We study a novel variant of the Black Hole Search (BHS) problem where the black hole, a node that silently destroys visiting agents, can appear at any time during execution, rather than being present initially as assumed in some of the previous works. Our focus is on ring networks using synchronous agents. We provide two solutions: a 4-agent algorithm for rings without the knowledge of n but when only a single link leading to the black hole is found and a 3-agent algorithm assuming known ring size. We also prove that the problem is not solvable with 2-agents (even with whiteboard and a home base that remains safe), making our 3-agent algorithm optimal.
In a cross-chain task, m active, untrustworthy parties attempt to trade assets using n passive but trustworthy smart contracts residing on multiple blockchains. The decentralized finance industry has developed a variety of tasks, including asset swaps, auctions, futures, etc.. In this paper, we show that there is a particular task, called cross-chain consensus, that is universal in the sense that any protocol that solves cross-chain consensus can be adapted to solve any well-formed cross-chain task. Like classical consensus, cross-chain consensus provides the basis for universal constructions and impossibility results. Nevertheless, cross-chain consensus differs structurally from its classical counterpart by splitting participants into two distinct roles: active, but untrustworthy (Byzantine) parties provide inputs, while passive but trustworthy smart contracts decide outputs.
Rekeying is an effective technique for protecting symmetric ciphers against side-channel and key-search attacks. Since its introduction, numerous rekeying schemes have been developed [16]. We introduce Post-Quantum Stateless Auditable Rekeying (PQ-STAR), a novel post-quantum secure [4] stateless rekeying scheme with audit support. PQ-STAR is presented in three variants of increasing security guarantees: (i) Plain PQ-STAR lets an authorized auditor decrypt and verify selected ciphertexts; (ii) Commitment-based PQ-STAR with the additional binding guarantee from the commitments, preventing a malicious sender from potentially claiming a random or wrong session key. (iii) Zero-knowledge PQ-STAR equips each session key with a signature-based zero-knowledge proof (ZKP), which proves that the session key was derived honestly, without ever revealing the secret preimage. We provide informal arguments that all variants achieve key-uniqueness, index-hiding, and forward-secrecy, even if a probabilistic polynomial-time (PPT) adversary arbitrarily learns many past session keys. PQ-STAR provides a verified, stateless, and audit-capable rekeying primitive that can be seamlessly integrated as a post-quantum upgrade for existing symmetric-key infrastructures.
A spectrum of solvability and unsolvability results for causal ordering of messages in the presence of Byzantine processes in asynchronous systems for unicast, multicast, and broadcast modes of communication have been shown. The possibility results implicitly assumed that the number of Byzantine processes f was less than n/3, where n is the total number of processes in the system. In this paper, we extend these results for the same system assumptions and parameters – mode of communication (unicast/broadcast/multicast), strong safety, weak safety, and liveness, and use of cryptography, to systems with f
We consider distributed systems of autonomous, punctiform, mobile robots that operate in the Euclidean plane by executing an infinite sequence of Look-Compute-Move cycles. Robots are anonymous, indistinguishable, homogeneous, and disoriented. In literature, four base models have been proposed to study four different memory-communication settings: 𝒪ℬℒ𝒪𝒯 (oblivious and silent), ℱ𝒮𝒯𝒜 (finite-state and silent), ℱ𝒞𝒪ℳ (oblivious and finite-communication), and ℒ𝒰ℳℐ (finite-state and finite-communication). In particular, the research has investigated how the computational power of these models is affected by considering three main classes of robot schedulers: FSYNCH (fully synchronous), SSYNCH (semi-synchronous), and ASYNCH (asynchronous). This paper focuses on a peculiar type of SSYNCH schedulers, the sequential ones, which activate only one robot at each round. We consider three subclasses: the general sequential scheduler (SEQ), the permutation scheduler (PERM), and the well-known round-robin (RROBIN). For each base model, we investigate how the robots’ computational power changes as the scheduler class varies, thus providing a first overview of the computational landscape of sequential schedulers.
