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Encrypted Root Filesystem HOWTO

Encrypted Root Filesystem HOWTO

Christophe Devine

Revision History
Revision v1.32005-03-13Revised by: cd
Updated the packages version.
Revision v1.22004-10-20Revised by: cd
Updated the packages version.
Revision v1.12003-12-01Revised by: cd
Added support for GRUB.
Revision v1.02003-09-24Revised by: cd
Initial release, reviewed by LDP.
Revision v0.92003-09-11Revised 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.


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

    
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