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. DOI: 10.61096/ijiarec.v5.iss2.2017.1402-1405. Disponível em: https://ijiarec.com/ijiarec/article/view/1505. Acesso em: 5 jul. 2026.