Sign XML file in python with a x509 certificate

x509_sign.py

# coding: utf-8

from lxml import etree
from signxml import xmldsig

cert = open('cert.pem').read()
key = open('key.pem').read()


doc = etree.parse('sample.xml').getroot()

root = etree.fromstring(
        etree.tostring(doc),
        parser=etree.XMLParser(encoding='ISO-8859-1'))


signed_root = xmldsig(root,
        digest_algorithm='sha1').sign(algorithm='rsa-sha1',
        key=key,
        cert=cert)


signed_root.xpath(
        '//ext:UBLExtensions/ext:UBLExtension/ext:ExtensionContent/ds:Signature',
        namespaces={
        'ext': 'urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2',
        'ds': 'http://www.w3.org/2000/09/xmldsig#'
        })[0].attrib['Id'] = 'SignSUNAT'


doc = etree.tostring(signed_root, encoding='ISO-8859-1')

with open('result.xml', 'w') as f:
    f.write(etree.tostring(signed_root, xml_declaration=True, encoding="ISO-8859-1", pretty_print=True))