  The unprecedented proliferation of smart devices together with novel
communication, computing, and control technologies have paved the way for the
Advanced Internet of Things~(A-IoT). This development involves new categories
of capable devices, such as high-end wearables, smart vehicles, and consumer
drones aiming to enable efficient and collaborative utilization within the
Smart City paradigm. While massive deployments of these objects may enrich
people's lives, unauthorized access to the said equipment is potentially
dangerous. Hence, highly-secure human authentication mechanisms have to be
designed. At the same time, human beings desire comfortable interaction with
their owned devices on a daily basis, thus demanding the authentication
procedures to be seamless and user-friendly, mindful of the contemporary urban
dynamics. In response to these unique challenges, this work advocates for the
adoption of multi-factor authentication for A-IoT, such that multiple
heterogeneous methods - both well-established and emerging - are combined
intelligently to grant or deny access reliably. We thus discuss the pros and
cons of various solutions as well as introduce tools to combine the
authentication factors, with an emphasis on challenging Smart City
environments. We finally outline the open questions to shape future research
efforts in this emerging field.
