June 8-12, Simons Institute for the Theory of Computing, University of California, Berkeley
Lower Bounds for Information-Theoretic MPC, Ivan Damgård, Aarhus University
Obfuscation: Past, Present, and Possible Futures, Amit Sahai, UCLA
In program obfuscation, a given program P1 is converted to another program P2 that represents the same functionality (i.e., the same I/O) while an adversary who can see P2 cannot understand its logic. From a cryptographic perspective, this is like having a software that can keep a secret. This leads to a new notion of program obfuscation called indistinguishability obfuscation (iO), where a polynomial-time adversary cannot distinguish between the obfuscations of two equivalent programs. The first mathematical construction of an indistinguishability obfuscator was proposed by Garg-Gentry-Halevi in 2013. The main idea to obfuscate programs using structured noise rather than just random noise. When the program is evaluated, the noise cancels out and the correct output is obtained.
To this end, a multilinear map can be used to encode field elements with specific algebraic manipulations and a Boolean function that returns Yes/No when the encoded value is zero or not. The program is first converted to a sequence matrices using a technique called oblivious matrix branching program. Then, the Kilian’s randomization technique is used to generate structured noise: the matrices are multiplied by a sequence of random matrices. The key challenge to the current obfuscation scheme is the input-mixing attack. A technique is required to enforce honest behavior in some way. Amit finished his talk by arguing that multilinear maps provide a new hardness generating technique at the level of Diffie-Hellman and learning with error hardness assumptions, but they have mainly been used for obfuscation. One open question is that can multilinear maps be used for solving other security problems?
Two-Round MPC via Multi-Key FHE, Daniel Wichs, Northeastern University
Efficient Multiparty Protocols via Log-Depth Threshold Formulae, Ron Rothblum, Weizmann Institute
- Design a protocol for a small number of parties (say, 3 or 4) which achieves security against a single corrupted party. Such protocols are typically easy to construct as they may employ techniques that do not scale well with the number of corrupted parties.
- Recursively compose with itself to obtain an efficient n-party protocol which achieves security against a constant fraction of corrupted parties.
The reduction idea is described in the following. First, consider a trivial n-party setting that includes a trusted party τ. The parties send their inputs to τ whom locally computes the MPC functionality and returns the output back to all parties. Now, replace the trusted party with a small constant number of virtual parties, say v1,v2,v3, which emulate the behavior of τ. The new protocol is secure as long as the adversary does not control a majority of the virtual parties. We now proceed by replacing v1 by with three new virtual parties w1,w2,w3. The new protocol is secure even if the adversary controls either one of v2,v3 and one of the w’s, or two of the w’s. Using this, Ron constructed a formula as shown in Figure 2↑, which can be used to build a formula for the entire protocol as shown in Figure 3↓, where the leaves are the real parties.
Rethinking Secure Computation – A Greedy Approach, Muthu Venkitasubramaniam, University of Rochester
The general framework for secure computation consists of two steps: (1) Compile a functionality into one of several standard representations such as a circuit (Boolean or arithmetic) or an oblivious RAM; (2) Use a generic scheme such as [Yao82, GMW87, BGW88, CCD88] to securely evaluate the functionality. However, for some certain problems such as Private Set Intersection, specific solutions can result in significantly more efficient solutions. Inspired by these, Muthu presented a new algorithmic approach for designing secure two-party computation protocols. In their model (join work with Abhi Shelat from UVA), they show that several problems such as convex hull, minimum spanning tree, unit job scheduling, and single source shortest path can be securely computed only by using secure comparison.
The underlying idea among all of these problems is that each of them has some known greedy algorithm. Accordingly, Muthu described a new framework called Greedy-Millionaire Framework, where a function f is defined secure greedy compatible if there exists it has the following three properties:
- Unique Solution: Given inputs U and V of Alice and Bob, f(U,V) is unique;
- Unique Order: There is a unique order in which the output is presented in the greedy algorithm;
- Local Updatability: The local greedy heuristic can be computed using a comparison operation over the function locally computed over each of the inputs.
Cryptocurrencies and Smart Contracts, Elaine Shi, University of Maryland
[BGT13] . Communication locality in secure multi-party computation: how to run sublinear algorithms in a distributed setting. Proceedings of the 10th theory of cryptography conference on Theory of Cryptography, pp. 356—376, 2013.
[DKMS12] . Brief announcement: breaking the // bit barrier, secure multiparty computation with a static adversary. Proceedings of the 2012 ACM symposium on Principles of distributed computing, pp. 227—228, 2012.
[SZ15] . Recent Results in Scalable Multi-Party Computation. In SOFSEM 2015: Theory and Practice of Computer Science (Italiano, GiuseppeF. and Margaria-Steffen, Tiziana and Pokorný, Jaroslav and Quisquater, Jean-Jacques and Wattenhofer, Roger, ed.). Springer Berlin Heidelberg, 2015. URL: http://dx.doi.org/10.1007/978-3-662-46078-8_3.
Filed under: Uncategorized | Tags: secure multiparty computation, theory, workshops
Last month I went to a workshop on secure multiparty computation that was organized by IARPA. I’ve had mixed luck with these types of workshops that are organized by funding agencies. However, this one was pretty good, because of the timeliness of the topic, well-prepared talks, and the fact that the participants were some of the big names in security and MPC.
This was also one first times I’ve visited and given a talk at MIT, which was a lot of fun. Many of the talk slides are included in the workshop link above.
Filed under: Uncategorized | Tags: conferences, distributed computing, secure multiparty computation, theory
Following is a very nice report of a Workshop on Applied Multiparty Computation at which my students Mahnush Movahedi and Mahdi Zamani gave talks. Enjoy. – Jared
Workshop on Applied Multi-Party Computation: A Summary
Mahnush Movahedi and Mahdi Zamani
Secure Computation in 2029: Boom, Bust, or Bonanza (keynote speech), David Evans, University of Virginia
In order to predict the development of MPC in 2029, David estimated the U.S. government investment on MPC in the past 1-2 years to be about $100 million, which is roughly about the annual investment spent by the government on art fields [A] . He then made three exciting predictions about MPC in 2029.
- Claim 1. The MPC industry should be bigger than anti-malware industry in 2029. David justified this by extrapolating investments on IT security and predicting that it will probably decrease by 2029 due to progress in developing secure software.
- Claim 2. High cost will no longer be the main impediment to the widespread use of MPC (at least for two-party computation). He explained this by estimating the cost of securely comparing two genomes (e.g. in genetic matchmaking) using MPC in 2004 (which is about $100 million) and estimating its cost in 2029 (which is about $0.001). For security against a malicious adversary, this cost in 2004 is estimated to be about $80 billion while in 2029 it is predicted to be about $0.005 per secure comparison.
- Claim 3. We don’t yet know what the “killer app” for MPC is and it’s probably not privacy. David argues that the amount of money people pay for privacy is usually very small. So, he predicts that the killer app for MPC will not just use MPC for privacy, and there are aspects of MPC that will probably be very useful in some applications. There was a panel on the business case for MPC after David’s talk, where people discussed more about the killer app for MPC. The panel’s video is available here.
- What if the outputs leak? In MPC, the output of computation is revealed to everyone. This output may leak some information to the adversary. Is there a way to measure or limit this leakage?
- How to ensure users accept and trust the MPC-based application?
- How to implement MPC with low (human) cost?
Broadcast (and Round) Efficient Secure Multiparty Computation, Juan Garay, Yahoo! Labs
In order to perform VSS in a point-to-point network, parties should have access to a broadcast channel, which ensures that all parties receive the same message broadcast by the sender. Juan argues that due to the lack of an efficient broadcast channel, VSS schemes should be measured in terms of broadcast complexity, and the goal should be to minimize the number of broadcasts in VSS. Juan defined a relaxed type of secret sharing called Weak Secret Sharing (WSS), where the recipients reconstruct either the value jointly held by them or a special symbol ⊥ indicating that the dealer is malicious. He then defines VSS by extending the definition of WSS such that the recipients always reconstruct the joint value even if there is cheating in the reconstruction phase.
Juan presented a linear VSS protocol for an unbounded static adversary (t<n/2 ). The protocol uses only two broadcasts in the sharing phase and none in the reconstruction—what he calls a (2,0)-broadcast VSS. This is achieved by increasing the number of rounds in the sharing phase, which makes their protocol (20,1)-round. He compares this to the state-of-the-art VSS protocol of Kumaresan et al.  that is a (2,2)-broadcast and (3,2)-round VSS. They first construct a (2,1)-broadcast WSS protocol and then use its sharing phase to build a (3,0)-broadcast, (9,1)-round VSS based on the construction of Rabin and Ben-Or . The number of broadcasts is reduced further to build a (2,0)-broadcast VSS using the moderated VSS of Katz and Koo , where the dealer acts as the moderator to simulate broadcast. Such a broadcast is called a modercast. At the end of his talk, Juan explained that their VSS scheme can be used for constructing efficient pseudosignatures, an information-theoretic analogue of digital signatures introduced by . Such signatures can be created in a setup phase to be used for simulating future invocations of broadcast or constructing efficient anonymous channels. Watch Juan’s presentation video here.
Asynchronous MPC with t<n/2 Using Non-Equivocation, Aniket Kate, MMCI, Saarland University
Aniket presented work on Asynchronous MPC (AMPC), where it is assumed that all messages sent by the parties are eventually delivered but with indefinite delays. In the synchronous setting, MPC has been solved with t<n/2 malicious parties (with cryptographic assumptions) but in the asynchronous setting, the best known algorithm tolerates t<n/3 malicious corruptions. These resiliency bounds are primarily due to the limitations of implementing a reliable broadcast channel that is necessary for constructing an Asynchronous Verifiable Secret Sharing (AVSS) protocol.
In order to verify correctness of a sharing, parties need to prevent the adversary from equivocation, which means making conflicting statements to different parties. A mechanism that makes equivocation impossible is called non-equivocation, which can also be transferable to allow a receiver to verifiably transfer the authenticated statement to other parties. Non-equivocation can be implemented using an increment-only counter and a digital signature oracle, which can be constructed using trusted hardware modules readily available in commodity computers and smartphone devices .
Quorums Quicken Queries: Efficient Asynchronous Secure Multiparty Computation, Mahnush Movahedi, University of New Mexico
Mahnush motivated her talk by the fact that guaranteeing synchrony in most real applications is difficult and even impractical. Thus, in order to use MPC in such applications, certain techniques are required to deal with asynchrony in a malicious setting. One can design a scalable MPC protocol in this setting by delegating computation of each gate to a logarithmic set of parties called a quorum, in which at most a certain fraction of parties are malicious. The quorum then computes the gate using any MPC protocol. Mahnush explained that distribution of communications and computations among several quorums requires incorporating extra coordination efforts between parties, which they handle using a number of efficient tools and techniques.
In a setup phase, a set of n quorums are created using the quorum building algorithm of King et al.  optimized with the Byzantine agreement scheme of Braud-Santoni et al. , which costs soft-O(1) bits per party. Mahnush argued that the parties in each quorum require a mechanism to coordinate with other quorums on when to start computation on asynchronous inputs. In other words, they need to wait for sufficient number of inputs to be received until they start the computation.
To this end, they propose to count the number of ready inputs using a distributed data structure called a τ -counter, where τ=n−t is the threshold on the number inputs to be received before the circuit is evaluated, and t<n/8 is the number of malicious parties. Using a τ -counter, MPC can be solved without assuming any synchrony points in the algorithm.
Secure Collaborative Statistics in Credit Rating Using Linear Programming, Tomas Toft, Aarhus University
At the start of his talk, Tomas described how MPC can be used to implement a secure collaborative credit rating for Danish dairy farmers who are willing to get loans. Credit rating is one of the interesting classical problems that is solved using MPC. Tomas described how this problem can be modeled by linear programming. A linear program consists of n variables and m constraints and the goal is to maximize an objective function. A solution for a linear program is an assignment to the variables such that the constraints hold.
To solve a linear program for n=285 variables and m=4 constraints, the presented protocol requires 2mn+6m2+n≃2700 multiplications and n+3m≃300comparisons. A Java implementation of their protocol that uses Amazon’s cloud services shows that the running time for this computation is around 5 minutes. The implementation is being demoed for actual banks using real data which allows bankers to jointly rank the farmers based on their credit scores. For more information about Tomas motivation for this problem see another blog post about his talk here.
Securely Solving Standard Network Flow Problems with Secure Multi-Party Computation, Abdelrahaman Aly, C.O.R.E., Univesité catholique de Louvain
Abdel introduced a new class of problems in which a graph is shared between parties as their inputs. The parties want to evaluate a function such as max-flow or shortest path over this shared graph. In most combinatorial problems such as various graph problems, the execution path depends on the input data. Thus, even if all input data are appropriately shared or encrypted among the parties, the execution path itself may reveal some information to the adversary.
MPC in Large Networks with an Application to Anonymous Broadcast, Mahdi Zamani, University of New Mexico
Mahdi proposed a method to reduce the communication cost of their protocol by performing local communications in logarithmic-size groups of parties called quorums, where the number of adversary-controlled parties in each quorum is at most a certain fraction. The quorums are created in an offline phase using the Byzantine agreement protocol of Santoni et al. . The offline phase also uses the fully homomorphic encryption protocols of Brakerski et al.  to evaluate a small-depth circuit. This is done to generated a sufficient number of Beaver  multiplication triples.
In the online phase, each input is secret shared in a quorum using Shamir’s secret sharing scheme. Each gate of the circuit is computed by a quorum over shares, where multiplication is performed using Beaver’s triples. Parties in the quorum send the result of this computation to all quorums associated with gates that need this result as input. It is necessary to securely send the output from one quorum to another without revealing any information to any individual party or to any coalition of adversarial parties. Mahdi solves this by creating fresh shares of the output for the target quorum, where the new shares are values of a new random polynomial that still evaluates to the (secret) output at 0.
MEVAL: A Practically Efficient System for Secure Multi-party Statistical Analysis, Koki Hamada, NTT Secure Platform Laboratories
Koki described an implementation of an MPC protocol called MEVAL (Multi-party EVALuator), which is optimized for statistical analyses. His talk described the techniques they have used to make their protocol efficient and the experiments conducted for evaluating the system. The computation in MEVAL is performed over values shared using Shamir’s secret-sharing scheme among three parties, where at most one of them is controlled by a passive adversary. MEVAL can also be used in a server-based setting, where all clients share their inputs between three servers. One application of such a setting is secure outsourcing of data storage.
 Zuzana Beerliová-Trubı́niová, Martin Hirt, Jesper Buus Nielsen: “On the Theoretical Gap Between Synchronous and Asynchronous MPC Protocols”, Proceedings of the 29th Symposium on Principles of Distributed Computing, pp. 211—218, 2010.
 Zvika Brakerski, Craig Gentry, Vinod Vaikuntanathan: “Fully Homomorphic Encryption Without Bootstrapping”, Proceedings of the 3rd Innovations in Theoretical Computer Science Conference, pp. 309—325, 2012.
 V. King, S. Lonergan, J. Saia, A. Trehan: “Load balanced Scalable Byzantine Agreement through Quorum Building, with Full Information”,International Conference on Distributed Computing and Networking (ICDCN), 2011.
 Ranjit Kumaresan, Arpita Patra, C.Pandu Rangan: The Round Complexity of Verifiable Secret Sharing: The Statistical Case in Advances in Cryptology – ASIACRYPT 2010 (Abe, Masayuki, ed.). Springer Berlin Heidelberg, 2010.
 Helger Lipmaa, Tomas Toft: Secure Equality and Greater-Than Tests with Sublinear Online Complexity in Automata, Languages, and Programming. Springer Berlin Heidelberg, 2013. URL http://dx.doi.org/10.1007/978-3-642-39212-2_56.
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Filed under: Uncategorized | Tags: algorithms, secure multiparty computation, security, theory
Attached is a paper on a problem we’ve been working on for a while: an efficient algorithm for Secure Multiparty Computation (SMPC) against a static adversary.
In the SMPC problem, n players each have a private input, and their goal is to compute the value of a n-ary function, f, over the inputs, without revealing any information about the inputs. The problem is complicated by the fact that a hidden subset of the players are controlled by an adversary that actively tries to subvert this goal.
SMPC abstracts several important problems in distributed security, and so, not surprisingly, there have been thousands of papers written
in the last several decades addressing it. However, there is a striking barrier that prevents wide-spread use: current algorithms to solve SMPC are not resource efficient. In particular, if there are n players involved in the computation and the function f can be computed by a circuit with m gates, then most algorithms require each player to send a number of messages and perform a number of computations that is
We describe scalable algorithms for SMPC against a static adversary. We assume a partially synchronous message passing communication model, but unlike most related work, we do not assume the existence of a broadcast channel. Our main result holds for the case where there are $n$ players, of which a fraction are controlled by an adversary, for any positive constant. We describe a SMPC algorithm for this model that requires each player to send messagesand perform computations to compute any function f, where m is the size of a circuit to compute f.
Filed under: Uncategorized | Tags: distributed computing, secure multiparty computation, security, theory
In this post, I want to talk about a research problem that I think even our new Republican legislatures can get excited about: Yao’s millionaire problem. In this problem, two millionaires want to determine who is richer, but neither wants to reveal their private net worth. Can we develop a protocol to help these millionaires? This problem is important because it kicked off the the study of secure multiparty computation.
Below is a picture of the protocol originally proposed by Yao. Alice and Bob are the two millionaires and i and j are the private net worths of Alice and Bob respectively. The protocol makes use of public key cryptography: Alice is assumed to be able to generate a public encryption key E_a() along with a private decryption key D_a(). To see that the protocol reveals who is richer, note that y_j = D_a(k) = x. Thus w_j = x if j<= i and w_j = x+1 if j>i. Showing that the protocol doesn’t reveal any information beyond who is richer is more challenging and is presented in detail in the paper.
I should point out that this protocol takes exponential time and that this run time has been improved in subsequent papers. However, there is a question about this problem I haven’t been able to answer. In the case where there are n players, do we need private channels or cryptographic assumptions to solve the problem? Are private channels needed even if we’re happy with a Monte Carlo solution? I’ve seen several papers that remove cryptographic assumptions, but none that seem to remove the need for private channels. Conversely, I’ve seen no papers that prove that private channels are necessary…