If you've specified the package attribute in the source AndroidManifest.xml, you can use the AGP Upgrade Assistant to migrate to the namespace value in the build file. Refer to https://d.android.com/r/tools/upgrade-assistant/agp-upgrade-assistant for general information about using the AGP Upgrade Assistant.

import 'package:cryptography/cryptography.dart';
import 'package:cryptography_flutter/cryptography_flutter.dart';

Future<(SecretKey, SimplePublicKey)> _getKeys(
    SimplePublicKey remotePublicKey) async {
  final cipher = FlutterX25519(X25519 as X25519);
  final localKeyPair = await cipher.newKeyPair();

  final sharedSecretKey = await cipher.sharedSecretKey(
      keyPair: localKeyPair, remotePublicKey: remotePublicKey);
  final localPublicKey = await localKeyPair.extractPublicKey();

  return (sharedSecretKey, localPublicKey);
}

Future<SecretBox> _symmetricEncrypt(String inString, SecretKey sKey) async {
  final cipher = FlutterChacha20.poly1305Aead();
  final eString = await cipher.encryptString(inString, secretKey: sKey);

  return eString;
}