Enhance Existing Kotlin APK Security

I have a production APK written in Kotlin that relies on Retrofit-based HTTP calls and Protocol Buffers for payload serialization. The core features are stable, but I now need precise, code-level adjustments focused on the client–server security layer and a few related Retrofit endpoints.

You will be working inside the current Android Studio project, so you must be fluent with Kotlin coroutines, Retrofit/OkHttp interceptors, protobuf message handling and contemporary Android security patterns such as TLS 1.3, certificate pinning and token refresh flows. I will point you directly to the classes that need attention; your job is to refactor or extend them so that data remains encrypted end-to-end, handshake validation is rock-solid and the updated APK installs cleanly on Android 13+.

Deliverables
• Updated source modules with clear comments
• A signed, install-ready APK reflecting the changes
• A brief technical note (version changes, new dependencies, security rationale)

Acceptance criteria: the project must compile without warnings, all existing unit tests pass, new security logic is verifiable with packet inspection (no plaintext sensitive data), and no functionality regressions are introduced.