When conducting a review of an electronic data interchange (EDI) application provided by a third-party service, the internal auditor should ensure several key aspects to maintain security and compliance:

Independent Review of Service Provider: Determine whether an independent review of the service provider's operations has been conducted. This review helps ensure that the service provider meets necessary standards and maintains adequate controls.

Contractual Clauses: Verify that the service provider's contracts include necessary clauses. These clauses should cover aspects like data security, confidentiality, compliance with standards, and performance metrics.

Ensuring encryption keys meet ISO standards and verifying the use of public-switched data networks are important but are more specific technical controls that might be part of broader reviews. The focus here should be on independent verification and robust contractual agreements