non guessable url with md5 generator

message_thread.rb

  def md5hash
    md5 = Digest::MD5.hexdigest(self.id.to_s + self.initial_sender_email + MD5_SECRET)
    logger.info "DEBUG: md5 = " + md5
    return self.id.to_s + "-" + md5
  end

  def check_md5(md5_to_check)
    md5 = Digest::MD5.hexdigest(self.id.to_s + self.initial_sender_email + MD5_SECRET)
    if md5 == md5_to_check
      return true
    else
      return false
    end
  end

messages_controller.rb

  def show
    if params[:md5].nil?
      message_not_found
      return
    end
    message_thread_array = params[:md5].split('-')
    if message_thread_array.size != 2
      message_not_found
      return
    end
    @message_thread = MessageThread.with_data.find(message_thread_array[0],
        :conditions => ['message_threads.type_id <> ?', MessageThread::CONTACT_TYPE],
        :include => :messages) rescue nil
    if @message_thread.nil?
      message_not_found
      return
    end
    unless @message_thread.check_md5(message_thread_array[1])
      message_not_found
      return
    end
    @message_thread.messages.each_with_index do |msg, index|
      msg.opened_by_sender! if msg.opened_by_sender == nil
      @message_thread.opened_by_sender!(msg.id) if index == (@message_thread.messages.size - 1)
    end
    @message = Message.new(:view_opened_time => DateTime.now.strftime("%Y-%m-%d %H:%M:%S %z"))
    @onload = "$('#message_body').focus();"
  end

notifier.rb

message.reply_url = messages_url(message_thread.md5hash)

routes.rb

  map.messages "messages/:md5", :controller => "messages", :action => "show",
      :conditions => { :method => :get }