Hello Muhammad
There are two ways to configure authentication. The first is to configure it on a per interface basis. In this case, you configure the commands as shown on the interfaces themsevles. This will enable authentication only between OSPF routers that are connected via interfaces that have been configured.
The second way to configure it is for all OSPF routers in the area. By enabling it under the OSPF configuration, all participating interfaces will attempt to authenticate with their neighbors that are in the same area. In this case, you don’t configure the ip ospf authentication message-digest command on each interface. Whether you do or not will not affect the end result.
I hope this has been helpful!
Laz