International Association
for Cryptologic Research

The common approach in secure communication channel protocols is to rely on ciphertexts arriving in-order and to close the connection upon any rogue ciphertext. Cryptographic security models for channels generally reflect such design. This is reasonable when running atop lower-level transport protocols like TCP ensuring in-order delivery, as for example, is the case with TLS or SSH. However, protocols like QUIC or DTLS which run over a non-reliable transport such as UDP, do not—and in fact cannot—close the connection if packets are lost or arrive in a different order. Those protocols instead have to carefully catch effects arising naturally in unreliable networks, usually by using a sliding-window technique where ciphertexts can be decrypted correctly as long as they are not misplaced too far. In order to be able to capture QUIC and the newest DTLS version 1.3, we introduce a generalized notion of robustness of cryptographic channels. This property can capture unreliable network behavior and guarantees that adversarial tampering cannot hinder ciphertexts that can be decrypted correctly from being accepted. We show that robustness is orthogonal to the common notion of integrity for channels, but together with integrity and chosen-plaintext security it provides a robust analog of chosen-ciphertext security of channels. In contrast to prior work, robustness allows us to study packet encryption in the record layer protocols of QUIC and of DTLS 1.3 and the novel sliding-window techniques both protocols employ. We show that both protocols achieve robust chosen-ciphertext security based on certain properties of their sliding-window techniques and the underlying AEAD schemes. Notably, the robustness needed in handling unreliable network messages requires both record layer protocols to tolerate repeated adversarial forgery attempts. This means we can only establish non-tight security bounds (in terms of AEAD integrity), a security degradation that was missed in earlier protocol drafts. Our bounds led the responsible IETF working groups to introduce concrete forgery limits for both protocols and the IRTF CFRG to consider AEAD usage limits more broadly.

Causal reasoning plays an important role in the comprehension of communication, but it has been elusive so far how causality should be properly preserved by instant messaging services. To the best of our knowledge, causality preservation is not even treated as a desired security property by most (if not all) existing secure messaging protocols like Signal. This is probably due to the intuition that causality seems already preserved when all received messages are intact and displayed according to their sending order. Our starting point is to notice that this intuition is wrong. Until now, for messaging channels (where conversations take place), both the proper causality model and the provably secure constructions have been left open. Our work fills this gap, with the goal to facilitate the formal understanding of causality preservation in messaging. First, we focus on the common two-user secure messaging channels and model the desired causality preservation property. We take the popular Signal protocol as an example and analyze the causality security of its cryptographic core (the double-ratchet mechanism). We show its inadequacy with a simple causality attack, then fix it such that the resulting Signal channel is causality-preserving, even in a strong sense that guarantees post-compromise security. Our fix is actually generic: it can be applied to any bidirectional channel to gain strong causality security. Then, we model causality security for the so-called message franking channels. Such a channel additionally enables end users to report individual abusive messages to a server (e.g., the service provider), where this server relays the end-to-end-encrypted communication between users. Causality security in this setting further allows the server to retrieve the necessary causal dependencies of each reported message, essentially extending isolated reported messages to message flows. This has great security merit for dispute resolution, because a benign message may be deemed abusive when isolated from the context. As an example, we apply our model to analyze Facebook’s message franking scheme. We show that a malicious user can easily trick Facebook (i.e., the server) to accuse an innocent user. Then we fix this issue by amending the underlying message franking channel to preserve the desired causality.

The Fast IDentity Online (FIDO) Alliance has developed the widely adopted FIDO2 protocol suite that allows for passwordless online authentication. Cryptographic keys stored on a user’s device (e.g. their smartphone) are used as credentials to authenticate to services by performing a challenge-response protocol. Yet, this approach leaves users unable to access their accounts in case their authenticator is lost. The device manufacturer Yubico thus proposed a FIDO2-compliant mech- anism that allows to easily create backup authenticators. Frymann et al. (CCS 2020) have first analyzed the cryptographic core of this pro- posal by introducing the new primitive of Asynchronous Remote Key Generation (ARKG) and accompanying security definitions. Later works instantiated ARKG both from classical and post-quantum assumptions (ACNS 2023, EuroS&P 2023). As we will point out in this paper, the security definitions put forward and used in these papers do not adequately capture the desired security requirements in FIDO2-based authentication and recovery. This issue was also identified in independent and concurrent work by Stebila and Wilson (AsiaCCS 2024), who proposed a new framework for the analy- sis of account recovery mechanisms, along with a secure post-quantum instantiation from KEMs and key-blinding signature schemes. In this work, we propose alternative security definitions for the primitive ARKG when used inside an account recovery mechanism in FIDO2. We give a secure instantiation from KEMs and standard signature schemes, which may in particular provide post-quantum security. Our solution strikes a middle ground between the compact, but (for this particular use case) inadequate security notions put forward by Frymann et al., and the secure, but more involved and highly tailored model introduced by Stebila and Wilson.

The Duplex construction, introduced by Bertoni~\emph{et al.} (SAC 2011), is the Swiss Army knife of permutation-based cryptography. It can be used to realise a variety of cryptographic objects---ranging from hash functions and MACs, to authenticated encryption and symmetric ratchets. Testament to this is the STROBE protocol framework which is a software cryptographic library based solely on the Duplex combined with a rich set of function calls. While prior works have typically focused their attention on specific uses of the Duplex, our focus here is its \emph{indifferentiability}. More specifically, we consider the indifferentiability of the Duplex construction from an \emph{online random oracle}---an idealisation which shares its same interface. As one of our main results we establish the indifferentiability of the Duplex from an online random oracle. However indifferentiability only holds for the standard Duplex construction and we show that the full-state variant of the Duplex cannot meet this notion. Our indifferentiability theorem provides the theoretical justification for the security of the Duplex in a variety of scenarios, amongst others, its use as a general-purpose cryptographic primitive in the STROBE framework. Next we move our attention to AEAD schemes based on the Duplex, namely SpongeWrap, which is the basis for NIST's Lightweight Cryptography standard Ascon. We harness the power of indifferentiability by establishing that SpongeWrap offers security against key-dependent message inputs, related-key attacks, and is also committing.

Extremely Lossy Functions (ELFs) are families of functions that, depending on the choice during key generation, either operate in injective mode or instead have only a polynomial image size. The choice of the mode is indistinguishable to an outsider. ELFs were introduced by Zhandry (Crypto 2016) and have been shown to be very useful in replacing random oracles in a number of applications. One open question is to determine the minimal assumption needed to instantiate ELFs. While all constructions of ELFs depend on some form of exponentially-secure public-key primitive, it was conjectured that exponentially-secure secret-key primitives, such as one-way functions, hash functions or one-way product functions, might be sufficient to build ELFs. In this work we answer this conjecture mostly negative: We show that no primitive, which can be derived from a random oracle (which includes all secret-key primitives mentioned above), is enough to construct even moderately lossy functions in a black-box manner. However, we also show that (extremely) lossy functions themselves do not imply public-key cryptography, leaving open the option to build ELFs from some intermediate primitive between the classical categories of secret-key and public-key cryptography.

In the Nostradamus attack, introduced by Kelsey and Kohno (Eurocrypt 2006), the adversary has to commit to a hash value y of an iterated hash function H such that, when later given a message prefix P, the adversary is able to find a suitable "suffix explanation" S with H(P||S)=y. Kelsey and Kohno show a herding attack with $2^{2n/3}$ evaluations of the compression function of H (with n bits output and state), locating the attack between preimage attacks and collision search in terms of complexity. Here we investigate the security of Nostradamus attacks for quantum adversaries. We present a quantum herding algorithm for the Nostradamus problem making approximately $\sqrt[3]{n}\cdot 2^{3n/7}$ compression function evaluations, significantly improving over the classical bound. We also prove that quantum herding attacks cannot do better than $2^{3n/7}$ evaluations for random compression functions, showing that our algorithm is (essentially) optimal. We also discuss a slightly less tight bound of roughly $2^{3n/7-s}$ for general Nostradamus attacks against random compression functions, where s is the maximal block length of the adversarially chosen suffix S.

Non-interactive zero-knowledge proofs or arguments allow a prover to show validity of a statement without further interaction. For non-trivial statements such protocols require a setup assumption in form of a common random or reference string (CRS). Generally, the CRS can only be used for one statement (single-theorem zero-knowledge) such that a fresh CRS would need to be generated for each proof. Fortunately, Feige, Lapidot and Shamir (FOCS 1990) presented a transformation for any non-interactive zero-knowledge proof system that allows the CRS to be reused any polynomial number of times (multi-theorem zero-knowledge). This FLS transformation, however, is only known to work for either computational zero-knowledge or requires a structured, non-uniform common reference string. In this paper we present FLS-like transformations that work for non-interactive statistical zero-knowledge arguments in the common random string model. They allow to go from single-theorem to multi-theorem zero-knowledge and also preserve soundness, for both properties in the adaptive and non-adaptive case. Our first transformation is based on the general assumption that one-way permutations exist, while our second transformation uses lattice-based assumptions. Additionally, we define different possible soundness notions for non-interactive arguments and discuss their relationships.

We give a cryptographic analysis of the Bluetooth Secure Connection Protocol Suite. Bluetooth supports several subprotocols such as numeric comparison, passkey entrance, and just works, in order to match the devices' different input/output capabilities. Previous analyses (e.g., Lindell, CT-RSA'09, or Troncoso and Hale, NDSS'21) often considered (and confirmed) the security of single subprotocols only. Recent practically verified attacks, however, such as the Method Confusion Attack (von Tschirschnitz et al., S&P 21) against Bluetooth's authentication and key secrecy property often exploit the bad interplay of different subprotocols. Even worse, some of these attacks show that one cannot show the Bluetooth protocol suite to be a secure authenticated key exchange protocol. We therefore aim at the best we can hope for, and show that the protocol still matches the common key secrecy requirements of a key-exchange protocol if one assumes a trust-on-first-use relationship. This means that the adversary needs to mount an active attack during the first connection, otherwise the subsequent reconnections remain secure. Investigating the cryptographic strength of the Bluetooth protocol we also look into the privacy mechanism of address randomization in Bluetooth (which is only available in the Low Energy version). We show that the cryptography indeed provides a decent level of address privacy, although this does not rule out identification of devices via other means, such as physical characteristics.

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.

OR-proofs enable a prover to show that it knows the witness for one of many statements, or that one out of many statements is true. OR-proofs are a remarkably versatile tool, used to strengthen security properties, design group and ring signature schemes, and achieve tight security. The common technique to build OR-proofs is based on an approach introduced by Cramer, Damgaard, and Schoenmakers (CRYPTO'94), where the prover splits the verifier's challenge into random shares and computes proofs for each statement in parallel. In this work we study a different, less investigated OR-proof technique, highlighted by Abe, Ohkubo, and Suzuki (ASIACRYPT'02). The difference is that the prover now computes the individual proofs sequentially. We show that such sequential OR-proofs yield signature schemes which can be proved secure in the non-programmable random oracle model. We complement this positive result with a black-box impossibility proof, showing that the same is unlikely to be the case for signatures derived from traditional OR-proofs. We finally argue that sequential-OR signature schemes can be proved secure in the quantum random oracle model, albeit with very loose bounds and by programming the random oracle.

The goal of white-box cryptography is to provide security even when the cryptographic implementation is executed in adversarially controlled environments. White-box implementations nowadays appear in commercial products such as mobile payment applications, e.g., those certified by Mastercard. Interestingly, there, white-box cryptography is championed as a tool for secure storage of payment tokens, and importantly, the white-boxed storage functionality is bound to a hardware functionality to prevent code-lifting attacks. In this paper, we show that the approach of using hardware-binding and obfuscation for secure storage is conceptually sound. Following security specifications by Mastercard and also EMVCo, we first define security for a white-box key derivation functions (WKDF) that is bound to a hardware functionality. WKDFs with hardware-binding model a secure storage functionality, as the WKDFs in turn can be used to derive encryption keys for secure storage. We then provide a proof-of-concept construction of WKDFs based on pseudorandom functions (PRF) and obfuscation. To show that our use of cryptographic primitives is sound, we perform a cryptographic analysis and reduce the security of our WKDF to the cryptographic assumptions of indistinguishability obfuscation and PRF-security. The hardware-functionality that our WKDF is bound to is a PRF-like functionality. Obfuscation helps us to hide the secret key used for the verification, essentially emulating a signature functionality as is provided by the Android key store. We rigorously define the required security properties of a hardware-bound white-box payment application (WPAY) for generating and encrypting valid payment requests. We construct a WPAY, which uses a WKDF as a secure building block. We thereby show that a WKDF can be securely combined with any secure symmetric encryption scheme, including those based on standard ciphers such as AES.

Ever since the foundational work of Goldwasser and Micali, simulation has proven to be a powerful and versatile construct for formulating security in various areas of cryptography. However security definitions based on simulation are generally harder to work with than game based definitions, often resulting in more complicated proofs. In this work we challenge this viewpoint by proposing new simulation-based security definitions for secure channels that in many cases lead to simpler proofs of security. We are particularly interested in definitions of secure channels which reflect real-world requirements, such as, protecting against the replay and reordering of ciphertexts, accounting for leakage from the decryption of invalid ciphertexts, and retaining security in the presence of ciphertext fragmentation. Furthermore we show that our proposed notion of channel simulatability implies a secure channel functionality that is universally composable. To the best of our knowledge, we are the first to study universally composable secure channels supporting these extended security goals. We conclude, by showing that the Dropbear implementation of SSH-CTR is channel simulatable in the presence of ciphertext fragmentation, and therefore also realises a universally composable secure channel. This is intended, in part, to highlight the merits of our approach over prior ones in admitting simpler security proofs in comparable settings.