(Swift 3,4,5...) Create Enveloping XML Digital Signature

This example creates an enveloping digital signature.
An enveloping signature is where the signed data is contained within the Signature within an Object element.
This example signs non-XML text data where the XML signature constitutes the entire output.

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

    // Let's use the ECDSA private key at https://www.chilkatsoft.com/exampleData/secp256r1-key.zip
    // for signing.
    let http = CkoHttp()!
    let zipFile = CkoBinData()!
    var keyUrl: String? = "https://www.chilkatsoft.com/exampleData/secp256r1-key.zip"

    var success: Bool = http.quickGetBd(keyUrl, binData: zipFile)
    if success != true {
        print("\(http.lastErrorText!)")
        return
    }

    let zip = CkoZip()!
    success = zip.openBd(zipFile)
    var zipEntry: CkoZipEntry? = zip.first(matchingEntry: "*.pem")
    let ecKey = CkoPrivateKey()!
    success = ecKey.loadPem(zipEntry!.unzip(toString: 0, srcCharset: "utf-8"))
    if success != true {
        print("\(ecKey.lastErrorText!)")
        zipEntry = nil
        return
    }

    zipEntry = nil

    // ----------------------------------------------------------------------------
    let gen = CkoXmlDSigGen()!

    // Provide the ECDSA key to the XML Digital Signature generator
    gen.setPrivateKey(ecKey)

    // Add an enveloped reference to the content to be signed.
    let sbContent = CkoStringBuilder()!
    sbContent.append("This is the content that is signed.")
    gen.addEnvelopedRef("abc123", content: sbContent, digestMethod: "sha256", canonMethod: "C14N", refType: "")

    // Generate the XML digital signature.
    // Notice that in other examples, the sbXml passed to CreateXmlDSigSb
    // already contains XML, and the XML signature is inserted at the location
    // specified by the SigLocation property.  In this case, both SigLocation
    // and sbXml are empty.  The result is that sbXml will contain just the Signature.
    let sbXml = CkoStringBuilder()!
    success = gen.createXmlDSigSb(sbXml)
    if success != true {
        print("\(gen.lastErrorText!)")
        return
    }

    // Examine the enveloped signature, where the data is contained within the XML Signature
    print("\(sbXml.getAsString()!)")

    // The Signature returned is compact and in a single line, like this:
    // <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#"><ds:SignedInfo><ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/><ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256"/><ds:Reference URI="#abc123"><ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/><ds:DigestValue>tEVrbXXjeTXjF3tIojul4/sgeEGN49E1dxr/GMs8GNE=</ds:DigestValue></ds:Reference></ds:SignedInfo><ds:SignatureValue>/pILUshwrzgdGc4bPgp85TDfbUiM9pn8EIPNRVWKuoVEtPsv4XRthUrv9aDDvajmyl2okLwTakANgtaxO1ULMw==</ds:SignatureValue><ds:KeyInfo><ds:KeyValue><ds:ECKeyValue xmlns="http://www.w3.org/2009/xmldsig11#"><ds:NamedCurve URI="urn:oid:1.2.840.10045.3.1.7" /><ds:PublicKey>BOVKaiLPKEDChhkA64UEBOXTv/VFHnhrUPN+bXqCvEl7rroAYpH5tKzbiGTtMSlp4JO9Pxg44zeX7EoWDvOrpD0=</ds:PublicKey></ds:ECKeyValue></ds:KeyValue></ds:KeyInfo><ds:Object Id="abc123">This is the content that is signed.</ds:Object></ds:Signature>

    // XML pretty-printed, the signature is as follows, but pretty-printing introductes whitespace that breaks the signature..

    // <?xml version="1.0" encoding="utf-8" ?>
    // <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    //     <ds:SignedInfo>
    //         <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
    //         <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256" />
    //         <ds:Reference URI="#abc123">
    //             <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" />
    //             <ds:DigestValue>tEVrbXXjeTXjF3tIojul4/sgeEGN49E1dxr/GMs8GNE=</ds:DigestValue>
    //         </ds:Reference>
    //     </ds:SignedInfo>
    //     <ds:SignatureValue>/pILUshwrzgdGc4bPgp85TDfbUiM9pn8EIPNRVWKuoVEtPsv4XRthUrv9aDDvajmyl2okLwTakANgtaxO1ULMw==</ds:SignatureValue>
    //     <ds:KeyInfo>
    //         <ds:KeyValue>
    //             <ds:ECKeyValue xmlns="http://www.w3.org/2009/xmldsig11#">
    //                 <ds:NamedCurve URI="urn:oid:1.2.840.10045.3.1.7" />
    //                 <ds:PublicKey>BOVKaiLPKEDChhkA64UEBOXTv/VFHnhrUPN+bXqCvEl7rroAYpH5tKzbiGTtMSlp4JO9Pxg44zeX7EoWDvOrpD0=</ds:PublicKey>
    //             </ds:ECKeyValue>
    //         </ds:KeyValue>
    //     </ds:KeyInfo>
    //     <ds:Object Id="abc123">This is the content that is signed.</ds:Object>
    // </ds:Signature>
    // 

}