ARM Security Technology Building a Secure System using ...

Introduction
1.3 What are the threats?
1.3.1 Market sector overview
Before exploring the details of the TrustZone hardware architecture it is important to
understand what is meant by security in this context and how the risks posed by the
attacks compare with the cost of prevention. Only with this information can a system
designer make justified design choices regarding what to protect and how much to
invest in hardware or software defenses.
There are many examples, spanning multiple applications and industries, of the costs
associated with the failure of embedded systems to resist attack. Some attacks, such as
payment fraud, cause a direct cost which has to be covered by the service provider,
while others, such as the addition of a modified hardware chip to a set-top box, can
cause a prolonged loss of revenues over the lifetime of the device.
The sensitive assets that each market sector tries to protect against attack are diverse.
For example, mobile handsets aim to protect the integrity of the radio network, while
television set-top boxes prevent unauthorized access to subscription channels. The
varied type and value of the assets being protected, combined with the different
underlying system implementations, mean that the attacks experienced by each are also
diverse. This section aims to outline a small part of the security history in some of the
market sectors where TrustZone technology might be deployed.
Mobile sector
Two critical parts of a GSM handset are the International Mobile Equipment Identity
(IMEI) code, a unique 15-digit code used to identify an individual handset when it
connects to the network, and the low level SIMLock protocol which is used to bind a
particular device to SIM cards of a particular network operator.
Both of these components are used to provide a security feature: the IMEI is used to
block stolen handsets from accessing the network, and the SIMLock protocol is used to
tie the device to the operator for the duration of a contract. On many handsets both of
these protection mechanisms can be bypassed with little effort, typically using a USB
cable and a reprogramming tool running on a desktop workstation.
The result of these insecurities in the implementation is an opportunity for fraud to be
committed on such a large scale that statistics reported by Reuters UK suggest it is
driving half of all street crime through mobile phone thefts, and costs the industry
billions of dollars every year.
1-6 Copyright © 2005-2009 ARM Limited. All rights reserved. PRD29-GENC-009492C
Non-Confidential Unrestricted Access