Shorter Double-Authentication Preventing Signatures for Small Address Spaces

A recent paper by Derler, Ramacher, and Slamanig (IEEE EuroS&P 2018) constructs double-authentication preventing signatures ( DAP signatures , a specific self-enforcement enabled variant of signatures where messages consist of an address and a payload) that have---if the supported address space is not too large---keys and signatures that are considerably more compact than those of prior work. We embark on their approach to restrict attention to small address spaces and construct novel DAP schemes that beat their signature size by a factor of five and reduce the signing key size from linear to constant (the verification key size remains almost the same). We construct our DAP signatures generically from identification protocols, using a transform similar to but crucially different from that of Fiat and Shamir. We use random oracles. We don\u27t use pairings

Generic Double-Authentication Preventing Signatures and a Post-Quantum Instantiation

Double-authentication preventing signatures (DAPS) are a variant of digital signatures which have received considerable attention recently (Derler et al. EuroS&P 2018, Poettering AfricaCrypt 2018). They are unforgeable signatures in the usual sense and sign messages that are composed of an address and a payload. Their distinguishing feature is the property that signing two different payloads with respect to the same address allows to publicly extract the secret signing key from two such signatures. DAPS are known in the factoring, the discrete logarithm and the lattice setting. The majority of the constructions are ad-hoc. Only recently, Derler et al. (EuroS&P 2018) presented the first generic construction that allows to extend any discrete logarithm based secure signatures scheme to DAPS. However, their scheme has the drawback that the number of potential addresses (the address space) used for signing is polynomially bounded (and in fact small) as the size of secret and the public keys of the resulting DAPS are linear in the address space. In this paper we overcome this limitation and present a generic construction of DAPS with constant size keys and signatures. Our techniques are not tailored to a specific algebraic setting and in particular allow us to construct the first DAPS without structured hardness assumptions, i.e., from symmetric key primitives, yielding a candidate for post-quantum secure DAPS

Efficient Ratcheting: Almost-Optimal Guarantees for Secure Messaging

In the era of mass surveillance and information breaches, privacy of Internet communication, and messaging in particular, is a growing concern. As secure messaging protocols are executed on the not-so-secure end-user devices, and because their sessions are long-lived, they aim to guarantee strong security even if secret states and local randomness can be exposed. The most basic security properties, including forward secrecy, can be achieved using standard techniques such as authenticated encryption. Modern protocols, such as Signal, go one step further and additionally provide the so-called backward secrecy, or healing from state exposures. These additional guarantees come at the price of a slight efficiency loss (they require public-key primitives). On the opposite side of the spectrum is the work by Jaeger and Stepanovs and by Poettering and Rösler, which characterizes the optimal security a secure-messaging scheme can achieve. However, their proof-of-concept constructions suffer from an extreme efficiency loss compared to Signal. Moreover, this caveat seems inherent. In this paper, we explore the area in between. That is, our starting point are the basic, efficient constructions. We then ask the question: how far can we go towards the optimal security without losing too much efficiency? We present a construction with guarantees much stronger than those achieved by Signal, and slightly weaker than optimal, yet its efficiency is closer to that of Signal (we only use standard public-key cryptography). On a technical level, achieving optimal guarantees inherently requires key-updating public-key primitives, where the update information is allowed to be public. We consider secret update information instead. Since a state exposure temporally breaks confidentiality, we carefully design such secretly-updatable primitives whose security degrades gracefully if the supposedly secret update information leaks

Multi-Device for Signal

Nowadays, we spend our life juggling with many devices such as smartphones, tablets or laptops, and we expect to easily and efficiently switch between them without losing time or security. However, most applications have been designed for single device usage. This is the case for secure instant messaging (SIM) services based on the Signal protocol, that implements the Double Ratchet key exchange algorithm. While some adaptations, like the Sesame protocol released by the developers of Signal, have been proposed to fix this usability issue, they have not been designed as specific multi-device solutions and no security model has been formally defined either. In addition, even though the group key exchange problematic appears related to the multi-device case, group solutions are too generic and do not take into account some properties of the multi-device setting.Indeed, the fact that all devices belong to a single user can be exploited to build more efficient solutions. In this paper, we propose a Multi-Device Instant Messaging protocol based on Signal, ensuring all the security properties of the original Signal

Continuous Group Key Agreement with Active Security

A continuous group key agreement (CGKA) protocol allows a long-lived group of parties to agree on a continuous stream of fresh secret key material. The protocol must support constantly changing group membership, make no assumptions about when, if, or for how long members come online, nor rely on any trusted group managers. Due to sessions\u27 long life-time, CGKA protocols must simultaneously ensure both post-compromise security and forward secrecy (PCFS). That is, current key material should be secure despite both past and future compromises. The work of Alwen et al. (CRYPTO\u2720), introduced the CGKA primitive and identified it as a crucial component for constructing end-to-end secure group messaging protocols (SGM) (though we believe there are certainly more applications given the fundamental nature of key agreement). The authors analyzed the TreeKEM CGKA, which lies at the heart of the SGM protocol under development by the IETF working group on Messaging Layer Security (MLS). In this work, we continue the study of CGKA as a stand-alone cryptographic primitive. We present $3$ new security notions with increasingly powerful adversaries. Even the weakest of the 3 (passive security) already permits attacks to which all prior constructions (including all variants of TreeKEM) are vulnerable. Going further, the 2 stronger (active security) notions additionally allow the adversary to use parties\u27 exposed states (and full network control) to mount attacks. These are closely related to so-called insider attacks, which involve malicious group members actively deviating from the protocol. Insider attacks present a significant challenge in the study of CGKA (and SGM). Indeed, we believe ours to be the first security notions (and constructions) to formulate meaningful guarantees (e.g. PCFS) against such powerful adversaries. They are also the first composable security notions for CGKA of any type at all. In terms of constructions, for each of the 3 security notions we provide a new CGKA scheme enjoying sub-linear (potentially even logarithmic) communication complexity in the number of group members. We prove each scheme optimally secure, in the sense that the only security violations possible are those necessarily implied by correctness

Security Analysis and Improvements for the IETF MLS Standard for Group Messaging

Secure messaging (SM) protocols allow users to communicate securely over untrusted infrastructure. In contrast to most other secure communication protocols (such as TLS, SSH, or Wireguard), SM sessions may be long-lived (e.g., years) and highly asynchronous. In order to deal with likely state compromises of users during the lifetime of a session, SM protocols do not only protect authenticity and privacy, but they also guarantee forward secrecy (FS) and post-compromise security (PCS). The former ensures that messages sent and received before a state compromise remain secure, while the latter ensures that users can recover from state compromise as a consequence of normal protocol usage. SM has received considerable attention in the two-party case, where prior work has studied the well-known double-ratchet paradigm in particular and SM as a cryptographic primitive in general. Unfortunately, this paradigm does not scale well to the problem of secure group messaging (SGM). In order to address the lack of satisfactory SGM protocols, the IETF has launched the message-layer security (MLS) working group, which aims to standardize an eponymous SGM protocol. In this work we analyze the TreeKEM protocol, which is at the core of the SGM protocol proposed by the MLS working group. On a positive note, we show that TreeKEM achieves PCS in isolation (and slightly more). However, we observe that the current version of TreeKEM does not provide an adequate form of FS. More precisely, our work proceeds by formally capturing the exact security of TreeKEM as a so-called continuous group key agreement (CGKA) protocol, which we believe to be a primitive of independent interest. To address the insecurity of TreeKEM, we propose a simple modification to TreeKEM inspired by recent work of Jost et al. (EUROCRYPT \u2719) and an idea due to Kohbrok (MLS Mailing List). We then show that the modified version of TreeKEM comes with almost no efficiency degradation but achieves optimal (according to MLS specification) CGKA security, including FS and PCS. Our work also lays out how a CGKA protocol can be used to design a full SGM protocol. Finally, we propose and motivate an extensive list of potential future research directions for the area

Registration-Based Encryption from Standard Assumptions

The notion of Registration-Based Encryption (RBE) was recently introduced by Garg, Hajiabadi, Mahmoody, and Rahimi [TCC\u2718] with the goal of removing the private-key generator (PKG) from IBE. Specifically, RBE allows encrypting to identities using a (compact) master public key, like how IBE is used, with the benefit that the PKG is substituted with a weaker entity called key curator who has no knowledge of any secret keys. Here individuals generate their secret keys on their own and then publicly register their identities and their corresponding public keys to the key curator. Finally, individuals obtain rare decryption-key updates from the key curator as the population grows. In their work, they gave a construction of RBE schemes based on the combination of indistinguishability obfuscation and somewhere statistically binding hash functions. However, they left open the problem of constructing RBE schemes based on standard assumptions. In this work, we resolve the above problem and construct RBE schemes based on standard assumptions (e.g., CDH or LWE). Furthermore, we show a new application of RBE in a novel context. In particular, we show that anonymous variants of RBE (which we also construct under standard assumptions) can be used for realizing abstracts forms of anonymous messaging tasks in simple scenarios in which the parties communicate by writing messages on a shared board in a synchronized way

Tightly-Secure Signatures from Five-Move Identification Protocols

We carry out a concrete security analysis of signature schemes obtained from five-move identification protocols via the Fiat-Shamir transform. Concretely, we obtain tightly-secure signatures based on the computational Diffie-Hellman (CDH), the short-exponent CDH, and the Factoring (FAC) assumptions. All our signature schemes have tight reductions to search problems, which is in stark contrast to all known signature schemes obtained from the classical Fiat-Shamir transform (based on three-move identification protocols), which either have a non-tight reduction to a search problem, or a tight reduction to a (potentially) stronger decisional problem. Surprisingly, our CDH-based scheme turns out to be (a slight simplification of) the Chevallier-Mames signature scheme (CRYPTO 05), thereby providing a theoretical explanation of its tight security proof via five-move identification protocols