Loopback Encrypted Filesystem HOWTO
Copyright by Ryan T. Rhea, rhear@cs.winthrop.edu
v1.1, 29 November 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.
Archived Document Notice: This document has been archived by the LDP
because it does not apply to modern Linux systems. It is no longer
being actively maintained.
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Table of Contents
1. Before you begin
2. Introduction
3. Summary
4. Details
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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
corrupted 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/>
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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>
Also, this document assumes that the ciphers are compiled directly
into the kernel. You may install them as modules, but the technique
is not discussed in this document. You will have to edit the file
process is discussed in detail in the kernel compilation HOWTO
referenced previously.
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 are
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 serpent, but try 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 device support', 'Use
relative block numbers as basis for transfer functions
(RECOMMENDED)', and 'General encryption support'. DO NOT select
the crypto 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 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
MCONFIG 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:
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/dev/loop0 /mnt/crypt ext2 user,noauto,rw,loop 0 0
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13.
Create the directory that will hold your filesystem, as in
14.
As the user, create your encrypted file as follows:
dd if=/dev/urandom of=/etc/cryptfile bs=1M count=10
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
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:
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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
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Suggestions:
Note that you could use any of the eight loopback devices, from 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.
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#!/usr/bin/perl -w
#
#minimal utility to setup loopback encryption filesystem
#Copyright 1999 by Ryan T. Rhea
`losetup -e serpent /dev/loop0 /etc/cryptfile`;
`mount /mnt/crypt`;
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Name the above script 'loop', and then you can be on your way with one
command ('loop') and a password.
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#!/usr/bin/perl -w
#
#minimal utility to deactivate loopback encryption filesystem
#Copyright 1999 by Ryan T. Rhea
`umount /mount/crypt`;
`losetup -d /dev/loop0`;
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Name the second one 'unloop', and then typing 'unloop' will quickly
deactivate your filesystem.
