1. L. von Ahn, M. Blum, N.J. Hopper, J. Langford, CAPTCHA: Using Hard AI Problems for Security. Computer Science Dept., Carnegie Mellon University, Pittsburgh PA 15213, USA. IBM T.J. WRC, Yorktown Heights NY 10598, USA
2. H.S. Baird, K. Popat, Human interactive proofs and document image analysis, in Proceeding of the 5th IAPR Workshop Document Analysis Systems, Princeton, NJ (2002)
3. A. Coates, H. Baird, R. Fateman, Pessimal print: a Reverse Turing test, in Proceeding of the IAPR 6th Int’l Conference on Document Analysis and Recognition, Seattle, WA, 2001, pp. 1154–1158
4. M.T. Banday, N.A. Shah, A study of CAPTCHAs for securing web services. IJSDIA Int. J. Secure Digital Inf. Age 1(2), 66–74 (2009). ISSN: 0975-1823
5. CAPTCHA website, Carnegie Mellon University,
http://www.captcha.net