Media type: E-Article Title: Lecture Notes in Computer Science: CAPTCHA: Using Hard AI Problems for Security Contributor: von Ahn, Luis; Blum, Manuel; Hopper, Nicholas J.; Langford, John imprint: Springer Berlin Heidelberg, 2003 Published in: Lecture Notes in Computer Science Language: Not determined DOI: 10.1007/3-540-39200-9_18 ISSN: 0302-9743 Origination: Footnote: