Luis von Ahn, Manuel Blum, Nicholas J. Hopper, John Langford. CAPTCHA: Using Hard AI Problems for Security. In Eli Biham, editor, Advances in Cryptology - EUROCRYPT 2003, International Conference on the Theory and Applications of Cryptographic Techniques, Warsaw, Poland, May 4-8, 2003, Proceedings. Volume 2656 of Lecture Notes in Computer Science, pages 294-311, Springer, 2003. [doi]
Abstract is missing.