Publications

2005
Rozenman, Eyal, and Salil Vadhan. “Derandomized squaring of graphs.” In Proceedings of the 8th International Workshop on Randomization and Computation (RANDOM '05), 3624:436-447. Berkeley, CA: Springer Verlag, Lecture Notes in Computer Science, 2005. Publisher's VersionAbstract

We introduce a “derandomized” analogue of graph squaring. This operation increases the connectivity of the graph (as measured by the second eigenvalue) almost as well as squaring the graph does, yet only increases the degree of the graph by a constant factor, instead of squaring the degree.

One application of this product is an alternative proof of Reingold’s recent breakthrough result that S-T Connectivity in Undirected Graphs can be solved in deterministic logspace.

2003
Sahai, Amit, and Salil Vadhan. “A complete problem for statistical zero knowledge.Journal of the ACM 50, no. 2 (2003): 196-249.Abstract
We present the first complete problem for SZK, the class of (promise) problems possessing statistical zero-knowledge proofs (against an honest verifier). The problem, called STATISTICAL DIFFERENCE, is to decide whether two efficiently samplable distributions are either statistically close or far apart. This gives a new characterization of SZK that makes no reference to interaction or zero knowledge.

We propose the use of complete problems to unify and extend the study of statistical zero knowledge. To this end, we examine several consequences of our Completeness Theorem and its proof, such as:

• A way to make every (honest-verifier) statistical zero-knowledge proof very communication efficient, with the prover sending only one bit to the verifier (to achieve soundness error 1/2).
• Simpler proofs of many of the previously known results about statistical zero knowledge, such as the Fortnow and Aiello--Håstad upper bounds on the complexity of SZK and Okamoto's result that SZK is closed under complement.
• Strong closure properties of SZK which amount to constructing statistical zero-knowledge proofs for complex assertions built out of simpler assertions already shown to be in SZK.
• New results about the various measures of "knowledge complexity," including a collapse in the hierarchy corresponding to knowledge complexity in the "hint" sense.
• Algorithms for manipulating the statistical difference between efficiently samplable distributions, including transformations which "polarize" and "reverse" the statistical relationship between a pair of distributions.
2002
Bender, Michael A., Antonio Fernández, Dana Ron, Amit Sahai, and Salil Vadhan. “The power of a pebble: exploring and mapping directed graphs.Information and Computation 176, no. 1 (2002): 1-21.Abstract
Exploring and mapping an unknown environment is a fundamental problem that is studied in a variety of contexts. Many results have focused on finding efficient solutions to restricted versions of the problem. In this paper, we consider a model that makes very limited assumptions about the environment and solve the mapping problem in this general setting.

We model the environment by an unknown directed graph G, and consider the problem of a robot exploring and mapping G. The edges emanating from each vertex are numbered from 1' to d', but we do not assume that the vertices of G are labeled. Since the robot has no way of distinguishing between vertices, it has no hope of succeeding unless it is given some means of distinguishing between vertices. For this reason we provide the robot with a "pebble" --- a device that it can place on a vertex and use to identify the vertex later.

In this paper we show:

1. If the robot knows an upper bound on the number of vertices then it can learn the graph efficiently with only one pebble.
2. If the robot does not know an upper bound on the number of vertices n, then Theta(loglog n) pebbles are both necessary and sufficient.
In both cases our algorithms are deterministic.
2001
Vadhan, Salil. “The complexity of counting in sparse, regular, and planar graphs.SIAM Journal on Computing 31, no. 2 (2001): 398-427.Abstract
We show that a number of graph-theoretic counting problems remain NP-hard, indeed #P-complete, in very restricted classes of graphs. In particular, we prove that the problems of counting matchings, vertex covers, independent sets, and extremal variants of these all remain hard when restricted to planar bipartite graphs of bounded degree or regular graphs of constant degree. We obtain corollaries about counting cliques in restricted classes of graphs and counting satisfying assignments to restricted classes of monotone 2-CNF formulae. To achieve these results, a new interpolation-based reduction technique which preserves properties such as constant degree is introduced.
1999
Sahai, Amit, and Salil Vadhan. “ Manipulating statistical difference.Randomization Methods in Algorithm Design (DIMACS Workshop, December 1997), volume 43 of DIMACS Series in Discrete Mathematics and Theoretical Computer Science 43 (1999): 251-270.Abstract

We give several efficient transformations for manipulating the statistical difference (variation distance) between a pair of probability distributions. The effects achieved include increasing the statistical difference, decreasing the statistical difference, "polarizing" the statistical relationship, and "reversing" the statistical relationship. We also show that a boolean formula whose atoms are statements about statistical difference can be transformed into a single statement about statistical difference. All of these transformations can be performed in polynomial time, in the sense that, given circuits which sample from the input distributions, it only takes polynomial time to compute circuits which sample from the output distributions.

By our prior work (see FOCS 97), such transformations for manipulating statistical difference are closely connected to results about SZK, the class of languages possessing statistical zero-knowledge proofs. In particular, some of the transformations given in this paper are equivalent to the closure of SZK under complement and under certain types of Turing reductions. This connection is also discussed briefly in this paper.

Wallner, D., E. Harder, and R. Agee. “Key management for multicast: Issues and architectures.Internet RFC 2627, no. June 1999 (1999).Abstract

This report contains a discussion of the difficult problem of key management for multicast communication sessions.  It focuses on two main areas of concern with respect to key management, which are, initializing the multicast group with a common net key and rekeying the multicast group.  A rekey may be necessary upon the compromise of a user or for other reasons (e.g., periodic rekey).  In particular, this report identifies a technique which allows for secure compromise recovery, while also being robust against collusion of excluded users.  This is one important feature of multicast key management which has not been addressed in detail by most other multicast key management proposals [1,2,4].  The benefits of this proposed technique are that it minimizes the number of transmissions required to rekey the multicast group and it imposes minimal storage requirements on the multicast group.

1998
Goldreich, Oded, Amit Sahai, and Salil Vadhan. “Honest-verifier statistical zero-knowledge equals general statistical zero-knowledge.Proceedings of the 30th Annual ACM Symposium on Theory of Computing (STOC ‘98) (1998): 399-408.Abstract
We show how to transform any interactive proof system which is statistical zero-knowledge with respect to the honest-verifier, into a proof system which is statistical zero-knowledge with respect to any verifier. This is done by limiting the behavior of potentially cheating verifiers, without using computational assumptions or even referring to the complexity of such verifier strategies. (Previous transformations have either relied on computational assumptions or were applicable only to constant-round public-coin proof systems.)

Our transformation also applies to public-coin (aka Arthur-Merlin) computational zero-knowledge proofs: We transform any Arthur-Merlin proof system which is computational zero-knowledge with respect to the honest-verifier, into an Arthur-Merlin proof system which is computational zero-knowledge with respect to any probabilistic polynomial-time verifier.

A crucial ingredient in our analysis is a new lemma regarding 2-universal hashing functions.