Taxonomy terms according to users permission

Dear all,

My goal is to build a hierarchie of downloadable documents, which
are on the one hand "public" and on the other hand "private".
What I already have, are the content types, to provide public and
private content.
I want to show certain taxonomy terms and vocabularies only to users,
that have the appropriate permission to see them.

I hope my question is not to stupid. Maybe I haven't have the
structural idea of taxonomy understood 100% yet, but I would
be so glad, if someone could help me with that issue.

Thank you!
Soezkan