cryptoapi hint, corrected

Christophe Devine devine at
Wed Feb 19 11:05:12 PST 2003

This version should be ok.

Christophe Devine
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TITLE:		Encrypted Filesystem Howto
AUTHOR:		Christophe Devine <devine at>

	Make your personal data secure by building your LFS system
	inside a filesystem encrypted with strong cryptography.



    0. Changelog

    1. Setting up the partition layout

    2. Enabling strong cryto in your current system
        2.1. Installing Linux-2.4.20
        2.2. Installing util-linux-2.11y

    3. Creating the encrypted partition

    4. Building the LFS system

    5. Setting up the boot partition

    6. Setting up the bootscripts

    0. Changelog

        2003-02-19 - some changes, thanks to Jari Ruusu for his comments

        2003-01-15 - switched to loop-aes, updated the packages version

        2002-10-19 - first version of the cryptoapi hint released

    1. Setting up the partition layout

Your hard disk should have at least three partitions:

  - one small (~ 8 Mb) unencrypted partition (let's say hda1),
    which will ask the password to mount your encrypted partition.

  - the encrypted partition holding the LFS system (hda2).

  - other temporary partitions for the host distribution.

    2. Enabling strong cryto in your current system

The host distribution you're using needs to have strong cryptography
support, which is probably not the case. Therefore, you must recompile
your kernel and parts of util-linux.

        2.1. Installing Linux-2.4.20

There are two main projects which add strong crypto support in the kernel:
CryptoAPI and loop-aes. This hint uses loop-aes, since it has a *FAST* and
highly optimized implementation of AES in assembly language, and therefore
provides maximum performance if you have an x86 CPU.

If necessary, download and unpack the kernel sources:

You also have to download and unpack:

Then you must patch the kernel:

linux-2.4.20 $ patch -Np1 -i ../loop-AES-v1.6i/kernel-2.4.20.diff

A small, but yet important bug has to be fixed (read Jari's comment
by patching the loop driver; download and apply the required patch:

linux-2.4.20 $ patch -Np1 -i ../loop-AES-v1.6i-bugfix.patch

Next, configure your kernel; make sure the following options are set:

    Block devices  --->

        <*> Loopback device support
        [*]   AES encrypted loop device support (NEW)

Finally compile the kernel, install it and reboot.

        2.2. Installing util-linux-2.11y

The losetup program, which is part of the util-linux package, must be
patched and recompiled in order to add strong cryptography support:

First of all, download and unpack:

Apply the patch provided with loop-aes:

util-linux-2.11y $ patch -Np1 -i ../loop-AES-v1.6i/util-linux-2.11y.diff

If you wish to use passwords that are less than 20 characters, enter:

util-linux-2.11y $ export CFLAGS="-O2 -DLOOP_PASSWORD_MIN_LENGTH=15"

If security is important, please do not enable passwords shorter than 20
characters. Security is not free, one has to 'pay' in form of long passwords.

Compile losetup and install it as root:

util-linux-2.11y $ ./configure && make lib mount
util-linux-2.11y # cp mount/losetup /sbin
util-linux-2.11y # cp mount/losetup.8 /usr/share/man/man8

    3. Creating the encrypted partition

Fill the target partition with random data:

# shred -n 1 -v /dev/hda2

Setup the encrypted loop device :

# losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2

For better security, it is recommended you use the -S xxxxxxxxxx option,
where "xxxxxxxxxx" is your chosen seed. This makes optimized dictionary
attacks much more difficult.

Make sure you don't forget you password ! For minimum security, it should
have at least 10 characters and contain letters (both uppercase and lower-
case), special characters and numbers.  Also, note that using 256-bit AES
would not be any more secure, since even 128-bit AES is almost impossible
to crack using brute-force (even with millions of CPU-years).

Now create the ext2 (or ext3 or reiserfs) filesystem and mount it:

# mke2fs /dev/loop0
# mount  /dev/loop0 /mnt/lfs

You can compare the encrypted and unencrypted data:

# xxd /dev/loop0 | less
# xxd /dev/hda2  | less

    4. Building the LFS system

Two steps in the book must be adapted :

    * Chapter 6, Installing util-linux :
        Use util-linux-2.11y, instead of the version used in the book,
        and apply the patch as described in section 2.2. of this hint.

    * Chapter 8, Making the LFS system bootable :

        Refer to section 5. below :

    5. Setting up the boot partition

The following instructions assume that you're still chrooted inside $LFS.

Create and mount the boot partition :

# mke2fs /dev/hda1
# mkdir /loader
# mount -t ext2 /dev/hda1 /loader

Create the filesystem hierarchy :

# mkdir /loader/{bin,boot,dev,etc,lib,mnt,sbin}

Copy the required files in it :

# cp /bin/{sh,mount,umount} /loader/bin/
# cp /boot/boot-text.b /loader/boot/boot.b
# cp -a /dev/{console,hda,hda1,hda2,loop0} /loader/dev/
# cp /lib/{,,,} /loader/lib/
# cp /sbin/{losetup,pivot_root} /loader/sbin
# cat > /loader/sbin/init << "EOF"

/sbin/losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2
/bin/mount -n -t ext2 /dev/loop0 /mnt

while [ $? -ne 0 ]
    /sbin/losetup -d /dev/loop0
    /sbin/losetup -e aes128 -S xxxxxxxxxx /dev/loop0 /dev/hda2
    /bin/mount -n -t ext2 /dev/loop0 /mnt

cd /mnt
/sbin/pivot_root . loader
exec /usr/sbin/chroot . /sbin/init
# chmod 755 /loader/sbin/init
# cat > /loader/etc/lilo.conf << EOF

Copy the kernel you've compiled in section 2.1. to /loader/vmlinuz and run:

# lilo -r /loader

    6. Setting up the bootscripts

Make sure your /etc/fstab contains:

/dev/loop0      /      ext2    defaults             0 1

Also, it is a good idea to check the boot partition integrity inside the
encrypted partition, in order to spot if someone, say a government agency
like the FBI or the NSA, has modified your boot partition so as to grab
your password. Add the following lines at the beginning of the system
initialisation script:

echo -n "Checking master boot record integrity: "
if [ "`dd if=/dev/hda count=1 2>/dev/null | md5sum`" = "e051a4532356709c73b86789acfbdbbd  -" ]
    echo "OK."
    echo -n "FAILED! press Enter to continue."

echo -n "Checking bootloader integrity: "
if [ "`dd if=/dev/hda1 2>/dev/null | md5sum`" = "f3686a17fac8a1090d962bef59c86d3b  -" ]
    echo "OK."
    echo -n "FAILED! press Enter to continue."

(you should replace the two md5sums above with the correct ones).

Now, if you're low on RAM you'll need some swap space. For example,
let's say hda3 will hold your encrypted swap partition; you need to
create the swap device first:

# losetup -e aes128 /dev/loop1 /dev/hda3
# mkswap /dev/loop1

Then add the following lines at the beginning of the system startup script:

echo "password chosen above" | losetup -p 0 -e ae128 /dev/loop1 /dev/hda3
swapon /dev/loop1

...and you're finally done.

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