(Swift 3,4,5...) SII Chile - FRMA Signature Computation and Add to XML

Compute the FRMA signature of a <DA> element enclosed inside a <CAF> element of the XML to be signed.

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

    // Also see:  Compute the FRMT Signature and Add to XML

    let xml = CkoXml()!

    // Load the unsigned XML that contains the following:

    // <DTE version="1.0">
    //   <Documento ID="F60T33">
    //         <TED version="1.0">
    //             <DD>
    // 		...
    //                 <CAF version="1.0">
    //                     <DA>
    // 			...
    //                     </DA>
    // 			... The FRMA will be added here ...
    //                 </CAF>
    //                 ...
    //             </DD>
    //             ... The FRMT will be added here in another example ...
    //         </TED>
    //   </Documento>
    // </DTE>

    var success: Bool = xml.loadFile("qa_data/xml_dsig/sii_cl/test_0.xml")
    if success == false {
        print("Failed to load initial XML file.")
        return
    }

    // Get a reference to the "DA" element
    var daXml: CkoXml? = xml.findChild("Documento|TED|DD|CAF|DA")
    if xml.lastMethodSuccess == false {
        print("Failed to find DA element")
        return
    }

    //  We need to get the "flattened" DA XML where:
    //    - No whitespace between elements.
    //    - The 5 pre-defined entities are converted.
    //    - The text is encoded in the ISO-8859-1 character set (Latin-1), 
    let sbFlattened = CkoStringBuilder()!
    daXml.emitCompact = true
    daXml.emitXmlDecl = false
    daXml!.getSb(sbFlattened)

    // Compute the SHA-1 message digest of the iso-8859-1 byte representation, 
    // and sign it with our RSA private key, getting the result in base64 format.

    let privKey = CkoPrivateKey()!
    success = privKey.loadAnyFormatFile("qa_data/rsa/rsaPrivKey_pkcs8.pem", password: "")
    if success == false {
        print("\(privKey.lastErrorText!)")
        return
    }

    let rsa = CkoRsa()!
    rsa.importPrivateKeyObj(privKey)

    rsa.encodingMode = "base64"
    rsa.charset = "iso-8859-1"
    var sig: String? = rsa.signStringENC(sbFlattened.getAsString(), hashAlg: "sha1")

    // Add the FRMA signature element to the XML.
    xml.updateChildContent("Documento|TED|DD|CAF|FRMA", value: sig)
    xml.updateAttr(at: "Documento|TED|DD|CAF|FRMA", autoCreate: true, attrName: "algoritmo", attrValue: "SHA1withRSA")

    daXml = nil

    // See what we have:
    xml.emitCompact = false
    xml.emitXmlDecl = true
    print("\(xml.getXml()!)")

    xml.save("qa_data/xml_dsig/sii_cl/test_1.xml")

}