Cross-Platform Utility App Development

I'm looking for an experienced app developer to create a utility/tools app for both iOS and Android platforms. The app will include user authentication functionalities.

Key Requirements:
- Cross-platform development expertise (iOS and Android)
- Experience in building utility/tools applications
- Strong knowledge in implementing secure user authentication

Ideal Skills and Experience:
- Proficiency in programming languages such as Swift, Kotlin, or React Native
- Prior experience with user authentication systems
- UI/UX design skills to ensure a user-friendly interface

Please provide a portfolio of similar work and an estimated timeline.