const ca = require('win-ca')
certsPem = []

ca({
  format: ca.der2.pem,
  store: "MY",
  ondata: cert => certsPem.push(cert)
})

var fs = require('fs')
var xml = fs.readFileSync("xmlFile.xml")
var key = fs.readFileSync("privateKey.key")

var SignedXml = require('xml-crypto').SignedXml
var sig = new SignedXml()
...
sig.signingKey = key
sig.computeSignature(xml)
...