C.Navamani, and S.Ruba. “A New Countersign in Graphical Captcha Based on Hard AI Problems”. International journal of intellectual advancements and research in engineering computations 5, no. 2 (July 31, 2017): 1402–1405. Accessed May 3, 2024. https://ijiarec.com/ijiarec/article/view/1505.