Payment channels have been a promising solution to blockchain scalability. While payment channels for script-empowered blockchains (such as Bitcoin and Ethereum) have been well studied, developing payment channels for scriptless blockchains (such as Monero) is considered challenging. In particular, nabling bidirectional payment on scriptless blockchains remains an open challenge.

We provide a systematic analysis of two related multiparty protocols, namely (Non-Interactive Fully) Distributed Verifiable Random Functions (DVRFs) and Decentralised Random Beacons (DRBs), including their syntax and definition of robustness and privacy properties. These two protocols are run by multiple network nodes where each node contributes with a partial evaluation and the collection of these partial values is used to evaluate a pseudorandom function. We refine current pseudorandomness definitions for distributed functions and show that the privacy provided by strong pseudorandomness, where an adversary is allowed to make partial function evaluation queries on the challenge value, is strictly better than that provided by standard pseudorandomness, where such adversarial queries are disallowed. We provide two new DVRF instantiations, named DDH-DVRF and GLOW-DVRF, that meet strong pseudorandomness under widely accepted cryptographic assumptions. We show the usefulness of our DRB formalism in two different ways. Firstly, we give a rigorous treatment of a folklore generic construction that builds a Decentralized Random Beacon from any DVRF instance and prove that it satisfies robustness and pseudorandomness provided that the original DVRF protocol is secure. Secondly, we capture several existing DRB protocols from academia and industry within our framework, which serves as an evidence of its wider applicability. Finally, we report on experimental evaluations of our newly introduced DVRFs with implementations under different cryptographic libraries, and we also report preliminary benchmark results on two of the DRBs obtained from the generic DVRF-to-DRB transformation. Our benchmarks can be independently verified as we provide an open source C++ reference implementation of the new DVRFs. Finally, we conclude that our new DRB instantiations are the most efficient instantiations currently available while enjoying strong and formally proven security properties.

Time-lock encryption is a method to encrypt a message such that it can only be decrypted after a certain deadline has passed. We propose a novel time-lock encryption scheme, whose main advantage over prior constructions is that even receivers with relatively weak computational resources should immediately be able to decrypt after the deadline, without any interaction with the sender, other receivers, or a trusted third party. We build our time-lock encryption on top of the new concept of computational reference clocks and an extractable witness encryption scheme. We explain how to construct a computational reference clock based on Bitcoin. We show how to achieve constant level of multilinearity for witness encryption by using SNARKs. We propose a new construction of a witness encryption scheme which is of independent interest: our scheme, based on Subset-Sum , achieves extractable security without relying on obfuscation. The scheme employs multilinear maps of arbitrary order and is independent of the implementations of multilinear maps.

Privacy is a core human need, but society sometimes has the requirement to do targeted, proportionate investigations in order to provide security. To reconcile individual privacy and societal security, we explore whether we can have surveillance in a form that is verifiably accountable to citizens. This means that citizens get verifiable proofs of the quantity and nature of the surveillance that actually takes place. In our scheme, governments are held accountable for the extent to which they exercise their surveillance power, and political parties can pledge in election campaigns their intention about reducing (or increasing) this figure. We propose a general idea of accountable escrow to reconciling and balancing the requirements of individual privacy and societal security. We design a balanced crypto system for asynchronous communication (e.g., email). We propose a novel method for escrowing the decryption capability in public-key cryptography. A government can decrypt it in order to conduct targeted surveillance, but doing so necessarily puts records in a public log against which the government is held accountable.