Software-Defined Vehicles (SDV) are modern automotive systems that incorporate complex software features, functional and control applications. In SDVs, software components are meant to replace electrical/electronic components that handle vehicle control functions. The complexity introduced by SDVs, together with safety requirements of automotive systems, raises security and trust concerns. This thesis proposes a system of security services to provide defense mechanisms against threats. The design of security services ensures protection of security primitives, allows secure distribution of long-term encryption and short-term authentication keys, message authentication tags can be aggregated under a single data structure and can be verified independently, the network is monitored with a rule-based stateful firewall and an intrusion detection system, and security alerts are securely logged. Formal correctness of the proposed system is demonstrated through formal proofs, and trust in security services is modeled through the ability of services to react and protect the system from malicious interventions.