1. WooKey project

1.1. Building trusted USB devices and IoTs

Securing the USB stack, and hence the USB hosts and devices, has been a growing concern since exploitable flaws have been revealed with the BadUSB threat [nohl2014badusb]. As a consequence: USB devices firmwares, operating systems, and user data are at risk!

The WooKey project aims at prototyping a secure and trusted USB mass storage device featuring user data encryption and strong user authentication, with fully open source and open hardware foundations.

The Wookey is a custom STM32 based USB thumb drive with mass storage capabilities designed for user data encryption and protection, with a full-fledged set of in-depth security defenses:

  • A secure DFU (Device Firmware Update) ensuring firmware integrity and authenticity
  • Up-to-date cryptography
  • An external and extractable authentication token embedding a secure element
  • EwoK microkernel, a secure microkernel implemented in Ada/SPARK
  • Memory confinement using the MPU (Memory Protection Unit), privilege separation, W^X principle, stack and heap anti-smashing
  • Tataouine, a versatile SDK developed to easily integrate user applications in C and Rust.
  • Open source and open hardware

Informations about the security concerns are detailed in the section Publications.

Even though the current WooKey focuses on the mass storage USB class, it’s easily portable to other USB device classes such as HID or CDC.

Beyond the mere USB oriented devices, the WooKey many defense in depth primitives can be ported and used in many IoT projects.

1.2. WooKey threat model

We consider that the adversary has logical or physical access to the device:

  • The adversary may try to read the data simply by connecting the device to a host
  • The adversary may try to physically reading the mass storage cells, for example when the device is lost or stolen.
  • The adversary may try to tamper with the device using logical attacks, for example when it is connected to an untrusted host, exploiting weaknesses in protocols used for external communication such as the USB stack or the external data storage buses.
  • Side-channel and fault injection attacks on the device during the pre-authentication phase.
  • The adversary may open the device to physically tamper with the internal storage, firmware, or any other component present on the actual device.

However, some threats are out of scope:

  • We will only consider physical attacks where the adversary does not possess the legitimate user PIN code of the external authentication token.
  • Side-channel and fault injection attacks on the device in a post-authentication phase are explicitly out of scope.
  • The data integrity is out of scope.

1.3. Security features

The WooKey provides the following main security features:

  • User data protection: all data at rest are encrypted, and their confidentiality protected.
  • Strong user authentication: the legitimate user must be present when data is decrypted. When a user PIN code is used, attack vectors to steal it must be limited.
  • Secure device software update (DFU): the device’s software is securely upgradable for system maintenance (e.g. security patches). Update files are authenticated and integrity is checked with no possible rollback to old versions. A software upgrade must be a voluntary and authenticated action. The firmware updates is reliable with no possible platform bricking.
  • Firmware robustness against software attacks: the firmware implements many security mechanisms and mitigation techniques to hinder an adversary attacking the exposed software surface (on the USB bus for instance) to be able to get a privileged access to the platform, and to gain access to the critical materials (such as sensitive cryptographic keys). The MPU (Memory Protection Unit) is used to confine software attacks in unprivileged and isolated containers.

References

[nohl2014badusb]BadUSB-On accessories that turn evil, Karsten Nohl and Jakob Lell, Black Hat USA, 2014