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08:30-09:00Coffee & Refreshments
09:00-10:30 Session 94B: Voting and Distributed Systems
Location: Taub 2
N-Tube: Formally Verified Secure Bandwidth Reservation in Path-Aware Internet Architectures
PRESENTER: Thilo Weghorn

ABSTRACT. We present N-Tube, a novel, provably secure, inter-domain bandwidth reservation algorithm that runs on a network architecture supporting path-based forwarding. N-Tube reserves global end-to-end bandwidth along network paths in a distributed, neighbor-based, and tube-fair way. It guarantees that benign bandwidth demands are granted available allocations that are immutable, stable, lower-bounded, and fair, even during adversarial demand bursts.

We formalize N-Tube and powerful adversaries as a labeled transition system, and inductively prove its safety and security properties. We also apply statistical model checking to validate our proofs and perform an additional quantitative assessment of N-Tube, providing strong guarantees for protection against DDoS attacks. We are not aware of any other complex networked system designs that have been subjected to a comparable analysis of both their qualitative properties (such as correctness and security) and their quantitative properties (such as performance).

Applying consensus and replication securely with FLAQR
PRESENTER: Priyanka Mondal

ABSTRACT. Availability is crucial to the security of distributed systems, but guaranteeing availability is hard, especially when participants in the system may act maliciously. Quorum replication protocols provide both integrity and availability: data and computation is replicated at multiple independent hosts, and a quorum of these hosts must agree on the output of all operations applied to the data. Unfortunately, these protocols have high overhead and can be difficult to calibrate for a specific application’s needs. Ideally, developers could use high-level abstractions for consensus and replication to write faulttolerant code by that is secure by construction.

This paper presents Flow-Limited Authorization for Quorum Replication (FLAQR), a core calculus for building distributed applications with heterogeneous quorum replication protocols while enforcing end-to-end information security. Our type system ensures that well-typed FLAQR programs cannot fail (experience an unrecoverable error) in ways that violate their typelevel specifications. We present noninterference theorems that characterize FLAQR’s confidentiality, integrity, and availability in the presence of consensus, replication, and failures, as well as a liveness theorem for the class of majority quorum protocols under a bounded number of faults.

How Efficient are Replay Attacks against Vote Privacy? A Formal Quantitative Analysis
PRESENTER: Johannes Müller

ABSTRACT. Replay attacks are among the most well-known attacks against vote privacy. Many e-voting systems have been proven vulnerable to replay attacks, including systems like Helios that are used in real practical elections. Despite their popularity, it is commonly believed that replay attacks are inefficient but the actual threat that they pose to vote privacy has never been studied formally. Therefore, in this paper, we precisely analyze for the first time how efficient replay attacks really are. We study this question from commonly used and complementary perspectives on vote privacy, showing as an independent contribution that a simple extension of a popular game-based privacy definition corresponds to a strong entropy-based notion. Our results demonstrate that replay attacks can be devastating for a voter’s privacy even when an adversary’s resources are very limited. We illustrate our formal findings by applying them to a number of real-world elections, showing that a modest number of replays can result in significant privacy loss. Overall, our work reveals that, contrary to a common belief, replay attacks can be very efficient and must therefore be considered a serious threat.

10:30-11:00Coffee Break
11:00-12:30 Session 96B: Cryptography 1
Location: Taub 2
Contingent payments from two-party signing and verification for abelian groups
PRESENTER: Sergiu Bursuc

ABSTRACT. The fair exchange problem has faced for a long time the bottleneck of a required trusted third party. The recent development of blockchains introduces a new type of party to this problem, whose trustworthiness relies on a public ledger and distributed computation. The challenge in this setting is to reconcile the minimalistic and public nature of blockchains with elaborate fair exchange requirements, from functionality to privacy. Zero-knowledge contingent payments (ZKCP) are a class of protocols that are promising in this direction, allowing the fair exchange of data for payment. We propose a new ZKCP protocol that, when compared to others, requires less computation from the blockchain and less interaction between parties. The protocol is based on two-party (weak) adaptor signatures, which we show how to instantiate from state of the art multiparty signing protocols. We improve the symbolic definition of ZKCP security and, for automated verification with Tamarin, we propose a general security reduction from the theory of abelian groups to the theory of exclusive or.

Collusion-Preserving Computation without a Mediator

ABSTRACT. Collusion-free (CF) and collusion-preserving (CP) protocols enrich the standard security offered by multi-party computation (MPC), to tackle settings where subliminal communication is undesirable. However, all existing solutions make arguably unrealistic assumptions on setups, such as physical presence of the parties, access to physical envelopes, or extreme isolation, where the only means of communication is a star-topology network. The above state of affairs remained a limitation of such protocols, which was even reinforced by impossibility results. Thus, for years, it has been unclear if and how the above setup assumptions could be relaxed towards more realistic scenarios. Motivated also by the increasing interest in using hardware tokens for cryptographic applications, in this work we provide the first solution to collusion preserving computation which uses weaker and more common assumptions than the state of the art, i.e., an authenticated broadcast functionality and access to honestly generated trusted hardware tokens. We prove that our protocol is collusion-preserving (in short, CP) secure as long as no parties abort. In the case of an aborting adversary, our protocol still achieves standard (G)UC security with identifiable (and unanimous) abort.

Leveraging the above identifiability property, we augment our protocol with a penalization scheme which ensures that it is not profitable to abort, thereby obtaining CP security against incentive-driven attackers. To define (and prove) this latter result, we combine the Rational Protocol Design (RPD) methodology by Garay et al. [FOCS 2013] with the CP framework of Alwen et al. [CRYPTO 2012] to derive a definition of security in the presence of incentive-driven local adversaries which can be of independent interest. Similar to existing CP/CF solutions, our protocol preserves, as a fallback, security against monolithic adversaries, even when the setup (i.e., the hardware tokens) is compromised or corrupted. In addition, our fallback solution achieves identifiable and unanimous abort, which we prove are impossible in previous CP solutions.

Bringing State-Separating Proofs to EasyCrypt - A Security Proof for Cryptobox
PRESENTER: Sabine Oechsner

ABSTRACT. Machine-checked cryptography aims to reinforce confidence in the primitives and protocols that underpin all digital security. However, machine-checked proof techniques remain in practice difficult to apply to real-world constructions. A particular challenge is structured reasoning about complex constructions at different levels of abstraction. The State-Separating Proofs (SSP) methodology for guiding cryptographic proofs by Brzuska, Delignat-Lavaud, Fournet, Kohbrok and Kohlweiss (ASIACRYPT'18) is a promising contestant to support such reasoning. In this work, we explore how SSPs can guide EasyCrypt formalisations of proofs for modular constructions. Concretely, we propose a mapping from SSP to EasyCrypt concepts which enables us to enhance cryptographic proofs with SSP insights while maintaining compatibility with existing EasyCrypt proof support. To showcase our insights, we develop a formal security proof for the cryptobox family of public-key authenticated encryption schemes based on non-interactive key exchange and symmetric authenticated encryption. As a side effect, we obtain the first formal security proof for NaCl's instantiation of cryptobox. Finally we discuss changes to the practice of SSP on paper and potential implications for future tool designers.

12:30-14:00Lunch Break

Lunches will be held in Taub lobby (CAV, CSF) and in The Grand Water Research Institute (DL, NMR, IJCAR, ITP).

14:00-15:30 Session 97B: Invited Talk 1
Location: Taub 2
Membership Inference Attacks against Classifiers

ABSTRACT. In membership inference (MI) attacks, the adversary is given access to a target classifier and a number of data instances, and aims to determine whether these data instances have been used when training the classifier. Given the close relationship between membership inference and differential privacy, the degree of vulnerability to MI attacks is an excellent empirical measure of a classifier's level of privacy.

MI attacks can be studied in different settings depending on how the adversary is allowed to access a model, e.g., blacbox access to the target classifier, access to model parameters in the classifier, access to gradients during the training in the classifier such as in a federated learning setting, and the ability to actively manipulate the training process to conduct membership inference.

In this talk, we will discuss existing results on MI attacks and defense mechanisms in these settings, and discuss open problems.

15:30-16:00Coffee Break
18:30-20:30 Walking tour (at Haifa)

pickup at 18:00 from the Technion (Tour at Haifa, no food will be provided)