A Complete Characterization of Game-Theoretically Fair, Multi-Party Coin Toss 📺
Cleve's celebrated lower bound (STOC'86) showed that a de facto strong fairness notion is impossible in 2-party coin toss, i.e., the corrupt party always has a strategy of biasing the honest party's outcome by a noticeable amount. Nonetheless, Blum's famous coin-tossing protocol (CRYPTO'81) achieves a strictly weaker "game-theoretic'' notion of fairness — specifically, it is a 2-party coin toss protocol in which neither party can bias the outcome towards its own preference; and thus the honest protocol forms a Nash equilibrium in which neither party would want to deviate. Surprisingly, an n-party analog of Blum's famous coin toss protocol was not studied till recently. The work by Chung et al.~(TCC'18) was the first to explore the feasibility of game-theoretically fair n-party coin toss in the presence of corrupt majority. We may assume that each party has a publicly stated preference for either the bit 0 or 0, and if the outcome agrees with the party's preference, it obtains utility 1; else it obtains nothing. A natural game-theoretic formulation is to require that the honest protocol form a coalition-resistant Nash equilibrium, i.e., no coalition should have incentive to deviate from the honest behavior. Chung et al. phrased this game-theoretic notion as “cooperative-strategy-proofness'' or ”CSP-fairness'' for short. Unfortunately, Chung et al.~showed that under (n-1)-sized coalitions, it is impossible to design such a CSP-fair coin toss protocol, unless all parties except one prefer the same bit. In this paper, we show that the impossibility of Chung et al.~is in fact not as broad as it may seem. When coalitions are majority but not $n-1$ in size, we can indeed get feasibility results in some meaningful parameter regimes. We give a complete characterization of the regime in which CSP-fair coin toss is possible, by providing a matching upper- and lower-bound. Our complete characterization theorem also shows that the mathematical structure of game-theoretic fairness is starkly different from the de facto strong fairness notion in the multi-party computation literature.
log∗-Round Game-Theoretically-Fair Leader Election
It is well-known that in the presence of majority coalitions, strongly fair coin toss is impossible. A line of recent works have shown that by relaxing the fairness notion to game theoretic, we can overcome this classical lower bound. In particular, Chung et al. (CRYPTO'21) showed how to achieve approximately (game-theoretically) fair leader election in the presence of majority coalitions, with round complexity as small as O(log log n) rounds. In this paper, we revisit the round complexity of game-theoretically fair leader election. We construct O(log* n) rounds leader election protocols that achieve (1-o(1))-approximate fairness in the presence of (1-o(1)) n-sized coalitions. Our protocols achieve the same round-fairness trade offs as Chung et al.'s and have the advantage of being conceptually simpler. Finally, we also obtain game-theoretically fair protocols for committee election which might be of independent interest.
Non-Interactive Anonymous Router 📺
Anonymous routing is one of the most fundamental online privacy problems and has been studied extensively for decades. Almost all known approaches that achieve anonymous routing (e.g., mix-nets, DC-nets, and numerous other systems) rely on multiple servers or routers to engage in some interactive protocol; and anonymity is guaranteed in the threshold model, i.e., if one or more of the servers/routers behave honestly. Departing from all prior approaches, we propose a novel non-interactive abstraction called a Non-Interactive Anonymous Router (NIAR), that works even with a single untrusted router. In a NIAR scheme, suppose that n senders each want to talk to a distinct receiver. A one-time trusted setup is performed such that each sender obtains a sending key, each receiver obtains a receiving key, and the router receives a token that “encrypts” the permutation mapping the senders to receivers. In every time step, the senders can each encrypt its message using its sender key, and the router can use its token to convert the n ciphertexts received from the senders to n transformed ciphertexts. Each transformed ciphertext is delivered to the corresponding receiver, and the receiver can decrypt the message using its receiver key. Imprecisely speaking, security requires that the untrusted router, even when colluding with a subset of corrupt senders and/or receivers, should not be able to break the privacy of honest parties, including who is talking to who, and the messages they exchange. We show how to construct a communication-efficient NIAR scheme with provable security guarantees based on the SXDH assumption in suitable bilinear groups and assuming Random Oracles (RO); further, the RO assumption can be removed if we allow a public key that is as large as the number of time steps supported. We also define a paranoid notion of security that achieves full insider protection, and show that if we additionally assume sub-exponentially secure Indistinguishability Obfuscation and as sub-exponentially secure one-way functions, one can construct a NIAR scheme with paranoid security. We show that a com- pelling application of NIAR is to realize a Non-Interactive Anonymous Shuffler (NIAS), where an untrusted server or data analyst can only de- crypt a shuffled version of the messages coming from n senders where the permutation is hidden. NIAS can be adopted to construct privacy- preserving surveys, differentially private protocols in the shuffle model, and pseudonymous bulletin boards.