Encrypted Root Filesystem HOWTO
Christophe Devine
This HOWTO is released under the GNU Free Documentation License Version 1.2.
Revision History
Revision v1.3 2005-03-13 cd
Updated the packages version.
Revision v1.2 2004-10-20 cd
Updated the packages version.
Revision v1.1 2003-12-01 cd
Added support for GRUB.
Revision v1.0 2003-09-24 cd
Initial release, reviewed by LDP.
Revision v0.9 2003-09-11 cd
Updated and converted to DocBook XML.
Abstract
This document explains how to make your personal data secure by encrypting your
Linux root filesystem using strong cryptography.
-------------------------------------------------------------------------------
Table of Contents
1._Preparing_the_system
1.1._Setting_up_the_partition_layout
1.2._Required_packages
1.3._Installing_Linux-2.4.29
1.4._Installing_Linux-2.6.10
1.5._Installing_util-linux-2.12p
2._Creating_the_encrypted_root_filesystem
3._Setting_up_the_boot_device
3.1._Creating_the_ramdisk
3.2._Booting_from_a_CD-ROM
3.3._Booting_from_a_HD_partition
4._Final_steps
5._About_this_HOWTO
1. Preparing the system
1.1. Setting up the partition layout
Your hard disk (hda) should contain at least three partitions:
* hda1: this small unencrypted partition will ask for a password in order to
mount the encrypted root filesystem.
* hda2: this partition will contain your encrypted root filesystem; make sure
it is large enough.
* hda3: this partition holds the current GNU/Linux system.
At this point, both hda1 and hda2 are unused. hda3 is where your Linux
distribution is currently installed; /usr and /boot must not be separated from
this partition.
Here's an example of what your partition layout might look like:
# fdisk -l /dev/hda
Disk /dev/hda: 255 heads, 63 sectors, 2432 cylinders
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hda1 1 1 8001 83 Linux
/dev/hda2 2 263 2104515 83 Linux
/dev/hda3 264 525 2104515 83 Linux
/dev/hda4 526 2047 12225465 83 Linux
1.2. Required packages
If you use Debian, the following packages are mandatory:
apt-get install gcc make libncurses5-dev patch bzip2 wget
To make copy & paste easier, you should also install:
apt-get install lynx gpm
1.3. Installing Linux-2.4.29
There are two main projects which add loopback encryption support in the
kernel: cryptoloop and loop-AES. This howto is based on loop-AES, since it
features an extremely fast and highly optimized implementation of Rijndael in
assembly language, and therefore provides maximum performance if you have an
IA-32 (x86) CPU. Besides, there are some security_concerns about cryptoloop.
First of all, download and unpack the loop-AES package:
cd /usr/src
wget http://loop-aes.sourceforge.net/loop-AES/loop-AES-v3.0b.tar.bz2
tar -xvjf loop-AES-v3.0b.tar.bz2
Then you must download and patch the kernel source:
wget http://ftp.kernel.org/pub/linux/kernel/v2.4/linux-2.4.29.tar.bz2
tar -xvjf linux-2.4.29.tar.bz2
cd linux-2.4.29
rm include/linux/loop.h drivers/block/loop.c
patch -Np1 -i ../loop-AES-v3.0b/kernel-2.4.28.diff
Setup the keyboard map:
dumpkeys | loadkeys -m - > drivers/char/defkeymap.c
Next, configure your kernel; make sure the following options are set:
make menuconfig
Block devices --->
<*> Loopback device support
[*] AES encrypted loop device support (NEW)
<*> RAM disk support
(4096) Default RAM disk size (NEW)
[*] Initial RAM disk (initrd) support
File systems --->
<*> Ext3 journalling file system support
<*> Second extended fs support
(important note: do not enable /dev file system support)
Compile the kernel and install it:
make dep bzImage
make modules modules_install
cp arch/i386/boot/bzImage /boot/vmlinuz
If grub is your bootloader, update /boot/grub/menu.lst or /boot/grub/grub.conf:
cat > /boot/grub/menu.lst << EOF
default 0
timeout 10
color green/black light-green/black
title Linux
root (hd0,2)
kernel /boot/vmlinuz ro root=/dev/hda3
EOF
Otherwise, update /etc/lilo.conf and run lilo:
cat > /etc/lilo.conf << EOF
lba32
boot=/dev/hda
prompt
timeout=60
image=/boot/vmlinuz
label=Linux
read-only
root=/dev/hda3
EOF
lilo
You may now restart the system.
1.4. Installing Linux-2.6.10
Proceed as described in the previous section, using loop-aes' kernel-
2.6.10.diff patch instead, and make sure cryptoloop support is not activated.
Note that modules support require that you have the module-init-tools package
installed.
1.5. Installing util-linux-2.12p
The losetup program, which is part of the util-linux package, must be patched
and recompiled in order to add strong cryptography support. Download, unpack
and patch util-linux:
cd /usr/src
wget http://ftp.kernel.org/pub/linux/utils/util-linux/util-linux-
2.12p.tar.bz2
tar -xvjf util-linux-2.12p.tar.bz2
cd util-linux-2.12p
patch -Np1 -i ../loop-AES-v3.0b/util-linux-2.12p.diff
To use passwords that are less than 20 characters, enter:
CFLAGS="-O2 -DLOOP_PASSWORD_MIN_LENGTH=8"; export CFLAGS
Security is certainly your major concern. For this reason, please do not enable
passwords shorter than 20 characters. Data privacy is not free, one has to
'pay' in form of long passwords.
Compile losetup and install it as root:
./configure && make lib mount
mv -f /sbin/losetup /sbin/losetup~
rm -f /usr/share/man/man8/losetup.8*
cd mount
gzip losetup.8
cp losetup /sbin
cp losetup.8.gz /usr/share/man/man8/
chattr +i /sbin/losetup
2. Creating the encrypted root filesystem
Fill the target partition with random data:
shred -n 1 -v /dev/hda2
Setup the encrypted loopback device:
losetup -e aes256 -S xxxxxx /dev/loop0 /dev/hda2
To prevent optimized dictionary attacks, it is recommended to add the -S xxxxxx
option, where "xxxxxx" is your randomly chosen seed (for example, you might
choose "gPk4lA"). Write down your seed on a piece of paper so that you don't
loose it afterwards. Also, in order to avoid boot-time problems with the
keyboard map, do not use non-ASCII characters (accents, etc.) in your password.
The Diceware site offers a simple way to create strong, yet easy to remember,
passphrases.
Now create the ext3 filesystem:
mke2fs -j /dev/loop0
Check that the password you entered is correct:
losetup -d /dev/loop0
losetup -e aes256 -S xxxxxx /dev/loop0 /dev/hda2
mkdir /mnt/efs
mount /dev/loop0 /mnt/efs
You can compare the encrypted and unencrypted data:
xxd /dev/hda2 | less
xxd /dev/loop0 | less
It's time to install your encrypted Linux system. If you use a GNU/Linux
distribution (such as Debian, Slackware, Gentoo, Mandrake, RedHat/Fedora, SuSE,
etc.), run the following command:
cp -avx / /mnt/efs
If you use the Linux From Scratch book, proceed as described in the manual,
with the modifications below:
* Chapter 6 - Installing util-linux:
Apply the loop-AES patch after unpacking the sources.
* Chapter 8 - Making the LFS system bootable:
Refer to the next section (Setting up the boot device).
3. Setting up the boot device
3.1. Creating the ramdisk
To begin with, chroot inside the encrypted partition and create the boot device
mount point:
chroot /mnt/efs
mkdir /loader
Then, create the initial ramdisk (initrd), which will be needed afterwards:
cd
dd if=/dev/zero of=initrd bs=1k count=4096
mke2fs -F initrd
mkdir ramdisk
mount -o loop initrd ramdisk
If you're using grsecurity, you may get a "Permission denied" error message; in
this case you'll have to run the mount command outside chroot.
Create the filesystem hierarchy and copy the required files in it:
mkdir ramdisk/{bin,dev,lib,mnt,sbin}
cp /bin/{bash,mount} ramdisk/bin/
ln -s bash ramdisk/bin/sh
mknod -m 600 ramdisk/dev/console c 5 1
mknod -m 600 ramdisk/dev/hda2 b 3 2
mknod -m 600 ramdisk/dev/loop0 b 7 0
cp /lib/{ld-linux.so.2,libc.so.6,libdl.so.2} ramdisk/lib/
cp /lib/{libncurses.so.5,libtermcap.so.2} ramdisk/lib/
cp /sbin/{losetup,pivot_root} ramdisk/sbin/
It's ok if you see a message like "/lib/libncurses.so.5: No such file or
directory", or "/lib/libtermcap.so.2: No such file or directory"; bash only
requires one of these two libraries. You can check which one is actually
required with:
ldd /bin/bash
Compile the sleep program, which will prevent the password prompt being flooded
by kernel messages (such as usb devices being registered).
cat > sleep.c << "EOF"
#include <unistd.h>
#include <stdlib.h>
int main( int argc, char *argv[] )
{
if( argc == 2 )
sleep( atoi( argv[1] ) );
return( 0 );
}
EOF
gcc -s sleep.c -o ramdisk/bin/sleep
rm sleep.c
Create the init script:
cat > ramdisk/sbin/init << "EOF"
#!/bin/sh
/bin/sleep 3
echo -n "Enter seed value: "
read SEED
/sbin/losetup -e aes256 -S $SEED /dev/loop0 /dev/hda2
/bin/mount -r -n -t ext3 /dev/loop0 /mnt
while [ $? -ne 0 ]
do
/sbin/losetup -d /dev/loop0
/sbin/losetup -e aes256 -S $SEED /dev/loop0 /dev/hda2
/bin/mount -r -n -t ext3 /dev/loop0 /mnt
done
cd /mnt
/sbin/pivot_root . loader
exec /usr/sbin/chroot . /sbin/init
EOF
chmod 755 ramdisk/sbin/init
Umount the loopback device and compress the initrd:
umount -d ramdisk
rmdir ramdisk
gzip initrd
mv initrd.gz /boot/
3.2. Booting from a CD-ROM
I strongly advise you to start your system with a read-only media, such as a
bootable CD-ROM.
Download and unpack syslinux:
wget http://ftp.kernel.org/pub/linux/utils/boot/syslinux/syslinux-
3.07.tar.bz2
tar -xvjf syslinux-3.07.tar.bz2
Configure isolinux:
mkdir bootcd
cp /boot/{vmlinuz,initrd.gz} syslinux-3.07/isolinux.bin bootcd
echo "DEFAULT /vmlinuz initrd=initrd.gz ro root=/dev/ram0" \
> bootcd/isolinux.cfg
Create and burn the bootable cd-rom iso image:
mkisofs -o bootcd.iso -b isolinux.bin -c boot.cat \
-no-emul-boot -boot-load-size 4 -boot-info-table \
-J -hide-rr-moved -R bootcd/
cdrecord -dev 0,0,0 -speed 4 -v bootcd.iso
rm -rf bootcd{,.iso}
3.3. Booting from a HD partition
The boot partition can come in handy if you happen to lose your bootable CD.
Remember that hda1 is a writable media and is thus insecure; use it only in
case of emergency!
Create and mount the ext2 filesystem:
dd if=/dev/zero of=/dev/hda1 bs=8192
mke2fs /dev/hda1
mount /dev/hda1 /loader
Copy the kernel and the initial ramdisk:
cp /boot/{vmlinuz,initrd.gz} /loader
If you use grub:
mkdir /loader/boot
cp -av /boot/grub /loader/boot/
cat > /loader/boot/grub/menu.lst << EOF
default 0
timeout 10
color green/black light-green/black
title Linux
root (hd0,0)
kernel /vmlinuz ro root=/dev/ram0
initrd /initrd.gz
EOF
grub-install --root-directory=/loader /dev/hda
umount /loader
If you use lilo:
mkdir /loader/{boot,dev,etc}
cp /boot/boot.b /loader/boot/
mknod -m 600 /loader/dev/hda b 3 0
mknod -m 600 /loader/dev/hda1 b 3 1
mknod -m 600 /loader/dev/hda2 b 3 2
mknod -m 600 /loader/dev/hda3 b 3 3
mknod -m 600 /loader/dev/hda4 b 3 4
mknod -m 600 /loader/dev/ram0 b 1 0
cat > /loader/etc/lilo.conf << EOF
lba32
boot=/dev/hda
prompt
timeout=60
image=/vmlinuz
label=Linux
initrd=/initrd.gz
read-only
root=/dev/ram0
EOF
lilo -r /loader
umount /loader
4. Final steps
Still inside chroot, modify /etc/fstab so that it contains:
/dev/loop0 / ext3 defaults 0 1
Delete /etc/mtab and exit from chroot. Finally, run "umount -d /mnt/efs" and
reboot. If something goes wrong, you can still boot your unencrypted partition
by entering "Linux root=/dev/hda3" at the LILO: prompt.
If everything went well, you can now re-partition your disk and encrypt hda3 as
well as hda4. In the following scripts, we assume that hda3 will hold the swap
device and hda4 will contain /home; you should initialize both partitions
first:
shred -n 1 -v /dev/hda3
shred -n 1 -v /dev/hda4
losetup -e aes256 -S xxxxxx /dev/loop1 /dev/hda3
losetup -e aes256 -S xxxxxx /dev/loop2 /dev/hda4
mkswap /dev/loop1
mke2fs -j /dev/loop2
Then create a script in the system startup directory and update fstab:
cat > /etc/init.d/loop << "EOF"
#!/bin/sh
if [ "`/usr/bin/md5sum /dev/hda1`" != \
"5671cebdb3bed87c3b3c345f0101d016 /dev/hda1" ]
then
echo -n "WARNING! hda1 integrity verification FAILED - press enter."
read
fi
echo "1st password chosen above" | \
/sbin/losetup -p 0 -e aes256 -S xxxxxx /dev/loop1 /dev/hda3
echo "2nd password chosen above" | \
/sbin/losetup -p 0 -e aes256 -S xxxxxx /dev/loop2 /dev/hda4
/sbin/swapon /dev/loop1
for i in `seq 0 63`
do
echo -n -e "\33[10;10]\33[11;10]" > /dev/tty$i
done
EOF
chmod 700 /etc/init.d/loop
ln -s ../init.d/loop /etc/rcS.d/S00loop
vi /etc/fstab
...
/dev/loop2 /home ext3 defaults 0 2
5. About this HOWTO
The Encrypted Root Filesystem HOWTO was first written in november 2002 for the
Linux_From_Scratch project. I'd like to thank the many people who have since
contributed to this document (in reverse chronological order): Micha Borrmann,
Dennis Lemckert, Oleg Vyushin, Ellen Bokhorst, Daczi László, Gaetano Zappulla,
Guillaume Lehmann, Claude Thomassin, Jean-Philippe Guérard, Luc Vo Van, Jacobus
Brink, Ernesto Pérez Estévez, Matthew Ploessel, Mike Lorek, Lars Bungum,
Michael Shields, Julien Perrot, Grant Stephenson, Cary W. Gilmer, James
Howells, Pedro Baez, Josh Purinton, Jari Ruusu and Zibeli Aton.
This HOWTO has been translated in various languages:
* French
* Italian
* Hungarian
* Dutch
Please send any comment to Christophe_Devine.