. Patch the kernel

  3. Run 'config' (or 'menuconfig' or 'xconfig') to configure your
     scattered.  First of all, before you will see any other options you
     must enable 'Prompt for development and/or incomplete code/drivers'
     under 'Code Maturity level options'.  Under 'Crypto options' enable
     'crypto ciphers' and whatever you want.  Remember that DES is known
     to be incompatible as of 2.2.10.4 - it may never be supported at
     all.  There are several important options to select under 'Block
     Devices'.  These include 'Loopback device support', 'Use relative
     block numbers as basis for transfer functions (RECOMMENDED)', and
     'General encryption support'.  DO NOT select 'cast 128' or options
     under the various network categories.  I will not go  any further
     into configuration of the kernel, it is out of the scope of this
     document and can be found at the LDP site.

  4. Compile the new kernel.

  5. Edit '/etc/lilo.conf' to add the new kernel image.  Run 'lilo -v'
     to add the kernel to the boot loader.

  6. Download the source for the newest 'util-linux' (I used

       <ftp://ftp.kernel.org/pub/linux/utils/util-linux/>

  7. Extract the 'util-linux' source.

  8. Apply the corresponding patch found in your

  9. CAREFULLY read the 'INSTALL' file!  This package contains the
     sources for many system dependent files (important tools such as
     file before compiling these sources have a boot disk and/or shotgun
     ready because your system will be quite confused.  Basically you
     want to set almost all of the 'HAVE_*' fields equal to yes so that
     the important authentication tools are not compiled and written
     over.  The tools you do want rebuilt are 'mount' and 'losetup' to
     accommodate the new encryption schemes.  I suggest that you refer
     to the ``'' section below for this step.

  10.
     Compile and install the 'util-linux' source

  11.
     Reboot the machine with the new kernel.

  12.
     Edit '/etc/fstab', adding an entry for your mount point as follows:

  ______________________________________________________________________
  /dev/loop0  /mnt/crypt  ext2  user,noauto,rw,loop 0 0
  ______________________________________________________________________

  13.
     Create the directory that will hold your filesystem, as in

  14.
     As the user, create your encrypted file as follows:

       dd if=/dev/zero of=/etc/cryptfile bs=1k count=1000

  15.
     Run losetup as follows:

       losetup -e serpent /dev/loop0 /etc/cryptfile

  You only have one chance to enter the password, be careful.  If you
  want to double-check your password, you can use the command:

       losetup -d /dev/loop0

  This will deactivate your loop device.  Next you will run losetup
  again to test your password, as follows:

       losetup -e serpent /dev/loop0 /etc/cryptfile

  16.
     Make your ext2 filesystem as follows:

       mkfs -t ext2 /dev/loop0 100000

  17.
     Now you can mount the encrypted filesystem with:

       mount -t ext2 /dev/loop0 /mnt/crypt

  18.
     When your done, you want to unmount and protect your filesystem as
     follows:

       umount /dev/loop0
       losetup -d /dev/loop0'

  4.  Details

  Kernel Patches:

  You can upgrade from '2.2.x' releases by patching.  Each patch that is
  released for '2.2.x' contains bugfixes.  New features will be added to
  the Linux '2.3.x' development kernel.  To install by patching, get all
  the newer patch files and do the following:

       cd /usr/src
       gzip -cd patchXX.gz  patch -p0

  Repeat xx for all versions bigger than the version of your current
  source tree, IN ORDER.

  The default directory for the kernel source is '/usr/src/linux'.  If
  your source is installed somewhere else, I would suggest using a
  symbolic link from '/usr/src/linux'.

  Editing 'MCONFIG' for the 'util-linux' package compilation:

  The following are excerpts from the 'MCONFIG' file I used to compile
  the 'util-linux' package.  Note that this is fairly specific for my
  setup, which is loosely based on RedHat 5.2.  The point is to make
  sure you don't overwrite any important system tools such as 'login',
  'getty', or 'passwd'.  Anyway, here are the important lines as
  follows:

       ______________________________________________________________________
       CPU=$(shell uname -m sed s/I.86/intel/)

       LOCALEDIR=/usr/share/locale

       HAVE_PAM=no

       HAVE_SHADOW=yes

       HAVE_PASSWD=yes

       REQUIRE_PASSWORD=yes

       ONLY_LISTED_SHELLS=yes

       HAVE_SYSVINIT=yes

       HAVE_SYSVINIT_UTILS=yes

       HAVE_GETTY=yes

       USE_TTY_GROUP=yes

       HAVE_RESET=yes

       HAVE_SLN=yes

       CC=gcc
       ______________________________________________________________________

  Suggestions:

  Note that you could use any of the eight loopback devices, from
  'dev/loop0' to '/dev/loop7'.  Use an inconspicuous directory for the
  mount point.  I would suggest creating a folder with 700 permissions
  inside your home folder.  The same goes for the file that holds the
  data.  I use a filename like 'sysfile' or 'config.data' inside the
  '/etc' folder.  This will usually get overlooked.

  I created very simple Perl scripts to mount and unmount the filesystem
  with one command.  Write these, make them executable (chmod u+x), and
  store them somewhere in your path.

  ______________________________________________________________________
  #!/usr/bin/perl -w
  #
  #simple utility to setup loopback encryption filesystem
  #Copyright 1999 by Ryan T. Rhea
  $cmd1 = `losetup -e serpent /dev/loop0 /etc/cryptfile`;
  $cmd2 = `mount /mnt/crypt`;
  print $cmd1;
  print $cmd2;
  ______________________________________________________________________

  Name the above script 'loop', and then you can be on your way with one
  command ('loop') and a password.

       ______________________________________________________________________
       #!/usr/bin/perl -w
       #
       #simple utility to deactivate loopback encryption filesystem
       #Copyright 1999 by Ryan T. Rhea
       $cmd1 = `umount /mount/crypt`;
       $cmd2 = `losetup -d /dev/loop0`;
       print $cmd1;
       print $cmd2;
       ______________________________________________________________________

  Name the second one 'unloop', and then typing 'unloop' will quickly
  deactivate your filesystem.

