The concept of reverse Turing tests, or more commonly known as CAPTCHAs, for distinguishing between humans and computers has been around for many years. The widespread use of CAPTCHAs these days has made them an integral part of the internet for providing online services, which are intended for humans, with some level of protection against automated abuse. Since their inception, much research has focused on investigating various issues surrounding the design and security of CAPTCHAs. A fundamental requirement of CAPTCHAs necessitates that they must be designed to be easy for humans but difficult for computers. However, it is well recognized that the trade-off between usability and security is difficult to balance. In addition, numerous attacks have been developed to defeat CAPTCHAs. In response to this, many different CAPTCHA design variants have been proposed over the years. Despite the fact that CAPTCHAs have been around for more than two decades, the future of CAPTCHAs remains an open question. This chapter presents an overview of research examining a wide range of issues that have been conducted on different types of CAPTCHAs.

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In recent years, the Internet of Things (IoT) devices have become increasingly deployed in many industries and generated a large amount of data that needs to be processed in a timely and efficient manner. Using aggregate signatures, it provides a secure and efficient way to handle large numbers of digital signatures with the same message. Recently, the privacy issue has been concerned about the topic of data sharing on the cloud. To provide the integrity, authenticity, authority, and privacy on the data sharing in the cloud storage, the notion of an aggregatable certificateless designated verifier signature scheme (ACLDVS) was proposed. ACLDVS also is a perfect tool to enable efficient privacy-preserving authentication systems for IoT and or the vehicular ad hoc networks (VANET). Our concrete scheme was proved to be secured underling of the Computational Diffie-Hellman assumption. Compared to other related schemes, our scheme is efficient, and the signature size is considerably short.