Following Attacks on Hardware can be planted by Attacker,

Physical Attack

Planned Attack

Stealing Secret Data

Types of Attack on Hardware.

Black Box Testing

This is an Non-Invasive type of attack. The attacker sends an input to the circuit and receives an output. Based on the I/O behavior, the attacker decide what kind of algorithm is used.  

Physical Probing

The attacker plants a probe into the chip itself and reads data off the chip. This is an Invasive attack, and requires sophisticated instrumentation.

Reverse Engineering

The attacker acquires the device and physically exposes the circuit. Each layer of the circuit is removed and high resolution photographs are taken. Invasive approach, and also requires very sophisticated instrumentation.  

Side Channel Analysis

The attacker measures sensitive parameters during normal operation of the circuit. Based on the Measurements, some secret value can be inferred. This is a Non-invasive kind of attack, and is a subject of intense research.

Countermeasures to prevent Hardware Attacks.