code: def sign(self):
          
          self.verify_integrity()
          
          if session.get('u2f_sign_required', False):
              if request.method == 'GET':
                  response = self.get_signature_challenge()
  
                  if response['status'] == 'ok':
                      return jsonify(response), 200
                  else:
                      return jsonify(response), 404
  
              elif request.method == 'POST':
                  response = self.verify_signature(request.json)
  
                  if response['status'] == 'ok':
                      return jsonify(response), 201
                  else:
                      return jsonify(response), 400
  
          return jsonify({'status': 'failed', 'error': 'Unauthorized!'}), 401
docs: Signature function