USB Persistence & Encrypted Persistence

In this workshop, we will examine the various features available to us when booting Kali Linux from USB devices. We will explore features such as persistence, creating LUKS encrypted persistence stores, and even dabble in “LUKS Nuking” our USB drive. The default Kali Linux ISOs (from 1.0.7 onwards) support USB encrypted persistence.

0x01 – Start by imaging the Kali ISO onto your USB stick (ours was /dev/sdb). Once done, you can inspect the USB partition structure using parted /dev/sdb print.

dd if=kali-linux-2016.2-amd64.iso of=/dev/sdb bs=1M

0x02 – Create and format an additional partition on the USB stick. In our example, we create a persistent partition of about 7 GB in size:

root@kali:~# parted
GNU Parted 2.3
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.

(parted) print devices                                                    
/dev/sda (480GB)
/dev/sdb (31.6GB)

(parted) select /dev/sdb                                                  
Using /dev/sdb

(parted) print                                                            
Model: SanDisk SanDisk Ultra (scsi)
Disk /dev/sdb: 31.6GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      32.8kB  2988MB  2988MB  primary               boot, hidden
 2      2988MB  3050MB  64.9MB  primary  fat16

(parted) mkpart primary 3050 10000                                        
(parted) quit                                                            
Information: You may need to update /etc/fstab.

0x04 – Encrypt the partition with LUKS:

cryptsetup --verbose --verify-passphrase luksFormat /dev/sdb3

0x05 – Open the encrypted partition:

cryptsetup luksOpen /dev/sdb3 my_usb

0x06 – Create an ext3 filesystem and label it.

mkfs.ext3 /dev/mapper/my_usb
e2label /dev/mapper/my_usb persistence

0x07 – Mount the partition and create your persistence.conf so changes persist across reboots:

mkdir -p /mnt/my_usb
mount /dev/mapper/my_usb /mnt/my_usb
echo "/ union" > /mnt/my_usb/persistence.conf
umount /dev/mapper/my_usb
cryptsetup luksClose /dev/mapper/my_usb

Now your USB stick is ready to plug in and reboot into Live USB Encrypted Persistence mode.

Multiple Persistence Stores

At this point we should have the following partition structure:

root@kali:~# parted /dev/sdb print

We can add additional persistence stores to the USB drive, both encrypted or not…and choose which persistence store we want to load, at boot time. Let’s create one more additional non-encrypted store. We’ll label and call it “work”.

0x01 – Create an additional, 4th partition which will hold the “work” data. We’ll give it another 5GB of space.

root@kali:~# parted /dev/sdb
GNU Parted 2.3
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            
Model: SanDisk SanDisk Ultra (scsi)
Disk /dev/sdb: 31.6GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
 1      32.8kB  2988MB  2988MB  primary               boot, hidden
 2      2988MB  3050MB  64.9MB  primary  fat16
 3      3050MB  10.0GB  6947MB  primary

(parted) mkpart primary 10000 15000                                      
(parted) quit                                                            
Information: You may need to update /etc/fstab.

0x02 – Format the fourth partition, label it “work”.

mkfs.ext3 /dev/sdb4
e2label /dev/sdb4 work

0x03 – Mount this new partition and create a persistence.conf in it:

mkdir -p /mnt/usb
mount /dev/sdb4 /mnt/usb
echo "/ union" > /mnt/usb/persistence.conf
umount /mnt/usb

Boot the computer, and set it to boot from USB. When the boot menu appears, edit the persistence-label parameter to point to your preferred persistence store!

Emergency Self Destruction of Data in Kali

As penetration testers, we often need to travel with sensitive data stored on our laptops. Of course, we use full disk encryption wherever possible, including our Kali Linux machines, which tend to contain the most sensitive materials.

root@kali:~# cryptsetup luksAddNuke /dev/sdb3
Enter any existing passphrase:
Enter new passphrase for key slot:

Now dump the keyslots to see the changes:

root@kali:~# cryptsetup luksDump /dev/sda5
Device /dev/sda5 doesn't exist or access denied.
root@kali:~# cryptsetup luksDump /dev/sdb3
LUKS header information for /dev/sdb3

Version:          1
Cipher name:      aes
Cipher mode:      xts-plain64
Hash spec:        sha1
Payload offset:   4096
MK bits:          256
MK digest:        f7 17 b9 a7 9f 7f 9b 21 f2 b9 40 78 c2 97 f5 f0 c2 bb 28 8b
MK salt:          f5 a4 80 02 e7 21 0d 7e 5a 64 f4 96 78 a3 15 3c
                  09 7b 3f 41 80 2b 5c bf c5 de 92 70 69 bb 34 b2
MK iterations:    64500
UUID:             96793acb-c2d3-45b7-aed9-1af952386556

Key Slot 0: ENABLED
  Iterations:             258064
  Salt:                   df 3c d6 03 4a 78 ce ef 62 fd f1 56 25 d4 c5 96
                          2a 12 bb 94 4b d7 cf c1 0a b5 27 47 09 ae 31 46
  Key material offset:    8
  AF stripes:             4000
Key Slot 1: ENABLED
  Iterations:             259108
  Salt:                   30 07 ff ef fc f5 74 65 04 f7 66 87 77 f1 74 4f
                          7d 2f 76 e2 71 e7 6a 9c 6d c1 c1 7b 80 53 cb c1
  Key material offset:    264
  AF stripes:             4000
Key Slot 2: DISABLED
Key Slot 3: DISABLED
Key Slot 4: DISABLED
Key Slot 5: DISABLED
Key Slot 6: DISABLED
Key Slot 7: DISABLED
root@kali:~#

Backup you LUKS keyslots and encrypt them:

cryptsetup luksHeaderBackup --header-backup-file luksheader.back /dev/sdb3
openssl enc -d -aes-256-cbc -in luksheader.back.enc -out luksheader.back

Now boot into your encrypted store, and give the Nuke password, rather than the real decryption password. This will render any info on the encrypted store useless. Once this is done, verify that the data is indeed inacessible.

Lets restore the data now. We’ll decrypt our backup of the LUKS keyslots, and restore them to the encrypted partition:

openssl enc -d -aes-256-cbc -in luksheader.back.enc -out luksheader.back
cryptsetup luksHeaderRestore --header-backup-file luksheader.back /dev/sdb3

Our slots are now restored. All we have to do is simply reboot and provide our normal LUKS password and the system is back to its original state.