International Association
for Cryptologic Research

<p>Space networking has become an increasing area of development with the advent of commercial satellite networks such as those hosted by Starlink and Kuiper, and increased satellite and space presence by governments around the world. Yet, historically such network designs have not been made public, leading to limited formal cryptographic analysis of the security offered by them. One of the few public protocols used in space networking is the Bundle Protocol, which is secured by Bundle Protocol Security (BPSec), an Internet Engineering Task Force (IETF) standard. We undertake a first analysis of BPSec under its default security context, building a model of the secure channel security goals stated in the IETF standard, and note issues therein with message loss detection. We prove BPSec secure, and also provide a stronger construction, one that supports the Bundle Protocol's functionality goals while also ensuring destination awareness of missing message components.</p>

WhatsApp provides end-to-end encrypted messaging to over two billion users. However, due to a lack of public documentation and source code, the specific security guarantees it provides are unclear. Seeking to rectify this situation, we combine the limited public documentation with information we gather through reverse-engineering its implementation to provide a formal description of the subset of WhatsApp that provides multi-device group messaging. We utilise this description to state and prove the security guarantees that this subset of WhatsApp provides. Our analysis is performed within a variant of the Device-Oriented Group Messaging model, which we extend to support device revocation. We discuss how to interpret these results, including the security WhatsApp provides as well as its limitations.

We introduce PQNoise, a post-quantum variant of the Noise framework. We demonstrate that it is possible to replace the Diffie-Hellman key-exchanges in Noise with KEMs in a secure way. A challenge is the inability to combine key pairs of KEMs, which can be resolved by certain forms of randomness-hardening for which we introduce a formal abstraction. We provide a generic recipe to turn classical Noise patterns into PQNoise patterns. We prove that the resulting PQNoise patterns achieve confidentiality and authenticity in the fACCE-model. Moreover we show that for those classical Noise-patterns that have been conjectured or proven secure in the fACCE-model our matching PQNoise-patterns eventually achieve the same security. Our security proof is generic and applies to any valid PQNoise pattern. This is made possible by another abstraction, called a hash-object, which hides the exact workings of how keying material is processed in an abstract stateful object that outputs pseudorandom keys under different corruption patterns. We also show that the hash chains used in Noise are a secure hash-object. Finally, we demonstrate the practicality of PQNoise delivering benchmarks for several base patterns.

Anonymity is an (abstract) security goal that is especially important to threatened user groups. Therefore, widely deployed communication protocols implement various measures to hide different types of information (i.e., metadata) about their users. Before actually defining anonymity, we consider an attack vector about which targeted user groups can feel concerned: continuous, temporary exposure of their secrets. Examples for this attack vector include intentionally planted viruses on victims' devices, as well as physical access when their users are detained. Ratcheted (or Continuous) Key Exchange (RKE) is a novel class of protocols that increase confidentiality and authenticity guarantees against temporary exposure of user secrets. For this, an RKE regularly renews user secrets such that the damage due to past and future exposures is minimized; this is called Post-Compromise Security and Forward-Secrecy, respectively. With this work, we are the first to leverage the strength of RKE for achieving strong anonymity guarantees under temporary exposure of user secrets. We extend existing definitions for RKE to capture attacks that interrelate ciphertexts, seen on the network, with secrets, exposed from users' devices. Although, at first glance, strong authenticity (and confidentiality) conflicts with strong anonymity, our anonymity definition is as strong as possible without diminishing other goals. We build strongly anonymity-, authenticity-, and confidentiality-preserving RKE and, along the way, develop new tools with applicability beyond our specific use-case: Updatable and Randomizable Signatures as well as Updatable and Randomizable Public Key Encryption. For both new primitives, we build efficient constructions.

Current messaging protocols are incapable of detecting active man-in-the-middle threats after a state compromise. Even strongly-secure protocols such as Signal, which offers forward secrecy and post-compromise security, are dependent on the adversary being passive immediately following state compromise, and healing guarantees are lost if the attacker is not. In addition, despite a great deal of research analyzing the confidentiality properties of secure messaging, entity authentication has largely been abstracted away. Modern messaging applications often rely on out-of-band communication to achieve entity authentication, with human users actively engaging with the protocol, verifying and attesting to long-term public keys. This is done primarily to reduce reliance on trusted third parties (by replacing that role with the user), but this implies that an accurate picture such messaging application's security must take this interaction into account. In this presentation, we examine these gaps by formalizing user-mediated entity authentication, introducing a security model for capturing user authentication in real-world ratcheted messaging protocols. We further demonstrate that the Signal application’s user-mediated authentication protocol cannot be proven secure in this strong model and suggest a new solution that allows the detection of an active state-compromising adversary. Our solution – the MoDUSA protocol – achieves active post-compromise entity authentication security, under certain assumptions on the out-of-band communication channel. These results have direct implications for existing and future ratcheted secure messaging applications.

Messaging schemes such as the Signal protocol rely on out-of-band channels to guarantee the authenticity of long-running communication. However those out-of-band checks may rarely be performed in practice. In this talk, we propose a method for performing continuous authentication during the communication, without needing an out-of-band channel. Leveraging the users' long-term secrets, our Authentication Steps extension guarantees authenticity as long as long-term secrets are not compromised, strengthening Signal's post-compromise security, and further allows to detect a potential compromise of long-term secrets after the fact via an out-of-band channel. Our protocol comes with a formal definition for continuous authentication and security proof, as well as a prototype implementation which seamlessly integrates on top of the official Signal Java library, together with bandwidth and storage overhead benchmarks.

We analyze the handshake protocol of the Transport Layer Security (TLS) protocol, version 1.3. We address both the full TLS 1.3 handshake (the one round-trip time mode, with signatures for authentication and (elliptic curve) Diffie–Hellman ephemeral ((EC)DHE) key exchange), and the abbreviated resumption/“PSK” mode which uses a pre-shared key for authentication (with optional (EC)DHE key exchange and zero round-trip time key establishment). Our analysis in the reductionist security framework uses a multi-stage key exchange security model, where each of the many session keys derived in a single TLS 1.3 handshake is tagged with various properties (such as unauthenticated versus unilaterally authenticated versus mutually authenticated, whether it is intended to provide forward security, how it is used in the protocol, and whether the key is protected against replay attacks). We show that these TLS 1.3 handshake protocol modes establish session keys with their desired security properties under standard cryptographic assumptions.

The Noise protocol framework is a suite of channel establishment protocols, of which each individual protocol ensures various security properties of the transmitted messages, but keeps specification, implementation, and configuration relatively simple. Implementations of the Noise protocols are themselves, due to the employed primitives, very performant. Thus, despite its relative youth, Noise is already used by large-scale deployed applications such as WhatsApp and Slack. Though the Noise specification describes and claims the security properties of the protocol patterns very precisely, there has been no computational proof yet. We close this gap. Noise uses only a limited number of cryptographic primitives which makes it an ideal candidate for reduction-based security proofs. Due to its patterns’ characteristics as channel establishment protocols, and the usage of established keys within the handshake, the authenticated and confidential channel establishment (ACCE) model (Jager et al. CRYPTO 2012) seems to perfectly fit for an analysis of Noise. However, the ACCE model strictly divides protocols into two non-overlapping phases: the pre-accept phase (i.e., the channel establishment) and post-accept phase (i.e., the channel). In contrast, Noise allows the transmission of encrypted messages as soon as any key is established (for instance, before authentication between parties has taken place), and then incrementally increases the channel’s security guarantees. By proposing a generalization of the original ACCE model, we capture security properties of such staged channel establishment protocols flexibly – comparably to the multi-stage key exchange model (Fischlin and Günther CCS 2014). We give security proofs for eight of the 15 basic Noise patterns in the full version (EPRINT 2019/436) and exemplify them by the proof of the  XK pattern in this article.

The Signal protocol is a cryptographic messaging protocol that provides end-to-end encryption for instant messaging in WhatsApp, Wire, and Facebook Messenger among many others, serving well over 1 billion active users. Signal includes several uncommon security properties (such as “future secrecy” or “post-compromise security”), enabled by a technique called ratcheting in which session keys are updated with every message sent. We conduct a formal security analysis of Signal’s initial extended triple Diffie–Hellman (X3DH) key agreement and Double Ratchet protocols as a multi-stage authenticated key exchange protocol. We extract from the implementation a formal description of the abstract protocol and define a security model which can capture the “ratcheting” key update structure as a multi-stage model where there can be a “tree” of stages, rather than just a sequence. We then prove the security of Signal’s key exchange core in our model, demonstrating several standard security properties. We have found no major flaws in the design and hope that our presentation and results can serve as a foundation for other analyses of this widely adopted protocol.