Do you use cryptography_flutter directly?

[rohintoncollins]

I am building a crypto library for an application I intend to release on iOS, macOS, Web, Android, Windows and Linux.
Having added cryptography and crypography_flutter to my new package, I am in confusion about how crypography_flutter should be used.

The overview says: "This is a Flutter plugin that enables pub.dev/packages/cryptography to use native APIs of Android, iOS, and Mac OS X."

The FlutterCryptography class has the following

import 'package:cryptography_flutter/cryptography_flutter.dart' show FlutterCryptography;

void main() {
  // Enables use of Flutter cryptography.
  //
  // You can call this anywhere in your application, but we recommend the
  // main function.
  FlutterCryptography.enable();
}

This implies that you add it to your project, along with the cryptography library, enable it with FlutterCryptography.enable(), but do not use the other classes in crypography_flutter directly. That they will be used by the cryptography library, as appropriate and depending on the runtime platform.

If that is the case, then that is great as I only need to concern myself with the cryptography API.

The alternative is that I need to use the cryptography_flutter classes, where available, and provide fallback objects in case they are not supported on the runtime platform. That wouldn't be so great, but is how I have approached coding so far - hence the question.

Which does lead me to a 2nd question.

If the answer to the above question is that we add and enable the cryptography_flutter package but do not use the classes in it directly, then would the following make sense:
I build a pure dart crypto package and only add the dint-dev/cryptography package as a dependency.
I build a flutter app
- add my cryptography package as a dependency which transitively pulls in dint-dev/cryptography
- add the cryptography_flutter package, and enable it somewhere in my app code.

[rohintoncollins]

Through testing:

1) My flutter app
  -> cryptography_flutter
  -> myDartCryptoPackage
       -> cryptography
2) My flutter app
  -> myFlutterCryptoPackage
       -> cryptography
       -> cryptography_flutter

Both the above setups result in FlutterChachaPoly1305 being used,

1. indirectly via Chacha20.poly1305Aead() which creates a FlutterChacha20 because cryptography_flutter is added as a dependency to my flutter package.
2. directly via FlutterChacha20.poly1305Aead()

So in summary, no you do not need to use the classes in cryptography_flutter directly. Just add cryptography_flutter as a dependency to your app, or to your flutter package, and the Flutter classes will be used on supported platforms.
This is good because it means that you can use setup 1) above where you can develop your own dart package with crypto functions based solely on the cryptography package and just add the cryptography_flutter dependency to your flutter app.

Bad news is, encrypting/decrypting a 1GB file:
C++ library based on Botan C++ library: ~ 7 seconds
Chacha20 (pure Dart implementation) ~ 90 seconds
FlutterChacha20 (supposed to be 100 times faster than pure Dart implementation) ~ 75 seconds

This was on a M1 Mac mini. Disappointing :(