Proofs of sequential communication delays from physical assumptions and their applications

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Authors: C. Baum, B. David, E. Pagnin, and A. Takahashi
In: Cryptography and Communications, 2025
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Abstract

Time-based cryptographic primitives unlock efficient realizations of several functionalities including Randomness Beacons, Proof of Replicated Storage, Encryption to the Future, and MultiParty Computation with partial fairness. Existing constructions derive time-delays from the average hardness of sequential computational problems, a measure that is susceptible to algorithmic and hardware improvements. Therefore time-based systems secure at deployment date are at constant risk to turn insecure. A way to combat this intrinsic drawback is to ground time-delays on assumptions that are not affected by scientific advancement such as trust (in a subset of parties) and physical communication delays. This paper builds on Baum et al.’s (SCN 2024) work on “CaSCaDE: (Time-Based) Cryptography from Space Communications DElay”, and provides concrete realizations and detailed security proofs of: a Time Lock Puzzle, a stateless Verifiable Random Function, a Delay Encryption scheme, and a Randomness Beacon from proofs of Sequential Communication Delays (SCD) and trust assumptions on subsets of parties. Notably, our SCD-based Delay Encryption construction constitutes the first alternative to existing supersingular isogenies Delay Encryption schemes.

Keywords: Universal Composability, Physical Assumptions, Sequential Delays.