Ensuring synchronization in the O-RAN Fronhaul S-Plane is crucial to accurately align timing between network elements and enable latency-sensitive features such as time division duplexing and carrier aggregation. The Precision Time Protocol (PTP) plays a key role in this context, but any vulnerability that compromises synchronization can cause serious impacts on network efficiency and quality of user experience. Although traditional solutions such as MACsec offer link-level encryption and authentication, they introduce a processing overhead that is potentially incompatible with the stringent requirements of O-RAN fronthaul. As an alternative, this work proposes an authentication-based security mechanism that combines the Timed Efficient Stream Loss-Tolerant Authentication (TESLA) protocol with ASCON, a lightweight cryptographic algorithm optimized for integrity.