# File lib/pusher-client/socket.rb, line 168
    def get_presence_auth(channel)
      return @auth_method.call(@socket_id, channel) if @auth_method

      string_to_sign = @socket_id + ':' + channel.name + ':' + channel.user_data
      signature = hmac(@secret, string_to_sign)
      "#{@key}:#{signature}"
    end