1. A. Basso, S. Sicco: Preventing massive automated access to web resources, Comput. Secur. 28(3/4), 174–188 (2009), doi:10.1016/j.cose.2008.11.002
2. M. Koster: Robots in the web: threat or treat?, ConneXions 9(4), 2–12 (1995)
3. A. Basso: Protecting web resources from massive automated access, Technical report RT-114/08, Computer Science Departement, University of Torino (2008), http://www.di.unito.it/basso/papers/captcha-RT114-08.pdf
4. A. Basso, F. Bergadano, I. Coradazzi, P.D. Checco: Lightweight security for internet polls, EGCDMAS (INSTICC Press, 2004) pp. 46–55
5. M. Blum, H. S. Baird: First workshop on human interactive proofs, Xerox Palo Alto Research Center, CA (2002) http://www2.parc.com/istl/groups/did/HIP2002/