Encrypting a Linux root partition with LUKS and DM-CRYPT

One of our customers needed to have his Linux laptop's root partition encrypted. We found a HOWTO on achieving this with RHEL5, and we adapted it for CentOS 5. The technique is based on LUKS and DM-CRYPT. Kudos to my colleague Chris Evans for going through the exercise of getting this to work on CentOS 5 and for producing the documentation that follows, which I'm posting here hoping that it will benefit somebody at some point.

* Boot off of a Live CD, I used Fedora Core 9 Preview
* Find out which disk is which; for me /dev/sda was the external usb, and /dev/sdb was the internal
sfdisk -d /dev/sdb | sfdisk /dev/sda
pvcreate --verbose /dev/sda2
vgextend --verbose VolGroup00 /dev/sda2
pvmove --verbose /dev/sdb2 /dev/sda2 # This takes ages
vgreduce --verbose VolGroup00 /dev/sdb2
pvremove --verbose /dev/sdb2
fdisk /dev/sdb
* Change the partition type to 83 for /dev/sdb2
* Here is when you get to choose the password that will protect your partition:
cryptsetup --verify-passphrase --key-size 256 luksFormat /dev/sdb2

cryptsetup luksOpen /dev/sdb2 cryptroot
pvcreate --verbose /dev/mapper/cryptroot
vgextend --verbose VolGroup00 /dev/mapper/cryptroot
pvmove --verbose /dev/sda2 /dev/mapper/cryptroot # This takes ages
vgreduce --verbose VolGroup00 /dev/sda2
pvremove --verbose /dev/sda2
mkdir /mnt/tmp
mount /dev/VolGroup00/LogVol00 /mnt/tmp
cp -ax /dev/* /mnt/tmp/dev # I said no to overwriting any files
chroot /mnt/tmp/
(chroot) # mount -t proc proc /proc
(chroot) # mount -t sysfs sysfs /sys
(chroot) # mount /boot
(chroot) # swapon -a
(chroot) # vgcfgbackup

For the initrd, the blog mentions /etc/sysconfig/mkinitrd as a file. CentOS had a directory, I tried doing their suggestion as a file in there, moving the directory out, and making the file as they suggested. Both failed. So I ran the following command:

(chroot) # mkinitrd -v /boot/initrd-2.6.18-53.el5.crypt.img --with=aes --with=sha256 --with=dm-crypt 2.6.18-53.el5

Now we need to modify the initrd so that it will decrypt the partition at boot time

(chroot) # cd /boot
(chroot) # mkdir /boot/initrd-2.6.18-53.el5.crypt.dir
(chroot) # cd /boot/initrd-2.6.18-53.el5.crypt.dir
(chroot) # gunzip < ../initrd-2.6.18-53.el5.crypt.img | cpio -ivd

Now, we need to modify init by adding the following lines after the line which reads “mkblkdevs” and before “echo Scanning and configuring dmraid supported devices.”:

echo Decrypting root device
cryptsetup luksOpen /dev/sda2 cryptroot
echo Scanning logical volumes
lvm vgscan --ignorelockingfailure
echo Activating logical volumes
lvm vgchange -ay --ignorelockingfailure vg00

Copy cryptsetup and lvm to be put into the initrd, the blog doesn't mention it, but I'm sure it needs it.

cp /sbin/cryptsetup bin/
cp /sbin/lvm bin/

Compress the new initrd

find ./ | cpio -H newc -o | gzip -9 > /boot/initrd-2.6.18-53.el5.crypt.img

Modify the grub.conf. Copy the grub entry for the current kernel, and change as follows

title Centos Encrypted Server (2.6.18-53.1.4.el5)
initrd /initrd-2.6.18-53.el5.crypt.img

Unmount the fs's in the chroot, and exit

cd /
umount /boot
umount /proc
umount /sys

NOTE: Don't upgrade the kernel without upgrading the initrd and grub.conf.

Reboot and test :)

At this point you have an encrypted root partition. You should be prompted for a password during the boot process (the boot partition is not encrypted). If somebody steals your laptop, they won't be able to mount the root partition without knowing the password.

After you have crypto setup, you can find out information about it (such as the crypto algorithm used) via this command:

# cryptsetup luksDump /dev/sda2
LUKS header information for /dev/sda2

Version: 1
Cipher name: aes
Cipher mode: cbc-essiv:sha256
Hash spec: sha1
Payload offset: 2056
MK bits: 256
MK digest: af 2e e6 39 3e 79 60 bb 4a 2b 33 05 1c 86 3a 83 bc a0 ef c1
MK salt: 79 b2 13 53 6f 52 72 a1 b5 3d dc d3 72 cd d6 f4
e3 25 3c 6e 08 00 f3 1d 44 1e 90 47 bc 43 e7 07
MK iterations: 10
UUID: 721abe52-5122-447b-8ed0-5ca3b2b32366

Key Slot 0: ENABLED
Iterations: 247223
Salt: 86 c7 53 6a 13 a9 77 81 89 ec 90 b3 e5 6a ea 8d
da 0c 6f ad ec 3e 3c 47 2d 6e 5f 59 28 4e 7c 63
Key material offset: 8
AF stripes: 4000
Key Slot 1: DISABLED
Key Slot 2: DISABLED
Key Slot 3: DISABLED
Key Slot 4: DISABLED
Key Slot 5: DISABLED


Anonymous said…
Hi, can you please elaborate the reasons for two drives (USB/internal), as well as where/how/when to install CentOS5? I see the referenced site seems to tell how to do this with three partitions, so it seems the idea here is to use one of the partitions on the USB drive (sda)?

Thanks to you and your helper, though!
Rob Lockhart said…
Hi, can you elaborate more regarding need for sda/sdb? Would be nice if there was a way to install directly to LUKS partition (as FC9) but I trust CentOS5 more for stability. Thanks for your work!
ACiD GRiM said…
I know this is old, but could you see if you can look for a key file on a uuid partition first, and then ask for a password?
Anonymous said…
Thanks for this. Please keep posting linux related how-to's!
Ben Yau said…
Hey 1st anonymous, guess you figured this one out already since your post was 2008. But you can plug in the USB, install the OS and use the USB as /boot and then install the rest of the OS on your internal drives (should be /dev/sdb, could be reversed and internal is /dev/sda and USB is /dev/sdb). With CentOS if you choose to encrypt your install, then /boot is not encrypted but everything else is. So your USB drive will be the unencrypted part of your OS install

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