IdP 3.0 question: Bag out of assertion creation depending on attribute value?

[Erdos, Marlena]

Dear Shibb Developers,

We'd like the IdP to cancel the assertion creation process if the user
doesn't have a particular attribute or attribute value.  And we'd like to
return a sensible error message to the user.

I can't tell from the existing IdP3 documentation how I'd do this (or
really, if it can be done (or rather, will be able to be done :-)).

Could you let me know?   I'll guess that Spring Web Flows might let me do
something interesting here (but I don't know SWFs so I really am just
guessing :-)).

Thank you!
Marlena Erdos

Harvard University Information Technology
Innovation & Architecture
Senior Technologist
(Mobile) +1 (617) 872-9736
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Cambridge, MA 02138