Two Papers Accepted to ACM CCS 2021

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The following two papers have been accepted to ACM Conference on Computer and Communications Security 2021:

OpenSquare: Decentralized Repeated Modular Squaring Service

Authors: Sri Aravinda Krishnan Thyagarajan, Adithya Bhat, Tiantian Gong, Aniket Kate and Dominique Schröder

Abstract: Repeated Modular Squaring is a versatile computational operation that has led to practical constructions of timed-cryptographic primitives like time-lock puzzles (TLP) and verifiable delay functions (VDF) that have a fast growing list of applications. While there is a huge interest for timed-cryptographic primitives in the blockchains area, we find two real-world concerns that need immediate attention towards their large-scale practical adoption: Firstly, the requirement to perform constantly computations seems unrealistic for most of the users. Secondly, choosing the parameters for the bound T seems complicated due to the lack of heuristics and experience.

We present OpenSquare, a decentralized repeated modular squaring service, that overcomes the above concerns. OpenSquare lets clients outsource their repeated modular squaring computation via smart contracts to any computationally powerful servers that offer computational services for rewards in an unlinkable manner.

OpenSquare naturally gives us publicly computable heuristics about a pre-specified number (T) and the corresponding reward amounts of repeated squarings necessary for a time period. Moreover, OpenSquare rewards multiple servers for a single request, in a sybil resistant manner to incentivise maximum server participation and is therefore resistant to censorship and single-points-of failures. We give game-theoretic analysis to support the mechanism design of OpenSquare: (1) incentivises servers to stay available with their services, (2) minimizes the cost of outsourcing for the client, and (3) ensures the client receives the valid computational result with high probability. To demonstrate practicality, we also implement OpenSquare’s smart contract in Solidity and report the gas costs for all of its functions. Our results show that the on-chain computational costs for both the clients and the servers are quite low, and therefore feasible for practical deployments and usage.

Efficient CCA Timed Commitments in Class Groups

Authors: Sri Aravinda Krishnan Thyagarajan, Guilhelm Castagnos, Fabian Laguillaumie and Giulio Malavolta

Abstract: Timed commitments [Boneh and Naor, CRYPTO 2000] are the timed analogue of standard commitments, where the commitment can be non-interactively opened after a pre-specified amount of time passes. Timed commitments have a large spectrum of applications, such as sealed bid auctions, fair contract signing, fair multi-party computation, and cryptocurrency payments. Unfortunately, all practical constructions rely on a (private-coin) trusted setup and do not scale well with the number of participants. These are two severe limiting factors that have hindered the widespread adoption of this primitive.

In this work, we set out to resolve these two issues and propose an efficient timed commitment scheme that also satisfies the strong notion of CCA-security. Specifically, our scheme has a transparent (i.e.\ public-coin) one-time setup and the amount of sequential computation is essentially independent of the number of participants. As a key technical ingredient, we propose the first (linearly) homomorphic time-lock puzzle with a transparent setup, from class groups of imaginary quadratic order. To demonstrate the applicability of our scheme, we use it to construct a new distributed randomness generation protocol, where n parties jointly sample a random string. Our protocol is the first to simultaneously achieve (1) high scalability in the number of participants, (2) transparent one-time setup, (3) lightning speed in the optimistic case where all parties are honest, and (4) ensure that the output random string is unpredictable and unbiased, even when the adversary corrupts n-1 parties. To substantiate the practicality of our approach, we implemented our protocol and our experimental evaluation shows that it is fast enough to be used in practice. We also evaluated a heuristic version of the protocol that is at least 3 orders of magnitude more efficient both in terms of communication size and computation time. This makes the protocol suitable for supporting hundreds of participants.