(PureBasic) Verify XML Digital Signature with an RSA Key

This example demonstrates how to verify an XML signature, where the RSA public key is embedded in the KeyInfo part of the Signature. When this is the case, nothing external is needed to verify the signature.

This example requires Chilkat v9.5.0.69 or greater.

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IncludeFile "CkHttp.pb"
IncludeFile "CkStringBuilder.pb"
IncludeFile "CkXmlDSig.pb"

Procedure ChilkatExample()

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

    ; The signed XML to be verified in this example contains the following:

    ; <?xml version="1.0" encoding="UTF-8"?>
    ; <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="#object">
    ;     <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
    ;     <DigestValue>OPnpF/ZNLDxJ/I+1F3iHhlmSwgo=</DigestValue>
    ;   </Reference>
    ; </SignedInfo>
    ; <SignatureValue>nihUFQg4mDhLgecvhIcKb9Gz8VRTOlw+adiZOBBXgK4JodEe5aFfCqm8WcRIT8GLLXSk8PsUP4//SsKqUBQkpotcAqQAhtz2v9kCWdoUDnAOtFZkd/CnsZ1sge0ndha40wWDV+nOWyJxkYgicvB8POYtSmldLLepPGMz+J7/Uws=</SignatureValue>
    ; <KeyInfo>
    ;   <KeyValue>
    ;     <RSAKeyValue><Modulus>4IlzOY3Y9fXoh3Y5f06wBbtTg94Pt6vcfcd1KQ0FLm0S36aGJtTSb6pYKfyX7PqCUQ8wgL6xUJ5GRPEsu9gyz8ZobwfZsGCsvu40CWoT9fcFBZPfXro1Vtlh/xl/yYHm+Gzqh0Bw76xtLHSfLfpVOrmZdwKmSFKMTvNXOFd0V18=</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>
    ;   </KeyValue>
    ; </KeyInfo>
    ; <Object Id="object">some text
    ;   with spaces and CR-LF.</Object>
    ; </Signature>

    ; The above XML is available at https://www.chilkatsoft.com/exampleData/signedSample1.xml
    ; Fetch the XML and then verify it..

    url.s = "https://www.chilkatsoft.com/exampleData/signedSample1.xml"
    http.i = CkHttp::ckCreate()
    If http.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    sbXml.i = CkStringBuilder::ckCreate()
    If sbXml.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success.i = CkHttp::ckQuickGetSb(http,url,sbXml)
    If success <> 1
        Debug CkHttp::ckLastErrorText(http)
        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbXml)
        ProcedureReturn
    EndIf

    dsig.i = CkXmlDSig::ckCreate()
    If dsig.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    ; First load the XML containing the signatures to be verified.
    ; Note that this particular Signature already contains the RSA public key that will be used
    ; for verification.
    success = CkXmlDSig::ckLoadSignatureSb(dsig,sbXml)
    If success <> 1
        Debug CkXmlDSig::ckLastErrorText(dsig)
        CkHttp::ckDispose(http)
        CkStringBuilder::ckDispose(sbXml)
        CkXmlDSig::ckDispose(dsig)
        ProcedureReturn
    EndIf

    ; The XML in this example contains only 1 signature.
    ; It's possible that an XML document can contain multiple signatures.
    ; Each can be verified as follows:
    i.i = 0
    While i < CkXmlDSig::ckNumSignatures(dsig)
        ; Select the Nth signature by setting the Selector property.
        CkXmlDSig::setCkSelector(dsig, i)

        ; The bVerifyReferenceDigests argument determines if we want
        ; to also verify each reference digest.  If set to 0,
        ; then only the SignedInfo part of the Signature is verified.
        bVerifyReferenceDigests.i = 1
        bVerified.i = CkXmlDSig::ckVerifySignature(dsig,bVerifyReferenceDigests)
        Debug "Signature " + Str(i + 1) + " verified = " + Str(bVerified)

        i = i + 1
    Wend


    CkHttp::ckDispose(http)
    CkStringBuilder::ckDispose(sbXml)
    CkXmlDSig::ckDispose(dsig)


    ProcedureReturn
EndProcedure