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1. Password-Authenticated TLS via OPAQUE and Post-Handshake Authentication

   https://doi.org/10.1007/978-3-031-30589-4_4  

   Hesse, Julia; Jarecki, Stanislaw; Krawczyk, Hugo; Wood, Christopher (April 2023, Lecture notes in computer science: Advances in Cryptology – EUROCRYPT 2023. EUROCRYPT 2023)

   Hazay, Carmit; Stam, Martin (Ed.)

   OPAQUE is an Asymmetric Password-Authenticated Key Exchange (aPAKE) protocol being standardized by the IETF (Internet Engineering Task Force) as a more secure alternative to the traditional “password-over-TLS” mechanism prevalent in current practice. OPAQUE defends against a variety of vulnerabilities of password-over-TLS by dispensing with reliance on PKI and TLS security, and ensuring that the password is never visible to servers or anyone other than the client machine where the password is entered. In order to facilitate the use of OPAQUE in practice, integration of OPAQUE with TLS is needed. The main proposal for standardizing such integration uses the Exported Authenticators (TLS-EA) mechanism of TLS 1.3 that supports post-handshake authentication and allows for a smooth composition with OPAQUE. We refer to this composition as TLS-OPAQUE and present a detailed security analysis for it in the Universal Composability (UC) framework. Our treatment is general and includes the formalization of components that are needed in the analysis of TLS-OPAQUE but are of wider applicability as they are used in many protocols in practice. Specifically, we provide formalizations in the UC model of the notions of post-handshake authentication and channel binding. The latter, in particular, has been hard to implement securely in practice, resulting in multiple protocol failures, including major attacks against prior versions of TLS. Ours is the first treatment of these notions in a computational model with composability guarantees. We complement the theoretical work with a detailed discussion of practical considerations for the use and deployment of TLS-OPAQUE in real-world settings and applications. 

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2. CompoSAT: Specification-Guided Coverage for Model Finding

   Porncharoenwase, Sorawee; Nelson, Tim; Krishnamurthi, Shriram (July 2018, Lecture notes in computer science)

   Model-finding tools like the Alloy Analyzer produce concrete examples of how a declarative specification can be satisfied. These formal tools are useful in a wide range of domains: software design, security, networking, and more. By producing concrete examples, they assist in exploring system behavior and can help find surprising faults. Specifications usually have many potential candidate solutions, but model- finders tend to leave the choice of which examples to present entirely to the underlying solver. This paper closes that gap by exploring notions of coverage for the model-finding domain, yielding a novel, rigorous metric for output quality. These ideas are realized in the tool CompoSAT, which interposes itself between Alloy’s constraint-solving and presentation stages to produce ensembles of examples that maximize coverage. We show that high-coverage ensembles like CompoSAT produces are useful for, among other things, detecting overconstraint—a particularly insidious form of specification error. We detail the underlying theory and implementation of CompoSAT and evaluate it on numerous specifications. 

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3. On Locally Decodable Codes in Resource Bounded Channels

   https://doi.org/10.4230/LIPIcs.ITC.2020.16  

   Blocki, J; Kulkarni, S; Zhou, S. (January 2020, Leibniz international proceedings in informatics)

   Yael Tauman Kalai and Adam D. Smith and Daniel Wichs (Ed.)

   Constructions of locally decodable codes (LDCs) have one of two undesirable properties: low rate or high locality (polynomial in the length of the message). In settings where the encoder/decoder have already exchanged cryptographic keys and the channel is a probabilistic polynomial time (PPT) algorithm, it is possible to circumvent these barriers and design LDCs with constant rate and small locality. However, the assumption that the encoder/decoder have exchanged cryptographic keys is often prohibitive. We thus consider the problem of designing explicit and efficient LDCs in settings where the channel is slightly more constrained than the encoder/decoder with respect to some resource e.g., space or (sequential) time. Given an explicit function f that the channel cannot compute, we show how the encoder can transmit a random secret key to the local decoder using f(⋅) and a random oracle 𝖧(⋅). We then bootstrap the private key LDC construction of Ostrovsky, Pandey and Sahai (ICALP, 2007), thereby answering an open question posed by Guruswami and Smith (FOCS 2010) of whether such bootstrapping techniques are applicable to LDCs in channel models weaker than just PPT algorithms. Specifically, in the random oracle model we show how to construct explicit constant rate LDCs with locality of polylog in the security parameter against various resource constrained channels. 

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4. A Hybrid Approach to Secure Function Evaluation using SGX

   https://doi.org/10.1145/3321705.3329835  

   Choi, Joseph I.; Tian, Dave; Hernandez, Grant; Patton, Christopher; Mood, Benjamin; Shrimpton, Thomas; Butler, Kevin R.; Traynor, Patrick (July 2019, ACM Asia Conference on Computer and Communications Security)

   A protocol for two-party secure function evaluation (2P-SFE) aims to allow the parties to learn the output of function f of their private inputs, while leaking nothing more. In a sense, such a protocol realizes a trusted oracle that computes f and returns the result to both parties. There have been tremendous strides in efficiency over the past ten years, yet 2P-SFE protocols remain impractical for most real-time, online computations, particularly on modestly provisioned devices. Intel's Software Guard Extensions (SGX) provides hardware-protected execution environments, called enclaves, that may be viewed as trusted computation oracles. While SGX provides native CPU speed for secure computation, previous side-channel and micro-architecture attacks have demonstrated how security guarantees of enclaves can be compromised. In this paper, we explore a balanced approach to 2P-SFE on SGX-enabled processors by constructing a protocol for evaluating f relative to a partitioning of f. This approach alleviates the burden of trust on the enclave by allowing the protocol designer to choose which components should be evaluated within the enclave, and which via standard cryptographic techniques. We describe SGX-enabled SFE protocols (modeling the enclave as an oracle), and formalize the strongest-possible notion of 2P-SFE for our setting. We prove our protocol meets this notion when properly realized. We implement the protocol and apply it to two practical problems: privacy-preserving queries to a database, and a version of Dijkstra's algorithm for privacy-preserving navigation. Our evaluation shows that our SGX-enabled SFE scheme enjoys a 38x increase in performance over garbled-circuit-based SFE. Finally, we justify modeling of the enclave as an oracle by implementing protections against known side-channels. 

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5. Breaking and Fixing Virtual Channels: Domino Attack and Donner

   Aumayr, Lukas; Moreno-Sanchez, Pedro; Kate, Aniket; Maffei, Matteo. (January 2023, Network and Distributed System Security (NDSS) Symposium)

   Payment channel networks (PCNs) mitigate the scalability issues of current decentralized cryptocurrencies. They allow for arbitrarily many payments between users connected through a path of intermediate payment channels, while requiring interacting with the blockchain only to open and close the channels. Unfortunately, PCNs are (i) tailored to payments, excluding more complex smart contract functionalities, such as the oracle-enabling Discreet Log Contracts and (ii) their need for active participation from intermediaries may make payments unreliable, slower, expensive, and privacy-invasive. Virtual channels are among the most promising techniques to mitigate these issues, allowing two endpoints of a path to create a direct channel over the intermediaries without any interaction with the blockchain. After such a virtual channel is constructed, (i) the endpoints can use this direct channel for applications other than payments and (ii) the intermediaries are no longer involved in updates. In this work, we first introduce the Domino attack, a new DoS/griefing style attack that leverages virtual channels to destruct the PCN itself and is inherent to the design adopted by the existing Bitcoin-compatible virtual channels. We then demonstrate its severity by a quantitative analysis on a snapshot of the Lightning Network (LN), the most widely deployed PCN at present. We finally discuss other serious drawbacks of existing virtual channel designs, such as the support for only a single intermediary, a latency and blockchain overhead linear in the path length, or a non-constant storage overhead per user. We then present Donner, the first virtual channel construction that overcomes the shortcomings above, by relying on a novel design paradigm. We formally define and prove security and privacy properties in the Universal Composability framework. Our evaluation shows that Donner is efficient, reduces the on-chain number of transactions for disputes from linear in the path length to a single one, which is the key to prevent Domino attacks, and reduces the storage overhead from logarithmic in the path length to constant. Donner is Bitcoin-compatible and can be easily integrated in the LN. 

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