Accountable Bulletin Boards: Definition and Provably Secure Implementation

Bulletin boards (BB) are important cryptographic building blocks that, at their core, provide a broadcast channel with memory. BBs are widely used within many security protocols, including secure multi-party computation protocols, e-voting systems, and electronic auctions. Even though the security of protocols crucially depends on the underlying BB, as also highlighted by recent works, the literature on constructing secure BBs is sparse. The so-far only provably secure BBs require trusted components and sometimes also networks without message loss, which makes them unsuitable for applications with particularly high security needs where these assumptions might not always be met. In this work, we fill this gap by leveraging the concepts of accountability and universal composability (UC). More specifically, we propose the first ideal functionality for accountable BBs that formalizes the security requirements of such BBs in UC. We then propose Fabric$^\ast_\text{BB}$ as a slight extension designed on top of Fabric$^\ast$, which is a variant of the prominent Hyperledger Fabric distributed ledger protocol, and show that Fabric$^\ast_\text{BB}$ UC-realizes our ideal BB functionality. This result makes Fabric$^\ast_\text{BB}$ the first provably accountable BB, an often desired, but so far not formally proven property for BBs, and also the first BB that has been proven to be secure based only on standard cryptographic assumptions and without requiring trusted BB components or network assumptions. Through an implementation and performance evaluation we show that Fabric$^\ast_\text{BB}$ is practical for many applications of BBs

AUC: Accountable Universal Composability

Accountability is a well-established and widely used security concept that allows for obtaining undeniable cryptographic proof of misbehavior, thereby incentivizing honest behavior. There already exist several general purpose accountability frameworks for formal game-based security analyses. Unfortunately, such game-based frameworks do not support modular security analyses, which is an important tool to handle the complexity of modern protocols. Universal composability (UC) models provide native support for modular analyses, including re-use and composition of security results. So far, accountability has mainly been modeled and analyzed in UC models for the special case of MPC protocols, with a general purpose accountability framework for UC still missing. That is, a framework that among others supports arbitrary protocols, a wide range of accountability properties, handling and mixing of accountable and non-accountable security properties, and modular analysis of accountable protocols. To close this gap, we propose AUC, the first general purpose accountability framework for UC models, which supports all of the above, based on several new concepts. We exemplify AUC in three case studies not covered by existing works. In particular, AUC unifies existing UC accountability approaches within a single framework