EDHOC is a lightweight authenticated key exchange protocol designed for constrained IoT devices. It currently supports asymmetric authentication, either using digital signatures or static Diffie-Hellman (DH) keys. Since many IoT deployments rely on Pre-Shared Keys (PSK) for authentication, a new PSK-based authentication method (EDHOC-PSK) is currently under standardization. This paper presents a symbolic analysis EDHOC-PSK (draft version 06) using SAPIC+, which compiles a single formal specification into multiple state-of-the-art verification tools, including Tamarin and ProVerif. Our model extends the typical Dolev-Yao (DY) adversary with additional capabilities, including leakage of ephemeral secrets, leakage of the long-term Pre-Shared Key leakage of the session key and a discrete-logarithm oracle. We verify the confidentiality, authentication, and key-agreement properties stated in the draft, and we refine the specification of identity protection by distinguishing anonymity and unlinkability. We show that EDHOC-PSK achieves anonymity for both parties against active attackers, while unlinkability holds only for the Initiator under passive attackers. Finally, we analyze a post-quantum Store-Now-Decrypt-Later (SNDL) adversary and find that all proven properties remain intact except Perfect Forward Secrecy (PFS), which cannot be preserved once DH secrets are recoverable.

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Subversion-resilient Authenticated key-exchange (AKE) aims to achieve the guarantees of secure AKE even in the presence of an adversary that has tampered with parts of the protocol's implementation. One way to achieve subversion-resilient AKE is the use of Reverse Firewalls (RFs), an untrusted third-party that can restore security. Recent work[17] highlights the challenges of designing RFs for practical secure channel-establishment.

The virtualization of network functions is a promising technology, which can enable mobile network operators to provide more flexibility and better resilience for their infrastructure and services. Yet, virtualization comes with challenges, as 5 G operators will require a means of verifying the state of the virtualized network components (e.g., Virtualized Network Functions (VNFs) or managing hypervisors) in order to fulfil security and privacy commitments. One such means is the use of attestation protocols. In this paper, we focus on Collective Remote Attestation (cRA), which is used to attest the state of a group of devices. Although cRA has been extensively studied in the context of IoT, it has not been used yet in virtualized mobile networks - a different use-case, with constraints of its own. In this paper, we propose the first protocol to efficiently and securely attest a group of Virtualized Network Functions which make up a VNF Forwarding Graph. Our protocol comes with strong and provable guarantees of: unforgeability of attestation, the linkability of attestations for related components, and the privacy of sensitive configuration details for the infrastructure provider. In particular, we are the first to formally define and analyze such properties for VNF-FG attestation. Finally, through our Proof-of-Concept implementation, we show that our construction is not only strongly secure, but also efficient.

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.

TLS (Transport Layer Security) is a widely deployed protocol that plays a vital role in securing Internet traffic. Given the numerous known attacks for TLS 1.2, it was imperative to change and even redesign the protocol in order to address them. In August 2018, a new version of the protocol, TLS 1.3, was standardized by the IETF (Internet Engineering Task Force). TLS 1.3 not only benefits from stronger security guarantees, but aims to protect the identities of the server and client by encrypting messages as soon as possible during the authentication. In this paper, we model the privacy guarantees of TLS 1.3 when parties execute a full handshake or use a session resumption, covering all the handshake modes of TLS. We build our privacy models on top of the one defined by Hermans et al. for RFIDs (Radio Frequency Identification Devices) that mostly targets authentication protocols. The enhanced models share similarities to the Bellare-Rogaway AKE (Authenticated Key Exchange) security model and consider adversaries that can compromise both types of participants in the protocol. In particular, modeling session resumption is non-trivial, given that session resumption tickets are essentially a state transmitted from one session to another and such link reveals information on the parties. On the positive side, we prove that TLS 1.3 protects the privacy of its users at least against passive adversaries, contrary to TLS 1.2, and against more powerful ones.

Much of Internet traffic nowadays passes through active proxies, whose role is to inspect, filter, cache, or transform data exchanged between two endpoints. To perform their tasks, such proxies modify channel-securing protocols, like TLS, resulting in serious vulnerabilities. Such problems are exacerbated by the fact that middleboxes are often invisible to one or both endpoints, leading to a lack of accountability. A recent protocol, called mcTLS, pioneered accountability for proxies, which are authorized by the endpoints and given limited read/write permissions to application traffic. Unfortunately, we show that mcTLS is insecure: the protocol modifies the TLS protocol, exposing it to a new class of middlebox-confusion attacks. Such attacks went unnoticed mainly because mcTLS lacked a formal analysis and security proofs. Hence, our second contribution is to formalize the goal of accountable proxying over secure channels. Third, we propose a provably-secure alternative to soon-to-be-standardized mcTLS: a generic and modular protocol-design that care- fully composes generic secure channel-establishment protocols, which we prove secure. Finally, we present a proof-of-concept implementation of our design, instantiated with unmodified TLS 1.3, and evaluate its overheads.

Distance-bounding protocols have been introduced to thwart relay attacks against contactless authentication protocols. In this context, verifiers have to authenticate the credentials of untrusted provers. Unfortunately, these protocols are themselves subject to complex threats such as terrorist-fraud attacks, in which a malicious prover helps an accomplice to authenticate. Provably guaranteeing the resistance of distance-bounding protocols to these attacks is complex. The classical solutions assume that rational provers want to protect their long-term authentication credentials, even with respect to their accomplices. Thus, terrorist-fraud resistant protocols generally rely on artificial extraction mechanisms, ensuring that an accomplice can retrieve the credential of his partnering prover, if he is able to authenticate. We propose a novel approach to obtain provable terrorist-fraud resistant protocols that does not rely on an accomplice being able to extract any long-term key. Instead, we simply assume that he can replay the information received from the prover. Thus, rational provers should refuse to cooperate with third parties if they can impersonate them freely afterwards. We introduce a generic construction for provably secure distance-bounding protocols, and give three instances of this construction: (1) an efficient symmetric-key protocol, (2) a public-key protocol protecting the identities of provers against external eavesdroppers, and finally (3) a fully anonymous protocol protecting the identities of provers even against malicious verifiers that try to profile them.

The Transport Layer Security (TLS) protocol is designed to allow two parties, a client and a server, to communicate securely over an insecure network. However, when TLS connections are proxied through an intermediate middlebox, like a Content Delivery Network (CDN), the standard endto- end security guarantees of the protocol no longer apply. In this paper, we investigate the security guarantees provided by Keyless SSL, a CDN architecture currently deployed by CloudFlare that composes two TLS 1.2 handshakes to obtain a proxied TLS connection. We demonstrate new attacks that show that Keyless SSL does not meet its intended security goals. These attacks have been reported to CloudFlare and we are in the process of discussing fixes. We argue that proxied TLS handshakes require a new, stronger, 3-party security definition. We present 3(S)ACCEsecurity, a generalization of the 2-party ACCE security definition that has been used in several previous proofs for TLS. We modify Keyless SSL and prove that our modifications guarantee 3(S)ACCE-security, assuming ACCE-security for the individual TLS 1.2 connections. We also propose a new design for Keyless TLS 1.3 and prove that it achieves 3(S)ACCEsecurity, assuming that the TLS 1.3 handshake implements an authenticated 2-party key exchange. Notably, we show that secure proxying in Keyless TLS 1.3 is computationally lighter and requires simpler assumptions on the certificate infrastructure than our proposed fix for Keyless SSL. Our results indicate that proxied TLS architectures, as currently used by a number of CDNs, may be vulnerable to subtle attacks and deserve close attention.

HB+ is a lightweight authentication scheme, which is secure against passive attacks if the Learning Parity with Noise Problem (LPN) is hard. However, HB+ is vulnerable to a key-recovery, man-in-the-middle (MiM) attack dubbed GRS. The HB+DB protocol added a distance-bounding dimension to HB+, and was experimentally proven to resist the GRS attack.

Contactless communications have become omnipresent in our daily lives, from simple access cards to electronic passports. Such systems are particularly vulnerable to relay attacks, in which an adversary relays the messages from a prover to a verifier. Distance-bounding protocols were introduced to counter such attacks. Lately, there has been a very active research trend on improving the security of these protocols, but also on ensuring strong privacy properties with respect to active adversaries and malicious verifiers.

Abstract Proposed by the 3rd Generation Partnership Project (3GPP) as a standard for 3G and 4G mobile-network communications, the AKA protocol is meant to provide a mutually-authenticated key-exchange between clients and associated network servers. As a result AKA must guarantee the indistinguishability from random of the session keys (key-indistinguishability), as well as client- and server-impersonation resistance. A paramount requirement is also that of client privacy, which 3GPP defines in terms of: user identity confidentiality, service untraceability, and location untraceability. Moreover, since servers are sometimes untrusted (in the case of roaming), the AKA protocol must also protect clients with respect to these third parties. Following the description of client-tracking attacks e.g. by using error messages or IMSI catchers, van den Broek et al. and respectively Arapinis et al. each proposed a new variant of AKA, addressing such problems. In this paper we use the approach of provable security to show that these variants still fail to guarantee the privacy of mobile clients. We propose an improvement of AKA, which retains most of its structure and respects practical necessities such as key-management, but which provably attains security with respect to servers and Man-in-the- Middle (MiM) adversaries. Moreover, it is impossible to link client sessions in the absence of client-corruptions. Finally, we prove that any variant of AKA retaining its mutual authentication specificities cannot achieve client-unlinkability in the presence of corruptions. In this sense, our proposed variant is optimal.

In distance-bounding authentication protocols, a verifier assesses that a prover is (1) legitimate and (2) in the verifier's proximity. Proximity checking is done by running time-critical exchanges between both parties. This enables the verifier to detect relay attacks (also called mafia fraud). While most distance-bounding protocols offer resistance to mafia, distance, and impersonation attacks, only few protect the privacy of the authenticating prover. One exception is the protocol due to Hermans, Peeters, and Onete, which offers prover untraceability with respect to a Man-in-the-Middle adversary. However in this protocol as well as in all other distance-bounding protocols, any legitimate verifier can identify, and thus track, the prover. In order to counter the threats of possible corruption or data leakage from verifiers, we propose a distance-bounding protocol providing strong prover privacy with respect to the verifier and deniability with respect to a centralized back-end server managing prover creation and revocation. In particular, we first formalize the notion of prover anonymity, which guarantees that even verifiers cannot trace provers, and deniability, which allows provers to deny that they were authenticated by a verifier. Finally, we prove that our protocol achieves these strong guarantees.

Distance-bounding protocols prevent man-in-the-middle attacks by measuring response times. The four attacks such protocols typically address, recently formalized in [10], are: (1) mafia fraud, where the adversary must impersonate to a verifier in the presence of an honest prover; (2) terrorist fraud, where the adversary gets some offline prover support to impersonate; (3) distance fraud, where provers claim to be closer to verifiers than they really are; and (4) impersonations, where adversaries impersonate provers during lazy phases. Durholz et al. [10] also formally analyzed the security of (an enhancement of) the Kim-Avoine protocol [14].

We propose a new distance bounding protocol, which builds upon the private RFID authentication protocol by Peeters and Hermans [25]. In contrast to most distance-bounding protocols in literature, our construction is based on public-key cryptography. Public-key cryptography (specifically Elliptic Curve Cryptography) can, contrary to popular belief, be realized on resource constrained devices such as RFID tags. Our protocol is wide-forward-insider private, achieves distance-fraud resistance and near-optimal mafia-fraud resistance. Furthermore, it provides strong impersonation security even when the number of time-critical rounds supported by the tag is very small. The computational effort for the protocol is only four scalar-EC point multiplications. Hence the required circuit area is minimal because only an ECC coprocessor is needed: no additional cryptographic primitives need to be implemented.