BC 4.6 and SAP Portals SSO Tickets


we have following installation @ out site:

SAP Portals calling the business connector
who in turn calls SAP via RFC’s …

no uncommon, but we want to authenticate
sap portal users via the BC to sap for
implementing a shop solution.

so we want to get the SAP SSO ticket from
the portal an pass it to the SAP backend.

we found out how to pass the ticket from
the bc to sap automatically - but we are not yet able to get in from the portal server …

any IDEAS ??

thanx for any help,