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https://github.com/rustybird/qubes-split-dm-crypt

Isolate secondary storage dm-crypt and LUKS1 header processing to Qubes DisposableVMs
https://github.com/rustybird/qubes-split-dm-crypt

disposablevm dm-crypt encryption luks qubes

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Isolate secondary storage dm-crypt and LUKS1 header processing to Qubes DisposableVMs

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# _Split dm-crypt_ for Qubes R4.0



**Isolates device-mapper based secondary storage encryption (i.e. not

the root filesystem) and LUKS1 header processing to DisposableVMs.**



Instead of directly attaching an encrypted LUKS1 partition from a source

VM such as sys-usb to a destination VM and decrypting it there, it works

like this:



1. The encrypted partition is attached from the source VM to an offline

   _device DisposableVM_ configured not to parse its content in any way:

   The kernel partition scanners, udev probes, and UDisks handling are

   disabled.



2. From there, the LUKS1 header is sent to a (short-lived) offline

   _header DisposableVM_ prompting for the password, and the encryption

   key is sent back to the device DisposableVM, which validates that it

   received an AES-XTS key and creates the dm-crypt mapping.



3. Finally, the decrypted partition is attached from the device

   DisposableVM to the destination VM.



**If the destination VM is compromised, it does not know the password or

encryption key. It also cannot easily exfiltrate decrypted data to the

disk in a form that would allow an attacker who seizes the disk contents

later to read it.** (But see below for caveats.)



## Usage



```

qvm-block-split attach|at|a [--ro] [] [] :

                detach|dt|d                         :



                overwrite-everything-with-random    :

                overwrite-header-with-random        :

                overwrite-header-with-format []  :

                overwrite-header-with-shell  []  :

                modify-header-with-shell     []  :



The  argument defaults to yet another DisposableVM.

 stands for an optional --key-file=[:] argument.

```



As seen above, **the `qvm-block-split` attach/detach commands accept a

subset of the familiar `qvm-block` syntax**, and some other commands are

included:



- Fully overwrite a device with random data



- Overwrite just the LUKS1 header with random data



- Format a new LUKS1 device with modern crypto parameters: AES-XTS with

  256+256 (instead of 128+128) bit keys, SHA512 (instead of SHA1) PBKDF2

  key derivation with 5 (instead of 0.1) seconds iteration time



## Remaining attacks



- After detaching, the password and/or key will linger in more RAM

  locations than without _Split dm-crypt_. Until there is a way to wipe

  the DisposableVMs' memory, and `qvm-block-split` is modified not to

  pass the key through dom0's memory, **power off your computer when

  memory forensics is a concern.**



- If both the destination VM and the source VM/disk are compromised,

  they could establish a covert channel using e.g. read and write access

  patterns, slowly saving some amount of decrypted data to the disk.



- If the source VM/disk is compromised and successfully exploits the

  header DisposableVM using a malicious LUKS1 header, a known AES-XTS

  key could be sent to the device DisposableVM and used to present

  malicious device content to the destination VM to potentially exploit

  it as well. **Be suspicious if you do not see the expected filesystem

  data in the destination VM. Or simply use a DisposableVM as the

  destination VM.**



- **Don't forget to overwrite your disk with random data before creating

  a LUKS1 volume on it.** Otherwise, a compromised destination VM could

  trivially save decrypted data to the disk in its free space, by

  encoding each bit as an unmodified (still empty or in some other way

  nonrandom-looking) or modified (random-looking) 128 bit AES block.



## Installation



1. Copy `vm/` to a DisposableVM Template's _TemplateVM_ (e.g.

   `fedora-XX`) - not to the DisposableVM Template _itself_ (e.g.

   `fedora-XX-dvm`).



   Inspect the code, and `sudo make install`; also install the `pv`

   (Pipe Viewer) package to be able to run the

   `overwrite-everything-with-random` command. Shut down the TemplateVM

   when finished.



2. Copy `dom0/bin/qvm-block-split` to dom0, e.g. into `~/bin/`, inspect

   the code extra carefully, and `chmod +x` the script.



3. Either make your DisposableVM Template from step 1 the system-wide

   default:



        qubes-prefs default_dispvm fedora-XX-dvm



   Or just let _Split dm-crypt_ know what it is:



        echo TEMPLATE_FOR_DISPVMS=fedora-XX-dvm >/etc/split-dm-crypt.conf



## Safety warning



The code's error handling is strict, and I haven't experienced any data

loss during development. Nevertheless, this is an early release. Please

**ensure you have a backup of all drives that are connected to your

computer.**



## Redistribution



_Split dm-crypt_ is under public domain equivalent license, see the

LICENSE-0BSD file for details.