code: def authenticate(self):
          
          if request.headers.get('Authorization', '').startswith('Negotiate '):
              in_token = base64.b64decode(request.headers['Authorization'][10:])
  
              try:
                  creds = current_app.extensions['gssapi']['creds']
              except KeyError:
                  raise RuntimeError('flask-gssapi not configured for this app')
  
              ctx = gssapi.SecurityContext(creds=creds, usage='accept')
  
              out_token = ctx.step(in_token)
  
              if ctx.complete:
                  username = ctx._inquire(initiator_name=True).initiator_name
                  return str(username), out_token
  
          return None, None
docs: Attempts to authenticate the user if a token was provided.