Hardening the Security of Server-Aided MPC Using Remotely Unhackable Hardware Modules

Garbling schemes are useful building blocks for enabling secure multi-party computation (MPC), but require considerable computational resources both for the garbler and the evaluator. Thus, they cannot be easily used in a resource-restricted setting, e.g. on mobile devices. To circumvent this problem, server-aided MPC can be used, where circuit garbling and evaluation are performed by one or more servers. However, such a setting introduces additional points of failure: The servers, being accessible over the network, are susceptible to remote hacks. By hacking the servers, an adversary may learn all secrets, even if the parties participating in the MPC are honest. In this work, we investigate how the susceptibility for such remote hacks in the server-aided setting can be reduced. To this end, we modularize the servers performing the computationally intensive tasks. By using data diodes, air-gap switches and other simple remotely unhackable hardware modules, we can isolate individual components during large parts of the protocol execution, making remote hacks impossible at these times. Interestingly, this reduction of the attack surface comes without a loss of efficiency

Reusing Tamper-Proof Hardware in UC-Secure Protocols

Universally composable protocols provide security even in highly complex environments like the Internet. Without setup assumptions, however, UC-secure realizations of cryptographic tasks are impossible. Tamper-proof hardware tokens, e.g. smart cards and USB tokens, can be used for this purpose. Apart from the fact that they are widely available, they are also cheap to manufacture and well understood. Currently considered protocols, however, suffer from two major drawbacks that impede their practical realization: - The functionality of the tokens is protocol-specific, i.e. each protocol requires a token functionality tailored to its need. - Different protocols cannot reuse the same token even if they require the same functionality from the token, because this would render the protocols insecure in current models of tamper-proof hardware. In this paper we address these problems. First and foremost, we propose formalizations of tamper-proof hardware as an untrusted and global setup assumption. Modeling the token as a global setup naturally allows to reuse the tokens for arbitrary protocols. Concerning a versatile token functionality we choose a simple signature functionality, i.e. the tokens can be instantiated with currently available signature cards. Based on this we present solutions for a large class of cryptographic tasks

Fortified Multi-Party Computation: Taking Advantage of Simple Secure Hardware Modules

In practice, there are numerous settings where mutually distrusting parties need to perform distributed computations on their private inputs. For instance, participants in a first-price sealed-bid online auction do not want their bids to be disclosed. This problem can be addressed using secure multi-party computation (MPC), where parties can evaluate a publicly known function on their private inputs by executing a specific protocol that only reveals the correct output, but nothing else about the private inputs. Such distributed computations performed over the Internet are susceptible to remote hacks that may take place during the computation. As a consequence, sensitive data such as private bids may leak. All existing MPC protocols do not provide any protection against the consequences of such remote hacks. We present the first MPC protocols that protect the remotely hacked parties’ inputs and outputs from leaking. More specifically, unless the remote hack takes place before the party received its input or all parties are corrupted, a hacker is unable to learn the parties’ inputs and outputs, and is also unable to modify them. We achieve these strong (privacy) guarantees by utilizing the fact that in practice parties may not be susceptible to remote attacks at every point in time, but only while they are online, i.e. able to receive messages. To this end, we model communication via explicit channels. In particular, we introduce channels with an airgap switch (disconnectable by the party in control of the switch), and unidirectional data diodes. These channels and their isolation properties, together with very few, similarly simple and plausibly remotely unhackable hardware modules serve as the main ingredient for attaining such strong security guarantees. In order to formalize these strong guarantees, we propose the UC with Fortified Security (UC#) framework, a variant of the Universal Composability (UC) framework

Fortified Universal Composability: Taking Advantage of Simple Secure Hardware Modules

Adaptive security is the established way to capture adversaries breaking into computers during secure computations. However, adaptive security does not prevent remote hacks where adversaries learn and modify a party’s secret inputs and outputs. We initiate the study of security notions which go beyond adaptive security. To achieve such a strong security notion, we utilize realistic simple remotely unhackable hardware modules such as air-gap switches and data diodes together with isolation assumptions. Such hardware modules have, to the best of our knowledge, not been used for secure multi-party computation so far. As a result, we are able to construct protocols with very strong composable security guarantees against remote hacks, which are not provided by mere adaptive security. We call our new notion Fortified UC security. Using only very few and very simple remotely unhackable hardware modules, we construct protocols where mounting remote attacks does not enable an adversary to learn or modify a party’s inputs and outputs unless he hacks a party via the input port before it has received its (first) input (or gains control over all parties). Hence, our protocols protect inputs and outputs against all remote attacks, except for hacks via the input port while a party is waiting for input. To achieve this level of security, the parties’ inputs and outputs are authenticated, masked and shared in our protocols in such a way that an adversary is unable to learn or modify them when gaining control over a party via a remote hack. It is important to note that the remotely unhackable hardware modules applied in this work are based on substantially weaker assumptions than the hardware tokens proposed by Katz at EUROCRYPT ‘07. In particular, they are not assumed to be physically tamper-proof, can thus not be passed to other (possibly malicious) parties, and are therefore not sufficient to circumvent the impossibility results in the Universal Composability (UC) framework. Our protocols therefore rely on well-established UC-complete setup assumptions in tandem with our remotely unhackable hardware modules to achieve composability

Composable Long-Term Security with Rewinding

Long-term security, a variant of Universally Composable (UC) security introduced by Müller-Quade and Unruh (JoC ’10), allows to analyze the security of protocols in a setting where all hardness assumptions no longer hold after the protocol execution has finished. Such a strict notion is highly desirable when properties such as input privacy need to be guaranteed for a long time, e.g. zero-knowledge proofs for secure electronic voting. Strong impossibility results rule out so-called long-term-revealing setups, e.g. a common reference string (CRS), to achieve long-term security, with known constructions for long-term security requiring hardware assumptions, e.g. signature cards. We circumvent these impossibility results by making use of new techniques, allowing rewinding-based simulation in a way that universal composability is possible. The new techniques allow us to construct a long-term-secure composable commitment scheme in the CRS-hybrid model, which is provably impossible in the notion of Müller-Quade and Unruh. We base our construction on a statistically hiding commitment scheme in the CRS-hybrid model with CCA-like properties. To provide a CCA oracle, we cannot rely on superpolynomial extraction techniques, as statistically hiding commitments do not define a unique value. Thus, we extract the value committed to via rewinding. However, even a CCA “rewinding oracle” without additional properties may be useless, as extracting a malicious committer could require to rewind other protocols the committer participates in. If this is e.g. a reduction, this clearly is forbidden. Fortunately, we can establish the well-known and important property of k-robust extractability, which guarantees that extraction is possible without rewinding k-round protocols the malicious committer participates in. While establishing this property for statistically binding commitment schemes is already non-trivial, it is even more complicated for statistically hiding ones. We then incorporate rewinding-based commitment extraction into the UC framework via a helper in analogy to Canetti, Lin and Pass (FOCS 2010), allowing both adversary and environment to extract statistically hiding commitments. Despite the rewinding, our variant of long-term security is universally composable. Our new framework provides the first setting in which a commitment scheme that is both statistically hiding and composable can be constructed from standard polynomial-time hardness assumptions and a CRS only. Unfortunately, we can prove that our setting does not admit long-term-secure oblivious transfer (and thus general two-party computations). Still, our long-term-secure commitment scheme suffices for natural applications, such as long-term secure and composable (commit-and-prove) zero-knowledge arguments of knowledge

PaaSword: A Data Privacy and Context-aware Security Framework for Developing Secure Cloud Applications - Technical and Scientific Contributions

Most industries worldwide have entered a period of reaping the benefits and opportunities cloud offers. At the same time, many efforts are made to address engineering challenges for the secure development of cloud systems and software.With the majority of software engineering projects today relying on the cloud, the task to structure end-to-end secure-by-design cloud systems becomes challenging but at the same time mandatory. The PaaSword project has been commissioned to address security and data privacy in a holistic way by proposing a context-aware security-by-design framework to support software developers in constructing secure applications for the cloud. This chapter presents an overview of the PaaSword project results, including the scientific achievements as well as the description of the technical solution. The benefits offered by the framework are validated through two pilot implementations and conclusions are drawn based on the future research challenges which are discussed in a research agenda