gistfile1.js

var SignedXml = require('xml-crypto').SignedXml
      , FileKeyInfo = require('xml-crypto').FileKeyInfo  
      , fs = require('fs')

var xml = "<library>" +
             "<book>" +
               "<name>Harry Potter</name>" +
             "</book>"
           "</library>"

var sig = new SignedXml()
sig.addReference("//*[local-name(.)='book']")    
sig.signingKey = fs.readFileSync("client.pem")
sig.computeSignature(xml)
fs.writeFileSync("signed.xml", sig.getSignedXml())