Remote full disk encryption on Linux
Using full disk encrytion means that you need to enter your passphrase before booting the computer. If the computer in question is in a remote location (such as a datacenter), this means that you don’t have access to the console and thus cannot enter the passphrase needed to unlock the disks. However, Linux offers the possibility of running a lightweight SSH server before fully booting the system so that you can enter the key.
We will be setting up LUKS (with dm-crypt) on Debian
Wheezy. According to the cryptsetup Google Code page, the cipher
mode aes-xts-plain should not be used for encrytped container size
larger than 2 TiB, aes-xts-plain64 should be used.
Drawbacks
- The ssh server keys for the small pre-boot ssh daemon need to be stored unencrypted. This means that an attacker could possibly access them and fake our server, tricking us into giving up the passphrase.
Preparation
Before proceeding, install the needed packages to run the small ssh server during the boot process2.
# apt-get install busybox dropbear
This is the default on Debian, but check if
/etc/initramfs-tools/initramfs.conf contains BUSYBOX=y. It’s also
important that it doesn’t contain DROPBEAR=n (the default is
y). Also verify that the keys for dropbear were generated, located
in /etc/initramfs-tools/etc/dropbear/. if not, use dropbearkey to
create RSA and DSS keys.
As the initramfs will not be encrypted, publickey authentication is
now assumed. Keys have probably been configured for you in
/etc/initramfs-tools/root/.ssh/. Following are the commands to
create the keys manually.
First we have to create a key in the format used by dropbear. We then
convert it to the format used by OpenSSH and extract the public part,
before adding it to authorized_keys.
# dropbearkey -t rsa -f /etc/initramfs-tools/root/.ssh/id_rsa.dropbear
# dropbearconvert dropbear openssh /etc/initramfs-tools/root/.ssh/id_rsa.dropbear /etc/initramfs-tools/root/.ssh/id_rsa
# dropbearkey -y -f /etc/initramfs-tools/root/.ssh/id_rsa.dropbear | grep "^ssh-rsa " > /etc/initramfs-tools/root/.ssh/id_rsa.pub
# cat /etc/initramfs-tools/root/.ssh/id_rsa.pub >> /etc/initramfs-tools/root/.ssh/
Later on, we will generate a new initramfs that includes dropbear and these keys. You should copy the ssh keys to your computer.
If your disks are already encrypted, you can stop here and just
generate a new initramfs with update-initramfs -u. Otherwise it’s
time to reboot into your particular variation of a rescue
OS to encrypt the disks. E.g. connecting via ssh:
Encrypting the disks remotely
Usually when renting dedicated servers, you won’t get to do the OS install yourself so you will have a server with unencrypted disks. Other hosting providers will let you have a remote console so you can set up your disks with encrypted disks.
This section deals with setting up FDE post-install. We need to do this as soon after install as possible, because we will have to keep the current OS setup in memeory while encrypting the disks.
localhost:~$ ssh -o "UserKnownHostsFile=~/.ssh/known_hosts.rescue" root@server
root@server's password:
root@rescue ~ #
Before encrypting the drives, we need to copy our old installation into memory since the disks will be wiped clean:
# mkdir /oldroot
# mount /dev/mapper/vg0-root /mnt/
# mount /dev/mapper/vg0-home /mnt/home/
# mount /dev/mapper/vg0-srv /mnt/srv/
# rsync -a /mnt/ /oldroot/
# umount /mnt/home/
# umount /mnt/srv/
# umount /oldroot
In order to encrypt the partition we need to remove the current LVM setup. We also backup the vg config to make life easier.
# vgcfgbackup vg0 -f vg0.freespace
# vgremove vg0
Now we can encrypt the disk drive. Since this is a server that will
boot relatively seldom, we specify --iter-time 6000 to determine the
number of iterations needed for the server to spend 6 seconds on the
key derivation function.
# cryptsetup luksFormat --cipher aes-xts-plain64 -s 256 --iter-time 6000 /dev/md1
# cryptsetup luksDump /dev/md1
# cryptsetup luksOpen /dev/md1 cryptroot
# ls /dev/mapper/cryptroot
Now we will re-create the LVM volume group, but we need to edit the
backup. Instead of the volume group using /dev/md1 as it’s physical
volume, it will now use /dev/mapper/cryptroot. We need to
find UID for cryptroot and edit the backup accordingly. The
settings are id and device under vg0.physical_volumes.pv0.
# blkid /dev/mapper/cryptroot
/dev/mapper/cryptroot: UUID="HEZqC9-zqfG-HTFC-PK1b-Qd2I-YxVa-QJt7xQ"
# cp vg0.freespace /etc/lvm/backup/vg0
# nano /etc/lvm/backup/vg0
[Edit file]
# vgcfgrestore vg0
Restored volume group vg0
# vgchange -a y vg0
The next task is to create filesystems on the logical volumes and then restore the system
# mkfs.ext4 /dev/vg0/root
# mkfs.ext4 /dev/vg0/home
# mkfs.ext4 /dev/vg0/srv
# mount /dev/vg0/root /mnt
# mkdir /mnt/home /mnt/srv
# mount /dev/vg0/home /mnt/home
# mount /dev/vg0/srv /mnt/srv
# rsync -a /oldroot/ /mnt/
Before we can change root into our system we need the special filesystems. We need to change root to fix some things
# mount /dev/md0 /mnt/boot
# mount --bind /dev /mnt/dev
# mount --bind /sys /mnt/sys
# mount --bind /proc /mnt/proc
# chroot /mnt
We need a crypttab to be able to boot from the encrypted filesystem. Since we installed LVM from the image, we can leave the fstab as-is.
# cat/etc/crypttab
cryptroot /dev/md1 none luks
# cat /etc/fstab
proc /proc proc defaults 0 0
/dev/md/0 /boot ext3 defaults 0 0
/dev/vg0/root / ext4 defaults 0 0
/dev/vg0/home /home ext4 defaults 0 0
/dev/vg0/srv /srv ext4 defaults 0 0
/dev/vg0/swap swap swap defaults 0 0
We just need to tell grub about the new root device
and regenerate the initramfs to include dropbear and busybox. This
will use the keys we generated for /etc/initramfs-tools/root/.ssh/.
# update-initramfs -u
# update-grub
# grub-install /dev/sda
# grub-install /dev/sdb
# echo "/sbin/ifdown --force eth0" >> /etc/rc.local
# echo "/sbin/ifup --force eth0" >> /etc/rc.local
And now, reboot and hope for the best
# exit
# umount /mnt/boot /mnt/home /mnt/srv /mnt/proc /mnt/sys /mnt/dev
# umount /mnt
# sync
# reboot
Unlocking remotely
localhost:~$ ssh -o "UserKnownHostsFile=~/.ssh/known_hosts.initramfs" -i ~/.ssh/id_rsa.initramfs root@server
~ # echo -ne $PASSPHRASE > /lib/cryptsetup/passfifo