Encrypted Root Filesystem HOWTO

Christophe Devine

Revision History                                                             
Revision v1.2            2004-10-20            Revised by: cd                
Updated the packages version.                                                
Revision v1.1            2003-12-01            Revised by: cd                
Added support for GRUB.                                                      
Revision v1.0            2003-09-24            Revised by: cd                
Initial release, reviewed by LDP.                                            
Revision v0.9            2003-09-11            Revised by: cd                
Updated and converted to DocBook XML.                                        


This document explains how to make your personal data secure by encrypting
your Linux root filesystem using strong cryptography.

This HOWTO is released under the GNU Free Documentation License Version 1.2.

-----------------------------------------------------------------------------
Table of Contents
1. Preparing the system
    1.1. Setting up the partition layout
    1.2. Installing Linux-2.4.27
    1.3. Installing Linux-2.6.8.1
    1.4. Installing util-linux-2.12b
   
   
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. Installing Linux-2.4.27

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 [http://groups.google.fr/groups?selm
=1emrG-1Ck-25%40gated-at.bofh.it] security concerns about cryptoloop.

First of all, download and unpack the loop-AES package:

+---------------------------------------------------------------------------+
|wget http://loop-aes.sourceforge.net/loop-AES/loop-AES-v2.2b.tar.bz2       |
|tar -xvjf loop-AES-v2.2b.tar.bz2                                           |
+---------------------------------------------------------------------------+

Then you must download and patch the kernel source:

+---------------------------------------------------------------------------+
|wget http://ftp.kernel.org/pub/linux/kernel/v2.4/linux-2.4.27.tar.bz2      |
|tar -xvjf linux-2.4.27.tar.bz2                                             |
|cd linux-2.4.27                                                            |
|rm include/linux/loop.h drivers/block/loop.c                               |
|patch -Np1 -i ../loop-AES-v2.2b/kernel-2.4.27.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=100                                                                |
|image=/boot/vmlinuz                                                        |
|    label=Linux                                                            |
|    read-only                                                              |
|    root=/dev/hda3                                                         |
|EOF                                                                        |
|lilo                                                                       |
+---------------------------------------------------------------------------+

You may now restart the system.
-----------------------------------------------------------------------------

1.3. Installing Linux-2.6.8.1

Proceed as described in the previous section, using loop-aes' 
kernel-2.6.8.1.diff patch instead. Note that modules support require that you
have the module-init-tools package installed.
-----------------------------------------------------------------------------

1.4. Installing util-linux-2.12b

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:

+-------------------------------------------------------------------------------+
|wget http://ftp.kernel.org/pub/linux/utils/util-linux/util-linux-2.12b.tar.bz2 |
|tar -xvjf util-linux-2.12b.tar.bz2                                             |
|cd util-linux-2.12b                                                            |
|patch -Np1 -i ../loop-AES-v2.2b/util-linux-2.12c.diff                          |
+-------------------------------------------------------------------------------+

To use passwords that are less than 20 characters, enter:

+---------------------------------------------------------------------------+
|CFLAGS="-O2 -DLOOP_PASSWORD_MIN_LENGTH=8"; export CFLAGS                   |
+---------------------------------------------------------------------------+

Security is probably one of your major concerns. 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/                                       |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------

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"). Also, in order to avoid boot-time problems with the
keyboard map, do not use non-ASCII characters (accents, etc.) in your
password. The [http://www.diceware.com/] 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 (don't forget to replace "xxxxxx" with your chosen
seed):

+---------------------------------------------------------------------------+
|cat > ramdisk/sbin/init << "EOF"                                           |
|#!/bin/sh                                                                  |
|                                                                           |
|/bin/sleep 3                                                               |
|/sbin/losetup -e aes256 -S xxxxxx /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 xxxxxx /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-2.10.tar.bz2 |
|tar -xvjf syslinux-2.10.tar.bz2                                                |
+-------------------------------------------------------------------------------+

Configure isolinux:

+---------------------------------------------------------------------------+
|mkdir bootcd                                                               |
|cp /boot/{vmlinuz,initrd.gz} syslinux-2.10/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/ram0 b 1 0                                        |
|cat > /loader/etc/lilo.conf << EOF                                         |
|lba32                                                                      |
|boot=/dev/hda                                                              |
|prompt                                                                     |
|timeout=100                                                                |
|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           |
+---------------------------------------------------------------------------+

Remove /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 [http://www.linuxfromscratch.org/lfs/news.html] Linux From Scratch
project. I'd like to thank the many people who have since helped me improve
this document (in reverse chronological order): 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:

  * [http://www.traduc.org/docs/HOWTO/lecture/
    Encrypted-Root-Filesystem-HOWTO.html] French
   
  * [http://www.linux.it/~gaetano/erfs/] Italian
   
  * [http://tldp.fsf.hu/HOWTO/Encrypted-Root-Filesystem-HOWTO-hu/] Hungarian
   

Please send any comment to [http://www.cr0.net:8040/about/] Christophe
Devine.
