Wednesday, April 25, 2012

Encrypt Partitions with dm-crypt LUKS

Encrypt Partitions with dm-crypt LUKS

These instructions use the Linux dm-crypt (device-mapper) facility available on the 2.6 kernel. In this example, lets encrypt the partition /dev/sdc1, it could be however any other partition or disk, or USB or a file based partition created with losetup. In this case we would use /dev/loop0. The device mapper uses labels to identify a partition. We use sdc1 in this example, but it could be any string.

dm-crypt with LUKS

LUKS with dm-crypt has better encryption and makes it possible to have "multiple passphrase" for the same partition or to change the password easily. To test if LUKS is available, simply type

# cryptsetup --help

if nothing about LUKS shows up, use the instructions below Without LUKS. First create a partition if necessary:

# fdisk /dev/sdc

Create encrypted partition

# dd if=/dev/urandom of=/dev/sdc1          # Optional. For paranoids only (takes days)
# cryptsetup -y luksFormat /dev/sdc1       # This destroys any data on sdc1
# cryptsetup luksOpen /dev/sdc1 sdc1
# mkfs.ext3 /dev/mapper/sdc1               # create ext3 file system
# mount -t ext3 /dev/mapper/sdc1 /mnt
# umount /mnt
# cryptsetup luksClose sdc1                # Detach the encrypted partition


# cryptsetup luksOpen /dev/sdc1 sdc1
# mount -t ext3 /dev/mapper/sdc1 /mnt


# umount /mnt
# cryptsetup luksClose sdc1

Using multiple passphrase for same partition

We can store another password in a file (say ~mitesh/temp/key_file.txt) and use it.

# cryptsetup luksAddKey /dev/sdc1 ~mitesh/temp/key_file.txt 
Enter any passphrase: 
# cryptsetup luksOpen --key-file  ~mitesh/temp/key_file.txt /dev/sdc1 sdc1

dm-crypt without LUKS

# cryptsetup -y create sdc1 /dev/sdc1      # or any other partition like /dev/loop0
# dmsetup ls                               # check it, will display: sdc1 (254, 0)
# mkfs.ext3 /dev/mapper/sdc1               # This is done only the first time!
# mount -t ext3 /dev/mapper/sdc1 /mnt
# umount /mnt/
# cryptsetup remove sdc1                   # Detach the encrypted partition

Do exactly the same (without the mkfs part!) to re-attach the partition. If the password is not correct, the mount command will fail. In this case simply remove the map sdc1 (cryptsetup remove sdc1) and create it again.