XML-Signatur (for SAML): Why is the certificate within the signatur???

Hey folks,

I have to implement single sign-on with SAML for my company's web site. One big part of the validation of a SAML message is the verification of the signature. Here is the signature of a SAML-Response example:
<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> <ds:SignedInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" xmlns:ds="http://www.w3.org/2000/09/xmldsig#"/> <ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" xmlns:ds="http://www.w3.org/2000/09/xmldsig#"/> <ds:Reference URI="#_2152811999472b94a0e9644dbc932cc3" xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> <ds:Transforms xmlns:ds="http://www.w3.org/2000/09/xmldsig#"& ...