Edith Elkind

Hybrid voting protocols and hardness of manipulation

This paper addresses the problem of constructing voting protocols that are hard to manipulate. We describe a general technique for obtaining a new protocol by combining two or more base protocols, and study the resulting class of (vote-once) hybrid voting protocols, which also includes most previously known manipulation-resistant protocols. We show that for many choices of underlying base protocols, including some that are easily manipulable, their hybrids are NP-hard to manipulate, and demonstrate that this method can be used to produce manipulation-resistant protocols with unique combinations of useful features.

Small coalitions cannot manipulate voting

We demonstrate how to make voting protocols resistant against manipulation by computationally bounded malicious voters, by extending the previous results of Conitzer and Sandholm in several important directions: we use one-way functions to close a security loophole that allowed voting officials to exert disproportionate influence on the outcome and show that our hardness results hold against a large fraction of manipulating voters (rather than a single voter). These improvements address important concerns in the field of secure voting systems. We also discuss the limitations of the current approach, showing that it cannot be used to achieve certain very desirable hardness criteria.

True costs of cheap labor are hard to measure: edge deletion and VCG payments in graphs

We address the problem of lowering the buyers expected payments in shortest path auctions, where the buyers goal is to purchase a path in a graph in which edges are owned by selfish agents. We show that by deleting some of the edges of the graph, one can reduce the total payment of the VCG mechanism by a factor of θ(n). However, we prove that it is NP-hard to find the best subset of edges to delete, even if the edge costs are small integers, or the graph has very simple structure; in the former case, this problem is hard to approximate, too. On the positive side, we describe a pseudopolynomial time algorithm for series-parallel graphs and fixed edge costs. Also, we discuss the applicability of this algorithm for the case of general (probabilistic) costs and derive a general lower bound on the performance of algorithms that are based on expected edge costs.

Interleaving Cryptography and Mechanism Design

We propose a new cryptographically protected multi-round auction mechanism for online auctions. This auction mechanism is designed to provide (in this order) security, cognitive convenience, and round-effectiveness. One can vary internal parameters of the mechanism to trade off bid privacy and cognitive costs, or cognitive costs and the number of rounds. We are aware of no previous work that interleaves cryptography explicitly with the mechanism design.