All candidates should:

have proficiency in Linux as a first-choice development platform (toolchains, containers/sandboxing, open embedded/yocto, CLI, any of: bash, ruby, python, etc.)
have a desire to take on responsibility and direct their own work with minimal overhead management.
have a B.S./M.S. Degree in CS or CE (Computer Science or Computer Engineering)
have systems level C++ experience (algorithms and data structures, knowledge of performance bottlenecks/optimization techniques, cross platform, good API design)

All candidates must:

be a U.S. Citizen
adhere to well established coding practices and open standards

Bonus domains of expertise:

micro-controller experience (ARM m0-m4, PIC, 8051)
structured concurrency/async-await frameworks
power optimization/low power devices
crypto, security (whitehat, anti-hacking, secure systems, secure protocol parsing, taint handling, fuzzing)
IP networking
neural networks (NN, DNN), machine learning (ML)
performance optimization (CPU, GPU, SIMD, etc.)
digital signal processing (DSP)
test driven development (TDD) or similar technique for delivering impeccable software quality