One complex coding challenge I faced involved implementing SSL pinning for secure API calls. Initially, we hard-coded the SSL certificate in the mobile app, but we encountered a problem when the domain's certificate was updated, causing the app to malfunction because it still referenced the old certificate.

To resolve this, we realized that requiring users to update the app every time the certificate changed was not a practical solution. Instead, we developed a dynamic approach to retrieve the certificate data from our backend server.

Here’s how we tackled the issue:

1. **Initial Setup**: We started by hard-coding the certificate in the app, which worked well until the server-side certificate was updated.

2. **Problem Identification**: We found that the app would stop functioning whenever the server's certificate was updated, as it continued to rely on the outdated certificate.

3. **Dynamic Fetching Mechanism**: To address this, we implemented a system where the app would dynamically fetch the latest certificate from the backend server. We introduced versioning for the certificate data, so whenever the server updated the certificate, it also updated the version number.

4. **API Call for Certificate**: The app was modified to make an API call to the backend server to verify the certificate version. If the version had changed, the app would retrieve the new certificate and replace the old one.

5. **Implementation**: We removed the hard-coded certificate and established logic to download and store the new certificate dynamically. This ensured that the app always had the latest certificate without requiring users to perform manual updates.

This solution provided a seamless and uninterrupted experience for users, eliminating the need for frequent app updates and ensuring that the app consistently utilized the correct certificate for secure API calls.