  Electronic Design Automation (EDA) industry heavily reuses third party IP
cores. These IP cores are vulnerable to insertion of Hardware Trojans (HTs) at
design time by third party IP core providers or by malicious insiders in the
design team. State of the art research has shown that existing HT detection
techniques, which claim to detect all publicly available HT benchmarks, can
still be defeated by carefully designing new sophisticated HTs. The reason
being that these techniques consider the HT landscape to be limited only to the
publicly known HT benchmarks, or other similar (simple) HTs. However the
adversary is not limited to these HTs and may devise new HT design principles
to bypass these countermeasures.
  In this paper, we discover certain crucial properties of HTs which lead to
the definition of an exponentially large class of Deterministic Hardware
Trojans $H_D$ that an adversary can (but is not limited to) design. The
discovered properties serve as HT design principles, based on which we design a
new HT called 'XOR-LFSR' and present it as a 'proof-of-concept' example from
the class $H_D$. These design principles help us understand the tremendous ways
an adversary has to design a HT, and show that the existing publicly known HT
benchmarks are just the tip of the iceberg on this huge landscape. This work,
therefore, stresses that instead of guaranteeing a certain (low) false negative
rate for a small constant set of publicly known HTs, a rigorous HT detection
tool should take into account these newly discovered HT design principles and
hence guarantee the detection of an exponentially large class (exponential in
number of wires in IP core) of HTs with negligible false negative rate.
