saml authentication handler configuration

I am trying to configure saml authentication handler using - http://docs.adobe.com/docs/en/aem/6-1/administer/security/saml-2-0-authenticationhandler.html

I have added the "idp_cert.binary" from the vendor to the administrator user too among other things

When I access the url protected with this handler, I am redirected to the sso logon page and then after succuessful login there, when the page comes back to aem, it just displays a message in browser - "saml authentication failed". In logs, the *WARN* message is "Private key of SP not provided: Cannot sign Authn request."

Any idea on what is missing

/Regards
Kanwal