C.NAVAMANI; S.RUBA. A new countersign in graphical captcha based on hard AI problems . International journal of intellectual advancements and research in engineering computations, [S. l.], v. 5, n. 2, p. 1402–1405, 2017. Disponível em: https://ijiarec.com/ijiarec/article/view/1505. Acesso em: 3 may. 2024.