  A finite impulse response (FIR) filter is a ubiquitous block in digital
signal processing applications. Its characteristics are determined by its
coefficients, which are the intellectual property (IP) for its designer.
However, in a hardware efficient realization, its coefficients become
vulnerable to reverse engineering. This paper presents a filter design
technique that can protect this IP, taking into account hardware complexity and
ensuring that the filter behaves as specified only when a secret key is
provided. To do so, coefficients are hidden among decoys, which are selected
beyond possible values of coefficients using three alternative methods. As an
attack scenario, an adversary at an untrusted foundry is considered. A reverse
engineering technique is developed to find the chosen decoy selection method
and explore the potential leakage of coefficients through decoys. An
oracle-less attack is also used to find the secret key. Experimental results
show that the proposed technique can lead to filter designs with competitive
hardware complexity and higher resiliency to attacks with respect to previously
proposed methods.
