Encrypting a Linux Partition Using LUKS

Sarath Pillai's picture
luks encryption

Red Hat started to implement encryption of the entire file system, from Red Hat Enterprise Linux 6 onwards. This encryption of the entire file system's is a much better way to secure the data on the disk. This is an added advantage for people who are prone to using mobile devices for their day to day work. The main advantage is that, even if the entire disk is lost, your data cannot be easily accessed as it is encrypted and requires a pass phrase key to access the data inside.

Red hat implements this through LUKS.


What is LUKS?


LUKS stands for Linux Unified Key Setup. LUKS enables the facility to encrypt a whole partition in Linux for security purposes. LUKS was initially created by Clemens Fruhwirth. The main added advantage of using LUKS for encryption over other encryption technologies is that it is platform independent. In short you can call LUKS as a standard for implementing encryption of file system's in Linux.


What does LUKS do?


Block devices can be encrypted using LUKS. And LUKS does not bother about the data inside the block device, because of which you can encrypt any kind of partition(even swap) using LUKS. In order to access the contents of an encrypted file system using LUKS the user is required to provide a pass phrase for the key which intern will be used to unlock the file system.

LUKS takes the help of dm-crypt module to access Linux Kernel Device Mapper.

In order to create an encrypted partition or access the encrypted partition, cryptsetup utility is used by the users.


What are the Limitations of LUKS?


  • Encryption upto the level of files cannot be done using LUKS
  • It has got a limit regarding the no of users, that can have different passwords in order to access the same block device.


How to encrypt a partition using LUKS?


Red Hat Enterprise linux provides you an interface to encrypt the partitions during the installation time, which is quite easy. In this tutorial we will be seeing how can we create a new partition and encrypt it using LUKS. 

I will be using CentOS 5.8 for this example tutorial. You can easily do the same in Red Hat enterprise Linux and also in Centos(and many other distributions.)

For this tutorial i will be creating a Logical Volume of 5gb and will encrypt that volume, with the help of LUKS.


[root@myvm ~]# lvcreate -L 5G -n myvolume VolGroup00
  Logical volume "myvolume" created

So my device which i just created is /dev/mapper/VolGroup00-myvolume. We will be encrypting this volume.

I have created a logical volume just for this example, you can use any of the newly created partition, you want. Like /dev/sda1, /dev/sda3 etc.


The next step that we need to do is to format the device with cryptsetup utility and make the device LUKS encrypted.


[root@myvm /]# cryptsetup luksFormat /dev/mapper/VolGroup00-myvolume
This will overwrite data on /dev/mapper/VolGroup00-myvolume irrevocably.
Are you sure? (Type uppercase yes): YES
Enter LUKS passphrase:
Verify passphrase:
Command successful.

You can replace "/dev/mapper/VolGroup00-myvolume" with whatever the partition you are going to encrypt with LUKS.

The above cryptsetup command will ask for a confirmation, because this will destroy any data you have on the partition. Then you are prompted for a passphrase.

Now lets see some detailed information about the encryption on our device /dev/mapper/VolGroup00-myvolume .


[root@myvm ~]# cryptsetup luksDump /dev/mapper/VolGroup00-myvolume
LUKS header information for /dev/mapper/VolGroup00-myvolume
Version:        1
Cipher name:    aes
Cipher mode:    cbc-essiv:sha256
Hash spec:      sha1
Payload offset: 1032
MK bits:        128
MK digest:      4f 4a 2e 9e 7e 04 44 e5 29 3e 6d d7 9e 56 17 2f 9f 5c bf 42
MK salt:        a1 e5 ba 61 ce e9 48 7b 60 7e f2 e3 c5 61 53 22
                f6 0b b2 8f ff 02 5f 56 62 0b 3d 3a 0f 7c c3 04
MK iterations:  10
UUID:           a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3
Key Slot 0: ENABLED
        Iterations:             311917
        Salt:                   d4 3e f3 37 5d 89 62 be d5 ab ee 27 de 17 b7 f8
                                cf 88 47 bf ab eb 2e 62 69 86 77 72 bc 26 a8 ed
        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
Key Slot 6: DISABLED
Key Slot 7: DISABLED

You can clearly see from the above output that we are using aes encryption with sha256.

Now lets see how are we going to access this newly encrypted device. Or in otherwords how will the kernel's device mapper recognize this encrypted partition of ours.

You can get the UUID of the newly encrypted device as shown below.

[root@myvm ~]# cryptsetup luksUUID /dev/mapper/VolGroup00-myvolume

Now We will assign a device mapping name to this device as shown below.


[root@myvm ~]# cryptsetup luksOpen /dev/mapper/VolGroup00-myvolume luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3
Enter LUKS passphrase for /dev/mapper/VolGroup00-myvolume:
key slot 0 unlocked.
Command successful.

Now You will have a device named "luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3" in /dev/mapper. Also the above command luksOpen will depcrypt the filesystem so that it can be accessed


[root@myvm mapper]# cd /dev/mapper/ ; ls
control                                    VolGroup00-LogVol01
luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3  VolGroup00-myvolume


Please do remember that the device is open and accessible as of now because we ran "cryptsetup luksopen" command at the time of giving it a device mapping name.

Now lets format our device in the same way we format our normal partitions.


[root@myvm mapper]# mke2fs -j /dev/mapper/luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3
mke2fs 1.39 (29-May-2006)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
655360 inodes, 1310591 blocks
65529 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=1342177280
40 block groups
32768 blocks per group, 32768 fragments per group
16384 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 32 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

Now Lets mount this device, as normal.


[root@myvm ~]# mount /dev/mapper/luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3 /mypartition/


Now lets check and confirm whether our LUKS formatted and encrypted device is mounted properly with df -h command.


[root@myvm ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
                       31G  2.7G   27G  10% /
/dev/sda1              99M   13M   82M  13% /boot
tmpfs                 252M     0  252M   0% /dev/shm
                      5.0G  139M  4.6G   3% /mypartition

We can clearly see from the last line of the above output that our newly mapped encrypted device is mounted under /mypartition

How to umount and Lock back the LUKS encrypted partition

First lest umount the partition as normal..and then lock back the filesystem through cryptsetup.


[root@myvm ~]# umount /dev/mapper/luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3
[root@myvm ~]# cryptsetup luksClose /dev/mapper/luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3

The first command above unmounts and the second command locks back the filesystem.

How to Unlock and mount LUKS encrypted Partition

Now you can again unlock and mount as shown below,whenever required. But will ask for the passphrase.

[root@myvm ~]# cryptsetup luksOpen /dev/mapper/VolGroup00-myvolume luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3
Enter LUKS passphrase for /dev/mapper/VolGroup00-myvolume:
key slot 0 unlocked.
Command successful.
[root@myvm ~]# mount /dev/mapper/luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3 


Note: luks-a8ac8a06-baf8-4dbc-9c2b-52d3080e9fe3 is just a convention that we follow for identifying the device with the UUID. You can use anything instead of that...But you will need to give that name whenever you mount.

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