How we implemented client-side encrypted streaming in Flutter with Dart Isolates

Over the past few months, we built and launched RonikCloud (https://ronikcloud.com/), a client-side encrypted cloud storage app built in Flutter across Web, Desktop (macOS, Windows, Linux), and Mobile (iOS, Android).

In this article, we share our architectural choices, lessons learned, and how we solved client-side encrypted streaming performance in Flutter.

The Problem: Cryptography vs 60/120 FPS UI

Client-side encryption requires running heavy authenticated cryptographic operations (AES-GCM-256 / XChaCha20-Poly1305) over multi-gigabyte files.

If executed on the main Dart UI thread, streaming encryption causes UI jank, frame drops, and frozen progress indicators.