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This work aims to argue that it is possible to address discrimination issues that naturally arise in contemporary audio CAPTCHA challenges and potentially enhance the effectiveness of audio CAPTCHA systems by adapting the challenges to the user characteristics.

A prototype has been designed, called PrivCAPTCHA, to offer privacy-preserving, user-centric CAPTCHA challenges. Anonymous credential proofs are integrated into the Session Initiation Protocol (SIP) protocol and the approach is evaluated in a real-world Voice over Internet Protocol (VoIP) environment.

The results of this work indicate that it is possible to create VoIP CAPTCHA services offering privacy-preserving, user-centric challenges while maintaining sufficient efficiency.

The proposed approach was evaluated through an experimental implementation to demonstrate its feasibility. Additional features, such as appropriate user interfaces and efficiency optimisations, would be useful for a commercial product. Security measures to protect the system from attacks against the SIP protocol would be useful to counteract the effects of the introduced overhead. Future research could investigate the use of this approach on non-audio CAPTCHA services.

PrivCAPTCHA is expected to achieve fairer, non-discriminating CAPTCHA services while protecting the user’s privacy. Adoption success relies upon the general need for employment of privacy-preserving practices in electronic interactions.

This approach is expected to enhance the quality of life of users, who will now receive CAPTCHA challenges closer to their characteristics. This applies especially to users with disabilities. Additionally, as a privacy-preserving service, this approach is expected to increase trust during the use of services that use it.

To the best of authors’ knowledge, this is the first comprehensive proposal for privacy-preserving CAPTCHA challenge adaptation. The proposed system aims at providing an improved CAPTCHA service that is more appropriate for and trusted by human users.

In order to enhance privacy protection during electronic transactions, the purpose of this paper is to propose, develop, and evaluate a personal data management framework called Polis that abides by the following principle: every individual has absolute control over his/her personal data that reside only at his/her own side.

This paper identifies representative electronic transactions that involve personal data and proposes Polis‐based protocols for them. The approach is evaluated on a Polis prototype both as a stand‐alone application and as part of a commercial database management system.

The results of this paper indicate that electronic transactions can remain both feasible and straightforward, while personal data remain only at the owner's side.

This paper describes a Polis‐approach implementing prototype, which is easy to deploy and friendly to current information management technologies. However, the usability of the prototype has to be enhanced with supporting tools for editing personal data and policies and a more intuitive user interface. Finally, the Polis‐platform enables a new class of user‐centered distributed applications, which it intends to investigate.

Even though the conditions for a personal data management approach like Polis are mature, and Polis can be progressively adopted, it still entails a major change in current business practices.

This paper proposes a new paradigm for the management of personal data, which admits individuals to have their personal data stored only at their own side. The new approach can be of mutual benefit to both individuals and companies.