public void Cryptography(XmlDocument doc, bool cryptographyMode)
 {
  RijndaelManaged key = null;
  try
  {
  // Create a new Rijndael key.
  key = new RijndaelManaged();
  const string passwordBytes = "Password1234"; //password here 
 
  byte[] saltBytes = Encoding.UTF8.GetBytes("SaltBytes");
  Rfc2898DeriveBytes p = new Rfc2898DeriveBytes(passwordBytes, saltBytes);
  // sizes are devided by 8 because [ 1 byte = 8 bits ] 
  key.IV = p.GetBytes(key.BlockSize/8);
  key.Key = p.GetBytes(key.KeySize/8);
 
  if (cryptographyMode)
  {
  Ecrypt(doc, "Content", key);
  }
  else
  {
  Decrypt(doc, key);
  }
 
  }
  catch (Exception ex)
  {
  MessageBox.Show(ex.Message);
  }
  finally
  {
  // Clear the key.
  if (key != null)
  {
  key.Clear();
  }
  }
 
 }
 
 private void Decrypt(XmlDocument doc, SymmetricAlgorithm alg)
 {
  // Check the arguments. 
  if (doc == null)
  throw new ArgumentNullException("Doc");
  if (alg == null)
  throw new ArgumentNullException("alg");
 
  // Find the EncryptedData element in the XmlDocument.
  XmlElement encryptedElement = doc.GetElementsByTagName("EncryptedData")[0] as XmlElement;
 
  // If the EncryptedData element was not found, throw an exception.
  if (encryptedElement == null)
  {
  throw new XmlException("The EncryptedData element was not found.");
  }
 
 
  // Create an EncryptedData object and populate it.
  EncryptedData edElement = new EncryptedData();
  edElement.LoadXml(encryptedElement);
 
  // Create a new EncryptedXml object.
  EncryptedXml exml = new EncryptedXml();
 
 
  // Decrypt the element using the symmetric key.
  byte[] rgbOutput = exml.DecryptData(edElement, alg); <---- I GET THE EXCEPTION HERE
  // Replace the encryptedData element with the plaintext XML element.
  exml.ReplaceData(encryptedElement, rgbOutput);
 
 }

Padding is invalid and cannot be removed