Security Engineering: a Guide to Building Dependable Distributed Systems Physical Tamper Resistance 14.1 Introduction

It is relatively easy to build an encryption system that is secure if it is working as intended and is used correctly but it is still very hard to build a system that does not compromise its security in situations in which it is either misused or one or more of its sub-components fails (or is 'encouraged' to misbehave) ... this is now the only area where the closed world is still a long way ahead of the open world and the many failures we see in commercial cryptographic systems provide some evidence for this. The techniques discussed in the previous few chapters—physical protection involving barriers, sensors, and alarms—are often used to protect critical information processing resources: • A bank's main servers will typically be kept in a guarded computer room. • The seismic sensor packages used to detect unlawful nuclear tests may be at the bottom of a borehole several hundred feet deep which is backfilled with concrete. • A hole-in-the-wall automatic teller machine is in effect a PC in a one-ton safe with a number of fancy peripherals. These include not just banknote dispensers but also temperature sensors to detect attempts to cut into the device, and ac-celerometers to detect if it's moved. An alarm should cause the immediate era-sure of all crypto key material in the device. But often it's inconvenient to use a massive construction, and this has spawned a market for portable tamper-resistant processors. These range from smartcards, which typically perform a limited set of operations in support of an application such as pay