Valery,
You can use the same OSPF password for authentication purposes on all OSPF-enabled interfaces. For example, assume that the interface Fa0/0 is in area 0 while the Fa0/1 is in area 1. The configuration for MD5 authentication would be performed as follows:
interface FastEthernet 0/0
ip ospf message-digest-key 1 md5 MySecretOSPFPassword
interface FastEthernet 0/1
ip ospf message-digest-key 1 md5 MySecretOSPFPassword
router ospf 1
area 0 authentication message-digest
area 1 authentication message-digest
Regarding the configuration of the E1 controller - I am not sure what you indicated by saying that you are associating a Loopback interface with it. In any case, you need to configure the E1 controller for proper framing and to bundle the individual timeslots into communication channels. These communication channels will be subsequently represented by virtual Serial interfaces in the IOS configuration that will need to be configured with IP addresses and the OSPF authentication, identically to the example above. I am not sure if your E1 controller is actually operational if you do not have configured the virtual Serial interfaces for individual timeslot bundles - and as far as I know, a Loopback interface cannot substitute them. Perhaps you are referring to the IP Unnumered feature...?
Best regards,
Peter
