qubes-doc/user/downloading-installing-upgrading/install-security.md

---
layout: doc
title: Installation Security
permalink: /doc/install-security/
redirect_from:
- /en/doc/install-security/
- /doc/InstallSecurity/
- /wiki/InstallSecurity/
---

# Installation Security Considerations #


## Verifying the Qubes ISO ##

You should [verify] the PGP signature on your Qubes ISO before you install
from it. However, if the machine on which you attempt the verification process
is already compromised, it could falsely claim that a malicious ISO has a good
signature. Therefore, in order to be certain that your Qubes ISO is trustworthy,
you require a trustworthy machine. But how can you be certain *that* machine is
trustworthy? Only by using another trusted machine, and so forth. This is a
[classic problem]. While various [solutions] have been proposed, the point is
that each user must ultimately make a choice about whether to trust that a file
is non-malicious.


## Choosing an Installation Medium ##

So, after taking some measures to verify its integrity and authenticity, you've
decided to trust your Qubes ISO. Great! Now you must decide what sort of medium
on which to write it so that you can install from it. From a Qubes-specific
security perspective, each has certain pros and cons.


### USB Drives ###

Pros:

 * Works via USB, including with a [USB qube].
 * Non-fixed capacity. (Easy to find one on which the ISO can fit.)

Cons:

 * Rewritable. (If the drive is mounted to a compromised machine, the ISO could
   be maliciously altered after it has been written to the drive.)
 * Untrustworthy firmware. (Firmware can be malicious even if the drive is new.
   Plugging a drive with rewritable firmware into a compromised machine can
   also [compromise the drive][BadUSB]. Installing from a compromised drive
   could compromise even a brand new Qubes installation.)


### Optical Discs ###

Pros:

 * Read-only available. (If you use read-only media, you don't have to worry
   about the ISO being maliciously altered after it has been written to the
   disc. You then have the option of verifying the signature on multiple
   different machines.)

Cons:

 * Fixed capacity. (If the size of the ISO is larger than your disc, it will be
   inconvenient.)
 * Passthrough recording (a.k.a., "burning") is not supported by Xen. (This
   mainly applies if you're upgrading from a previous version of Qubes.)
   Currently, the only options for recording optical discs (e.g., CDs, DVDs,
   BRDs) in Qubes are:
   1. Use a USB optical drive.
   2. Attach a SATA optical drive to a secondary SATA controller, then assign
      this secondary SATA controller to an AppVM.
   3. Use a SATA optical drive attached to dom0.

   (Option 3 violates the Qubes security model since it entails transferring an
   untrusted ISO to dom0 in order to burn it to disc, which leaves only the
   other two options.)

Considering the pros and cons of each, perhaps a USB drive with non-rewritable
(or at least cryptographically-signed) firmware and a physical write-protect
switch might be the option.


[verify]: /security/verifying-signatures/
[classic problem]: https://www.ece.cmu.edu/~ganger/712.fall02/papers/p761-thompson.pdf
[solutions]: https://www.dwheeler.com/trusting-trust/
[USB qube]: /doc/usb-qubes/#creating-and-using-a-usb-qube
[BadUSB]: https://srlabs.de/badusb/