  Reconfigurable intelligent surface (RIS) and ambient backscatter
communication (AmBC) have been envisioned as two promising technologies due to
their high transmission reliability as well as energy-efficiency. This paper
investigates the secrecy performance of RIS assisted AmBC networks. New
closed-form and asymptotic expressions of secrecy outage probability for
RIS-AmBC networks are derived by taking into account both imperfect successive
interference cancellation (ipSIC) and perfect SIC (pSIC) cases. On top of
these, the secrecy diversity order of legitimate user is obtained in high
signal-to-noise ratio region, which equals \emph{zero} and is proportional to
the number of RIS elements for ipSIC and pSIC, respectively. The secrecy
throughput and energy efficiency are further surveyed to evaluate the secure
effectiveness of RIS-AmBC networks. Numerical results are provided to verify
the accuracy of theoretical analyses and manifest that: i) The secrecy outage
behavior of RIS-AmBC networks exceeds that of conventional AmBC networks; ii)
Due to the mutual interference between direct and backscattering links, the
number of RIS elements has an optimal value to minimise the secrecy system
outage probability; and iii) Secrecy throughput and energy efficiency are
strongly influenced by the reflecting coefficient and eavesdropper's
wiretapping ability.
