Since the beginning of the Covid-19 Pandemic, Contact Tracing Apps have been implemented in many countries as a way to detect if someone has been in contact with a patient within a minimum amount of time. However, most existing solutions only consider users in pairs. Since many people meet at the same time in real-life scenarios, those applications aren't able to accurately reflect the situation. Moreover, extending current schemes to a multi-party setting could cause scaling problems and place a heavier load on the device. In this paper, we propose a new Contact Tracing protocol that works in a multi-party setting. We evaluate our scheme to show its efficiency.

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Social media (SM) has become a primary communication tool in the modern world, with an ever-increasing volume of users. Many SM users use anonymous nicknames as their public usernames. However, Zhang et al. (2018) were able to demonstrate an attack that can identify users from the contents of their posts. This attack is caused by the fact that two different posts can be guessed to be the same user. Such linking of different posts is called a linkable feature. On the other hand, usually post under an anonymous nickname, but when a post is thrust into the limelight, we may want to claim the post as our own. Unfortunately, however, current SM offers only two options: using an anonymous nickname or publishing under our own name. In other words, the function of disclosure, which is to make some posts public even though they are usually anonymous, has not been realized in existing SM. In this paper, we propose a SM with unlinkability and disclosure simultaneously, which is achieved by applying a commitment scheme. A commitment scheme consists of commitment and decommitment phases. As for unlinkability, we newly introduce a one-time post name, which is a commitment value of nickname and post. As for disclosure, we use a decommitment phase to one-time post name. We also have demonstrated that our SM is practically feasible.