Debian Administration

Encrypting an existing Debian lenny installation

Once in a time, I get to travel to places that make me worry about the data on my laptop. This time, it is not the US, but another openly democratic country where they kill you for a joint, let alone nude pictures. Enough politics, though.

I have a laptop with the /boot in a separate partition, followed by a /root partition and a /swap. Having a separate /boot is mandatory, as the BIOS has to load an unencrypted kernel and its initrd image before being able to access the encrypted partition. Another option is to keep /boot on a USB stick, but this setup can take a whole other post.

First things first, let us install software for managing encrypted disks and updating the initrd image:

#aptitude install cryptsetup initramfs-tools

We have to make sure that the encryption modules are present in the initrd image, so I add the following three modules to the initrd config:

#echo aes-i586 >>/etc/initramfs-tools/modules
#echo dm-crypt >>/etc/initramfs-tools/modules
#echo dm-mod >>/etc/initramfs-tools/modules

This step is probably unnecessary as initramfs-update is able to figure out the modules needed by parsing /etc/crypttab and /etc/fstab and by checking the loaded modules.

Next step is to inform cryptsetup and inittab of the partition mapping between /dev/hda2 (the physical device) and /dev/mapper/root (its encryption interface).

#echo "root /dev/hda2 none luks" >>/etc/crypttab
#sed -i 's#/dev/hda2#/dev/mapper/root#' /etc/fstab

We also have to change the root device for grub the same way we did it for inittab:

#sed -i 's#/dev/hda2#/dev/mapper/root#' /boot/grub/menu.lst

Now, recreate the initrd image by issuing

#update-initramfs -k all -u

We are now ready to shutdown and to boot from a LiveCD in order to make a backup, create an encrypted partition and copy back the root filesystem contents on an already encrypted partition. I leave the reader at the exercise of choosing available backup options. A simple "cp -ax /mnt/root/* /mnt/backup" command will be enough to backup, though.

Once the backup is ready, erase the data on the partition by issuing

#shred -n1 -v /dev/hda2

and then create the encrypted partition with

#cryptsetup luksFormat /dev/hda2
#cryptsetup luksOpen /dev/hda2 root

After the encrypted device is set up and open, create a filesystem, mount it and copy the backup of the root partition to the encrypted device.

#mkfs.ext3 /dev/mapper/root
#mount /dev/mapper/root /mnt/root
#cp -ax /mnt/backup/* /mnt/root/

You are now ready to boot into the encrypted root partition.

Once the root encryption works, addding swap encryption is a piece of cake. Just add it to /etc/crypttab and modify the /etc/fstab accordingly:

#echo "swap /dev/hda3 /dev/random swap" >>/etc/crypttab
#sed -i 's#/dev/hda3#/dev/mapper/swap#' /etc/fstab

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