Uwe Hermann | A slightly paranoid Debian developer

This is what I set up for backups recently using a cheap USB-enclosure which can house 2 SATA disks and shows them as 2 USB mass-storage devices to my system (using only one USB cable). Without any further introduction, here goes the HOWTO:

First, create one big partition on each of the two disks (/dev/sdc and /dev/sdd in my case) of the exact same size. The cfdisk details are omitted here.

  $ cfdisk /dev/sdc
  $ cfdisk /dev/sdd

Then, create a new RAID array using the mdadm utility:

  $ mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdc1 /dev/sdd1

The array is named md0, consists of the two devices (--raid-devices=2) /dev/sdc1 and /dev/sdd1, and it's a RAID-1 array, i.e. data is simply mirrored on both disks so if one of them fails you don't lose data (--level=1). After this has been done the array will be synchronized so that both disks contain the same data (this process will take a long time). You can watch the current status via:

  $ cat /proc/mdstat
  Personalities : [raid1]
  md0 : active raid1 sdd1[1] sdc1[0]
        1465135869 blocks super 1.1 [2/2] [UU]
        [>....................]  resync =  0.0% (70016/1465135869) finish=2440.6min speed=10002K/sec
  unused devices: 

Some more info is also available from mdadm:

  $ mdadm --detail --scan
  ARRAY /dev/md0 metadata=1.01 name=foobar:0 UUID=1234578:1234578:1234578:1234578

  $ mdadm --detail /dev/md0
  /dev/md0:
          Version : 1.01
    Creation Time : Sat Feb  6 23:58:51 2010
       Raid Level : raid1
       Array Size : 1465135869 (1397.26 GiB 1500.30 GB)
    Used Dev Size : 1465135869 (1397.26 GiB 1500.30 GB)
     Raid Devices : 2
    Total Devices : 2
      Persistence : Superblock is persistent
      Update Time : Sun Feb  7 00:03:21 2010
            State : active, resyncing
   Active Devices : 2
  Working Devices : 2
   Failed Devices : 0
    Spare Devices : 0
   Rebuild Status : 0% complete
             Name : foobar:0  (local to host foobar)
             UUID : 1234578:1234578:1234578:1234578
           Events : 1
      Number   Major   Minor   RaidDevice State
         0       8       33        0      active sync   /dev/sdc1
         1       8       49        1      active sync   /dev/sdd1

Next, you'll want to create a big partition on the RAID device (cfdisk details omitted)...

  $ cfdisk /dev/md0

...and then encrypt all the (future) data on the device using dm-crypt+LUKS and cryptsetup:

  $ cryptsetup --verbose --verify-passphrase luksFormat /dev/md0p1
  Enter your desired pasphrase here (twice)
  $ cryptsetup luksOpen /dev/md0p1 myraid

After opening the encrypted container with cryptsetup luksOpen you can create a filesystem on it (ext3 in my case):

  $ mkfs.ext3 -j -m 0 /dev/mapper/myraid

That's about it. In future you can access the RAID data by using the steps below.

Starting the RAID and mouting the drive:

  $ mdadm --assemble /dev/md0 /dev/sdc1 /dev/sdd1
  $ cryptsetup luksOpen /dev/md0p1 myraid
  $ mount -t ext3 /dev/mapper/myraid /mnt

Shutting down the RAID:

  $ umount /mnt
  $ cryptsetup luksClose myraid
  $ mdadm --stop /dev/md0

That's all. Performance is shitty due to all the data being shoved out over one USB cable (and USB itself being too slow for these amounts of data), but I don't care too much about that as this setup is meant for backups, not performance-critical stuff.

Update 04/2011: Thanks to Bohdan Zograf there's a Belorussian translation of this article now!

A few weeks ago I published a small HOWTO for using loop-aes to encrypt your hard drive, usb thumb drive etc.

As I have bought a new 300 GB external USB disk drive on Friday, I have tried something new this time: disk encryption using dm-crypt / LUKS. It has been suggested to me multiple times that dm-crypt is superior to loop-aes, however I didn't get a real reason. Yes, it doesn't require any kernel patches and is easier to setup. But has any serious cryptographer looked at it sharply, yet? Did it withhold his eye contact?

Anyways, here's how I encrypted my 300 GB drive. I largely followed the guide at the EncryptedDeviceUsingLUKS wiki page...

1. Make sure you run Linux 2.6.16 or better. Previous versions suffer from an implementation problem which affects the security of dm-crypt, see Linux Kernel dm-crypt Local Cryptographic Key Disclosure.
2. Enable the following options in your kernel:
   • Code maturity level options
     • Prompt for development and/or incomplete code/drivers
   • Device Drivers -> Multi-device support (RAID and LVM)
     • Device mapper support
     • Crypt target support
   • Cryptographic options
     • AES cipher algorithms
3. Overwrite the whole drive with random data in order to slow down attacks on the encryption. At the same time perform a bad blocks scan to make sure the hard drive is not going to die too soon:
   badblocks -c 10240 -s -w -t random -v /dev/sdb
   Replace /dev/sdb with whatever is correct on your system. If you're really paranoid, and are willing to wait one or two days, do this:
   dd if=/dev/urandom of=/dev/sdb
4. Install the required packages:
   apt-get install cryptsetup
   The current cryptsetup in Debian unstable already supports LUKS, which was not the case a while ago, if I'm not mistaken. So Debian testing or stable will most probably not work!
5. Create one or more partitions on the drive:
   cfdisk /dev/sdb
   I created one big 300 GB partition, /dev/sdb1.
6. Setup LUKS:
   cryptsetup --verbose --verify-passphrase luksFormat /dev/sdb1
   Enter a good passphrase here. Don't spoil the whole endeavour by chosing a stupid or short passphrase.
7. Open the encrypted device and assign it to a virtual /dev/mapper/samsung300gb device:
   cryptsetup luksOpen /dev/sdb1 samsung300gb
8. Create a filesystem on the encrypted device:
   mkfs.ext3 -j -m 1 -O dir_index,filetype,sparse_super /dev/mapper/samsung300gb
   I used ext3 with some optimizations, see mke2fs(8).
9. Mount the encrypted partition:
   mkdir /mnt/samsung300gb
mount /dev/mapper/samsung300gb /mnt/samsung300gb
   That's it. Everything you write to /mnt/samsung300gb will be encrypted transparently.
10. For unmounting use:
    umount /mnt/samsung300gb
cryptsetup luksClose /dev/mapper/samsung300gb

After unmounting, nobody will be able to see your data without knowing the correct passphrase. Drive is stolen? No problem. Drive is broken, and you want to send it in for repair without the guys there poking in your data? No problem. You leave the USB drive at home and some jerk breaks into your house, steals your drive, rapes your wife, and kills your kids? No problem. Well, sort of, but you get the idea ;-)

There's more things you can do, thanks to LUKS: have multiple passphrases which unlock your data, change/add/remove passphrases as you see fit, etc.

Comments?

Update 2006-04-17: You have to use cryptsetup from unstable if you want LUKS support. cryptsetup in testing does not support this (thanks Ariel).