  The goal of physical layer security (PLS) is to make use of the properties of
the physical layer, including the wireless communication medium and the
transceiver hardware, to enable critical aspects of secure communications. In
particular, PLS can be employed to provide i) node authentication, ii) message
authentication, and, iii) message confidentiality. Unlike the corresponding
classical cryptographic approaches which are all based on computational
security, PLS's added strength is that it is based on information theoretic
security, in which no limitation with respect to the opponent's computational
power is assumed and is therefore inherently quantum resistant. In this survey,
we review the aforementioned fundamental aspects of PLS, starting with node
authentication, moving to the information theoretic characterization of message
integrity, and finally, discussing message confidentiality both in the secret
key generation from shared randomness and from the wiretap channel point of
view. The aim of this review is to provide a comprehensive roadmap on important
relevant results by the authors and other contributors and discuss open issues
on the applicability of PLS in sixth generation systems.
