It doesn't work that way. The POST data is local to the SP, and is not
shared with the IdP. (The only purpose of saving it is to preserve it
until the user has logged in. The typical use case is to avoid losing
changes made to a wiki page due to session timeout.)
There is no general way to transfer data from an SP to an IdP. You can
implement it in a non-standard way if you want to. See the following
thread for some hints about how to do that with simpleSAMLphp on both
ends:
https://groups.google.com/d/topic/simplesamlphp/klbp57ZqlKc/discussion
Of course, that is entirely non-standard.
Best regards,
Olav Morken
UNINETT / Feide
I think you have got it the wrong way around. The SP does generally not
provide data to the IdP. Instead, the IdP authenticates the user, and
provides attributes about the user to the SP.
Therefore the SP authproc entry executes after the IdP provides it's
response to the SP. (Its purpose is to allow the SP to postprocess
the response from the IdP.)
> Can anybody help me out, how to put an
> attribute to the AuthnRequest so that the IDP can work on it?
There is no space for SAML 2.0 attributes in the authentication
request. Instead you must use a non-standard extension if you need
to transport extra data in the authentication request.