L. Von Ahn, B. Maurer, C. McMillen, D. Abraham, and
M. Blum, "reCaptcha: Human-based character
recognition via web security measures," Science, vol.
321, pp. 1465-1468, 2008.
 ElieBursztein , Matthieu Martin , John Mitchell, Textbased
CAPTCHA strengths and weaknesses,
Proceedings of the 18th ACM conference on Computer
and communications security, October 17-21, 2011,
Chicago, Illinois, USA .
 Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and
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.
 Turing Test, http://en.wikipedia.org/wiki/Turing_test,
visited Date: 05/05/2019.
 What is reCAPTCHA?,
 J. Elson, J. R. Doucerur, J. Howell, and J. Saul.Asirra: A
Captcha that exploits interest-aligned manual image
categorization. In Proceedings of the 14th ACM
conference on Computer and communications security,
CCS ’07, pages 366–374, New York, NY, USA, 2007.
 Nucaptcha,www.nucaptcha.com, visited date:
 Kluever, K.A., and Zanibbi, R. (2009) Balancing
Usability and Security in a Video CAPTCHA. Paper
presented at the 5th Symposium on Usable Privacy and
Security. July 2009. Mountain View, California.
 How we broke the NuCaptcha video scheme and what
we propose to fix it, http://elie.im/blog/security/how-webroke-
to-fix-it/, Visited Date: 25/04/2012.
 It’s 1999. Lowe, David G. (1999). "Object recognition
from local scale-invariant features". Proceedings of the
International Conference on Computer Vision. 2. pp.
 Von Ahn, L.; Dabbish, L. (2004)."Labeling images
with a computer game".Proceedings of the 2004
conference on Human factors in computing systems -
CHI '04. pp. 319–326. DOI:10.1145/985692.985733.