6 research outputs found
VCG Under Sybil (False-name) Attacks -- a Bayesian Analysis
VCG is a classical combinatorial auction that maximizes social welfare.
However, while the standard single-item Vickrey auction is false-name-proof, a
major failure of multi-item VCG is its vulnerability to false-name attacks.
This occurs already in the natural bare minimum model in which there are two
identical items and bidders are single-minded. Previous solutions to this
challenge focused on developing alternative mechanisms that compromise social
welfare. We re-visit the VCG auction vulnerability and consider the bidder
behavior in Bayesian settings. In service of that we introduce a novel notion,
termed the granularity threshold, that characterizes VCG Bayesian resilience to
false-name attacks as a function of the bidder type distribution. Using this
notion we show a large class of cases in which VCG indeed obtains Bayesian
resilience for the two-item single-minded setting.Comment: This is an extended version of an article to appear in AAAI-2020.
Supporting code for generating the article's figures can be found at
https://github.com/yotam-gafni/vcg_bayesian_fn
Optimal Mechanism Design for Agents with DSL Strategies: The Case of Sybil Attacks in Combinatorial Auctions
In robust decision making under uncertainty, a natural choice is to go with
safety (aka security) level strategies. However, in many important cases, most
notably auctions, there is a large multitude of safety level strategies, thus
making the choice unclear. We consider two refined notions:
(i) a term we call DSL (distinguishable safety level), and is based on the
notion of ``discrimin'', which uses a pairwise comparison of actions while
removing trivial equivalencies. This captures the fact that when comparing two
actions an agent should not care about payoffs in situations where they lead to
identical payoffs.
(ii) The well-known Leximin notion from social choice theory, which we apply
for robust decision-making. In particular, the leximin is always DSL but not
vice-versa.
We study the relations of these notions to other robust notions, and
illustrate the results of their use in auctions and other settings. Economic
design aims to maximize social welfare when facing self-motivated participants.
In online environments, such as the Web, participants' incentives take a novel
form originating from the lack of clear agent identity -- the ability to create
Sybil attacks, i.e., the ability of each participant to act using multiple
identities. It is well-known that Sybil attacks are a major obstacle for
welfare-maximization. Our main result proves that when DSL attackers face
uncertainty over the auction's bids, the celebrated VCG mechanism is
welfare-maximizing even under Sybil attacks. Altogether, our work shows a
successful fundamental synergy between robustness under uncertainty, economic
design, and agents' strategic manipulations in online multi-agent systems.Comment: In Proceedings TARK 2023, arXiv:2307.0400
Greedy Transaction Fee Mechanisms for (Non-)myopic Miners
Decentralized cryptocurrencies are payment systems that rely on aligning the
incentives of users and miners to operate correctly and offer a high quality of
service to users. Recent literature studies the mechanism design problem of the
auction serving as a cryptocurrency's transaction fee mechanism (TFM). We
present a general framework that captures both myopic and non-myopic settings,
as well as different possible strategic models for users. Within this general
framework, when restricted to the myopic case, we show that while the mechanism
that requires a user to "pay-as-bid", and greedily chooses among available
transactions based on their fees, is not dominant strategy incentive-compatible
for users, it has a Bayesian-Nash equilibrium where bids are slightly shaded.
Relaxing this incentive compatibility requirement circumvents the impossibility
results proven by previous works, and allows for an approximately revenue and
welfare optimal, myopic miner incentive-compatible (MMIC), and
off-chain-agreement (OCA)-proof mechanism. We prove these guarantees using
different benchmarks, and show that the pay-as-bid greedy auction is the
revenue optimal Bayesian incentive-compatible, MMIC and 1-OCA-proof mechanism
among a large class of mechanisms. We move beyond the myopic setting explored
in the literature, to one where users offer transaction fees for their
transaction to be accepted, as well as report their urgency level by specifying
the time to live of the transaction, after which it expires. We analyze
pay-as-bid mechanisms in this setting, and show the competitive ratio
guarantees provided by the greedy allocation rule. We then present a
better-performing non-myopic rule, and analyze its competitive ratio. The above
analysis is stated in terms of a cryptocurrency TFM, but applies to other
settings, such as cloud computing and decentralized "gig" economy, as well.Comment: 38 pages, 3 figure
Worst-case Bounds on Power vs. Proportion in Weighted Voting Games with Application to False-Name Manipulation
Weighted voting games apply to a wide variety of multi-agent settings. They enable the formalization of power indices which quantify the coalitional power of players. We take a novel approach to the study of the power of big vs. small players in these games. We model small (big) players as having single (multiple) votes. The aggregate relative power of big players is measured w.r.t. their votes proportion. For this ratio, we show small constant worst-case bounds for the Shapley-Shubik and the Deegan-Packel indices. In sharp contrast, this ratio is unbounded for the Banzhaf index. As an application, we define a false-name strategic normal form game where each big player may split its votes between false identities, and study its various properties. Together, our results provide foundations for the implications of players’ size, modeled as their ability to split, on their relative power.</p
Unified Fair Allocation of Goods and Chores via Copies
We consider fair allocation of indivisible items in a model with goods, chores, and copies, as a unified framework for studying: (1) the existence of EFX and other solution concepts for goods with copies; (2) the existence of EFX and other solution concepts for chores. We establish a tight relation between these issues via two conceptual contributions: First, a refinement of envy-based fairness notions that we term envy without commons (denoted EFX WC when applied to EFX). Second, a formal duality theorem relating the existence of a host of (refined) fair allocation concepts for copies to their existence for chores. We demonstrate the usefulness of our duality result by using it to characterize the existence of EFX for chores through the dual environment, as well as to prove EFX existence in the special case of leveled preferences over the chores. We further study the hierarchy among envy-freeness notions without commons and their α-MMS guarantees, showing, for example, that any EFX WC allocation guarantees at least 11 4 -MMS for goods with copies.</p