Auth is generally a small subset of user and profile. it is also store in the Session which is not generally available via the API since the Session is only valid for the current transaction (other than for internal counters like number of records).
If your custom fields are implemented as profile fields, you should have them accessible from the Users entity of the API. But you will have to reference them by index given they are not stored/presented as associative arrays.
EZ Merchant Solutions: Custom (USA based) B2B Development, Consulting, Development and Special Projects (get a quote here).
Commercial addons, payment methods and modifications to meet your business and operations needs.