Pg_catalog reference

Dear all,
i'm trying to create tables using pg_user (pg_authid) as a foreign key
for my table. I need to log and control that only registered users can
modify data and i want to control data changes via logging triggers. I
need to know who exactly was modifying data. To be more exact i want to
create a trigger that can log user information into the a table. I want
to have a way to make rollbacks of high level data (documents...whatever).

But when i'm trying to create a constraint referencing pg_catalog tables
- i'm getting an error.

What you consider to be a solution in thi case?

Any help appriciated. Thanks.

Best regards,
Anton Nikiforov