There are many techniques that are used to make hardware tamper-resistant (see Question 7.21). Some of these techniques are intended to thwart direct attempts at opening a device and reading information out of its memory; others offer protection against subtler attacks, such as timing attacks and induced hardware-fault attacks.
At a very high level, a few of the general techniques currently in use to make devices tamper-resistant are:
- Employing sensors of various types (for example, light, temperature,
and resistivity sensors) in attempt to detect occurrences of malicious
probing.
- Packing device circuitry as densely as possible (dense circuitry
makes it difficult for attackers to use a logic probe effectively).
- Using error-correcting memory.
- Making use of non-volatile memory so that the device can tell if
it has been reset (or how many times it has been reset).
- Using redundant processors to perform calculations, and ensuring that all the calculated answers agree before outputting a result.
