AES encryption in iOS - NullReferenceException

I'm recently trying to upgrade my iOS project to support 64-bit, my unity version is v4.6.6p2. When we export an Xcode project using il2cpp and run the App on the iPad, it always catch a NullReferenceException on the AES.create() line.

I have tried 32-bit and 64-bit iPad, they are all the same. But the code run perfectly on the editor and mono project.

here is my code:

     using UnityEngine;
     using System;
     using System.Collections;
     using System.Collections.Generic;
     using System.Text.RegularExpressions;
     using System.Security.Cryptography;
     using System.Text;
     using System.IO;
     
     public class testEncrypt : MonoBehaviour {
         public UILabel label;
         private string _Key = "somekeystring";
     
     
         // Use this for initialization
         void Start () {
             try {
             label.text = Encrypt("testing");
             }
             catch (Exception ex) {
                 Debug.Log("error: " + ex.ToString());
             }
         }
         
         // Update is called once per frame
         void Update () {
         
         }
     
     
             
         private string createKey(string key) {
             MD5 _md5 = null;
             try {
                 _md5 = MD5.Create();
             }
             catch (Exception ex) {
                 Debug.Log("md5 error: " + ex.ToString());
             }
             byte[] source = Encoding.Default.GetBytes(key);
             byte[] crypto = null;
             try {
                 crypto = _md5.ComputeHash(source);
             }
             catch (Exception ex) {
                 Debug.Log("computeHash: " + ex);
             }
             string result = Convert.ToBase64String(crypto);
             return result;
         }
     
         private static byte[] GetKey(byte[] suggestedKey, SymmetricAlgorithm p) {
             byte[] kRaw = suggestedKey;
             System.Collections.Generic.List<byte> kList = new System.Collections.Generic.List<byte>();
             
             for (int i = 0; i < p.LegalKeySizes[0].MinSize; i += 8) {
                 kList.Add(kRaw[(i / 8) % kRaw.Length]);
             }
             byte[] k = kList.ToArray();
             return k;
         }
         
         private string Encrypt(string clearText) {
             string EncryptionKey = _Key;
             byte[] clearBytes = Encoding.ASCII.GetBytes(clearText);
                 
             try {
             //    Aes encryptor = Aes.Create();
             using (Aes encryptor = Aes.Create()) {
                 //Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] {/* hide */});
                 encryptor.BlockSize = /* hide */;
                 encryptor.KeySize = /* hide */;
                 encryptor.Mode = CipherMode.CBC;
                 encryptor.Padding = PaddingMode.PKCS7;
                 encryptor.IV = new byte[] {/* hide */};
                 encryptor.Key = GetKey(Encoding.Default.GetBytes(_Key), encryptor);
                     
                 using (MemoryStream ms = new MemoryStream()) {
                     using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write)) {
                         cs.Write(clearBytes, 0, clearBytes.Length);
                         cs.Close();
                     }
                     clearText = Convert.ToBase64String(ms.ToArray());
                 }
             }
             }
             catch (Exception ex) {
                 Debug.Log("aes error: " + ex);
             }
             return clearText;
         }    
     }
     

Exception catch: aes error: System.NullReferenceException: A null value was found where an object instance was required.

I have googled for sometime and to my surprise I can only find very very few question/report on this issue. I'm now stuck and don't know what to do. Is it a bug that I need to file or is it something I did wrong to run into this problem? Is there any work around or sample that I can use?