(Visual Basic 6.0) 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.

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

' 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 New ChilkatXmlDSig
Dim http As New ChilkatHttp

' First load the signed XML
Dim sbSignedXml As New ChilkatStringBuilder
Dim success As Long
success = sbSignedXml.LoadFile("qa_data/xml_dsig_verify/signedWithExternalUrlRefs.xml","utf-8")
If (success = 0) Then
    Debug.Print "Failed to load signed XML."
    Exit Sub
End If

success = verifier.LoadSignatureSb(sbSignedXml)
If (success = 0) 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 New ChilkatStringBuilder
Dim bd As New ChilkatBinData
Dim numRefs As Long
numRefs = verifier.NumReferences
Dim i As Long
i = 0
Do While i < numRefs
    If (verifier.IsReferenceExternal(i) = 1) Then
        sbRefUri.Clear 
        success = sbRefUri.Append(verifier.ReferenceUri(i))
        If (sbRefUri.StartsWith("https://",0) = 1) 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 = 0) Then
                Debug.Print http.LastErrorText
                Exit Sub
            End If

            success = verifier.SetRefDataBd(i,bd)
            If (success = 0) 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..
Dim bVerified As Long
bVerified = verifier.VerifySignature(1)
If (bVerified = 0) Then
    Debug.Print verifier.LastErrorText
End If

Debug.Print "Signature verified = " & bVerified