Accountability is a fundamental after-the-fact approach to detect and punish illegal actions during the execution of a warrant for accessing users’ sensitive data. To achieve accountability in a security protocol, a trusted authority is required, denoted as judge, to faithfully cooperate with the rest of the entities in the system. However, malicious judges or uncooperative protocol participants may void the accountability mechanism in practice, for example by fabricating fake evidence or by refusing to provide any evidence at all. To provide remediation to these issues, in this paper we propose Fialka, a novel accountable decryption system based on privacy-preserving smart contracts (PPSC). The neutrality that is inherent to a secure blockchain platform is inherited by PPSC which are then used in our approach as an accountable key manager as well as a transparent judge. To the best of our knowledge, we present the first PPSC-based accountable decryption system to increase the transparency of warrant execution with formal definitions and proofs. Furthermore, we provide and evaluate a prototype implementation using the PPSC-enabled platform Oasis Devnet, which additionally demonstrates the feasibility of Fialka.

Anonymity revocation is an essential component of credential issuing systems since unconditional anonymity is incompatible with pursuing and sanctioning credential misuse. However, current anonymity revocation approaches have shortcomings with respect to the auditability of the revocation process. In this paper, we propose a novel anonymity revocation approach based on privacy-preserving blockchain-based smart contracts, where the code self-execution property ensures availability and public ledger immutability provides auditability. We describe an instantiation of this approach, provide an implementation thereof and conduct a series of evaluations in terms of running time, gas cost and latency. The results show that our scheme is feasible and efficient.

Employers often want to verify an applicant's academic credentials. Currently, this requires manual processes by institutions to trace records and verify degrees. This approach is slow, subject to individual institutions' record-keeping, and exposes graduates to a lack of privacy concerning when and by whom their credentials are being authenticated. Blockchain offers one solution to this problem. As an "append-only whiteboard in the sky", the blockchain ledger is a verifiable source for cryptographically proving the existence of official documents. Employers can be given an access code by a graduate that enables them to authenticate credentials without having to involve the awarding institution. Similarly, the institutions need only add their records to the Blockchain, saving time and resources. While a blockchain system is designed for transparency, this system also adds a component of privacy for graduates, as their former institution need not be made aware that potential employers are making a request to validate their certificates. For sensitive applications, and for enhancing privacy in general while also improving functionality, the blockchain approach is an effective solution to the problem of credential authentication.