解密encryptedData

以下C#代码是解密 API wx.getUserInfo 返回的encryptedData ，但是会出现持续性的解密出来是一串乱码，如解密成功了，持续的全部解密成功， 直到下次有解密失败的，又持续的解密失败。大伙有没有更好的办法吗？

  private static byte[] AES_decrypt(string input, string iv, string key)
        {
            RijndaelManaged aes = new RijndaelManaged();
            aes.KeySize = 256;
            aes.BlockSize = 128;
            aes.Mode = CipherMode.CBC;
            aes.Padding = PaddingMode.None;
            aes.Key = Convert.FromBase64String(key);
            aes.IV = Convert.FromBase64String(iv);
            var decrypt = aes.CreateDecryptor(aes.Key, aes.IV);
            byte[] xBuff = null;
            using (var ms = new MemoryStream())
            {
                using (var cs = new CryptoStream(ms, decrypt, CryptoStreamMode.Write))
                {
                    byte[] xXml = Convert.FromBase64String(input);
                     byte[] msg = new byte[xXml.Length + 32 - xXml.Length % 32];
                     Array.Copy(xXml, msg, xXml.Length);
                    cs.Write(xXml, 0, xXml.Length);
                }
                xBuff = decode2(ms.ToArray());
              
            }
            return xBuff;
        }       

 private static byte[] decode2(byte[] decrypted)
        {
            int pad = (int)decrypted[decrypted.Length - 1];
            if (pad < 1 || pad > 32)
            {
                pad = 0;
            }
            byte[] res = new byte[decrypted.Length - pad];
            Array.Copy(decrypted, 0, res, 0, decrypted.Length - pad);
            return res;
        }