Alarm alerts for doors over mqtt

Configuration
1

How would I configure alarm for a door sensor?
My wish is to make an alarm that will send me a message when my alarm is active and door signals an opening.

My alarm is a mqtt switch:

- platform: mqtt
  state_topic: "home/alarmswitch"
  command_topic: "home/alarmswitch/set"
  name: Alarm
  friendly_name: Alarm
  icon: mdi:alarm-bell
  payload_on: 'alarm_on'
  payload_off: 'alarm_off'

And my sensor is mqtt sensor that currently receives all values that I should somehow filter it out.

- platform: mqtt
  name: "RF sensor"
  state_topic: "rfswitch/rf"

How would I proceed to do this? All of the examples I found only have a single state alarm.

2

The automation would look something like this:

- alias: 'door alarm'
  trigger:
  - platform: state
    entity_id: binary_sensor.door
    to: 'on'
  condition:
    condition: state
    entity_id: switch.alarm
    state: 'on'
  action:
  - service: switch.turn_on
    entity_id: switch.siren

Your challenge is to extract the door’s status (binary_sensor.door) from the single MQTT topic rfswitch/rf that “currently receives all values”.

I suggest you review the following post where I described two ways to handle the case where a single MQTT topic contains information for different sensors:
Sonoff RF Bridge. Strategies for receiving data