  Visible Light Communication (VLC) is one the most promising enabling
technology for future 6G networks to overcome Radio-Frequency (RF)-based
communication limitations thanks to a broader bandwidth, higher data rate, and
greater efficiency. However, from the security perspective, VLCs suffer from
all known wireless communication security threats (e.g., eavesdropping and
integrity attacks). For this reason, security researchers are proposing
innovative Physical Layer Security (PLS) solutions to protect such
communication. Among the different solutions, the novel Reflective Intelligent
Surface (RIS) technology coupled with VLCs has been successfully demonstrated
in recent work to improve the VLC communication capacity. However, to date, the
literature still lacks analysis and solutions to show the PLS capability of
RIS-based VLC communication. In this paper, we combine watermarking and jamming
primitives through the Watermark Blind Physical Layer Security (WBPLSec)
algorithm to secure VLC communication at the physical layer. Our solution
leverages RIS technology to improve the security properties of the
communication. By using an optimization framework, we can calculate RIS phases
to maximize the WBPLSec jamming interference schema over a predefined area in
the room. In particular, compared to a scenario without RIS, our solution
improves the performance in terms of secrecy capacity without any assumption
about the adversary's location. We validate through numerical evaluations the
positive impact of RIS-aided solution to increase the secrecy capacity of the
legitimate jamming receiver in a VLC indoor scenario. Our results show that the
introduction of RIS technology extends the area where secure communication
occurs and that by increasing the number of RIS elements the outage probability
decreases.
