How to use the xmlsec.findNode function in xmlsec

def encrypt_file(tmpl_file, key_file, data, dataSize):
    assert(tmpl_file)
    assert(key_file)
    assert(data)

    # Load template
    doc = libxml2.parseFile(tmpl_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % tmpl_file
        return cleanup(doc)
    
    # Find start node
    node = xmlsec.findNode(doc.getRootElement(), xmlsec.NodeEncryptedData,
                           xmlsec.EncNs)
    if node is None:
	print "Error: start node not found in \"%s\"" % tmpl_file
        return cleanup(doc)

    # Create encryption context, we don't need keys manager in this example
    enc_ctx = xmlsec.EncCtx(None)
    if enc_ctx is None:
        print "Error: failed to create encryption context"
        return cleanup(doc)
        
    # Load DES key, assuming that there is not password
    key = xmlsec.keyReadBinaryFile(xmlsec.keyDataDesId(), key_file)
    if key is None:
        print "Error failed to load DES key from binary file \"%s\"" % key_file
        return cleanup(doc, enc_ctx)

def verify_file(xml_file, key_file):
    assert(xml_file)
    assert(key_file)

    # Load XML file
    if not check_filename(xml_file):
        return -1
    doc = libxml2.parseFile(xml_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % tmpl_file
        return cleanup(doc)

    # Find start node
    node = xmlsec.findNode(doc.getRootElement(),
                           xmlsec.NodeSignature, xmlsec.DSigNs)

    # Create signature context, we don't need keys manager in this example
    dsig_ctx = xmlsec.DSigCtx()
    if dsig_ctx is None:
        print "Error: failed to create signature context"
        return cleanup(doc)

    # Load private key, assuming that there is not password
    key = xmlsec.cryptoAppKeyLoad(key_file, xmlsec.KeyDataFormatPem,
                                  None, None, None)
    if key is None:
        print "Error: failed to load private pem key from \"%s\"" % key_file
        return cleanup(doc, dsig_ctx)
    dsig_ctx.signKey = key

def decrypt_file(mngr, enc_file):
    assert(mngr)
    assert(enc_file)

    # Load template
    if not check_filename(enc_file):
        return -1
    doc = libxml2.parseFile(enc_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % enc_file
        return cleanup(doc)
    
    # Find start node
    node = xmlsec.findNode(doc.getRootElement(), xmlsec.NodeEncryptedData,
                           xmlsec.EncNs)
    if node is None:
	print "Error: start node not found in \"%s\"" % tmpl_file
        return cleanup(doc)
    
    # Create encryption context
    enc_ctx = xmlsec.EncCtx(mngr)
    if enc_ctx is None:
        print "Error: failed to create encryption context"
        return cleanup(doc)

    # Decrypt the data
    if enc_ctx.decrypt(node) < 0 or enc_ctx.result is None:
        print "Error: decryption failed"
        return cleanup(doc, enc_ctx)

def verify_file(mngr, xml_file):
    assert(mngr)
    assert(xml_file)

    # Load XML file
    if not check_filename(xml_file):
        return -1
    doc = libxml2.parseFile(xml_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % tmpl_file
        return cleanup(doc)

    # Find start node
    node = xmlsec.findNode(doc.getRootElement(),
                           xmlsec.NodeSignature, xmlsec.DSigNs)
    if node is None:
        print "Error: start node not found in \"%s\"", xml_file

    # Create signature context
    dsig_ctx = xmlsec.DSigCtx(mngr)
    if dsig_ctx is None:
        print "Error: failed to create signature context"
        return cleanup(doc)

    # Verify signature
    if dsig_ctx.verify(node) < 0:
        print "Error: signature verify"
        return cleanup(doc, dsig_ctx)

    # Print verification result to stdout

def verify_file(mngr, xml_file):
    assert(mngr)
    assert(xml_file)

    # Load XML file
    if not check_filename(xml_file):
        return -1
    doc = libxml2.parseFile(xml_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % tmpl_file
        return cleanup(doc)

    # Find start node
    node = xmlsec.findNode(doc.getRootElement(),
                           xmlsec.NodeSignature, xmlsec.DSigNs)
    if node is None:
        print "Error: start node not found in \"%s\"", xml_file

    # Create signature context
    dsig_ctx = xmlsec.DSigCtx(mngr)
    if dsig_ctx is None:
        print "Error: failed to create signature context"
        return cleanup(doc)

    # Verify signature
    if dsig_ctx.verify(node) < 0:
        print "Error: signature verify"
        return cleanup(doc, dsig_ctx)

    # Print verification result to stdout

def sign_file(tmpl_file, key_file):
    assert(tmpl_file)
    assert(key_file)

    # Load template
    doc = libxml2.parseFile(tmpl_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % tmpl_file
        return -1
    
    # Find start node
    node = xmlsec.findNode(doc.getRootElement(), xmlsec.NodeSignature,
                           xmlsec.DSigNs)
    if node is None:
	print "Error: start node not found in \"%s\"" % tmpl_file
        return cleanup(doc)
        
    # Create signature context, we don't need keys manager in this example
    dsig_ctx = xmlsec.DSigCtx()
    if dsig_ctx is None:
        print "Error: failed to create signature context"
        return cleanup(doc)

    # Load private key, assuming that there is not password
    key = xmlsec.cryptoAppKeyLoad(key_file, xmlsec.KeyDataFormatPem,
                                  None, None, None)
    if key is None:
        print "Error: failed to load private pem key from \"%s\"" % key_file

def decrypt_file(enc_file, key_file):
    assert(enc_file)
    assert(key_file)

    # Load template
    doc = libxml2.parseFile(enc_file)
    if doc is None or doc.getRootElement() is None:
	print "Error: unable to parse file \"%s\"" % enc_file
        return cleanup(doc)
    
    # Find start node
    node = xmlsec.findNode(doc.getRootElement(), xmlsec.NodeEncryptedData,
                           xmlsec.EncNs)
    if node is None:
	print "Error: start node not found in \"%s\"" % tmpl_file
        return cleanup(doc)

    # Create encryption context, we don't need keys manager in this example
    enc_ctx = xmlsec.EncCtx(None)
    if enc_ctx is None:
        print "Error: failed to create encryption context"
        return cleanup(doc)
        
    # Load DES key
    key = xmlsec.keyReadBinaryFile(xmlsec.keyDataDesId(), key_file)
    if key is None:
        print "Error failed to load DES key from binary file \"%s\"" % key_file
        return cleanup(doc, enc_ctx)