Trusted Execution Environments in Android

Continuing on from my post about TrustZone it seems that there is a lot of interest in hardware-backed security for Android and what you can do with it. One of the most interesting things that a hardware-isolated area can do for devices, whether that be a dedicated co-processor or technology such as TrustZone, is to provide a trusted enviroment dedicated to protecting your most valuable assets and the operations that are performed on them. Installing something like a micro operating system in this divide can give you a lot of features that the main OS just cannot gain access to and is the thrust of standards bodies such as Global Platform 1. This micro OS, or to use the popular parlance: a Trusted Execution Environment (TEE), is becoming more important in a world of one-click / swipe / wave-a-device payments and device authorisation and over the coming years will see a surge in popularity not only from independant vendors but from the large OS vendors too. But lets take a step back.

The concept of a Trusted Execution Environment is to provide a secure area of the main processor, memory, and peripherals, that can be used to perform privileged operations. First defined by the Open Mobile Terminal Platform (OMTP) forum in their Advanced Trusted Environment:OMTP TR1 standard 2 and later adopted by Global Platform in their standardisation effort, the TEE has become a bridge between pure software security mechanisms and hardware-only solutions. The TEE uses the concept of isolation that technologies such as TrustZone enable to execute in the processors Secure World mode.

The TEE can be a fully-functional operating system offering software developers the opportunity to create Trusted Applications: applications that reside in the Secure World and perform security-critical functions outside of the control of the main operating system running in the Normal World. An example of such a Trusted Application can be a Trusted User Interface (TUI) - a display that is presented to the user completely protected by the Secure World and inaccessible to the main operating system such as Android. The interface could display sensitive information such as passwords and be confident that attacks such as screen scraping or video buffer capture would not reveal anything.

It is clear that the popularity of TEEs is increasing. Based on one commercial TEE vendors press releases the adoption rate of the Trustonic TEE is reported to be over 100m devices every 6 months (source: http://www.trustonic.com - figures from February 2014 to July 2014) although wide-spread utilisation by third-party developers is yet to be exploited. Ekberg et al 3 attribute this to a lack of access to the TEE stating that “Despite TEE’s large-scale deployment, there’s been no widely available means for application developers to benefit from its functionality as mobile device manufacturers have restricted TEE access to their internal use cases.”, but also admit that standardisation could potentially solve this issue. Recent announcements by companies such as Linaro point to a more open access model 4 but we are yet to see commercial devices with OP-TEE technology.

In short, TEEs are here to stay and I expect that the likes of Apple and Android will open up access to this trusted area for more developers to enhance the security of their applications in the near future.

What are you passionate about?

I have recently been reading the book entitled Talk Like TED Carmine Gallo which promises to bestow the virtues of great public speaking upon all who read it. Early on in the book there is a rather salient point that got me thinking, a point that starts with a simple question, “What are you passionate about”. Now there are quite a few things I am passionate about but in the context of Software Engineering, my chosen career path, it is something that underpins all the great projects that over time I have really enjoyed working on. What is it? Data.

I am passionate about data, specifically the conclusions you can draw from it. This is not to say the actual gathering of data, although that can be quite interesting in itself: constructing tools and processes as you squirrel away the nuts of information that together paint a picture that no one individual data point can allude to. I am more passionate about the ‘Wheres Wally’ dance: the finding of that little something you’ve been looking for in a sea of noise, the epiphany, the moment, the unveiling. The answer to the puzzle that is something you intrinsically know is just outside your grasp and that with the data, that collection of measurements and information, the answer will magically appear. The puzzle that is made up of a thousand pieces and by putting them all together it becomes clear. That is what I’m passionate about. I guess my career has always followed that route of problem solving.

Software Engineering is a great field to be in if you enjoy problem solving: you get to create a solution based upon parts constructed with only your imagination, a programming language, and your favourite text editor. In my experience, the first solution you produce is often not quite what you were looking for, and the itch remains. You continue to iterate, introduce bugs, fix bugs, thinking of new and novel ways to answer your initial questions and finally you have something that not only works, it satisfies that itch. When you employ this process to scratch a larger itch, a higher-level more abstract problem that requires the gathering and analysis of data I find there is satisfaction from the initial problem solving during development plus the benefit of discovering that pattern or snippet of information that maybe you only thought was there before but now is proven with the data. Maybe this explains why I have an affinity with Pervasive Computing and, in its latest incarnation as a buzz word - Internet of Things (IoT). The topic of Data Inference, that is what I really enjoy.

I’ve gathered much data over the years: email achives and usage data, energy monitoring and the subsequent discovery of inefficient appliances, health data with Fitbit and Garmin or lifestyle monitoring with Slogger, it can all be combined to do wonderful things. But there is a tendancy to gather data just of the sake of it and I have certainly been guilty of that but I am starting to take a step back and trust the data more - to make informed decisions based upon it - so lets see how that goes this year. Big data is definately here, but the more important point everyone should be asking is “What do we do with all that data and how can it benefit humanity?”.

TrustZone For Android Mobile Security

Recently I was asked to provide a quick, high-level introduction to TrustZone and how it could potentially improve the security on Android platforms. Any response to this is tricky: TrustZone is just a mechanism built in to a platform that if unused can do very little for device security but when utilised to its fullest, can create a totally seperate environment dedicated to protecting your most important secrets. But first a bit of background.

According to Bloomberg 1 ARM’s chip designs are found in 99% of the world’s smartphones and tablets; 2013 alone saw ARM’s partners ship over 10 billion chips (source: ARM Strategic Report 2013). Popular devices such as the Apple iPhone and iPad, Amazon’s Kindle, and Samsung’s flagship Galaxy series all use a Central Processing Unit (CPU) based on an ARM design. In 2004 ARM released its design for a hardware-enforced parallel execution environment for the PB1176 and ARMv7 architectures that was adopted into all later application processor designs.

TrustZone itself is an implementation of device-level security utilizing extensions to the CPU and Advanced Microcontroller Bus Architecture (AMBA), or memory bus. By connecting all these components together in a homogeneous architecture it is possible to contruct two distinct ‘worlds’, a “Secure World” and a “Non-Secure World” (or “Normal World”) 2. The two modes are orthogonal to each other with the Secure World enjoying full access to all memory regions and priviledged CPU areas whereas the Normal World can be restricted. This arrangement is configured during the boot process. The interface between the two worlds is governed by a special Secure Monitor Mode, accessible via an interrupt instigated with the Secure Monitor Call (SMC) instruction. Identification of which world the processor is currently executing it is possible by the use of a extra ‘flag’ known as the NS, or Non-Secure bit. All components that wish to use the functionality provided by TrustZone must beaware of this flag.

With TrustZone it is possible to isolate an area of the CPU, memory, and peripherals for use by a trusted software component called a Trusted Execution (TEE) 3 or other such privileged software. For example, Android’s implementation of the core crytographical keystore functionality, KeyChain, can use hardware components such as TrustZone, Sim Card, or Trusted Platform Module (TPM), to enhance overall security. By using TrustZone a device can provice secure software functionalty, backed up by the hardware it is running on.

It is clear that with more widespread use TrustZone could benefit an increasingly mobile society who expect to do the most secure of operations with their devices.


  1. http://www.bloomberg.com/bw/articles/2014-02-04/arm-chips-are-the-most-used-consumer-product-dot-where-s-the-money] [return]
  2. J. Winter. Trusted computing building blocks for embedded linux-based arm trust- zone platforms. In Proceedings of the 3rd ACM workshop on Scalable trusted com- puting, pages 21–30. ACM, 2008. [return]
  3. J. Winter. Trusted computing building blocks for embedded linux-based arm trust- zone platforms. In Proceedings of the 3rd ACM workshop on Scalable trusted com- puting, pages 21–30. ACM, 2008. [return]

Getting back into blogging

Its been a while, in fact it has been around a year since I updated this site (to be fair I did write a few posts on another blog during that period … excuses, excuses) which I attribute to a increasingly busy schedule but more to a lack of enthusiasm. So, in an attempt to get back into this blogging lark I thought it would be a good opportunity to redesign the site with Hugo, a static, but more importantly Markdown-based web engine, and put up a few articles on something dear to my heart, Software Engineering. So expect more development related posts interspersed with running, triathlon, travel, and other randomness as I attept to do this on a semi-regular basis.

Oh, and if you are looking for any of my past entries from 2007 onwards, they will be back up shortly as I figure out how to convert WordPress content to Hugo and still keep some form resemblence to the original post.