'Validate signature on XML document.
The parameters actually used in this CryptoBackend
implementation are :
:param signedtext: The signed XML data as string
:param cert_file: xmlsec key_spec string(), filename,
"pkcs11://" URI or PEM data
:param cert_type: string, must be \'pem\' for now
:returns: True on successful validation, False otherwise'

def validate_signature(self, signedtext, cert_file, cert_type, node_name, node_id, id_attr):

    if (cert_type != 'pem'):
        raise Unsupported('Only    PEM    certs    supported    here')
    import xmlsec
    xml = xmlsec.parse_xml(signedtext)
    try:
        return xmlsec.verify(xml, cert_file)
    except xmlsec.XMLSigException:
        return False
