Verifying RSA

[Matthias Niggemeier]

Hi there,
I have a problem verifying a signed xml doc. My doc looks like this:

... xml snipped ...
</Product>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#";>
<SignedInfo>
<CanonicalizationMethod 
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"; />
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"; />
<Reference URI="">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"; />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"; />
<DigestValue>abcde</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>abcdefg</SignatureValue>
</Signature>

When I try to verify, I get an error

Reference URI="" failed to verify

What is wrong? The sender of the xml doc persists that the signature is
valid. My code to verify:

bool LoadValidateRSA(tstring sURI)
{
    xercesc::DOMDocument *doc = NULL;   
    XSECProvider prov;
    DSIGSignature* sig = NULL;
    XERCES_CPP_NAMESPACE::DOMBuilder* builder = NULL;
        WinCAPICryptoKeyRSA* pRSA = NULL;
        WinCAPICryptoProvider WCProv;

        builder = 
GetImplementationLS()->createDOMBuilder(xercesc::DOMImplementationLS::MODE_SYNCHRONOUS,0);
      builder->setFeature(xercesc::XMLUni::fgDOMNamespaces, true);
      doc = builder->parseURI(X(sURI));

        pRSA = (WinCAPICryptoKeyRSA*)WCProv.keyRSA();
        sig = prov.newSignatureFromDOM(doc); 
        char cModulus[] = "sAGXf2IjqajW1F7m"; // snipped
        char cExponent[] = "ABCD";
        
        pRSA->loadPublicExponentBase64BigNums(cExponent, (unsigned int) 
strlen(cExponent));
        pRSA->loadPublicModulusBase64BigNums(cModulus, (unsigned int) 
strlen(cModulus));
        
        sig->load();
        sig->setSigningKey(pRSA);
        bVerifyOK = sig->verify();
}

I looked at the samples, but they didn't help me further than this :-(

Regards

Matthias