Overlapping variants of sequenced tagged captcha (STC): Generation and their comparative analysis

Sophisticated and evolving CAPTCHA breaking algorithms as mentioned in [(K. Chellapilla et al., 2004), (G. Mori et al., 2003), (J. Yan et al., 2008)] have successfully broken currently employed CAPTCHAs. This has resulted in a continuous requirement for more robust CAPTCHAs. In light of this, STC generation scheme was introduced in (A. Gupta et al., 2009) to stay secure for a long time. In this paper, we focus on the generation and comparative study of four variations in STC. These variations occur in overlapping of characters and/or Tagged numbers in the STC and further expand the STC concept. Further, we have identified two parameters viz. readability and security in this paper. These parameters have been used to compare and quantitatively analyze these schemes in order to find a suitably balanced one with an optimum combination of both.