ORA-12638: credential retrieval failed

The Oracle docs note this on the ora-12638 error:

ORA-12638: Credential retrieval failed

Cause: The authentication service failed to retrieve the credentials of a user.

Action: Enable tracing to determine the exact error.

I found following line in sqlnet.ora file from $ORACLE_HOME/network/admin

SQLNET.AUTHENTICATION_SERVICES= (NTS)

Then i modified the above line as below and it started working

SQLNET.AUTHENTICATION_SERVICES= (NONE)

Reason : 

Oracle client attempt to use your current Windows domain credentials to authenticate you with the Oracle server. This could fail for a couple of reasons:

- The Oracle server is not configured to support Windows authentication
- The credentials you use to login to your local machine are not sufficient to allow you to login to the server.

In my case, it was the later. This failed because I was logged on to my local machine using my normal domain credentials rather than my administrator account.