Implementing the auth handler

I am looking for some help on how to implement the auth handler. As per the protocol http://docs.oasis-open.org/kmip/spec/v1.2/os/kmip-spec-v1.2-os.html there are 
```

Credential | 420023
Credential Type | 420024
Credential Value | 420025
```

I see in auth.go I have a CREDENTIAL_TYPE_USERNAME_AND_PASSWORD but I am not sure how to use this in server.go