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An emerging direction for authenticating people is the adoption of biometric authentication systems. Biometric credentials are becoming increasingly popular as a means of authenticating people due to the wide range of advantages that they provide with respect to classical authentication methods (e.g., password-based authentication). The most characteristic feature of this authentication method is the naturally strong bond between a user and her biometric credentials. This very same advantageous property, however, raises serious security and privacy concerns in case the biometric trait gets compromised. In this article, we present the most challenging issues that need to be taken into consideration when designing secure and privacypreserving biometric authentication protocols. More precisely, we describe the main threats against privacy-preserving biometric authentication systems and give directions on possible countermeasures in order to design secure and privacy-preserving biometric authentication protocols.

Homomorphic authenticators (HAs) enable a client to authenticate a large collection of data elements m1,. .. , mt and outsource them, along with the corresponding authenticators, to an untrusted server. At any later point, the server can generate a short authenticator σ f,y vouching for the correctness of the output y of a function f computed on the outsourced data, i.e., y = f (m1,. .. , mt). Recently researchers have focused on HAs as a solution, with minimal communication and interaction, to the problem of delegating computation on outsourced data. The notion of HAs studied so far, however, only supports executions (and proofs of correctness) of computations over data authenticated by a single user. Motivated by realistic scenarios (ubiquitous computing, sensor networks, etc.) in which large datasets include data provided by multiple users, we study the concept of multi-key homomorphic authenticators. In a nutshell, multi-key HAs are like HAs with the extra feature of allowing the holder of public evaluation keys to compute on data authenticated under different secret keys. In this paper, we introduce and formally define multi-key HAs. Secondly, we propose a construction of a multi-key homomorphic signature based on standard lattices and supporting the evaluation of circuits of bounded polynomial depth. Thirdly, we provide a construction of multi-key homomorphic MACs based only on pseudorandom functions and supporting the evaluation of low-degree arithmetic circuits. Albeit being less expressive and only secretly verifiable, the latter construction presents interesting efficiency properties.

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Authentication for resource-constrained devices is seen as one of the major challenges in current wireless communication networks. The HB + protocol by Juels and Weis provides device authentication based on the learning parity with noise (LPN) problem and is appropriate for resource-constrained devices, but it has been shown to be vulnerable to a simple man-in-the-middle attack. Subsequent work has focused on modifying the cryptographic properties of the original protocol to mitigate this problem. We propose that this attack could be mitigated using physical layer measures from distance-bounding protocols and simple modifications to devices' radio receivers. We take the HB + as a reference protocol and combine it with distance-bounding techniques. This hybrid solution, the HB + DB protocol is shown to provide resistance against the man-in-the-middle attacks on HB + as a result of the additional physical-layer mechanisms. We analyze the security of the proposed HB + DB protocol against active man-in-the-middle attacks and present experiments showing how it is practically possible to limit the success of a practical man-inthe-middle attack. We also briefly discuss the possibility that HB + DB could provide some resistance to basic threats scenarios meant to be mitigated by distance-bounding protocols. We make a practical implementation to verify that our proposed method is feasible. Finally, we discuss a proof-of-concept channel for our scheme implemented on a platform equivalent in resources to a contactless smart card/NFC device.

Authentication for resource-constrained devices is seen as one of the major challenges in current wireless communication networks. The HB + protocol performs device authentication based on the learning parity with noise (LPN) problem and simple computational steps, that renders it suitable for resource-constrained devices such as radio frequency identification (RFID) tags. However, it has been shown that the HB + protocol as well as many of its variants are vulnerable to a simple man-in-the-middle attack. We demonstrate that this attack could be mitigated using physical layer measures from distance-bounding and simple modifications to devices' radio receivers. Our hybrid solution (HB + DB) is shown to provide both effective distance-bounding using a lightweight HB + -based response function, and resistance against the man-in-the-middle attack to HB + . We provide experimental evaluation of our results as well as a brief discussion on practical requirements for secure implementation.

As messaging applications are becoming increasingly popular, it is of utmost importance to analyze their security and mitigate existing weaknesses. This paper focuses on one of the most acclaimed messaging applications: Signal.Signal is a protocol that provides end-to-end channel security, forward secrecy, and post-compromise security. These features are achieved thanks to a key-ratcheting mechanism that updates the key material at every message. Due to its high security impact, Signal's key-ratcheting has recently been formalized, along with an analysis of its security.In this paper, we revisit Signal, describing some attacks against the original design and proposing SAID: Signal Authenticated and IDentity-based. As the name indicates, our protocol relies on an identity-based setup, which allows us to dispense with Signal's centralized server. We use the identity-based long-term secrets to obtain persistent and explicit authentication, such that SAID achieves higher security guarantees than Signal.We prove the security of SAID not only in the Authenticated Key Exchange (AKE) model (as done by previous work), but also in the Authenticated and Confidential Channel Establishment (ACCE) model, which we adapted and redefined for SAID and asynchronous messaging protocols in general into a model we call identity-based Multistage Asynchronous Messaging (iMAM). We believe our model to be more faithful in particular to the true security of Signal, whose use of the message keys prevents them from achieving the composable guarantee claimed by previous analysis.

Abstract-Distance-bounding protocols allow devices to cryptographically verify the physical proximity of two parties and is a prominent secure neighbour detection method. We describe how existing distance-bounding protocols could be modified to verify the proximity of both next-hop and two-hop neighbours. This approach allows a node to verify that another node is a physical next-hop neighbour, and also detects legitimate neighbours who make dishonest claims as to who their neighbours are. This approach could prevent dishonest neighbours from hoarding traffic as the result of advertising false two-hop routes.Index Terms-Wireless sensor network, distance-bounding, secure neighbour discovery, wormhole attack.