Consuming a SAML 2.0 assertion with ColdFusion - What do I do with a public key (.pem) file?

Question

I am tasked with getting our ColdFusion 9 app to receive a SAML assertion for single sign-on. We are the service provider. Thus far, I have used the only real source of information about ColdFusion and SAML at the following URL for guidance: http://blog.tagworldwide.com/?p=19

I have a sample SAML XML assertion from the identity provider and it looks very similar to the following example from Salesforce.com.

<samlp:Response ID="_257f9d9e9fa14962c0803903a6ccad931245264310738" 
   IssueInstant="2009-06-17T18:45:10.738Z" Version="2.0">
<saml:Issuer Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity">
   https://www.salesforce.com
</saml:Issuer>

<samlp:Status>
   <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
</samlp:Status>

<saml:Assertion ID="_3c39bc0fe7b13769cab2f6f45eba801b1245264310738" 
   IssueInstant="2009-06-17T18:45:10.738Z" Version="2.0">
   <saml:Issuer Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity">
      https://www.salesforce.com
   </saml:Issuer>

   <saml:Signature>
      <saml:SignedInfo>
         <saml:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
         <saml:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
         <saml:Reference URI="#_3c39bc0fe7b13769cab2f6f45eba801b1245264310738">
            <saml:Transforms>
               <saml:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
               <saml:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">
                  <ec:InclusiveNamespaces PrefixList="ds saml xs"/>
               </saml:Transform>
            </saml:Transforms>
            <saml:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
            <saml:DigestValue>vzR9Hfp8d16576tEDeq/zhpmLoo=
            </saml:DigestValue>
         </saml:Reference>
      </saml:SignedInfo>
      <saml:SignatureValue>
         AzID5hhJeJlG2llUDvZswNUrlrPtR7S37QYH2W+Un1n8c6kTC
         Xr/lihEKPcA2PZt86eBntFBVDWTRlh/W3yUgGOqQBJMFOVbhK
         M/CbLHbBUVT5TcxIqvsNvIFdjIGNkf1W0SBqRKZOJ6tzxCcLo
         9dXqAyAUkqDpX5+AyltwrdCPNmncUM4dtRPjI05CL1rRaGeyX
         3kkqOL8p0vjm0fazU5tCAJLbYuYgU1LivPSahWNcpvRSlCI4e
         Pn2oiVDyrcc4et12inPMTc2lGIWWWWJyHOPSiXRSkEAIwQVjf
         Qm5cpli44Pv8FCrdGWpEE0yXsPBvDkM9jIzwCYGG2fKaLBag==
      </saml:SignatureValue>
      <saml:KeyInfo>
         <saml:X509Data>
            <saml:X509Certificate>
               MIIEATCCAumgAwIBAgIBBTANBgkqhkiG9w0BAQ0FADCBgzELM
               [Certificate truncated for readability...]
            </saml:X509Certificate>
         </saml:X509Data>
      </saml:KeyInfo>
   </saml:Signature>

   <saml:Subject>
      <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified">
         saml01@salesforce.com
      </saml:NameID>

      <saml:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
      <saml:SubjectConfirmationData NotOnOrAfter="2009-06-17T18:50:10.738Z" 
         Recipient="https://login.www.salesforce.com"/>
      </saml:SubjectConfirmation>
   </saml:Subject>

   <saml:Conditions NotBefore="2009-06-17T18:45:10.738Z" 
      NotOnOrAfter="2009-06-17T18:50:10.738Z">

      <saml:AudienceRestriction>
         <saml:Audience>https://saml.salesforce.com</saml:Audience>
      </saml:AudienceRestriction>
   </saml:Conditions>

   <saml:AuthnStatement AuthnInstant="2009-06-17T18:45:10.738Z">
      <saml:AuthnContext>
         <saml:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:unspecified
         </saml:AuthnContextClassRef>
      </saml:AuthnContext>
   </saml:AuthnStatement>

   <saml:AttributeStatement>

      <saml:Attribute Name="portal_id">
         <saml:AttributeValue xsi:type="xs:anyType">060D00000000SHZ
         </saml:AttributeValue>
      </saml:Attribute>

      <saml:Attribute Name="organization_id">
         <saml:AttributeValue xsi:type="xs:anyType">00DD0000000F7L5
         </saml:AttributeValue>
      </saml:Attribute>

      <saml:Attribute Name="ssostartpage" 
         NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified">

         <saml:AttributeValue xsi:type="xs:anyType">
            http://www.salesforce.com/security/saml/saml20-gen.jsp
         </saml:AttributeValue>
      </saml:Attribute>

      <saml:Attribute Name="logouturl" 
         NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri">

         <saml:AttributeValue xsi:type="xs:string">
            http://www.salesforce.com/security/del_auth/SsoLogoutPage.html
         </saml:AttributeValue>
      </saml:Attribute>
   </saml:AttributeStatement>
</saml:Assertion>
</samlp:Response>

The Identify Provider also gave us a .pem file that apparently contains two certificate blocks. I don't know what I am supposed to do with this file. If I pull out the X509Certificate block in the assertion and decode it using a website like http://www.sslshopper.com/certificate-decoder.html, I can see the info it contains.

I have example code running so far that tests whether or not the XML signature and cert is valid. It currently is not. However, nothing in the code refers to the .pem file.

So what am I supposed to do with the *.pem file that has been provided? Any hints?

UPDATE:

Okay, I am continuing to try to solve my own problem. As I mentioned, the .pem file I have contains 2 cert blocks, so I decoded them to see the contents. The first one contains info about the Identity Provider. The second contains info about DigiCert Inc.

I am beginning to think that the purpose of the IDP cert in the .pem file is simply to match against the one sent within the XML assertion itself as a primary security check. Is that right?