ERASMUS: Efficient Remote Attestation via Self- Measurement for Unattended Settings

Remote attestation (RA) is a popular means of detecting malware in embedded and IoT devices. RA is usually realized as an interactive protocol, whereby a trusted party -- verifier -- measures integrity of a potentially compromised remote device -- prover. Early work focused on purely software-based and fully hardware-based techniques, neither of which is ideal for low-end devices. More recent results have yielded hybrid (SW/HW) security architectures comprised of a minimal set of features to support efficient and secure RA on low-end devices. All prior RA techniques require on-demand operation, i.e, RA is performed in real time. We identify some drawbacks of this general approach in the context of unattended devices: First, it fails to detect mobile malware that enters and leaves the prover between successive RA instances. Second, it requires the prover to engage in a potentially expensive (in terms of time and energy) computation, which can be harmful for critical or real-time devices. To address these drawbacks, we introduce the concept of self-measurement where a prover device periodically (and securely) measures and records its own software state, based on a pre-established schedule. A possibly untrusted verifier occasionally collects and verifies these measurements. We present the design of a concrete technique called ERASMUS : Efficient Remote Attestation via Self-Measurement for Unattended Settings, justify its features and evaluate its performance. In the process, we also define a new metric -- Quality of Attestation (QoA). We argue that ERASMUS is well-suited for time-sensitive and/or safety-critical applications that are not served well by on-demand RA. Finally, we show that ERASMUS is a promising stepping stone towards handling attestation of multiple devices (i.e., a group or swarm) with high mobility

Retraction: Eldefrawy, M.H.; Khan, M.K.; Alghathbar, K.; Tolba, A.S.; Kim, K.J. Authenticated Key Agreement with Rekeying for Secured Body Sensor Networks. Sensors 2011, 11, 5835–5849.

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Communication-Efficient Proactive MPC for Dynamic Groups with Dishonest Majorities

International audienceSecure multiparty computation (MPC) has recently been increasingly adopted to secure cryptographic keys in enterprises, cloud infrastructure, and cryptocurrency and blockchain-related settings such as wallets and exchanges. Using MPC in blockchains and other distributed systems highlights the need to consider dynamic settings. In such dynamic settings, parties, and potentially even parameters of underlying secret sharing and corruption tolerance thresholds of sub-protocols, may change over the lifetime of the protocol. In particular, stronger threat models-in which mobile adversaries control a changing set of parties (up to t out of n involved parties at any instant), and may eventually corrupt all n parties over the course of a protocol's execution-are becoming increasingly important for such real world deployments; secure protocols designed for such models are known as Proactive MPC (PMPC). In this work, we construct the first efficient PMPC protocol for dynamic groups (where the set of parties changes over time) secure against a dishonest majority of parties. Our PMPC protocol only requires O(n 2) (amortized) communication per secret, compared to existing PMPC protocols that require O(n 4) and only consider static groups with dishonest majorities. At the core of our PMPC protocol is a new efficient technique to perform multiplication of secret shared data (shared using a bivariate scheme) with O(n √ n) communication with security against a dishonest majority without requiring pre-computation. We also develop a new efficient bivariate batched proactive secret sharing (PSS) protocol for dishonest majorities, which may be of independent interest. This protocol enables multiple dealers to contribute different secrets that are efficiently shared together in one batch; previous batched PSS schemes required all secrets to come from a single dealer

On Regenerating Codes and Proactive Secret Sharing: Relationships and Implications

We look at two basic coding theoretic and cryptographic mechanisms developed separately and investigate relationships between them and their implications. The first mechanism is Proactive Secret Sharing (PSS), which allows randomization and repair of shares using information from other shares. PSS enables constructing secure multi-party computation protocols that can withstand mobile dynamic attacks. This self-recovery and the redundancy of uncorrupted shares allows a system to overcome recurring faults throughout its lifetime, eventually finishing the computation (or continuing forever to maintain stored data). The second mechanismis Regenerating Codes (RC) which were extensively studied and adopted in distributed storage systems. RC are error correcting (or erasure handling) codes capable of recovering a block of a distributively held codeword from other servers\u27 blocks. This self-healing nature enables more robustness of a code distributed over different machines. Given that the two mechanisms have a built-in self-healing (leading to stabilizing) and that both can be based on Reed Solomon Codes, it is natural to formally investigate deeper relationships between them. We prove that a PSS scheme can be converted into an RC scheme, and that under some conditions RC can be utilized to instantiate a PSS scheme. This allows us, in turn, to leverage recent results enabling more efficient polynomial interpolation (due to Guruswami and Wooters) to improve the efficiency of a PSS scheme. We also show that if parameters are not carefully calibrated, such interpolation techniques (allowing partial word leakage) may be used to attack a PSS scheme over time. Secondly, the above relationships give rise to extended (de)coding notions. Our first example is mapping the generalized capabilities of adversaries (called generalized adversary structures) from the PSS realm into the RC one. Based on this we define a new variant of RC we call Generalized-decoding Regenerating Code (GRC) where not all network servers have a uniform sub-codeword (motivated by non-uniform probability of attacking different servers case). We finally highlight several interesting research directions due to our results, e.g., designing new improved GRC, and more adaptive RC re-coding techniques

Standard Model Time-Lock Puzzles: Defining Security and Constructing via Composition

The introduction of time-lock puzzles initiated the study of publicly “sending information into the future.” For time-lock puzzles, the underlying security-enabling mechanism is the computational complexity of the operations needed to solve the puzzle, which must be tunable to reveal the solution after a predetermined time, and not before that time. Time-lock puzzles are typically constructed via a commitment to a secret, paired with a reveal algorithm that sequentially iterates a basic function over such commitment. One then shows that short-cutting the iterative process violates cryptographic hardness of an underlying problem. To date, and for more than twenty-five years, research on time-lock puzzles relied heavily on iteratively applying well-structured algebraic functions. However, despite the tradition of cryptography to reason about primitives in a realistic model with standard hardness assumptions (often after initial idealized assumptions), most analysis of time-lock puzzles to date still relies on cryptography modeled (in an ideal manner) as a random oracle function or a generic group function. Moreover, Mahmoody et al. showed that time-lock puzzles with superpolynomial gap cannot be constructed from random-oracles; yet still, current treatments generally use an algebraic trapdoor to efficiently construct a puzzle with a large time gap, and then apply the inconsistent (with respect to Mahmoody et al.) random-oracle idealizations to analyze the solving process. Finally, little attention has been paid to the nuances of composing multi-party computation with timed puzzles that are solved as part of the protocol. In this work, we initiate a study of time-lock puzzles in a model built upon a realistic (and falsifiable) computational framework. We present a new formal definition of residual complexity to characterize a realistic, gradual time-release for time-lock puzzles. We also present a general definition of timed multi-party computation (MPC) and both sequential and concurrent composition theorems for MPC in our model

Efficient, Reusable Fuzzy Extractors from LWE

A fuzzy extractor (FE), proposed for deriving cryptographic keys from biometric data, enables reproducible generation of high-quality randomness from noisy inputs having sufficient min-entropy. FEs rely in their operation on a public helper string that is guaranteed not to leak too much information about the original input. Unfortunately, this guarantee may not hold when multiple independent helper strings are generated from correlated inputs as would occur if a user registers their biometric data with multiple servers; reusable FEs are needed in that case. Although the notion of reusable FEs was introduced in 2004, it has received relatively little attention since then. We first analyze an FE proposed by Fuller et al. (Asiacrypt 2013) based on the learning-with-errors (LWE) assumption, and show that it is not reusable. We then show how to adapt their construction to obtain a weakly reusable FE. We also show a generic technique for turning any weakly reusable FE to a strongly reusable one, in the random-oracle model. Finally, we give a direct construction of a strongly reusable FE based on the LWE assumption, that does not rely on random oracles