For a fintech app, "secure storage" isn't the finish line. You want keys the operating system itself can't export, gated behind a fingerprint or Face ID, usable to bind an account to a device and to sign transactions so they can't be repudiated. That means the real hardware: the iOS Secure Enclave and Android's hardware-backed Keystore / StrongBox — a separate secure chip, not your app's memory. Flutter doesn't expose any of this in Dart, so this talk is how to reach it. We'll generate a non-exportable key inside the Secure Enclave / StrongBox via platform channels, gate its use behind biometrics, sign a server challenge for device binding and step-up auth, and use key attestation to prove to your backend that the key is genuinely hardware-backed — not a software fake. Then the sharp edges nobody warns you about: keys invalidating when the user re-enrolls a fingerprint, wildly varying device support, and what to safely fall back to. You'll leave able to add hardware-rooted trust to a Flutter app instead of just hoping the sandbox holds.