Add pbkdf2 and exceptions support

lib/src/native_crypto.dart

@@ -4,6 +4,7 @@ import 'dart:async';
 import 'dart:typed_data';
 
 import 'package:flutter/services.dart';
+import 'package:native_crypto/exceptions.dart';
 
 /// [Sources]
 /// Plugin class.
@@ -16,14 +17,38 @@ class NativeCrypto {
   static const MethodChannel _channel =
       const MethodChannel('native.crypto.helper');
 
+  /// PBKDF2.
+  ///
+  /// [keyLength] is in Bytes. 
+  /// It returns an `Uint8List`.
+  Future<Uint8List> pbkdf2(String password, String salt, {int keyLength: 32, int iteration: 10000}) async {
+    Uint8List key;
+    try {
+      key = await _channel.invokeMethod('pbkdf2', <String, dynamic>{
+      'password': password,
+      'salt': salt,
+      'keyLength': keyLength,
+      'iteration': iteration,
+    });
+    } on PlatformException catch (e) {
+      throw e;
+    }
+    return key;
+  }
+  
   /// Generates AES key.
   ///
   /// [size] is in bits, 128, 192 or 256. 
   /// It returns an `Uint8List`.
   Future<Uint8List> symKeygen(int size) async {
-    final Uint8List aesKey = await _channel.invokeMethod('symKeygen', <String, dynamic>{
-      'size': size
+    Uint8List aesKey;
+    try {
+      aesKey = await _channel.invokeMethod('symKeygen', <String, dynamic>{
+      'size': size,
     });
+    } on PlatformException catch (e) {
+      throw e;
+    }
     return aesKey;
   }
 
@@ -48,11 +73,15 @@ class NativeCrypto {
   /// with encrypted cipher as first and IV as second member.
   Future<Uint8List> symDecrypt(
       List<Uint8List> payloadbytes, Uint8List aesKey) async {
-    final Uint8List decryptedPayload =
-        await _channel.invokeMethod('symDecrypt', <String, dynamic>{
+    Uint8List decryptedPayload;
+    try {
+      decryptedPayload = await _channel.invokeMethod('symDecrypt', <String, dynamic>{
       'payload': payloadbytes,
       'aesKey': aesKey,
     });
+    } on PlatformException catch (e) {
+      throw DecryptionException(e.message);
+    }
     return decryptedPayload;
   }
 }