Introduction to analysing full disk encryption solutions

I’ve written a couple of times on the subject of boot loaders and full disk encryption, but I haven’t really explored it in more detail. With this blog post I hope to dive a bit deeper into how to actually start performing these type of analysis and why they are useful to perform. I’ll start with the usefulness first and then go into the part on how to do it, but will not be fully reversing a disk encryption boot loader. I won’t be doing a lot of hard-core reversing like finding vulnerabilities within the cryptographic operations or reversing custom filesystem implementations, but hopefully provide enough information to get started in the area of reversing unknown boot loaders.

The type of products with which you can use the approaches and techniques described in this blog post are the most useful when applied to full disk encryption (FDE) solutions that are configured to not require pre-boot authentication. The reason being, that you then could potentially obtain the disk decryption key. If the solution requires pre-boot authentication, the information that you can obtain, might be reduced to meta-data or ‘deleted’ files. Which brings us to the whole, why are these type of analysis useful?

The reason of why this is useful, I didn’t fully realise until a couple of years ago when a colleague introduced me to the wonders of all the (hidden) information that FDE solutions may contain. Let’s look at the type of information that you may encounter while investigating these solutions:

  • (encrypted) Hidden file systems
  • (obfuscated) Encryption keys
  • Usernames
  • (hashed/encrypted) Passwords
  • Windows domain credentials
  • Configuration information of the FDE solution
  • Files marked for deletion
  • Finding 0days and bypassing encryption

Based on the above list of items we can pretty much conclude that analysing FDE solutions is useful from an offensive as well as from a defensive point of view. It can either help us to breach a target network or obtain sensitive information as well as collect forensic evidence or aid us into understanding the specific cryptographic implementation to enable us to decrypt the disk and analyse it. The helper tools I’ve used in this blog post can be found here. Keep on reading if you want to know the rest of all the details and the process I usually follow. I’ll try to describe the following steps:

  • Creating a (partial) copy of the disk
  • Analysing the disk
  • Static & dynamic boot analysis

Since I don’t have easy access to disk encryption software with the exact features I’d like to analyse I’ll be using DiskCryptor as an example product.

For some reason it seems that the products with the most interesting features to reverse engineer have a horrendous ‘request trial’ process as well as not providing trials to a random researcher on the internet :( sad panda :(

The other reason to use DiskCryptor is the fact that it is open source, thus enabling people that want to get started with type of stuff to more easily understand difficult snippets of assembly. My personal approach to a lot of reversing challenges usually revolves around finding a similar open source variant first or finding the open source components used in the proprietary solution if applicable. Reason being that it makes your life a lot easier to understand not only general concepts, but also specific code quirks. A very nice explanation on finding as much information as possible before your start reversing is given by Alex Ionescu in his offensive con keynote ‘Reversing without reversing’.

Oh and there is no specific goal, besides just explaining my general thought process. As a side note I am no reverse engineering expert, so feel free to correct me :-)

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attacking encrypted systems with qemu and volatility

Lately I’ve had to deal with setups which had transparent full disk encryption and were pretty hardened. If you are wondering what ‘transparent full disk encryption’  means, that’s how I call solutions that encrypt your hard disk, but don’t require any interaction from the user to boot into the operating system. They usually accomplish this because they:

  • use secure boot and a TPM with key sealing (good)
  • they use proprietary software-only obfuscation to hide the key (bad)
  • use an external hardware device to store the keys without secure boot or key sealing (bad)

Most of the time the goal is to break out of a preconfigured application and the usual tricks like these ones, don’t really work:

However getting access to safe mode / start up repair does partially work for some of these setups:

Partially, because most of the options were not present and those that were present only gave me a cmd.exe which was disabled with a local group policy. An interesting approach the defence side took was replacing explorer.exe with an executable which did nothing. Even if you managed to break out of their application you still had nothing, no desktop, no menu, no buttons etc. For a few setups where the ‘startup-repair’ options seemed to work the encryption drivers did not load, resulting in an environment with no access to the target disk. In case you were wondering about network attacks, those were a no go as well, since the firewalls were strictly configured for ingress and egress traffic, based on ip/port/application and yes the connection themselves used TLS with client certificates and not vulnerable to man in the middle attacks.

Usually when I encounter these environment it still is possible to perform a variety of Direct Memory Access (DMA) attacks using tools like inception or pcileech. In these cases however this was physically not possible, either because there were no DMA ports available or just because I didn’t have the correct hardware with me to perform the attacks.

A common issues with all those setups however was the fact that the disk encryption software did not seal the encryption keys to a hardware security device like a TPM. This enables an attacker to create an image from the hard disk and boot this image on another computer. If the attacker also got a hold of the enclosure (USB key, smart card, obfuscated algorithm, unencrypted partition) holding the encryption keys it becomes possible to boot the disk image and fully control the victim disk in an untrusted environment.

In this blog article we are going to have a look at some of the things that you can do when you can boot a disk image of an otherwise unpenetrable environment. Please keep in mind that in part we are reinventing the wheel for two reasons:

  • Learning the nitty gritty details
  • Having a portable and understandable solution

There are solutions available that probably would enable you to achieve the same result, but for my personal taste I prefer to have something much more lightweight that can be easily ported between QEMU versions. Additionally you could also achieve the same result with the quick & dirty approach of booting the image in VMWare, pausing the machine, editing the memory file, resuming the machine. However I prefer QEMU since it allows full control over the entire process, due to the build in GDB server as well as customising the inner workings by editing/adding code and recompiling it. The following existing projects already wrap QEMU with cool and handy features if you want to use these type of setups to analyse malware or other applications:

Enough introduction of what we are going to do, let’s dive in and start elevating our shells to SYSTEM ;)
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Reversing, grasping the big picture

So no reversing section but still a reversing post. My personal opinion is that reversing is part of a forensic research in some way or an other…you could state that reversing is like a very specific forensic investigator. Most people asociate reversing with copyright infringements and bypassing security measures to access forbidden goodies(game cheats for example). Reversing can also be used for legal purposes just to name a few:

  • perform a blackbox audit on an executable
  • perform a investigation on a piece of malware
  • help develop a quick patch until the official one is released
  • learn and understand compiler optimization

I love reversing, I also hate reversing. Yet I keep practicing it and trying to learn. Why ? Because it really is a beautifull way to learn new things and to relax(this depends on the person reversing of course).

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