(Mono C#) Verify XML Signature with External URL References

Demonstrates how to verify an XML digital signature that includes references to URLs where the data to be digested is on a web server.

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// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// The signed XML we wish to verify contains external references such as this:

//     <ds:Reference Id="xmldsig-e7ae7ce2-9133-4d56-bd97-0a6aef738cc2-ref0" URI="https://www.chilkatsoft.com/images/starfish.jpg">
//       <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
//       <ds:DigestValue>AOU810yJV5Np/DnO29qpObqiTSTTCDvxGsX5ayiTYXI=</ds:DigestValue>
//     </ds:Reference>
//     <ds:Reference Id="xmldsig-e7ae7ce2-9133-4d56-bd97-0a6aef738cc2-ref1" URI="https://www.chilkatsoft.com/hamlet.xml">
//       <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
//       <ds:DigestValue>4sRRyWOzC7EOic4fQ9+Op1pa10DbgoBGjBvkq09LZmE=</ds:DigestValue>
//     </ds:Reference>

Chilkat.XmlDSig verifier = new Chilkat.XmlDSig();
Chilkat.Http http = new Chilkat.Http();

// First load the signed XML
Chilkat.StringBuilder sbSignedXml = new Chilkat.StringBuilder();
bool success = sbSignedXml.LoadFile("qa_data/xml_dsig_verify/signedWithExternalUrlRefs.xml","utf-8");
if (success == false) {
    Debug.WriteLine("Failed to load signed XML.");
    return;
}

success = verifier.LoadSignatureSb(sbSignedXml);
if (success == false) {
    Debug.WriteLine(verifier.LastErrorText);
    return;
}

// Iterate over each reference.  If it is an external URL reference, download the data and provide it to the verifier.
Chilkat.StringBuilder sbRefUri = new Chilkat.StringBuilder();
Chilkat.BinData bd = new Chilkat.BinData();
int numRefs = verifier.NumReferences;
int i = 0;
while (i < numRefs) {
    if (verifier.IsReferenceExternal(i) == true) {
        sbRefUri.Clear();
        sbRefUri.Append(verifier.ReferenceUri(i));
        if (sbRefUri.StartsWith("https://",false) == true) {
            Debug.WriteLine("External URL Reference: " + sbRefUri.GetAsString());

            // Download the data at the URL and provide to the verifier.
            success = http.DownloadBd(sbRefUri.GetAsString(),bd);
            if (success == false) {
                Debug.WriteLine(http.LastErrorText);
                return;
            }

            success = verifier.SetRefDataBd(i,bd);
            if (success == false) {
                Debug.WriteLine(verifier.LastErrorText);
                return;
            }

        }

    }

    i = i + 1;
}

// Now that we have the external data, verify the signature..
bool bVerified = verifier.VerifySignature(true);
if (bVerified == false) {
    Debug.WriteLine(verifier.LastErrorText);
}

Debug.WriteLine("Signature verified = " + Convert.ToString(bVerified));