Abstract. In mining and integrating data from multiple sources, there are many privacy and security issues. In several different contexts, the security of the full privacy-preserving data mining protocol depends on the security of the underlying private scalar product protocol. We show that two of the private scalar product protocols, one of which was proposed in a leading data mining conference, are insecure. We then describe a provably private scalar product protocol that is based on homomorphic encryption and improve its efficiency so that it can also be used on massive datasets. Keywords: Privacy-preserving data mining, private scalar product protocol, vertically partitioned frequent pattern mining 1

Abstract. In voting based on homomorphic threshold encryption, the voter encrypts his vote and sends it in to the authorities that tally the votes. If voters can send in arbitrary plaintexts then they can cheat. It is therefore important that they attach an argument of knowledge of the plaintext being a correctly formed vote. Typically, these arguments are honest verifier zero-knowledge arguments that are made non-interactive using the Fiat-Shamir heuristic. Security is argued in the random oracle model. The simplest case is where each voter has a single vote to cast. Practical solutions have already been suggested for the single vote case. However, as we shall see homomorphic threshold encryption can be used for a variety of elections, in particular there are many cases where voters can cast multiple votes at once. In these cases, it remains important to bring down the cost of the NIZK argument. We improve on state of the art in the case of limited votes, where each voter can vote a small number of times. We also improve on the state of the art in shareholder elections, where each voter may have a large number of votes to spend. Moreover, we improve on the state of the art in Borda voting. Finally, we suggest a NIZK argument for correctness of an approval vote. To the best of our knowledge, approval voting has not been considered before in the cryptographic literature. 1

Efficient zero-knowledge proofs of knowledge (ZK-PoK) are basic building blocks of many practical cryptographic applications such as identification schemes, group signatures, and secure multiparty computation. Currently, first applications that critically rely on ZK-PoKs are being deployed in the real world. The most prominent example is Direct Anonymous Attestation (DAA), which was adopted by the Trusted Computing Group (TCG) and implemented as one of the functionalities of the cryptographic chip Trusted Platform Module (TPM). Implementing systems using ZK-PoK turns out to be challenging, since ZK-PoK are, loosely speaking, significantly more complex than standard crypto primitives, such as encryption and signature schemes. As a result, implementation cycles of ZK-PoK are time-consuming and error-prone, in particular for developers with minor or no cryptographic skills. In this paper we report on our ongoing and future research vision with the goal to bring ZK-PoK to practice by automatically generating sound ZK-PoK protocols and make them accessible to crypto and security engineers. To this end we are developing protocols and compilers that support and automate the design and generation of secure and efficient implementation of ZK-PoK protocols.

The notion of Zero Knowledge Proofs (of knowledge) [ZKP] is central to cryptography; it provides a set of security properties that proved indispensable in concrete protocol design. These properties are defined for any given input and also for any auxiliary verifier private state, as they are aimed at any use of the protocol as a subroutine in a bigger application. Many times, however, moving the theoretical notion to practical designs has been quite problematic. This is due to the fact that the most efficient protocols fail to provide the above ZKP properties for all possible inputs and verifier states. This situation has created various problems to protocol designers who have often either introduced imperfect protocols with mistakes or with lack of security arguments, or they have been forced to use much less efficient protocols in order to achieve the required properties. In this work we address this issue by introducing the notion of “protocol portability, ” a property that identifies input and verifier state distributions under which a protocol becomes a ZKP when called as a subroutine in a sequential execution of a larger application. We then concentrate on the very efficient and heavily employed “Generalized Schnorr Proofs ” (GSP) and identify the portability of such protocols. We also point to previous protocol weaknesses and errors that have been made in numerous applications throughout the years, due to employment of GSP instances while lacking the notion of portability (primarily in the case of unknown order

Abstract. Zero-knowledge proofs of knowledge (ZK-PoK) are important building blocks for numerous cryptographic applications. Although ZK-PoK have very useful properties, their real world deployment is typically hindered by their significant complexity compared to other (noninteractive) crypto primitives. Moreover, their design and implementation is time-consuming and error-prone. We contribute to overcoming these challenges as follows: We present a comprehensive specification language and a certifying compiler for ZK-PoK protocols based on Σ-protocols and composition techniques known in literature. The compiler allows the fully automatic translation of an abstract description of a proof goal into an executable implementation. Moreover, the compiler overcomes various restrictions of previous approaches, e.g., it supports the important class of exponentiation homomorphisms with hidden-order co-domain, needed for privacy-preserving applications such as idemix. Finally, our compiler is certifying, in the sense that it automatically produces a formal proof of security (soundness) of the compiled protocol (currently covering special homomorphisms) using the Isabelle/HOL theorem prover.

Abstract. Efficient zero-knowledge proofs of knowledge (ZK-PoK) are basic building blocks of many practical cryptographic applications such as identification schemes, group signatures, and secure multiparty computation. Currently, first applications that critically rely on ZK-PoKs are being deployed in the real world. The most prominent example is Direct Anonymous Attestation (DAA), which was adopted by the Trusted Computing Group (TCG) and implemented as one of the functionalities of the cryptographic Trusted Platform Module (TPM) chip. Implementing systems using ZK-PoK turns out to be challenging, since ZK-PoK are, loosely speaking, significantly more complex than standard crypto primitives, such as encryption and signature schemes. As a result, implementation cycles of ZK-PoK are time-consuming and error-prone, in particular for developers with minor or no cryptographic skills. In this paper we report on our ongoing and future research vision with the goal to bring ZK-PoK to practice by making them accessible to crypto and security engineers. To this end we are developing compilers and related tools that support and partially automate the design, implementation, verification and secure implementation of ZK-PoK protocols. 1

Abstract. 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.

We survey the contributions of the entire theoretical computer science/cryptography community during 1975-2002 that impact the question of how to run verifiable elections with secret ballots. The approach based on homomorphic encryptions is the most successful; one such scheme is sketched in detail and argued to be feasible to implement. It is explained precisely what these ideas accomplish but also what they do not accomplish, and a short history of election fraud throughout history is included.

We present homomorphic trapdoor commitments to group elements. In contrast, previous homomorphic trapdoor commitment schemes only allow the messages to be exponents. Our commitment schemes are length-reducing, we can make a short commitment to many group elements at once, and they are perfectly hiding and computationally binding. The commitment schemes are based on groups with a bilinear map. We can commit to elements from a base group, whereas the commitments belong to the target group. We present two constructions based on simple computational intractability assumptions, which we call respectively the double pairing assumption and the simultaneous triple pairing assumption. While the assumptions are new, we demonstrate that they are implied by well-known assumptions; respectively the decision Diffie-Hellman assumption and the decision linear assumption.

Abstract. Zero-knowledge proofs of knowledge (ZK-PoK) play an important role in many cryptographic applications. Direct anonymous attestation (DAA) and the identity mixer anonymous authentication system are first real world applications using ZK-PoK as building blocks. But although being used for many years now, design and implementation of sound ZK-PoK remains challenging. In fact, there are security flaws in various protocols found in literatur. Especially for non-experts in the field it is often hard to design ZK-PoK, since a unified and easy to use theoretical framework on ZK-PoK is missing. With this paper we overcome important challenges and facilitate the design and implementation of efficient and sound ZK-PoK in practice. First, Camenisch et al. have presented at EUROCRYPT 2009 a first unified and modular theoretical framework for ZK-PoK. This is compelling, but makes use of a rather inefficient 6-move protocol. We extend and improve their framework in terms of efficiency and show how to realize it using efficient 3-move Σ-protocols. Second, we perform an exact security and efficiency analysis for our new protocol and various protocols found in the literature. The analysis yields novel- and perhaps surprising- results and insights. It reveals for instance that using a 2048 bit RSA modulus, as specified in the DAA standard, only guarantees an upper bound on the success probability of a malicious prover between 1/2 4 and 1/2 24. Also, based on that analysis we show how to select the most efficient protocol to realize a given proof goal. Finally, we also provide low-level support to a designer by presenting a compiler realizing our framework and optimization techniques, allowing easy implementation of efficient and sound protocols.