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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook V4.1//EN">
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<article class="whitepaper" id="Encrypted-Root-Filesystem-HOWTO">
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<articleinfo>
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<title>Encrypted Root Filesystem HOWTO</title>
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<author>
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<firstname>Christophe</firstname>
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<surname>Devine</surname>
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</author>
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<abstract>
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<para>
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This brief document explains how to make your personal data secure
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by encrypting your Linux root filesystem using strong cryptography.
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</para>
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</abstract>
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<legalnotice>
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<para>
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This HOWTO is released under the GNU Free Documentation License
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Version 1.2.
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</para>
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</legalnotice>
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<revhistory>
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<revision>
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<revnumber>v1.0</revnumber>
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<date>2003-09-24</date>
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<authorinitials>cd</authorinitials>
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<revremark>Initial release, reviewed by LDP.</revremark>
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</revision>
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<revision>
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<revnumber>v0.9</revnumber>
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<date>2003-09-11</date>
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<authorinitials>cd</authorinitials>
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<revremark>Updated and converted to DocBook XML.</revremark>
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</revision>
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</revhistory>
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</articleinfo>
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<sect1 id="partition-layout">
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<title>Setting up the partition layout</title>
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<para>
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Your hard disk (hda) should have at least three partitions:
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<itemizedlist>
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<listitem><para>
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hda1: this small (~4 Mb) unencrypted partition will ask for
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a password in order to mount the encrypted root filesystem.
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</para></listitem>
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<listitem><para>
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hda2: this encrypted partition will contain your root filesystem.
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</para></listitem>
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<listitem><para>
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hda3: this partition holds the current GNU/Linux system.
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</para></listitem>
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</itemizedlist>
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</para>
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</sect1>
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<sect1 id="enable-strong-crypto">
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<title>Enabling strong crypto in your system</title>
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<sect2 id="install-kernel">
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<title>Installing Linux-2.4.22</title>
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<para>
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There are two main projects which add strong crypto support in the
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kernel: CryptoAPI and loop-aes. This howto uses loop-aes, since it
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has an extremely fast and highly optimized implementation of Rijndael
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in assembly language, and therefore provides maximum performance if
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you have an IA-32 (x86) CPU.
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</para><para>
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First of all, download and unpack the kernel sources:
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</para><para>
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<ulink url="ftp://ftp.kernel.org/pub/linux/kernel/v2.4/linux-2.4.22.tar.bz2"></ulink>
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</para><para>
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You also have to download and unpack:
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</para><para>
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<ulink url="http://loop-aes.sourceforge.net/loop-AES/loop-AES-v1.7e.tar.bz2"></ulink>
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</para><para>
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Then you must patch the kernel:
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</para><para>
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<screen>linux-2.4.22 $ patch -Np1 -i ../loop-AES-v1.7e/kernel-2.4.22.diff</screen>
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</para><para>
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Next, configure your kernel; make sure the following options are set:
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</para><para>
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<screen> Block devices --->
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<*> Loopback device support
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[*] AES encrypted loop device support (NEW)
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<*> RAM disk support
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(4096) Default RAM disk size (NEW)
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[*] Initial RAM disk (initrd) support</screen>
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</para><para>
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Setup the keyboard map:
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</para><para>
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<screen>linux-2.4.22 $ dumpkeys | loadkeys -m - > drivers/char/defkeymap.c</screen>
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</para><para>
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Compile the kernel, install it and reboot.
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</para>
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</sect2>
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<sect2 id="install-util-linux">
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<title>Installing util-linux-2.12pre</title>
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<para>
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The losetup program, which is part of the util-linux package, must be
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patched and recompiled in order to add strong cryptography support.
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</para><para>
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Download and unpack:
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</para><para>
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<ulink url="http://ftp.cwi.nl/aeb/util-linux/util-linux-2.12pre.tar.gz"></ulink>
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</para><para>
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cd into util-linux-2.12pre and apply this patch:
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</para><para>
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<screen>$ patch -Np1 -i ../loop-AES-v1.7e/util-linux-2.12pre.diff</screen>
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</para><para>
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To use passwords that are less than 20 characters, enter:
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</para><para>
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<screen>$ CFLAGS="-O2 -DLOOP_PASSWORD_MIN_LENGTH=12"; export CFLAGS</screen>
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</para><para>
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If security is important, please do not enable passwords shorter than
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20 characters. Security is not free, one has to 'pay' in form of long
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passwords.
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</para><para>
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Compile losetup and install it as root:
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</para><para>
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<screen>$ ./configure && make lib mount
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# cp mount/losetup /sbin
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# rm -f /usr/share/man/man8/losetup.8.gz
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# cp mount/losetup.8 /usr/share/man/man8</screen>
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</para>
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</sect2>
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</sect1>
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<sect1 id="encrypt-root-filesystem">
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<title>Creating the encrypted root filesystem</title>
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<para>
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Fill the target partition with random data:
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</para><para>
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<screen># shred -n 1 -v /dev/hda2</screen>
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</para><para>
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Setup the encrypted loopback device:
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</para><para>
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<screen># losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2</screen>
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</para><para>
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For better security, it is recommended to use the -S xxxxxxxxxx
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option, where "xxxxxxxxxx" is your (randomly) chosen seed. This
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prevents optimized dictionary attacks.
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</para><para>
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Also, note that using 256-bit AES would not be any more secure,
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since even 128-bit AES is impossible to crack by brute-force.
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Furthermore, AES-256 is about 25% slower than AES-128.
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</para><para>
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Now create the ext2 (or ext3 or reiserfs) filesystem:
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</para><para>
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<screen># mke2fs /dev/loop0</screen>
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</para><para>
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Check that you correctly entered the password:
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</para><para>
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<screen># losetup -d /dev/loop0
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# losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2
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Password:
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# mount /dev/loop0 /mnt</screen>
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</para><para>
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You can compare the encrypted and unencrypted data:
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</para><para>
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<screen># xxd /dev/loop0 | less
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# xxd /dev/hda2 | less</screen>
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</para>
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</sect1>
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<sect1 id="system-install">
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<title>Installing your encrypted Linux system</title>
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<sect2 id="regular-distro">
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<title>If you use a GNU/Linux distribution</title>
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<para>
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This command works on all distributions, including Debian, Gentoo,
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Mandrake and Redhat:
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</para><para>
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<screen># cp -avx / /mnt</screen>
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</para>
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</sect2>
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<sect2 id="linux-from-scratch">
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<title>If you use the Linux From Scratch book</title>
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<para>
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Proceed as described in the manual, with the modifications below:
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<itemizedlist>
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<listitem>
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<para>Chapter 6 - Installing util-linux:</para>
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<para>Apply the loop-AES patch after unpacking the sources.</para>
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</listitem><listitem>
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<para>Chapter 8 - Making the LFS system bootable:</para>
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<para>Refer to Chapter 5 of this howto.</para>
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</listitem>
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</itemizedlist>
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</para>
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</sect2>
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</sect1>
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<sect1 id="boot-device">
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<title>Setting up the boot device</title>
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<sect2 id="initial-ramdisk">
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<title>Creating the ramdisk</title>
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<para>
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To begin with, chroot inside the encrypted partition and create
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the boot device mount point:
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</para><para>
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<screen>chroot /mnt
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mkdir /loader</screen>
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</para><para>
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Then, create the initial ramdisk (initrd), which will be needed
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afterwards:
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</para><para>
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<screen>dd if=/dev/zero of=initrd bs=1k count=4096
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mke2fs -F initrd
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mkdir ramdisk
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mount -o loop initrd ramdisk</screen>
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</para><para>
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Create the filesystem hierarchy and copy the required files in it:
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</para><para>
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<screen>mkdir ramdisk/{bin,dev,lib,mnt,sbin}
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cp /bin/{sh,mount,umount} ramdisk/bin/
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cp -a /dev/{console,hda2,loop0} ramdisk/dev/
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cp /lib/{ld-linux.so.2,libc.so.6,libdl.so.2,libncurses.so.5} \
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ramdisk/lib/
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cp /sbin/{losetup,pivot_root} ramdisk/sbin/</screen>
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</para><para>
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Create the init script (don't forget to replace "xxxxxxxxxx"
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with your chosen seed):
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</para><para>
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<screen>cat > ramdisk/sbin/init << "EOF"
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#!/bin/sh
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/sbin/losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2
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/bin/mount -n -t ext2 /dev/loop0 /mnt
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while [ $? -ne 0 ]
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do
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/sbin/losetup -d /dev/loop0
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/sbin/losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2
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/bin/mount -n -t ext2 /dev/loop0 /mnt
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done
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cd /mnt
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/sbin/pivot_root . loader
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exec /usr/sbin/chroot . /sbin/init
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EOF
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chmod 755 ramdisk/sbin/init</screen>
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</para><para>
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Umount the loopback device and compress the initrd:
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</para><para>
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<screen>umount -d ramdisk
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rmdir ramdisk
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gzip initrd</screen>
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</para>
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</sect2>
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<sect2 id="boot-partition">
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<title>Setting up the boot partition</title>
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<para>
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Create and mount the ext2 filesystem:
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</para><para>
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<screen>mke2fs /dev/hda1
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mount -t ext2 /dev/hda1 /loader</screen>
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</para><para>
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Copy the kernel compiled in Chapter 2.1 and the initial ramdisk:
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</para><para>
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<screen>cp /path/to/vmlinuz /loader/
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cp /path/to/initrd.gz /loader/</screen>
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</para><para>
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Configure and run LILO:
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</para><para>
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<screen>mkdir /loader/{boot,dev,etc}
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cp /boot/boot.b /loader/boot/
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cp -a /dev/{hda,hda1,ram0} /loader/dev/
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cat > /loader/etc/lilo.conf << EOF
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lba32
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boot=/dev/hda
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root=/dev/ram0
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vga=4
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read-only
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image=/vmlinuz
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label=Linux
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initrd=/initrd.gz
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EOF
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lilo -r /loader</screen>
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</para>
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</sect2>
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<sect2 id="bootable-cd">
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<title>Booting with a CD-ROM</title>
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<para>
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You may also choose not to use /dev/hda1 as a boot device at all,
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but instead burn the kernel and the ramdisk on a bootable cd-rom.
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Download and unpack syslinux:
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</para><para>
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<ulink url="ftp://ftp.kernel.org/pub/linux/utils/boot/syslinux/syslinux-2.06.tar.gz"></ulink>
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</para><para>
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Configure isolinux:
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</para><para>
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<screen>mkdir bootcd
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cp /path/to/vmlinuz bootcd/
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cp /path/to/initrd.gz bootcd/
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cp syslinux-2.06/isolinux.bin bootcd/
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echo "DEFAULT vmlinuz initrd=initrd.gz root=/dev/ram0 vga=4" \
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> bootcd/isolinux.cfg</screen>
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</para><para>
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Create and burn the bootable cd-rom iso image:
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</para><para>
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<screen>mkisofs -o bootcd.iso -b isolinux.bin -c boot.cat \
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-no-emul-boot -boot-load-size 4 -boot-info-table \
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-J -hide-rr-moved -R bootcd/
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cdrecord -dev 0,0,0 -speed 16 -v bootcd.iso</screen>
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</para>
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</sect2>
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</sect1>
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<sect1 id="bootscripts">
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<title>Setting up the bootscripts</title>
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<para>
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Make sure your /etc/fstab contains:
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</para><para>
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<screen>/dev/loop0 / ext2 defaults 0 1</screen>
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</para><para>
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At this point, hda3 is not needed anymore, so you can create an
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encrypted filesystem on this partition and use it as a backup.
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</para><para>
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Also, it is a good idea to check the boot partition integrity
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inside the encrypted partition, in order to spot if a government
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agency like the FBI or the NSA has modified your boot partition
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so as to grab your password. Add the following script, which can
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be called for example S00checkloader, in the system startup
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directory (/etc/rcS.d/ under Debian):
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</para><para>
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<screen>#!/bin/sh
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echo -n "Checking master boot record integrity: "
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if [ "`dd if=/dev/hda count=1 2>/dev/null | md5sum`" = \
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"e051a4532356709c73b86789acfbdbbd -" ]
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then
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echo "OK."
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else
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echo -n "FAILED! press Enter to continue."
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read
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fi
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echo -n "Checking boot partition integrity: "
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if [ "`dd if=/dev/hda1 2>/dev/null | md5sum`" = \
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"f3686a17fac8a1090d962bef59c86d3b -" ]
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then
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echo "OK."
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else
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echo -n "FAILED! press Enter to continue."
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read
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fi</screen>
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</para><para>
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(you should replace the two md5sums above with the correct ones).
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</para><para>
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Now, if you're low on RAM you'll need some swap space. Let's
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suppose hda4 will hold your encrypted swap partition; you must
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create the swap device first:
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</para><para>
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<screen># shred -n 1 -v /dev/hda4
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# losetup -e aes128 /dev/loop1 /dev/hda4
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# mkswap /dev/loop1</screen>
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</para><para>
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Then add the following lines at the end of S00checkloader:
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</para><para>
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<screen>echo "password chosen above" | \
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losetup -p 0 -e aes128 /dev/loop1 /dev/hda4
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swapon /dev/loop1</screen>
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</para>
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</sect1>
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<sect1 id="about">
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<title>About this HOWTO</title>
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<para>
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||||
The Encrypted Root Filesystem HOWTO was first written in november 2002
|
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for the <ulink url="http://www.linuxfromscratch.org/lfs/news.html">Linux
|
||||
>From Scratch</ulink> project. I'd like to thank the people who have since
|
||||
helped me improve this howto (in reverse chronological order): Josh
|
||||
Purinton, Jari Ruusu and Zibeli Aton.
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||||
</para><para>
|
||||
Please send any comment to <devine (at) cr0.net>.
|
||||
</para><para>
|
||||
The latest version of this document is located at:
|
||||
</para><para>
|
||||
<ulink url="http://www.cr0.net:8040/code/crypto/efs-howto.php"></ulink>
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</para>
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</sect1>
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</article>
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||||
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Reference in New Issue