  Intelligent transportation systems increasingly depend on wireless
communication, facilitating real-time vehicular communication. In this context,
message authentication is crucial for establishing secure and reliable
communication. However, security solutions must consider the dynamic nature of
vehicular communication links, which fluctuate between line-of-sight (LoS) and
non-line-of-sight (NLoS). In this paper, we propose a lightweight cross-layer
authentication scheme that employs public-key infrastructure-based
authentication for initial legitimacy detection while using keyed-based
physical-layer re-authentication for message verification. However, the
latter's detection probability (P_d) decreases with the reduction of the
signal-to-noise ratio (SNR). Therefore, we examine using Reconfigurable
Intelligent Surface (RIS) to enhance the SNR value directed toward the
designated vehicle and consequently improve the P_d, especially for NLoS
scenarios. We conducted theoretical analysis and practical implementation of
the proposed scheme using a 1-bit RIS, consisting of 64 x 64 reflective units.
Experimental results show a significant improvement in the P_d, increasing from
0.82 to 0.96 at SNR = - 6 dB for an orthogonal frequency division multiplexing
system with 128 subcarriers. We also conducted informal and formal security
analyses, using Burrows-Abadi-Needham (BAN)-logic, to prove the scheme's
ability to resist passive and active attacks. Finally, the computation and
communication comparisons demonstrate the superior performance of the proposed
scheme compared to traditional crypto-based methods.
