refactor: move pbkdf2 derive bits off the main-thread (#233)

Author: HamdaanAliQuatil

lib/src/impl_ffi/impl_ffi.pbkdf2.dart

@@ -71,7 +71,7 @@ final class _Pbkdf2SecretKeyImpl implements Pbkdf2SecretKeyImpl {
 
     return _Scope.async((scope) async {
       final out = scope<ffi.Uint8>(lengthInBytes);
-      _checkOpIsOne(ssl.PKCS5_PBKDF2_HMAC(
+      _checkOpIsOne(await _PKCS5_PBKDF2_HMAC(
         scope.dataAsPointer(_key),
         _key.length,
         scope.dataAsPointer(salt),
@@ -85,3 +85,35 @@ final class _Pbkdf2SecretKeyImpl implements Pbkdf2SecretKeyImpl {
     });
   }
 }
+
+/// Helper function to run `ssl.PKCS5_PBKDF2_HMAC` in an [Isolate]
+/// using [Isolate.run].
+///
+/// This offloads the computationally expensive PBKDF2 operation to a
+/// separate isolate to avoid blocking the main isolate.
+///
+/// Using this auxiliary function to wrap the call should reduce the risk that
+/// unnecessary variables are copied into the closure passed to [Isolate.run].
+Future<int> _PKCS5_PBKDF2_HMAC(
+  ffi.Pointer<ffi.Char> key,
+  int keyLength,
+  ffi.Pointer<ffi.Uint8> salt,
+  int saltLength,
+  int iterations,
+  ffi.Pointer<EVP_MD> md,
+  int lengthInBytes,
+  ffi.Pointer<ffi.Uint8> out,
+) async =>
+    await Isolate.run(
+      () => ssl.PKCS5_PBKDF2_HMAC(
+        key,
+        keyLength,
+        salt,
+        saltLength,
+        iterations,
+        md,
+        lengthInBytes,
+        out,
+      ),
+      debugName: 'PKCS5_PBKDF2_HMAC',
+    );