author = { von Ahn, Luis AND Blum, Manuel AND Hopper, Nicholas J. AND Langford, John },

title = { Lecture Notes in Computer Science: CAPTCHA: Using Hard AI Problems for Security },

publisher = {Springer Berlin Heidelberg},

isbn = {0302-9743},

year = {2003},

url = { http://slubdd.de/katalog?TN_libero_mab2 }