CAPTCHAs are a standard security mechanism used on many websites to protect online services against abuse by automated programs, or bots. The purpose of a CAPTCHA is to distinguish whether an online transaction is being carried out by a human or a bot. Unfortunately, to date many existing CAPTCHA schemes have been found to be vulnerable to automated attacks. It is widely accepted that state-of-...

Many web-based services such as email, search engines, and polling sites are being abused by spammers via computer programs known as bots. This problem has bred a new research area called Human Interactive Proofs (HIP) and a testing device called CAPTCHA, which aims to protect services from malevolent attacks by distinguishing bots from human users. In the past decade, researchers have focused ...

Image CAPTCHA, aiming at effectively distinguishing human users from malicious script attacks, has been an important mechanism to protect online systems from spams and abuses. Despite the increasing interests in developing and deploying image CAPTCHAs, the usability aspect of those CAPTCHAs has hardly been explored systematically. In this paper, the universal design factors of image CAPTCHAs, s...

A Captcha is a puzzle that is easy for humans but hard to solve for computers. A formal framework, modelling Captcha puzzles (as hard AI problems), was introduced by Ahn, Blum, Hopper, and Langford ([ABHL03], Eurocrypt 2003). Despite their attractive features and wide adoption in practice, the use of Captcha puzzles for general cryptographic applications has been limited. In this work, we explo...

CAPTCHA (Completely Automated Public Turing Test to Tell Computers and Humans Apart) is a simple test that is easy for humans but extremely difficult for computers to solve. CAPTCHA has been widely used in commercial websites such as web-based email providers, TicketMaster, GoDaddy, and Facebook to protect their resources from attacks initiated by automatic scripts. By design, CAPTCHA is unable...

Completely automated public turing test to tell computers and humans apart (CAPTCHA) is widely used prevent malicious attacks on various online services. Text- image-CAPTCHAs have shown broader acceptability due usability security factors. However, recent progress in deep learning implies that text-CAPTCHAs can easily be exposed fraudulent attacks. Thus, are getting research attention enhance s...

CAPTCHAs have become a very popular security mechanism used to prevent automated abuse of online services intended for humans. Different flavors of CAPTCHA can be seen on Internet. However, a wide variety of CAPTCHAs have been successfully attacked by automated programs. This has made CAPTCHA design an interesting area for research. Among various flavors of CAPTCHA text based are most preferabl...

Since their inception, captchas have been widely used for preventing fraudsters from performing illicit actions. Nevertheless, economic incentives have resulted in an arms race, where fraudsters develop automated solvers and, in turn, captcha services tweak their design to break the solvers. Recent work, however, presented a generic attack that can be applied to any text-based captcha scheme. F...

A reading-based CAPTCHA, called ‘ScatterType,’ designed to resist character–segmentation attacks, is described. Its challenges are pseudorandomly synthesized images of text strings rendered in machine-print typefaces: within each image, characters are fragmented using horizontal and vertical cuts, and the fragments are scattered by vertical and horizontal displacements. This scattering is desig...

CAPTCHA stands for Completely Automated Public Turing Test to Tell Computer and Human Apart. It is the test that human can pass easily but computer cannot. For example, humans can read distorted text, but current computer programs cannot. For many years, CAPTCHAs have proven very useful for many reputable, Web-based email and application service providers, including social networking sites and ...