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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<article id="Disk-Encryption-HOWTO">
<articleinfo>
<title>Disk Encryption HOWTO</title>
<author>
<firstname>David</firstname>
<surname>Braun</surname>
<affiliation>
<address><email>cruhmoz602 [at] sneakemale [period] com (rewrite &#39;male&#39; as &#39;mail&#39;)</email></address>
</affiliation>
</author>
<pubdate>2004-11-17</pubdate>
<revhistory id="revhistory">
<revision>
<revnumber>1.5</revnumber>
<date>2004-11-17</date>
<authorinitials>DB</authorinitials>
<revremark>added warning about dm-crypt</revremark>
</revision>
<revision>
<revnumber>1.4</revnumber>
<date>2004-08-17</date>
<authorinitials>DB</authorinitials>
<revremark>bug fixes, hints toward plausible deniability and dm-crypt</revremark>
</revision>
<revision>
<revnumber>1.3</revnumber>
<date>2003-12-18</date>
<authorinitials>DB</authorinitials>
<revremark>added link to Diceware</revremark>
</revision>
<revision>
<revnumber>1.2</revnumber>
<date>2003-10-09</date>
<authorinitials>DB</authorinitials>
<revremark>added idle logout and Gentoo instructions</revremark>
</revision>
<revision>
<revnumber>1.1</revnumber>
<date>2003-09-13</date>
<authorinitials>DB</authorinitials>
<revremark>added step to zero out keychain</revremark>
</revision>
<revision>
<revnumber>1.0</revnumber>
<date>2003-08-28</date>
<authorinitials>tmm</authorinitials>
<revremark>Initial release, reviewed by LDP</revremark>
</revision>
<revision>
<revnumber>0.92</revnumber>
<date>2003-08-27</date>
<authorinitials>DB</authorinitials>
<revremark>first submission to LDP</revremark>
</revision>
</revhistory>
<abstract>
<para>A method is described for encrypting a hard disk, either in whole
or in part, with the encryption key stored on an external medium for
increased security.</para>
</abstract>
<copyright>
<year>2004</year>
<holder>David Braun</holder>
</copyright>
</articleinfo>
<sect1 id="Introduction">
<title>Introduction</title>
<para>I&#39;ve got a
laptop computer running Linux and I don&#39;t want to
worry about someone reading the personal information it contains, in case
it gets lost or stolen. My log on password may stop someone from booting
it, but it won&#39;t prevent an attacker from removing the hard disk and
reading its data. I need stronger protection.</para>
<para>Fortunately, it&#39;s relatively easy to use encryption so the hard
disk data would be unreadable if it were to fall into the wrong hands.
Encryption&#39;s not only useful for portable computers like laptops&mdash;it
can be used to protect any computer with personal information. I protect
my computer&#39;s files with encryption for the same reason I lock my
filing cabinet at home. For further motivation, you may be interested in
reading Michael Crawford&#39;s <ulink
url="http://www.goingware.com/encryption/">Why You Should Use Encryption</ulink>.</para>
<para>I could encrypt only certain files, such as those in my home
directory. This would protect the files but then I&#39;d have to worry
about information leaking out of them to other, unencrypted places on the
disk. Instead I encrypt the whole disk so I don&#39;t have to manage this
problem.</para>
<para>There are many encryption algorithms to choose from. I chose <ulink
url="http://csrc.nist.gov/CryptoToolkit/aes/">AES</ulink> because it has
been approved by the US government&#39;s <ulink url="http://www.nist.gov/">National
Institute of Standards and Technology</ulink> and is well regarded by the
cryptography community. I want my use of it to be resistant to dictionary
attacks, so I use a long, randomly generated key. There&#39;s no way
I&#39;m going to memorize such a key so I keep it in a form I can carry
with me easily: on a USB flash drive on my keychain. I encrypt the key
with a passphrase so my data is protected in two ways: by a) what I have
(the USB flash drive) and b) what I know (the passphrase). I can even give
a friend access to my computer without giving away my passphrase&mdash;she uses
her own USB flash drive and her own passphrase.</para>
<para>The operating system keeps the data encrypted on the disk at all
times and decrypts it in RAM only as it&#39;s used. This way if the
computer loses power suddenly the data will remain protected. The
decryption key is loaded into RAM at boot time and kept there while the
computer is on, so I don&#39;t need to keep the USB flash drive plugged in
after starting the computer.</para>
<para>The procedure outlined in this HOWTO is written for version 2.4 of
the Linux kernel. It will become less complicated with the release of
Linux 2.6, which will have built-in support for encryption and do a better
job of managing partitions within loopback devices.</para>
<para>This document assumes the reader has a moderate level of experience
with Linux (you should be comfortable <ulink
url="http://www.tldp.org/HOWTO/Kernel-HOWTO/index.html">patching and
compiling kernels</ulink> as well as <ulink
url="http://www.tldp.org/HOWTO/Multi-Disk-HOWTO.html">partitioning,
mounting, and unmounting disks</ulink>).</para>
<sect2 id="TechnicalSummary">
<title>Technical Summary</title>
<para>The encryption is implemented through a special kind of
<glossterm>loopback device</glossterm>. A loopback device doesn&#39;t
store any data itself; instead it takes all the data storage and
retrieval requests it receives and passes them along to a real storage
device, such as a disk or a file. As the data passes through, it can be
filtered, and in our case the filter used is encryption.</para>
<para>When the system is deployed, a removable medium (USB flash drive)
boots using GRUB, a kernel, and an initrd. Both the key and the kernel
are selected from the GRUB menu, allowing a single removable medium to
be used with multiple computers. The initrd contains just enough tools
to ask for a passphrase, set up an encrypted loopback device, and mount
it. After mounting, <literal>pivot_root</literal> is used to resume the
boot process from the encrypted device. Loopback device offsets are
used, instead of partitions, to access separate swap and root file
system spaces within the encrypted loopback device because the 2.4
kernel doesn&#39;t provide access to partitions within loopback devices.
The offset method does not generalize to multiple partitions
(unfortunately) because the maximum offset understood by
<literal>losetup</literal> is 2GB.</para>
</sect2>
<sect2 id="CopyrightAndLicense">
<title>Copyright and License</title>
<para>Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
copy of the license is included in <xref linkend="gfdl" />.</para>
<para><trademark class="registered">Linux is a registered trademark of
Linus Torvalds.</trademark></para>
</sect2>
<sect2 id="Disclaimer">
<title>Disclaimer</title>
<para>No liability for the contents of this document can be accepted.
Use the concepts, examples and information at your own risk. There may
be errors and inaccuracies that could be damaging to your system and you
may lose important data. Proceed with caution, and although this is
highly unlikely, the author does not take any responsibility.</para>
<para>All copyrights are held by their by their respective owners,
unless specifically noted otherwise. Use of a term in this document
should not be regarded as affecting the validity of any trademark or
service mark. Naming of particular products or brands should not be seen
as endorsements.</para>
<para>I know you hate reading directions and want to skip to the meaty
bit right away, but I advise you to read the whole document first before
touching anything. I know all the HOWTOs say that, but I really mean it
for this one. It&#39;s worth it; trust me. You may also want to run
through the procedure first on a test system before tackling a
production system.</para>
</sect2>
<sect2 id="Acknowledgments">
<title>Acknowledgments</title>
<para>Thanks to Linus Torvalds, Jari Ruusu, and all the developers who
contributed to their software, without which this HOWTO would have been
impossible.</para>
<para>Thanks to the <ulink url="http://www.nist.gov/">National Institute
of Standards and Technology</ulink> for carefully selecting a strong,
open encryption algorithm.</para>
<para>Thanks to Mark Garboden and others on the <ulink
url="http://mail.nl.linux.org/linux-crypto/">linux-crypto</ulink>
mailing list and <ulink
url="http://www.tldp.org/mailinfo.html#maillists">The Linux
Documentation Project mailing lists</ulink> who took the time to
critique my writing and offer suggestions.</para>
<para>Thanks to alert readers Ladislao Bastetti and Norris Pouhovitch for
struggling through unusual hardware configurations, finding mistakes in
the HOWTO, and suggesting good ideas.</para>
</sect2>
<sect2 id="Feedback">
<title>Feedback</title>
<para>Feedback is solicited for this document. Please send additions,
comments, and criticisms to the author.</para>
</sect2>
<sect2 id="Approaches">
<title>Approaches</title>
<para>There are three different approaches we can take to encrypt the
disk: encrypt the whole thing, a single partition, or a single file. I
strongly recommend the first approach for best security. The first two
approaches assume you&#39;ll be booting from removable media, such as a
USB flash drive or a business card size CD-ROM. If you don&#39;t want to
do this, you may modify the method to boot from the disk instead by
making a small, unencrypted boot partition. If you want to use a USB
flash drive to boot your computer, be sure your motherboard can do it
first. At the time of this writing many cannot.</para>
<para>To avoid having to enumerate all three approaches everywhere
I&#39;m going to refer to what you&#39;re protecting as the
<glossterm>asset</glossterm>. I will refer to the removable medium used
to store the key as the <glossterm>keychain</glossterm>. I call it the
keychain instead of the key because we can store lots of keys, each for
different computers, on the same medium.</para>
<sect3>
<title>Whole Disk</title>
<para>A problem with keeping data secret with encryption is that the
data likes to move around. Imagine the encryption is like a fence
around your data. While the data&#39;s inside the fence, it&#39;s
safe. To be most useful, however, data likes to be transmitted on
networks, put on removable disks like CD-ROMs, and shared with
friends. Any time your data leaves the fenced area it&#39;s
unprotected. We can&#39;t put an encryption fence around all possible
locations where our data might play but we do want to make the fence
as large as practical. By putting the encryption fence around your
whole hard disk, you won&#39;t have to worry about data becoming
unprotected if it jumps to another part of the disk.</para>
<warning>
<para>In this approach, we create one swap
space and one root file system. Some people want more than a single
encrypted partition for the root file system. Unfortunately, the
method detailed here relies on the offset parameter of
<literal>losetup</literal> to create &#34;subpartitions&#34; within
the asset. The offset parameter is limited to a maximum value of 2GB,
limiting the size of all but the last partition to 2GB. This works
nicely for swap, which is already limited to 2GB on the i386
architecture, but I&#39;m guessing it won&#39;t be practical for other
uses. Using it to create multiple partitions smaller than 2GB is left
as an exercise for the reader.</para>
<para>Another way to handle multiple partitions is to encrypt each partition separately (using the same key) to avoid the technical limitation above. This isn't secure as encrypting the whole disk because the partition table is exposed. When an attacker attempts to break encryption, the first thing he does is try to figure out what it's encrypting. A partition table listing Linux partitions is a big hint. For this reason I discourage encrypting multiple partitions separately, but arguably it's a good compromise for getting around the current <literal>losetup</literal> limitation. Another option is simply to wait for the release of Linux 2.6 because it is expected to make the offset parameter unnecessary.</para>
</warning>
</sect3>
<sect3>
<title id="Partition">Partition (for multiboot systems)</title>
<para>Encrypting the whole disk is fine if Linux is the only operating
system on it, but this won&#39;t work for people who have set up their
computer to boot multiple operating systems, e.g., <ulink
url="http://www.kernel.org">Linux</ulink>, <ulink
url="http://www.netbsd.org/">NetBSD</ulink>, and <ulink
url="http://developer.apple.com/darwin/">Darwin</ulink>. In this case
we can encrypt just the Linux partition and leave the others alone.
Since we&#39;re booting from a removable medium, we won&#39;t even
need to include the Linux partition in the multiboot menu with the
others. To see why this isn&#39;t as secure as encrypting the whole
disk, read <xref linkend="Attack_Tree" />.</para>
</sect3>
<sect3>
<title>File (for home directories)</title>
<para>You may want to encrypt only a file on a file system. Once
you&#39;ve encrypted it you can put into it whatever you want,
including other file systems. You might want to use this approach to
encrypt only your home directory, for example. This is the least
secure of the three approaches and not recommended. If you choose this
approach you will notice instructions below to skip whole sections.
This is because I&#39;m assuming you&#39;ve already booted an
operating system and have your swap issues handled, so those sections
don&#39;t apply to you. This HOWTO may be overkill for your needs and
you can probably get away with just reading the fine
<literal><ulink url="http://loop-aes.sourceforge.net/loop-AES.README">README</ulink></literal>
that comes with <ulink url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink>.
If you do, be sure to read <xref linkend="ThreatModel" /> before you
finish here.</para>
</sect3>
</sect2>
<sect2 id="ThreatModel">
<title>Threat Model</title>
<para>In order to protect our asset well, we must first understand what
we&#39;re protecting it against. The general idea is that you&#39;ve got
a laptop which is vulnerable to being stolen or lost, and have a USB
flash drive on your keychain that isn&#39;t, so this system is designed
to handle the case that your laptop is stolen. I&#39;m guessing your
keychain won&#39;t be as easily stolen because it&#39;s in your pocket,
and because an attacker won&#39;t know that it&#39;s important. If you
pull your USB flash drive out of your pocket and someone non-technical
exclaims, &#34;What&#39;s that?&#34;, tell them it&#39;s a <ulink
url="http://www.pez.com/">Pez dispenser</ulink>.</para>
<note>
<para>This system falls short when it comes to <glossterm>plausible
deniability</glossterm>, which means there&#39;s no way to hide the
fact that your personal data is encrypted. This is like locking your
jewels in a safe and keeping the safe in plain sight in the middle of
your living room. Only you can open the safe, but a man with a gun can
tell you to open the safe for him. So if you&#39;re worried about your
<ulink url="http://zdnet.com.com/2100-11-502433.html?legacy=zdnn">computer
being subpoenaed</ulink> and being told to hand over your laptop,
keychain, and passphrase, you&#39;d better look at other solutions
such as <ulink url="http://stegfs.sourceforge.net/">StegFS</ulink>.</para>
<para>The following solution to the deniability problem has been suggested by Norris Pouhovitch.
It should be possible to install a minimal Windows partition at the front
of the disk and to encrypt the remainder. When the computer
is turned on without the keychain, it boots Windows normally. When the
keychain is booted, it skips the Windows partition, decrypts the remainder of
the disk, and boots Linux.</para>
<para>The advantage of this scheme is that if the laptop is stolen and turned
on, it will look like what a casual attacker is expecting to see (a Windows
computer). On the other hand, a serious attacker could notice the unusually
small partition and become suspicious. I will flesh out this idea further in
a future version of the HOWTO.</para>
</note>
<table id="Attack_Tree">
<title>Attack Tree</title>
<tgroup cols="6">
<colspec colname="A1" />
<colspec colname="A2" />
<colspec colname="A3" />
<colspec colname="A4" />
<colspec align="center" />
<thead>
<row>
<entry align="center" nameend="A4" namest="A1">Attack</entry>
<entry>Reaction</entry>
<entry align="center">Notes</entry>
</row>
</thead>
<tbody>
<row valign="middle">
<entry morerows="3">attacker steals laptop</entry>
<entry nameend="A4" namest="A2">while it is on</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
<entry>The asset is unprotected while the computer is running because the encryption key is in RAM. You can lower the risk by using an idle logout (<xref linkend="Idle_Logout"/>), but if you think your laptop is about to be stolen, turn off the power immediately and quickly read the <ulink url="http://www.aikidofaq.com/">Aikido HOWTO</ulink>.</entry>
</row>
<row valign="middle">
<entry morerows="2">while it is off</entry>
<entry nameend="A4" namest="A3">attacker doesn&#39;t steal
keychain</entry>
<entry><glossterm linkend="new_key">new key</glossterm></entry>
</row>
<row valign="middle">
<entry morerows="1">attacker steals keychain</entry>
<entry>attacker knows your passphrase</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
</row>
<row valign="middle">
<entry>attacker doesn&#39;t know your passphrase</entry>
<entry><glossterm linkend="new_key">new key</glossterm></entry>
</row>
<row valign="middle">
<entry morerows="2">attacker steals keychain but doesn&#39;t
have laptop</entry>
<entry nameend="A4" namest="A2">attacker knows passphrase</entry>
<entry><glossterm linkend="new_key">new key</glossterm></entry>
<entry>Your asset is at risk because the attacker can decrypt
it.</entry>
</row>
<row valign="middle">
<entry morerows="1">attacker doesn&#39;t know passphrase</entry>
<entry nameend="A4" namest="A3">you&#39;re feeling lazy or
you&#39;re convinced the keychain was lost, not stolen</entry>
<entry><glossterm linkend="new_passphrase">new passphrase</glossterm></entry>
<entry>You&#39;re probably OK without changing the asset key
because the attacker can&#39;t decrypt the asset without the
passphrase.</entry>
</row>
<row valign="middle">
<entry nameend="A4" namest="A3">you&#39;re feeling paranoid</entry>
<entry><glossterm linkend="new_key">new key</glossterm></entry>
</row>
<row valign="middle">
<entry nameend="A4" namest="A1">attacker convinces you to send
data over network</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
</row>
<row valign="middle">
<entry nameend="A4" namest="A1">attacker convinces you to copy
data to removable medium</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
</row>
<row valign="middle">
<entry nameend="A4" namest="A1">you encrypt only a partition and
a process writes data to a different partition</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
</row>
<row valign="middle">
<entry nameend="A4" namest="A1">you encrypt only a file and a
process copies data from RAM to the unencrypted swap, or to a
file in <literal>/tmp</literal>, or elsewhere on the unencrypted disk</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
</row>
<row valign="middle">
<entry nameend="A4" namest="A1">attacker demands you hand over
laptop, keychain, and passphrase while waving a rubber hose
menacingly</entry>
<entry><glossterm linkend="SOL">SOL</glossterm></entry>
<entry>There is no plausible deniability built into the system.</entry>
</row>
</tbody>
</tgroup>
</table>
<glosslist id="reactions" xreflabel="Corrective Reactions">
<glossentry id="new_passphrase">
<glossterm>new passphrase</glossterm>
<glossdef>
<para>Restore the keychain backup and choose a new passphrase.</para>
</glossdef>
</glossentry>
<glossentry id="new_key">
<glossterm>new key</glossterm>
<glossdef>
<para>Generate a new random key to re-encrypt the asset, choose a
new passphrase, and restore the asset backup.</para>
</glossdef>
</glossentry>
<glossentry id="SOL">
<glossterm><acronym>SOL</acronym></glossterm>
<glossdef>
<para>Sorry Over your Loss</para>
</glossdef>
</glossentry>
</glosslist>
</sect2>
<sect2 id="Caveats">
<title>Caveats</title>
<para><itemizedlist><listitem><para>This method won&#39;t work (yet)
with <ulink url="http://swsusp.sourceforge.net/">Software Suspend for
Linux</ulink>.</para></listitem><listitem><para>Encrypting the disk will
undoubtedly slow it down. I don&#39;t know by how much. If anyone has
done some benchmarks, please send them to me.</para></listitem><listitem><para>There
is nothing in this method to support <glossterm>plausible deniability</glossterm>
(see <xref linkend="ThreatModel" />).</para></listitem><listitem><para>It
won&#39;t prevent information leaks via networks and removable disks.</para></listitem><listitem><para>Encrypting
backups is beyond the scope of this HOWTO.</para></listitem></itemizedlist></para>
</sect2>
<sect2 id="Requirements">
<title>Requirements</title>
<itemizedlist>
<listitem>
<para>a computer with an easily accessible removable medium reader
(such as a USB port or a CD-ROM drive)</para>
</listitem>
<listitem>
<para>a motherboard which supports booting from removable media
(check carefully for USB, not all do)</para>
</listitem>
<listitem>
<para>removable medium (such as a <glossterm>USB flash drive</glossterm>)
to be used as the <glossterm>keychain</glossterm></para>
</listitem>
<listitem>
<para><ulink url="http://www.kernel.org/">Linux</ulink> 2.4</para>
</listitem>
<listitem>
<para><ulink url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink></para>
</listitem>
</itemizedlist>
<sect3>
<title>A Digression about USB Flash Drives</title>
<para>There are many choices on the market. When I bought mine, I
found one which fit the following requirements:</para>
<itemizedlist>
<listitem>
<para>physically small (I carry it on my physical keychain)</para>
</listitem>
<listitem>
<para>supports USB 2.0 at full speed</para>
</listitem>
<listitem>
<para>has a write-protect switch, so I don&#39;t clobber my
encryption keys by accident</para>
</listitem>
</itemizedlist>
<para>You might be tempted to get one with a fingerprint reader. I
strongly encourage you not to. It might initially seem like a good
idea, because by adding the biometric, your security protection
expands to:</para>
<itemizedlist>
<listitem>
<para>something you have (the USB flash drive)</para>
</listitem>
<listitem>
<para>something you know (the passphrase)</para>
</listitem>
<listitem>
<para>something you are (your fingerprint, or whatever)</para>
</listitem>
</itemizedlist>
<para>However, suppose something goes wrong. If you are now asking
yourself, &#34;What could go wrong?&#34;, then why are you reading
this HOWTO? If something goes wrong, you make a change (see <xref
linkend="reactions" />):</para>
<itemizedlist>
<listitem>
<para>Change what you have by using a different USB flash drive.</para>
</listitem>
<listitem>
<para>Change what you know by learning a new passphrase.</para>
</listitem>
<listitem>
<para><emphasis>You can&#39;t change what you are.</emphasis></para>
</listitem>
</itemizedlist>
<para>Stop and ponder that last line for a while.</para>
</sect3>
</sect2>
<sect2 id="LookingToTheFuture">
<title>Looking to the Future</title>
<para>I wrote this document while using the 2.4 kernel. Linux 2.6 introduces the <ulink url="http://sources.redhat.com/dm/">Device-mapper</ulink> which we will be able to use to avoid playing games with
losetup offsets. Linux 2.6 also introduces <ulink url="http://www.saout.de/misc/dm-crypt/">dm-crypt</ulink>, an
encryption layer for the Device-mapper which looks quite elegant. Unfortunately, <ulink url="http://mareichelt.de/pub/texts.cryptoloop.php">it's not safe!</ulink> Hopefully someday
it will be fixed, but in the mean time the best course is to stick with loop-AES.</para>
<para>A future version of this HOWTO will explain how to use the Device-mapper with Linux 2.6.</para>
</sect2>
</sect1>
<sect1 id="Procedure">
<title>Procedure</title>
<para>This method is designed to erase the contents of the asset before
encrypting it. If you already have data on the disk you intend to encrypt,
you should copy it somewhere else temporarily and then move it back once
the encryption is set up. It is possible to encrypt data in place, but for
now I consider such magic too advanced for this HOWTO. See <ulink
url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink>&#39;s
<ulink url="http://loop-aes.sourceforge.net/loop-AES.README">README</ulink>
for more details if you&#39;re interested in that method.</para>
<para>To do the following operations you will need to be running a system
which has a <ulink url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink>
capable kernel. If you don&#39;t have one already, I recommend using
<ulink url="http://www.knoppix.com/">KNOPPIX</ulink>. It boots off a
CD-ROM and doesn&#39;t need to be installed, so it&#39;s very little
hassle.</para>
<para>For simplicity these instructions assume you&#39;ll be preparing the
keychain and the asset on the same computer, but this needn&#39;t be the
case. Adapt the instructions to whatever&#39;s convenient for you.</para>
<sect2 id="PrepareTheKeychain">
<title>Prepare the Keychain</title>
<para>If you&#39;re taking the approach of encrypting only a file
instead of a disk or a partition, you may skip this section and proceed
directly to <xref linkend="PrepareTheAsset"/>.</para>
<para>In the ideal setup you will use a bootable keychain device, such
as a <glossterm>USB flash drive</glossterm> or a business card size
CD-ROM. This is because we want to expose as little of your disk as
possible, but we&#39;re going to have to expose a minimal boot process
or the computer will never start. Since the boot process will be
necessarily unencrypted, it&#39;s better to have it away from your
computer (on your keychain). If you can&#39;t or don&#39;t want to use a
bootable keychain for some reason, then follow these instructions anyway
but instead apply them to a small boot partition on your disk instead of
the keychain.</para>
<para>In the following example the keychain shows up as the first SCSI
drive <literal>/dev/sda</literal>. Replace <literal>/dev/sda</literal>
with the device for your drive as appropriate.</para>
<para>The first step&mdash;zeroing out the keychain&mdash;is technically
unnecessary, but it will make the keychain backup smaller if you back
it up as an image as I suggest in <xref linkend="TestingAndBackup"/>.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>dd if=/dev/zero of=/dev/sda</userinput></screen>
</blockquote>
<para>Next, partition the keychain as you would any bootable
disk. See the <ulink
url="http://www.tldp.org/HOWTO/Partition/index.html">Linux Partition
HOWTO</ulink> if you need help with partitioning.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>cfdisk /dev/sda</userinput></screen>
</blockquote>
<para>Put a file system on the first partition.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkfs /dev/sda1</userinput></screen>
</blockquote>
<para>Mount the keychain.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkdir /tmp/keychain</userinput>
<prompt>bash# </prompt><userinput>mount /dev/sda1 /tmp/keychain</userinput>
<prompt>bash# </prompt><userinput>cd /tmp/keychain</userinput></screen>
</blockquote>
<sect3>
<title>Build the Kernel</title>
<para>If you use the keychain with multiple computers you may
want to build a different kernel for each one.</para>
<para>You probably need to build a custom kernel for your keychain so
you can ensure two things:</para>
<itemizedlist>
<listitem>
<para>It has been patched correctly with <ulink
url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink>
and encryption support is turned on.</para>
</listitem>
<listitem>
<para>All the device drivers necessary to boot your computer and
make the asset accessible have been compiled in instead of loaded
as modules.</para>
</listitem>
</itemizedlist>
<para>You can load device drivers as modules, since we&#39;re using an
initrd, but I chose to compile them into the kernel in order to keep
the boot disk as simple as possible. Feel free to do differently.</para>
<para>For help building a custom kernel read <ulink
url="http://www.tldp.org/HOWTO/Kernel-HOWTO/index.html">The Linux
Kernel HOWTO</ulink>. Be sure to set <literal>CONFIG_BLK_DEV_RAM</literal>
in the kernel configuration so it can boot using an initrd.</para>
<para>Follow the directions that come with <ulink
url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink> to
build the new loop driver. Also follow the directions to rebuild
the <ulink url="http://www.kernel.org/pub/linux/utils/util-linux/">util-linux</ulink>
tools, some of which we&#39;ll copy to the keychain later. Your distribution may have
already built them for you (e.g., see the <literal>loop-aes-utils</literal> and <literal>loop-aes-source</literal> packages in Debian).</para>
<para>Once you&#39;ve built the kernel, copy it to the keychain.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkdir boot</userinput>
<prompt>bash# </prompt><userinput>cp arch/i386/boot/bzImage boot/vmlinuz-laptop</userinput></screen>
</blockquote>
<para>Install <ulink url="http://www.gnu.org/software/grub/grub.html">GRUB</ulink>
or your favorite boot loader.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>grub-install --root-directory=. /dev/sda</userinput></screen>
</blockquote>
<para>Here is a sample <filename>menu.lst</filename> for GRUB. It has
entries for two computers named <systemitem class="systemname">laptop</systemitem>
and <systemitem class="systemname">desktop</systemitem>.</para>
<important>
<para>It is required to pass the name of the key (I suggest you name
it after the computer) as the first parameter to <xref
linkend="linuxrc" />.</para>
</important>
<example>
<title><filename>/tmp/keychain/boot/grub/menu.lst</filename></title>
<programlisting>title laptop
root (hd0,0)
kernel /boot/vmlinuz-laptop root=/dev/ram0 init=/linuxrc laptop
initrd /boot/initrd
title desktop
root (hd0,0)
kernel /boot/vmlinuz-desktop root=/dev/ram0 init=/linuxrc desktop
initrd /boot/initrd.old</programlisting>
</example>
</sect3>
<sect3>
<title>Make the initrd</title>
<para>We boot the keychain using an initrd so we can remove it after
the boot process starts (who wants a USB flash drive hanging out of
their laptop while trying to look cool in a caf&eacute;?). To gain access to
the asset we create a loopback device attached to the initrd&#39;s
<filename>/dev/loop0</filename>. Putting the device file on the initrd
means the initrd will have to stay mounted while the asset is mounted (not a big deal).</para>
<para>To learn all about making initial RAM disks you&#39;re welcome
to read <ulink
url="http://www.tldp.org/HOWTO/Bootdisk-HOWTO/index.html">The Linux
Bootdisk HOWTO</ulink> and Linux&#39;s <ulink
url="http://linux.bkbits.net:8080/linux-2.4/anno/Documentation/initrd.txt@1.2?nav=index.html%7Csrc/%7Csrc/Documentation"><filename>Documentation/initrd.txt,</filename></ulink>
or don&#39;t bother and just follow along.</para>
<para>We start by choosing 4MB for the size of the initial RAM disk,
all of which we won&#39;t need, but it&#39;s the conventional maximum
size (and it won&#39;t hurt) so that&#39;s one less decision to make.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>head -c 4m /dev/zero &#62; boot/initrd</userinput>
<prompt>bash# </prompt><userinput>mke2fs -F -m0 -b 1024 boot/initrd</userinput></screen>
</blockquote>
<para>Mount the initrd so we can work on it.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkdir /tmp/initrd</userinput>
<prompt>bash# </prompt><userinput>mount -o loop=/dev/loop3 boot/initrd /tmp/initrd</userinput>
<prompt>bash# </prompt><userinput>cd /tmp/initrd</userinput></screen>
</blockquote>
<para>Create the minimal directory structure we&#39;ll need.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkdir -p {bin,dev,lib,mnt/{keys,new-root},usr/sbin,sbin}</userinput></screen>
</blockquote>
<para>Create the minimal set of devices we&#39;ll need. Note that
<filename>tty</filename> is necessary for the password prompt. This
command assumes your asset is the drive <filename>/dev/hda</filename>.
Change it as appropriate.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>cp -a /dev/{console,hda,loop0,loop1,tty} dev</userinput></screen>
</blockquote>
<para>We&#39;ll copy the six programs we&#39;ll need.</para>
<tip>
<para>You can use <literal>which</literal> to find a program&#39;s
full pathname, e.g.:</para>
<screen><prompt>bash# </prompt><userinput>which mount</userinput>
<computeroutput>/bin/mount</computeroutput></screen>
</tip>
<para>Copy the programs:</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>cp /bin/{mount,sh,umount} bin</userinput>
<prompt>bash# </prompt><userinput>cp /sbin/{losetup,pivot_root} sbin</userinput>
<prompt>bash# </prompt><userinput>cp /usr/sbin/chroot usr/sbin</userinput></screen>
</blockquote>
<para>Use <literal>ldd</literal> to find out which shared libraries are
used by each program:</para>
<blockquote><screen><prompt>bash# </prompt><userinput>ldd /bin/{mount,sh,umount} /sbin/{losetup,pivot_root} /usr/sbin/chroot</userinput>
<computeroutput>/bin/mount:
libc.so.6 =&#62; /lib/libc.so.6 (0x40023000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)
/bin/sh:
libncurses.so.5 =&#62; /lib/libncurses.so.5 (0x40020000)
libdl.so.2 =&#62; /lib/libdl.so.2 (0x4005c000)
libc.so.6 =&#62; /lib/libc.so.6 (0x4005f000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)
/bin/umount:
libc.so.6 =&#62; /lib/libc.so.6 (0x40023000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)
/sbin/losetup:
libc.so.6 =&#62; /lib/libc.so.6 (0x40023000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)
/sbin/pivot_root:
libc.so.6 =&#62; /lib/libc.so.6 (0x40023000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)
/usr/sbin/chroot:
libc.so.6 =&#62; /lib/libc.so.6 (0x40023000)
/lib/ld-linux.so.2 =&#62; /lib/ld-linux.so.2 (0x40000000)</computeroutput>
</screen></blockquote>
<para>Copy the libraries. On my system I copied these libraries (yours may be different):</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>cp /lib/{ld-linux.so.2,libc.so.6,libdl.so.2,libncurses.so.5} lib</userinput></screen>
</blockquote>
</sect3>
</sect2>
<sect2 id="PrepareTheAsset">
<title>Prepare the Asset</title>
<para>It&#39;s possible to repeat these steps as many times as you want
to handle multiple computers using the same keychain. Each computer will
have its own key and probably its own kernel. The instructions here
assume the computer&#39;s name is <systemitem class="systemname">laptop</systemitem>;
substitute the name of the computer you&#39;re working with each time
you repeat the steps.</para>
<para>First, back up your data. See the <ulink
url="http://www.tldp.org/HOWTO/Linux-Complete-Backup-and-Recovery-HOWTO/index.html">Linux
Complete Backup and Recovery HOWTO</ulink>.</para>
<para>No, stop, listen to me. Back up your data. Really. It&#39;s no fun
to have an encrypted hard disk if you can&#39;t decrypt it because of
some mistake you made. These tools are powerful magic; if you blow it
you can&#39;t just call up <emphasis>Computer Gurus Are Us</emphasis>
and expect them to get your data back for you. That&#39;s the whole
point of this exercise.</para>
<para>If you are encrypting your whole disk (recommended), replace
<filename>/dev/hda</filename> with the device for your disk.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>ln -s /dev/hda /tmp/asset</userinput></screen>
</blockquote>
<para>If you are encrypting a partition (multiboot case), replace
<filename>/dev/hda3</filename> with the device for your partition.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>ln -s /dev/hda3 /tmp/asset</userinput></screen>
</blockquote>
<para>If you are encrypting a file only, replace <filename>~/encrypted</filename>
with the name of the file and create a link named <filename>/tmp/keychain</filename>
that points to where you decide to store your key file (an already
prepared removable medium, e.g., <filename>/mnt/cf</filename>).</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>ln -s ~/encrypted /tmp/asset</userinput>
<prompt>bash# </prompt><userinput>ln -s /mnt/cf /tmp/keychain</userinput></screen>
</blockquote>
<para>Initialize the asset with random data. This will make it less
obvious to the attacker which parts are free space.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>shred -n 1 -v /tmp/asset</userinput></screen>
</blockquote>
<para>Here we create an encrypted file system to hold the keys. More
encryption, you say? Yes, in case your keychain is stolen (see <xref
linkend="Attack_Tree" />), you don&#39;t want your keys to be exposed. I
chose one megabyte as the size of the file system because it&#39;s a
round number. There&#39;s no way we&#39;re going to need that much space
for keys so feel free to chose a smaller size if you like (each key file
will be 61 bytes long).</para>
<para>Again, initialize with random data.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>cd /tmp/initrd</userinput>
<prompt>bash# </prompt><userinput>head -c 1m /dev/urandom &#62; keys</userinput></screen>
</blockquote>
<para>To make the passphrase resistant to dictionary attacks we&#39;ll
generate a seed. Whenever you see the symbol <replaceable>&#60;seed&#62;</replaceable>
be sure to replace it with the one you generated. The following command
will display a random seed on the screen.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>head -c 15 /dev/random | uuencode -m - | head -2 | tail -1</userinput></screen>
</blockquote>
<para>Set up the loopback device using the seed. This is where you
choose your passphrase, which must be at least 20 characters in length. Choose one with care that you know you won&#39;t forget. You may want to use the <ulink url="http://world.std.com/~reinhold/diceware.html">Diceware method</ulink> for choosing a secure passphrase.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>losetup -e AES128 -C 100 -S <replaceable>&#60;seed&#62;</replaceable> -T /dev/loop1 keys</userinput></screen>
</blockquote>
<para>Format and mount the keys file system (the <xref
linkend="decrypt.sh" /> script assumes you use the <literal>ext2</literal>
file system here).</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mke2fs /dev/loop1</userinput>
<prompt>bash# </prompt><userinput>mkdir /tmp/keys</userinput>
<prompt>bash# </prompt><userinput>mount /dev/loop1 /tmp/keys</userinput></screen>
</blockquote>
<para>Now for the actual asset key, 45 bytes as random as your computer
can make them. Try a dictionary attack against that, attacker! Ha! We
name the key after the computer with which it will be used (<systemitem
class="systemname">laptop</systemitem>). Substitute the name of your
computer instead.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>head -c 45 /dev/random | uuencode -m - | head -2 | tail -1 &#62; /tmp/keys/laptop</userinput></screen>
</blockquote>
<para>Set up a loopback device with the key for encrypted access to the
asset.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>losetup -e AES128 -p 0 /dev/loop0 /tmp/asset &#60; /tmp/keys/laptop</userinput></screen>
</blockquote>
<para>Unmount the keys file system.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>umount /tmp/keys</userinput>
<prompt>bash# </prompt><userinput>losetup -d /dev/loop1</userinput></screen>
</blockquote>
<sect3>
<title>Swap Partition</title>
<para>Skip this section if you&#39;re encrypting only a file.</para>
<para>It&#39;s critical to give <literal>mkswap</literal> a size
parameter here because we&#39;re not handing it a dedicated partition.
Choose whatever size you want; I chose 2GB.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkswap /dev/loop0 $((2*1024*1024))</userinput>
<computeroutput>mkswap: warning: truncating swap area to 2097144kB
Setting up swapspace version 1, size = 2147471360 bytes</computeroutput></screen>
</blockquote>
</sect3>
<sect3>
<title>Root File System</title>
<para>If you&#39;re encrypting only a file, format it with a file
system like this and skip to <xref linkend="Scripts" />.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkfs /dev/loop0</userinput></screen>
</blockquote>
<para>We&#39;ll create the root &#34;partition&#34; after the swap
space. I put the word &#39;partition&#39; in quotes because it&#39;s
not a real partition. We&#39;re faking it using the offset argument of
<literal>losetup</literal>.</para>
<para>Notice how <literal>mkswap</literal> told us the actual size of
the swapspace, which is not necessarily the size requested. Use the
actual size (which was 2147471360 in the above example) when
specifying the offset to begin the root file system.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>losetup -o <replaceable>&#60;root offset&#62;</replaceable> /dev/loop1 /dev/loop0</userinput></screen>
</blockquote>
<para>If the asset is the whole disk or the last partition on the
disk, then we needn&#39;t worry about specifying a size for the file
system. If this applies to you, do the following and skip to <xref
linkend="initrd_Mount_Point" />.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkfs /dev/loop1</userinput></screen>
</blockquote>
<para>Since the asset isn&#39;t the last partition on the disk, we
must give <literal>mkfs</literal> a size limitation or it will write
all over whatever partitions are between this one and the end of the
disk. I repeat, <emphasis>if you don&#39;t give <literal>mkfs</literal>
the correct size parameter here, you may lose data</emphasis>.
<literal>mkfs</literal> is actually just a front end, so to be as
careful as possible we&#39;ll choose an actual file system maker, in
this case <literal>mke2fs</literal>.</para>
<para>It&#39;s possible to limit the size of the file system by
specifying its size in blocks, but <literal>mke2fs</literal> chooses
the block size based on the size of the file system. A classic
Catch-22! We can ask it to do a dry run on the rest of the disk (more
than we want) to see what block size it would chose.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mke2fs -n -j /dev/loop1</userinput>
<computeroutput>mke2fs 1.34-WIP (21-May-2003)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
9781248 inodes, 19544448 blocks
977222 blocks (5.00%) reserved for the super user
First data block=0
597 block groups
32768 blocks per group, 32768 fragments per group
16384 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000, 7962624, 11239424</computeroutput></screen>
</blockquote>
<para>In this case it chose 4096. Whatever it chooses is probably
close enough for our file system. Calculate the correct size in
blocks.</para>
<blockquote>
<para>file system size = (size of partition &minus; size of swap space) &divide;
block size</para>
</blockquote>
<para>Suppose the size of the partition is 10GB and the size of the
swap is 2GB. The correct size for <literal>mke2fs</literal> is (10 &minus;
2) &times; 2<superscript>30</superscript> &divide; 4096 = 2097152. Don&#39;t get this wrong! Make backups!
Measure twice, cut once!</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mke2fs -j /dev/loop1 2097152</userinput></screen>
</blockquote>
<sect4 id="initrd_Mount_Point">
<title>initrd Mount Point</title>
<para>Mount the new root file system and create the
<filename>initrd</filename> mount point. This is necessary for the
<xref linkend="linuxrc" /> script&#39;s call to <literal>pivot_root</literal>.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mount /dev/loop1 mnt/new-root</userinput>
<prompt>bash# </prompt><userinput>mkdir mnt/new-root/initrd</userinput>
<prompt>bash# </prompt><userinput>umount mnt/new-root</userinput></screen>
</blockquote>
</sect4>
</sect3>
</sect2>
<sect2 id="Scripts">
<title>Scripts</title>
<para>We have enough information to create the decryption script. Change
the variables at the beginning to reflect your setup (including the seed
you generated earlier).</para>
<para>If you&#39;re encrypting the whole disk or a partition, set
<varname>ROOT_OFFSET</varname> to the size you got from
<literal>mkswap</literal>. Put the script in <filename>/tmp/initrd</filename>
and name it <filename>decrypt.sh</filename>.</para>
<para>If you&#39;re encrypting only a file then this script can live
anywhere. In this case be sure to set <varname>ROOT_OFFSET</varname> to
zero and set <varname>MOUNT</varname> to a convenient mount point
(probably not <filename>/mnt/new-root</filename>).</para>
<figure id="decrypt.sh" xreflabel="decrypt.sh">
<title><filename>/tmp/initrd/decrypt.sh</filename></title>
<programlisting>#!/bin/sh
SEED=<replaceable>&#60;seed&#62;</replaceable>
ASSET=/dev/hda
ROOT_OFFSET=<replaceable>&#60;root offset&#62;</replaceable>
ROOT_TYPE=ext3
MOUNT=/mnt/new-root
KEY=&#34;$1&#34;
# Ask for a passphrase to open the keys (this prevents exposure of the keys in
# case the owner loses the keychain). Give the user three tries to get the
# passphrase right.
for ((FAILED=1, TRY=1; ($FAILED != 0) &#38;&#38; (TRY &#60;= 3); TRY++))
do
mount -n -t ext2 -o loop=/dev/loop1,encryption=AES128,itercountk=100,pseed=$SEED keys /mnt/keys
FAILED=$?
done
if [ $FAILED -ne 0 ]; then
echo &#34;Sorry, you get only three attempts to guess the password.&#34;
exit 1
fi
# Use the key to decrypt the asset.
losetup -e AES128 -p 0 /dev/loop0 $ASSET &#60; &#34;/mnt/keys/$KEY&#34;
# Close the keys.
umount -n /mnt/keys
losetup -d /dev/loop1
# Set up the root &#34;partition&#34; device.
losetup -o $ROOT_OFFSET /dev/loop1 /dev/loop0
# Mount the root file system (read-only, so it can be checked with fsck).
mount -n -r -t $ROOT_TYPE /dev/loop1 $MOUNT</programlisting>
</figure>
<para>Make the script executable.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>chmod +x decrypt.sh</userinput></screen>
</blockquote>
<para>If you&#39;re encrypting only a file, skip to <xref
linkend="TestingAndBackup" />. Otherwise, save the following boot
script as <filename>linuxrc</filename> and place it in
<filename>/tmp/initrd</filename>.</para>
<figure id="linuxrc" xreflabel="linuxrc">
<title><filename>/tmp/initrd/linuxrc</filename></title>
<programlisting>#!/bin/sh
# Decrypt the asset
source decrypt.sh &#34;$1&#34;
# Pivot to the asset&#39;s root file system.
cd $MOUNT
/sbin/pivot_root . initrd
# Pass control to init.
shift 1
exec chroot . /sbin/init $* &#60;dev/console &#62;dev/console 2&#62;&#38;1</programlisting>
</figure>
<para>Make the script executable.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>chmod +x linuxrc</userinput></screen>
</blockquote>
<para>Okay, the keychain and asset are now ready. Unmount everything.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>umount /tmp/{initrd,keychain}</userinput></screen>
</blockquote>
<para>You now have an empty, encrypted file system. Hurray!</para>
</sect2>
<sect2 id="TestingAndBackup">
<title>Testing and Backup</title>
<para>Test your system by booting the keychain or executing the <xref
linkend="decrypt.sh" /> script as appropriate (give it the name of the
key you want to use as a parameter). After booting there may be a
complaint about a nonexistent <filename>/sbin/init</filename> but
that&#39;s okay for now.</para>
<para>Check to make sure your root file system mounted successfully.
When you&#39;re confident everything is working, back up your keychain.
In fact, make lots of backups. You might ask, &#34;But isn&#39;t it
insecure to have a copy of my keychain somewhere?&#34; The answer is
yes, it is, but not as insecure as losing your only keychain, if you
define security as also meaning &#34;securing access to my data&#34;.</para>
<para>Because my keychain is small I decided to back up the whole image
so it&#39;s easy to restore:</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>bzip2 -c /dev/sda &#62; keychain.img.bz2</userinput></screen>
</blockquote>
<para>If you&#39;re encrypting only a file, you can pat yourself on your
back at this point because you&#39;ve finished.</para>
</sect2>
<sect2 id="RescueDisk">
<title>Rescue Disk</title>
<para>Rescue disks are useful when a system isn&#39;t behaving properly
and/or refuses to boot. Check to make sure your rescue disk has <ulink
url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink> support
in its kernel and has the correctly patched <application><ulink
url="http://www.kernel.org/pub/linux/utils/util-linux/">util-linux</ulink></application>
tools such as <literal>losetup</literal> and <literal>mount</literal>,
otherwise it will be worthless with your newly encrypted asset. In the
future, all rescue disks will include this support because it will come
standard with the 2.6 kernel. In the meantime, <ulink
url="http://www.knoppix.com/">KNOPPIX</ulink> (for example) already has
all the necessary support and can be used as a rescue disk.</para>
<para>After booting an appropriate rescue disk, mount your keychain and
execute the <xref linkend="decrypt.sh" /> script.</para>
<blockquote>
<screen><prompt>bash# </prompt><userinput>mkdir /tmp/{keychain,initrd}</userinput>
<prompt>bash# </prompt><userinput>mount /dev/sda1 /tmp/keychain</userinput>
<prompt>bash# </prompt><userinput>mount -o loop=/dev/loop3 /tmp/keychain/boot/initrd /tmp/initrd</userinput>
<prompt>bash# </prompt><userinput>pushd /tmp/initrd</userinput>
<prompt>bash# </prompt><userinput>./decrypt.sh laptop</userinput>
<prompt>bash# </prompt><userinput>popd</userinput>
<prompt>bash# </prompt><userinput>umount /tmp/{initrd,keychain}</userinput></screen>
</blockquote>
<para>You can now access your asset through the mount point you
specified in <xref linkend="decrypt.sh" />.</para>
</sect2>
<sect2 id="InstallingLinux">
<title>Installing Linux</title>
<para>Your final task is to install Linux to your new encrypted file system. As you do this make sure the entries in your <filename>/etc/fstab</filename> for the root and swap look like those below:</para>
<informalexample>
<programlisting># /etc/fstab: static file system information.
#
# &#60;file system&#62; &#60;mount point&#62; &#60;type&#62; &#60;options&#62; &#60;dump&#62; &#60;pass&#62;
/dev/loop0 none swap sw 0 0
/dev/loop1 / ext3 errors=remount-ro 0 1</programlisting>
</informalexample>
<para>If you already have an installation elsewhere, read the <ulink
url="http://www.tldp.org/HOWTO/Hard-Disk-Upgrade/">Hard Disk Upgrade
Mini How-To</ulink> to learn how to copy it over.</para>
<para>The procedure for a fresh installation of Linux is different for
each distribution. Please send me
instructions for distributions not listed below and I will include them here.</para>
<sect3>
<title><ulink url="http://www.debian.org">Debian</ulink></title>
<orderedlist>
<listitem>
<para>Boot from a rescue disk by following the instructions in <xref linkend="RescueDisk"/>.</para>
</listitem>
<listitem>
<para>Install using the method <ulink
url="http://www.debian.org/releases/stable/i386/ch-preparing.en.html#s-linux-upgrade">3.7
Installing Debian GNU/Linux from a Unix/Linux System</ulink>.</para>
</listitem>
</orderedlist>
</sect3>
<sect3>
<title><ulink url="http://www.gentoo.org/">Gentoo</ulink></title>
<orderedlist>
<listitem>
<para>Boot from a rescue disk (Gentoo's Live CD 1.4 won't work) by following the instructions in <xref linkend="RescueDisk"/>.</para>
</listitem>
<listitem>
<para>Activate the swap partition if you created one.</para>
<blockquote><screen><prompt>bash# </prompt><userinput>swapon /dev/loop0</userinput></screen></blockquote>
</listitem>
<listitem>
<para>Point <filename>/mnt/gentoo</filename> to the root file system.</para>
<blockquote><screen><prompt>bash# </prompt><userinput>ln -s new-root /mnt/gentoo</userinput></screen></blockquote>
</listitem>
<listitem>
<para>Skip to <ulink
url="http://www.gentoo.org/doc/en/gentoo-x86-install.xml#doc_chap8">Chapter 8. Stage tarballs and chroot</ulink> in the <ulink
url="http://www.gentoo.org/doc/en/gentoo-x86-install.xml">Gentoo Linux 1.4 Installation Instructions</ulink>.</para>
</listitem>
</orderedlist>
</sect3>
<sect3 id="Idle_Logout">
<title>Idle Logout</title>
<para>Once your system is up and running, consider configuring it to log out automatically after a period of inactivity. This will lessen (but not eliminate) the risk of exposing your asset if the laptop is stolen while on (see <xref linkend="Attack_Tree"/>).</para>
</sect3>
</sect2>
</sect1>
<sect1 id="MoreInformation">
<title>More Information</title>
<itemizedlist>
<listitem>
<para>The <filename><ulink
url="http://loop-aes.sourceforge.net/loop-AES.README">README</ulink></filename>
that comes with <application><ulink
url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink></application>
explains how to use it in multiple scenarios.</para>
</listitem>
<listitem>
<para><ulink
url="http://www.tldp.org/HOWTO/Encrypted-Root-Filesystem-HOWTO/">Encrypted Root Filesystem HOWTO</ulink></para>
</listitem>
<listitem>
<para>The <ulink url="http://www.gentoo.org/proj/en/hardened/">Hardened Gentoo Project</ulink>'s <ulink url="http://www.sdc.org/~leila/usb-dongle/readme.html">A Structured Approach to Hard Disk Encryption</ulink> is more comprehensive and is targeted to <ulink url="http://www.gentoo.org/">Gentoo</ulink> users.</para>
</listitem>
</itemizedlist>
</sect1>
<glossary id="Glossary">
<title>Glossary</title>
<glossentry>
<glossterm><acronym><ulink url="http://csrc.nist.gov/CryptoToolkit/aes/">AES</ulink></acronym></glossterm>
<glossdef>
<para>Advanced Encryption Standard, a strong, well-regarded
<glossterm>encryption</glossterm> algorithm chosen by the United
States <ulink url="http://www.nist.gov/">National Institute of
Standards and Technology</ulink></para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>asset</glossterm>
<glossdef>
<para>the data being protected by <glossterm>encryption</glossterm>&mdash;either
a disk, partition, or a file</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>encryption</glossterm>
<glossdef>
<para>a mathematical means of scrambling data so that it&#39;s
unintelligible unless decrypted using a specific
<glossterm>key</glossterm></para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>key</glossterm>
<glossdef>
<para>the small piece of data necessary to make encrypted data
intelligible</para>
</glossdef>
</glossentry>
<glossentry id="keychain">
<glossterm>keychain</glossterm>
<glossdef>
<para>the physical medium (such as a <glossterm>USB flash drive)
</glossterm>used to hold the encryption <glossterm>key</glossterm>
(and possibly the beginning of the boot process)</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>loopback device</glossterm>
<glossdef>
<para>a Linux block device which appears to store data (by using
another device)</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm><ulink url="http://sourceforge.net/projects/loop-aes/">loop-AES</ulink></glossterm>
<glossdef>
<para>software written by Jari Ruusu that implements the
<glossterm>AES</glossterm> algorithm using a <glossterm>loopback
device</glossterm></para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>plausible deniability</glossterm>
<glossdef>
<para>a means to avoid being coerced into decrypting one&#39;s own
data for an attacker</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>USB flash drive</glossterm>
<glossdef>
<para>a small electronic device containing a memory chip and a USB
interface</para>
</glossdef>
</glossentry>
</glossary>
<appendix id="gfdl">
<title>GNU Free Documentation License</title>
<subtitle>Version 1.2, November 2002</subtitle>
<blockquote id="fsf-copyright">
<para>Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 59
Temple Place, Suite 330, Boston, MA 02111-1307 USA Everyone is permitted
to copy and distribute verbatim copies of this license document, but
changing it is not allowed.</para>
</blockquote>
<section id="gfdl-0">
<title>PREAMBLE</title>
<para>The purpose of this License is to make a manual, textbook, or
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freedom: to assure everyone the effective freedom to copy and
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responsible for modifications made by others.</para>
<para>This License is a kind of &#34;copyleft&#34;, which means that
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sense. It complements the GNU General Public License, which is a
copyleft license designed for free software.</para>
<para>We have designed this License in order to use it for manuals for
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program should come with manuals providing the same freedoms that the
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can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.</para>
</section>
<section id="gfdl-1">
<title>APPLICABILITY AND DEFINITIONS</title>
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</section>
<section id="gfdl-2">
<title>VERBATIM COPYING</title>
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<section id="gfdl-3">
<title>COPYING IN QUANTITY</title>
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</section>
<section id="gfdl-4">
<title>MODIFICATIONS</title>
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<orderedlist id="gfdl-modif-cond" numeration="upperalpha">
<listitem>
<simpara>Use in the Title Page (and on the covers, if any) a title
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<listitem>
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<listitem>
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<listitem>
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</section>
<section id="gfdl-5">
<title>COMBINING DOCUMENTS</title>
<para>You may combine the Document with other documents released under
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</section>
<section id="gfdl-6">
<title>COLLECTIONS OF DOCUMENTS</title>
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</section>
<section id="gfdl-7">
<title>AGGREGATION WITH INDEPENDENT WORKS</title>
<para>A compilation of the Document or its derivatives with other
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<section id="gfdl-8">
<title>TRANSLATION</title>
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<section id="gfdl-9">
<title>TERMINATION</title>
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<section id="gfdl-10">
<title>FUTURE REVISIONS OF THIS LICENSE</title>
<para>The Free Software Foundation may publish new, revised versions of
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http://www.gnu.org/copyleft/.</para>
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<section id="gfdl-addendum">
<title>ADDENDUM: How to use this License for your documents</title>
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<para>Copyright (c) YEAR YOUR NAME. Permission is granted to copy,
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the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is
included in the section entitled &#34;GNU Free Documentation
License&#34;.</para>
</blockquote>
<para>If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the &#34;with...Texts.&#34; line with this:</para>
<blockquote id="inv-cover-sample">
<para>with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being
LIST.</para>
</blockquote>
<para>If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.</para>
<para>If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.</para>
</section>
</appendix>
</article>
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