Sequenced picture captcha: generation and its strength analysis

CAPTCHAs are employed on web systems to differentiate between human users and automated programs which indulge in spamming and other fraudulent activities. CAPTCHAs currently in use have been broken and rendered ineffective as a result of continuous evolution in CAPTCHA breaking. Thus, there is a need to employ stronger CAPTCHAs to keep these breaking attacks at bay while retaining ease of implementation on websites and ease of use for humans. In this paper, we introduce Sequenced Picture Captcha (SPC) which comprises of object pictures, each of which is accompanied by a Tag. The user is required to determine the logical sequence of the displayed object pictures based on the Tags. Hence, the user is required to identify both the object pictures and the Tags. Thus, Tagging introduces two levels of security. Moreover, being a Picture CAPTCHA, SPC inherits the advantages of high user convenience and simplicity of operation.