Many cryptographic protocols secure against malicious players use specially designed cryptographic tools. Essentially, these special tools function much like less-expensive tools, but give extra ‘powers’ to a reduction or simulation algorithm. Using these powers, cryptographers can construct a proof of security using standard techniques. However, these powers are not available to either the honest parties or the adversary. In a large class of protocols, by replacing the expensive, specially designed cryptographic tool with a corresponding less-expensive tool, we can improve the protocol’s efficiency without changing the functionality available to either the adversary or the honest parties. The key motivating question we address in this paper is whether the new, ‘substituted’ protocol is still secure. We introduce a framework for reasoning about this question. Our framework uses translators: special purpose oracles that map outputs of one cryptographic tool to corresponding outputs of a different tool. Translators are similar to, but generally weaker than, the “angels” of Prabhakaran and Sahai [29]. We introduce the notion of substitution-friendly protocols and show that such protocols remain secure after substitution in our framework. We also leverage existing proofs of security; there is no need to re-prove security from scratch. We demonstrate our framework with a non-interactive non-malleable bit commitment protocol.
Many cryptographic protocols secure against malicious players use specially designed cryptographic tools. Essentially, these special tools function much like less-expensive tools, but give extra ‘powers’ to a reduction or simulation algorithm. Using these powers, cryptographers can construct a proof of security using standard techniques. However, these powers are not available to either the honest parties or the adversary. In a large class of protocols, by replacing the expensive, specially designed cryptographic tool with a corresponding less-expensive tool, we can improve the protocol’s efficiency without changing the functionality available to either the adversary or the honest parties. The key motivating question we address in this paper is whether the new, ‘substituted’ protocol is still secure. We introduce a framework for reasoning about this question. Our framework uses translators: special purpose oracles that map outputs of one cryptographic tool to corresponding outputs of a different tool. Translators are similar to, but generally weaker than, the “angels” of Prabhakaran and Sahai [29]. We introduce the notion of substitution-friendly protocols and show that such protocols remain secure after substitution in our framework. We also leverage existing proofs of security; there is no need to re-prove security from scratch. We demonstrate our framework with a non-interactive non-malleable bit commitment protocol.
In cryptographic protocols, honest parties would prefer that their security is assured even in presence of adversarial parties who have unbounded computational power. Information theoretic secure realization of cryptographic primitives provides such guarantees; but for most tasks such strong security guarantees cannot be provided for any reasonable notion of security. The standard technique used in cryptography is to assume the existence of some puzzle, whose hard instances are easy to generate but no efficient algorithm can solve them. Such assumptions, which define intractable problems for efficient algorithms, are called computational intractability assumptions.
In this work, we motivate a study of computational intractability assumptions which is goal driven and lends support to the fundamental nature of some of the traditional assumptions beyond being historical accidents. Secure multi-party computation deals with the study of constructing secure protocols for general cryptographic tasks which conform to various notions of security. Inspired by complexity theory, we use the notion of reduction to further our understanding of computational intractability assumptions. Our framework explores the hardness of natural assumptions of the form: “Task F can be securely computed given an ideally secure facility for computing G ”.
Formally, we characterize the minimal computational intractability assumption which is sufficient to securely realize a functionality using trusted copies of some other functionality. A functionality F reduces to G , similar to the complexity-theoretic notion of reduction, if F can be securely realized when parties can access trusted copies of G . Thus, if F does not reduce to G information-theoretically, we are interested in characterizing the minimal assumption, which is sufficient, to securely realize the reduction of F to G .
In our framework, we explore the relative strengths of these minimal computational assumptions. The fundamental problem, then, is to establish relations, like implication, equivalence and separation, among the assumptions which correspond to various reductions. In this work, we further our understanding of the minimal computational assumptions corresponding to such reductions by showing the following results:
1. We identify a spectrum for the set of assumptions that correspond to reduction between a pair of tasks under different security notions. These reductions are implied by the computational intractability assumption: “semi-honest secure protocol for oblivious transfer exists” (sh-OT assumption). Additionally, the information theoretically impossible reductions considered in this thesis imply the assumption: “one-way functions exist” (OWF assumption). We conjecture that OWF assumption is necessary for these reductions. In fact, the reductions considered in this thesis are either information theoretically true, false, equivalent to OWF assumption or equivalent to sh-OT assumption [MPR10a].
We expand our study to encompass reductions involving randomized functionalities and study the consequences of providing parties with a trusted source of common unbiased coins. For a strong notion of security they turn out to be useless and unless sh-OT assumption holds these randomness sources are not useful [MOPR11, MP11].
2. We [MPS10] seek to lend additional support to the widely believed premise that “non-trivial cryptography entails the existence of one-way functions”. We show that (constant round) weak coin-tossing protocols, imply OWF assumption. For the general problem, i.e. secure weak coin-tossing protocols with polynomial round complexity, we show a slightly weaker implication NP n BPP.
3. Finally, we indicate the possibility of a wide range of intractability assumptions in our spectrum, intermediate to OWF assumption and sh-OT assumption. We show that a class of natural assumptions corresponding to reductions that are relativistically separated from OWF assumption. Similar techniques have been used to show their separation from the assumption that “public-key encryption exists” (PKE assumption) [MMP11]; but these assumptions are not known to imply sh-OT assumption. We conjecture that, in fact, sh-OT assumption is relativistically separated from these assumptions.
The notion of secure computation is central to cryptography. Introduced in the seminal works of Yao [FOCS'82, FOCS'86] and Goldreich, Micali and Wigderson [STOC'87], secure multiparty computation allows a group of (mutually) distrustful parties to jointly compute any functionality over their individual private inputs in such a manner that the honest parties obtain the correct outputs and no group of malicious parties learn anything beyond their inputs and the prescribed outputs. General feasibility results for secure computation were given by Yao [FOCS'86] and Goldreich et al. [STOC'87] more than two decades ago. Subsequent to these works, designing secure computation protocols that can tolerate more powerful adversaries and satisfy stronger notions of security has been a very active area of research. In this dissertation, we study two such new frontiers in the area of secure computation.
In the first part of this dissertation, we initiate a study of designing leakage-resilient interactive protocols. Specifically, we consider the scenario where an adversary, in addition to corrupting a subset of parties, can leak (potentially via physical attacks) arbitrary information from the secret state of any honest party. This is in contrast to the standard notion of secure computation where it is assumed that the adversary only has black-box'' access to the honest parties. In particular, we formalize a meaningful definition of leakage-resilient zero knowledge proof systems and provide constructions that satisfy our notion. We also discuss various applications of our results.
The second part of this dissertation concerns with the general question of whether it is possible to securely run cryptographic protocols over an insecure network environment such as the Internet. It is well-known that the standard notion of secure computation is only relevant to the stand-alone setting where a single protocol is being executed in isolation; as such it does not guarantee security when multiple protocol sessions may be executed concurrently under the control of an adversary who is present across all sessions. We consider the open problem of constructing secure password-based authenticated key exchange protocols in such a setting in the plain model (i.e., without assuming any trusted infrastructure or random oracles). We give the first construction of such a protocol based on standard cryptographic assumptions. Our results are in fact much more general, and extend to other functionalities w.r.t. a (necessarily) weakened notion of concurrently secure computation.
The results presented in this dissertation stem from two papers which are joint works with Sanjam Garg and Amit Sahai, and with Vipul Goyal and Rafail Ostrovsky, respectively.
In this work, we investigate the security of interactive computations. The main emphasis is on the mathematical methodology that is needed to formalise and analyse various security properties. Differently from many classical treatments of secure multi-party computations, we always quantify security in exact terms. Although working with concrete time bounds and success probabilities is technically more demanding, it also has several advantages. As all security guarantees are quantitative, we can always compare different protocol designs. Moreover, these security guarantees also have a clear economical interpretation and it is possible to compare cryptographic and non-cryptographic solutions. The latter is extremely important in practice, since cryptographic techniques are just one possibility to achieve practical security. Also, working with exact bounds makes reasoning errors more apparent, as security proofs are less abstract and it is easier to locate false claims. The choice of topics covered in this thesis was guided by two principles. Firstly, we wanted to give a coherent overview of the secure multi-party computation that is based on exact quantification of security guarantees. Secondly, we focused on topics that emerged from the author's own research. In that sense, the thesis generalises many methodological discoveries made by the author. As surprising as it may seem, security definitions and proofs mostly utilise principles of hypothesis testing and analysis of stochastic algorithms. Thus, we start our treatment with hypothesis testing and its generalisations. In particular, we show how to quantify various security properties, using security games as tools. Next, we review basic proof techniques and explain how to structure complex proofs so they become easily verifiable. In a nutshell, we describe how to represent a proof as a game tree, where each edge corresponds to an elementary proof step. As a result, one can first verify the overall structure of a proof by looking at the syntactic changes in the game tree and only then verify all individual proof steps corresponding to the edges. The remaining part of the thesis is dedicated to various aspects of protocol design. Firstly, we discuss how to formalise various security goals, such as input-privacy, output-consistency and complete security, and how to choose a security goal that is appropriate for a specific setting. Secondly, we also explore alternatives to exact security. More precisely, we analyse connections between exact and asymptotic security models and rigorously formalise a notion of subjective security. Thirdly, we study in which conditions protocols preserve their security guarantees and how to safely combine several protocols. Although composability results are common knowledge, we look at them from a slightly different angle. Namely, it is irrational to design universally composable protocols at any cost; instead, we should design computationally efficient protocols with minimal usage restrictions. Thus, we propose a three-stage design procedure that leads to modular security proofs and minimises usage restrictions.
An important security concern for crypto-graphic protocols is the extent to which they adversely affect the security of the systems in which they run. In particular, can we rule out the possibility that introducing a new protocol to a system might, as a "side effect", break the security of unsuspecting protocols in that system? Universally Composable (UC) security rules out such adverse side effects. However, many functionalities of interest provably cannot be realized with UC security unless the protocol participants are willing to put some trust in external computational entities. We propose a notion of security that: (a) allows realizing practically any functionality by protocols in the plain model without putting trust in any external entity; (b) guarantees that secure protocols according to this notion have no adverse side-effects on existing protocols in the system -- as long as the security of these existing protocols is proven via the traditional methodology of black box reduction to a game-based cryptographic hardness assumption with bounded number of rounds. Our security notion builds on the angel-based security notion of Prabhakaran and Sahai. A key part in our analysis is to come up with a CCA-secure commitment scheme that (a) cannot be proven secure via a black box reduction to a game-based assumption, but (b) can be proven secure using a non-black-box reduction. To the best of our knowledge, this is the first time that the interplay between black-box provability and unprovability is used to demonstrate security properties of protocols.
In this paper, we study the question of whether or not it is possible to construct protocols for general secure computation in the setting of malicious adversaries and no honest majority that use the underlying primitive (e.g., enhanced trapdoor permutation) in a black-box way only. Until now, all known general constructions for this setting were inherently non-blackbox since they required the parties to prove zero-knowledge statements that are related to the computation of the underlying primitive. Our main technical result is a fully black-box reduction from oblivious transfer with security against malicious parties to oblivious transfer with security against semi-honest parties. As a corollary, we obtain the first constructions of general multiparty protocols (with security against malicious adversaries and without an honest majority) which only make a black-box use of semi-honest oblivious transfer, or alternatively a black-box use of lower-level primitives such as enhanced trapdoor permutations or homomorphic encryption. In order to construct this reduction we introduce a new notion of security called privacy in the presence of defensible adversaries. This notion states that if an adversary can produce (retroactively, after the protocol terminates) an input and random tape that make
Key exchange protocols, first introduced by Diffie and Hellman in 1976, are one of the most widely-deployed cryptographic protocols. They allow two parties, that have never interacted before, to establish shared secrets. These shared cryptographic keys may subsequently be used to establish a secure communication channel. Use cases include the classic client-server setting that is for example at play when browsing the internet, but also chats via end-to-end-encrypted instant messaging applications. Security-wise, we generally demand of key exchange protocols to achieve key secrecy and authentication. While, informally, authentication ensures that the communicating parties have confidence in the identity of their peers, key secrecy ensures that any shared cryptographic key that is established via the key exchange protocol is only known to the participants in the protocol and can be used securely in cryptographic protocols, i.e., is sufficiently random. In 1993, Bellare and Rogaway gave a first formalization of key exchange protocol security that captures these properties with respect to powerful adversaries with full control over the network. Their model constitutes the basis of the many subsequent treatments of authenticated key exchange security, including the models presented in this thesis. The common methodological approach underlying all of these formalizations is the provable security paradigm, which has become a standard tool in assessing the security of cryptographic protocols and primitives. So-called security models specify the expected security guarantees of the scheme in question with regards to a well-defined class of adversaries. Proofs that validate these security claims do so by reducing the security of the overall scheme to the security of the underlying cryptographic primitives and hardness assumptions. However, advances in computational power and more sophisticated cryptanalytic capabilities often render exactly these components insecure. Especially the advent of quantum computers will have a devastating effect on much of today's public key cryptography. This is especially true for key exchange protocols since they rely crucially on public-key algorithms. In this thesis, our focus in future-proofing key exchange protocols is two-fold. First, we focus on extending security models for key exchange protocols to capture the (un)expected break of cryptographic primitives and hardness assumptions. The aim is to gain assurances with respect to future adversaries and to investigate the effects of primitive failures on key exchange protocols. More specifically, we explore how key exchange protocols can be safely transitioned to new, post-quantum secure algorithms with hybrid techniques. Hybrids combine classical and post-quantum algorithms such that the overall key agreement scheme remains secure as long as one of the two base schemes remains secure. For this, we introduce security notions for key encapsulation mechanisms that account for adversaries with varying levels of quantum capabilities and present three new constructions for hybrid key encapsulation mechanisms. Our hybrid designs are practice-inspired and for example capture draft proposals for hybrid modes in the Transport Layer Security (TLS) protocol, which is one of the most widely-deployed cryptographic protocols that enables key agreement. Furthermore, our notion of breakdown resilience for key exchange protocols allows to gauge the security of past session keys in the event of a failure of a cryptographic component in the key exchange. We exercise our model on variants of the post-quantum secure key exchange protocol NewHope by Alkim et al. Thereby, we confirm the intuition that, in order to guard against adversaries that only have access to quantum computing power in the (more distant) future, it is sufficient to use classically-secure authentication mechanisms alongside post-quantum key agreement to achieve authenticated key exchange. As with any mathematical statement, theorems in the provable security paradigm are only as valid as the underlying assumptions. A careful consideration of any newly made assumption is thus essential to ensure the meaningfulness of the statement itself and make the assumption a viable tool for future analyses. Thus, secondly, we systematically classify the PRF-ODH assumption, a complexity-theoretic hardness assumption that has been used in key exchange security analyses of such prominent protocols as TLS, Signal, and Wireguard. In particular, we give a unified, parametrized definition of the assumption encompassing different variants that are present in the literature. We relate the resulting parametrized notions in terms of their strength and show where these assumptions fit in the collection of well-understood related hardness assumptions. We finally sketch our result on the impossibility of instantiating this assumption in the standard model, thereby disposing of the uncertainty in the community whether PRF-ODH is in fact a standard model assumption, i.e., removes the usage of some idealized assumptions in key exchange protocol proofs.