# A Blockchain-Based Framework to Enhance Anonymous Services with Accountability Guarantees
## 2 Related Work and Background  
In the scientific literature, there exist several papers dealing with the general issue of balancing anonymity and accountability in different fields. Refs. [12; 13; 14] rely on anonymous credential systems and related schemes. Anonymous credential systems allow users to interact with a service provider in an anonymous yet accountable way [15; 16; 17]. Ref. [12] is the first paper proposing an anonymous credential system, in which, to prevent the misuse of anonymity, the anonymity property can be revoked for particular transactions. Ref. [13] introduces an approach relying on anonymous credentials that allows access control systems to offer fully anonymous access to resources along with strong accountability guarantees. It is worth noting that the proposed approach relies on a trusted third party to build a mechanism to escrow the identity of the user. Most of the solutions present in the literature assume a client-server architecture in which only the clients care about their privacy. On the contrary, Ref. [18] aims to reach the right balance between privacy and accountability in P2P systems, where both clients and servers are peer users. In [19], the authors deal with the issue of accountability in anonymous publication and storage services where malicious servers can make documents unrecoverable. To discourage this kind of behavior, the authors propose the creation of a \"buddy system\" that creates an association between pairs of shares from a given document. Therefore, a server holding a given share is responsible for detecting any anomaly regarding its buddy. Ref. [20] proposes a framework to provide an anonymous mutual authentication protocol in wireless mesh networks. The proposed framework utilizes group signatures, where the private key and the credentials of the users are generated through a secure three-party protocol. User accountability is implemented via a user revocation protocol, whose execution can be carried out by two semi-trusted authorities, one of which is the network operator. Refs. [21; 22] deal with the issue of accountability in anonymous communication networks. Indeed, Ref. [22] highlights that both anonymity and accountability requirements should be satisfied to gain support for the deployment of large-scale anonymity infrastructures. To achieve this, Ref. [21] presents a mechanism that provides practical repudiation for the proxy nodes by tracing back a selected traffic to the predecessor node through a cryptographically verifiable chain. A proposal that includes some similarities to our work is presented in [23]. Indeed, the authors proposed a solution that leverages the Bitcoin blockchain as a platform to manage and determine ownership of users' access credentials. The authors design an authentication scheme that is able to provide anonymity and accountability without relying on any trusted third party. Regardless, the proposed solution achieves accountability only in the sense that the service provider can blacklist the misbehaving credentials related to a user. Indeed, as highlighted by [24], accountability can still be obtained by eliminating the reliance on a third trusted party for credentials revocation, but this comes with a cost: the revoked user remains anonymous. Another solution relying on blockchain to support anonymous authentication in VANET is provided in [25]. We conclude this section by discussing some proposals about anonymous social networks [6]. Indeed, these appear as services that might take advantage of adopting our solution. Anonymous social networks completely shift the traditional social networks paradigm. Indeed, while the latter put the user identity and its social link first, anonymous social networks encourage communication between strangers and allow users to express themselves without fear of bullying or retaliation [5]. Among the most famous anonymous social networks, we found Whisper [5] and Yik Yak [26].