How should calls to a custom Apex web service be secured to ensure only secure connections from an ETL tool?

Securing calls to a custom Apex web service is critical to maintaining the integrity and confidentiality of the data being transferred. Choosing two-way SSL (Secure Sockets Layer) is particularly effective because it establishes a secure communication channel between the ETL tool and the web service, ensuring that both the client and server authenticate each other.

With two-way SSL, the server presents its certificate to the client, and the client must also present its certificate to the server. This mutual authentication prevents unauthorized access by ensuring that only clients with valid certificates can successfully communicate with the Apex web service. It adds a robust layer of security beyond just encrypting the data in transit, as it verifies the identities of both parties involved in the transaction.

Other options may provide some level of security, but they do not match the comprehensive protection offered by two-way SSL. Using a VPN can secure the connection at a network level but might not provide the necessary level of authentication for the individual parties involved. Profile security pertains to user access management in Salesforce and does not specifically address securing web service calls. IP whitelisting restricts access based on IP addresses, but it does not guarantee that the client communicating with the service is who they claim to be, since IP addresses can be spoofed or accessed