(Excel) 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.

Download Excel Class Modules Chilkat Excel Class Modules

' 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>

Dim verifier As Chilkat.XmlDSig
Set verifier = Chilkat.NewXmlDSig
Dim http As Chilkat.Http
Set http = Chilkat.NewHttp

' First load the signed XML
Dim sbSignedXml As Chilkat.StringBuilder
Set sbSignedXml = Chilkat.NewStringBuilder

success = sbSignedXml.LoadFile("qa_data/xml_dsig_verify/signedWithExternalUrlRefs.xml","utf-8")
If (success = False) Then
    Debug.Print "Failed to load signed XML."
    Exit Sub
End If

success = verifier.LoadSignatureSb(sbSignedXml)
If (success = False) Then
    Debug.Print verifier.LastErrorText
    Exit Sub
End If

' Iterate over each reference.  If it is an external URL reference, download the data and provide it to the verifier.
Dim sbRefUri As Chilkat.StringBuilder
Set sbRefUri = Chilkat.NewStringBuilder
Dim bd As Chilkat.BinData
Set bd = Chilkat.NewBinData

numRefs = verifier.NumReferences

i = 0
Do While i < numRefs
    If (verifier.IsReferenceExternal(i) = True) Then
        sbRefUri.Clear 
        Dim success As Boolean
        success = sbRefUri.Append(verifier.ReferenceUri(i))
        If (sbRefUri.StartsWith("https://",False) = True) Then
            Debug.Print "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) Then
                Debug.Print http.LastErrorText
                Exit Sub
            End If

            success = verifier.SetRefDataBd(i,bd)
            If (success = False) Then
                Debug.Print verifier.LastErrorText
                Exit Sub
            End If

        End If

    End If

    i = i + 1
Loop

' Now that we have the external data, verify the signature..

bVerified = verifier.VerifySignature(True)
If (bVerified = False) Then
    Debug.Print verifier.LastErrorText
End If

Debug.Print "Signature verified = "; bVerified