code: func (e *Extractor) Extract(token *jwt.Token) (interface{}, error) {
  	e.Token = token
  
  	if _, ok := token.Method.(*jwt.SigningMethodRSA); !ok {
  		return nil, errors.New("invalid signing method")
  	}
  
  	claims, ok := token.Claims.(*KiteClaims)
  	if !ok {
  		return nil, fmt.Errorf("no kontrol key found")
  	}
  
  	e.Claims = claims
  
  	return jwt.ParseRSAPublicKeyFromPEM([]byte(claims.KontrolKey))
  }
docs: // Extract is a keyFunc argument for jwt.Parse function.