J. King, A. Santos, and C. Xuan.
"KHAP: Using Keyed Hard AI Problems to Secure Human Interfaces", in <em>Anais do IV Workshop em Segurança de Sistemas Computacionais</em>, Gramado, 2004, pp. 232-243, doi: <a href="https://doi.org/10.5753/sbseg.2004.21241">https://doi.org/10.5753/sbseg.2004.21241</a>.