No-Match Attacks and Robust Partnering Definitions – Defining Trivial Attacks for Security Protocols is Not Trivial

An essential cornerstone of the definition of security for key exchange protocols is the notion of partnering. It defines when two protocol instances can be considered to have communicated with each other and thus share important secret information. In all existing security definitions this serves as an important tool to exclude trivial attacks. The de-facto standard definition of partnering is that of (partial) matching conversations (MC), which essentially states that two processes are partnered if every message sent by the first is actually received by the second and vice versa. We show that proving security under MC-based definitions is error-prone. In particular, we provide several examples of protocols that claim to be secure under a MC-based security definition but where the security proof is actually flawed. To this end, we introduce no-match attacks, a new class of attacks that renders many existing security proofs invalid. Interestingly, no-match attacks do not seem to constitute practical attacks against the protocols in the sense that they compromise the secrecy of confidential parameters in real life applications. However, they propose serious, sometimes unsolvable obstacles to proofs in traditional security models. We show that no-match attacks are often hard to avoid in MC-based security definitions without a) modifications of the original protocol or b) resorting to the use of cryptographic primitives with special properties. Finally, we show several ways to thwart no-match attacks. Most notably and as one of our major contributions, we provide a conceptually new definition of partnering that circumvents the problems of a MC-based partnering notion while preserving all its advantages. Our new notion of partnering not only makes security definitions for key exchange model practice much more closely. In contrast to many other security notions of key exchange it also adheres to the high standards of good cryptographic definitions: it is general, supports cryptographic intuition, allows for efficient falsification, and provides a fundamental composition property that MC-based notions lack

On the Security of TLS-DH and TLS-RSA in the Standard Model

TLS is the most important cryptographic protocol in the Internet. At CRYPTO 2012, Jager et al. presented the first proof of the unmodified TLS with ephemeral Diffie-Hellman key exchange (TLS-DHE) for mutual authentication. Since TLS cannot be proven secure under the classical definition of authenticated key exchange (AKE), they introduce a new security model called authenticated and confidential channel establishment (ACCE) that captures the security properties expected from TLS in practice. We extend this result in two ways. First we show that the cryptographic cores of the remaining ciphersuites, RSA encrypted key transport (TLS-RSA) and static Diffie-Hellman (TLS-DH), can be proven secure for mutual authentication in an extended ACCE model that also allows the adversary to register new public keys. In our security analysis we show that if TLS-RSA is instantiated with a CCA secure public key cryptosystem and TLS-DH is used in scenarios where a) the knowledge of secret key assumption holds or b) the adversary may not register new public keys at all, both ciphersuites can be proven secure in the standard model under standard security assumptions. Next, we present new and strong definitions of ACCE (and AKE) for server-only authentication which fit well into the general framework of Bellare-Rogaway-style models. We show that all three ciphersuites families do remain secure in this server-only setting. Our work identifies which primitives need to be exchanged in the TLS handshake to obtain strong security results under standard security assumptions (in the standard model) and may so help to guide future revisions of the TLS standard and make improvements to TLS\u27s extensibility pay off

Generic Authenticated Key Exchange in the Quantum Random Oracle Model

We propose FO-AKE a generic construction of two-message authenticated key exchange (AKE) from any passively secure public key encryption (PKE) in the quantum random oracle model (QROM). Whereas previous AKE constructions relied on a Diffie-Hellman key exchange or required the underlying PKE scheme to be perfectly correct, our transformation allows arbitrary PKE schemes with non-perfect correctness. Furthermore, we avoid the use of (quantum-secure) digital signature schemes which are considerably less efficient than their PKE counterparts. As a consequence, we can instantiate our AKE transformation with any of the submissions to the recent NIST post-quantum competition, e.g., ones based on codes and lattices. FO-AKE can be seen as a generalization of the well known Fujisaki-Okamoto transformation (for building actively secure PKE from passively secure PKE) to the AKE setting. Therefore, as a helper result, we also provide a security proof for the Fujisaki-Okamoto transformation in the QROM for PKE with non-perfect correctness. Our reduction fixes several gaps in a previous proof (CRYPTO 2018), is tighter, and tolerates a larger correctness error

Tightly-Secure Authenticated Key Exchange, Revisited

We introduce new tightly-secure authenticated key exchange (AKE) protocols that are extremely efficient, yet have only a constant security loss and can be instantiated in the random oracle model both from the standard DDH assumption and a subgroup assumption over RSA groups. These protocols can be deployed with optimal parameters, independent of the number of users or sessions, without the need to compensate a security loss with increased parameters and thus decreased computational efficiency. We use the standard “Single-Bit-Guess” AKE security (with forward secrecy and state corruption) requiring all challenge keys to be simultaneously pseudo-random. In contrast, most previous papers on tightly secure AKE protocols (Bader et al., TCC 2015; Gjøsteen and Jager, CRYPTO 2018; Liu et al., ASIACRYPT 2020) concentrated on a non-standard “Multi-Bit-Guess” AKE security which is known not to compose tightly with symmetric primitives to build a secure communication channel. Our key technical contribution is a new generic approach to construct tightly-secure AKE protocols based on non-committing key encapsulation mechanisms. The resulting DDH-based protocols are considerably more efficient than all previous constructions

On the Impossibility of Tight Cryptographic Reductions

The existence of tight reductions in cryptographic security proofs is an important question, motivated by the theoretical search for cryptosystems whose security guarantees are truly independent of adversarial behavior and the practical necessity of concrete security bounds for the theoretically-sound selection of cryptographic parameters. At Eurocrypt 2002, Coron described a meta-reduction technique that allows to prove the impossibility of tight reductions for certain digital signature schemes. This seminal result has found many further interesting applications. However, due to a technical subtlety in the argument, the applicability of this technique beyond digital signatures in the single-user setting has turned out to be rather limited. We describe a new meta-reduction technique for proving such impossibility results, which improves on known ones in several ways. First, it enables interesting novel applications. This includes a formal proof that for certain cryptographic primitives (including public-key encryption/key encapsulation mechanisms and digital signatures), the security loss incurred when the primitive is transferred from an idealized single-user setting to the more realistic multi-user setting is impossible to avoid, and a lower tightness bound for non-interactive key exchange protocols. Second, the technique allows to rule out tight reductions from a very general class of non-interactive complexity assumptions. Third, the provided bounds are quantitatively and qualitatively better, yet simpler, than the bounds derived from Coron\u27s technique and its extensions

Authenticated Key Exchange and Signatures with Tight Security in the Standard Model

We construct the first authenticated key exchange protocols that achieve tight security in the standard model. Previous works either relied on techniques that seem to inherently require a random oracle, or achieved only “Multi-Bit-Guess” security, which is not known to compose tightly, for instance, to build a secure channel. Our constructions are generic, based on digital signatures and key encapsulation mechanisms (KEMs). The main technical challenges we resolve is to determine suitable KEM security notions which on the one hand are strong enough to yield tight security, but at the same time weak enough to be efficiently instantiable in the standard model, based on standard techniques such as universal hash proof systems. Digital signature schemes with tight multi-user security in presence of adaptive corruptions are a central building block, which is used in all known constructions of tightly-secure AKE with full forward security. We identify a subtle gap in the security proof of the only previously known efficient standard model scheme by Bader et al. (TCC 2015). We develop a new variant, which yields the currently most efficient signature scheme that achieves this strong security notion without random oracles and based on standard hardness assumptions

Report on evaluation of KpqC candidates

This report analyzes the 16 submissions to the Korean post-quantum cryptography (KpqC) competition

MoniPoly---An Expressive qq-SDH-Based Anonymous Attribute-Based Credential System

Modern attribute-based anonymous credential (ABC) systems benefit from special encodings that yield expressive and highly efficient show proofs on logical statements. The technique was first proposed by Camenisch and Groß, who constructed an SRSA-based ABC system with prime-encoded attributes that offers efficient AND, OR and NOT proofs. While other ABC frameworks have adopted constructions in the same vein, the Camenisch-Groß ABC has been the most expressive and asymptotically most efficient proof system to date, even if it was constrained by the requirement of a trusted message-space setup and an inherent restriction to finite-set attributes encoded as primes. In this paper, combining a new set commitment scheme and a SDH-based signature scheme, we present a provably secure ABC system that supports show proofs for complex statements. This construction is not only more expressive than existing approaches, it is also highly efficient under unrestricted attribute space due to its ECC protocols only requiring a constant number of bilinear pairings by the verifier; none by the prover. Furthermore, we introduce strong security models for impersonation and unlinkability under adaptive active and concurrent attacks to allow for the expressiveness of our ABC as well as for a systematic comparison to existing schemes. Given this foundation, we are the first to comprehensively formally prove the security of an ABC with expressive show proofs. Specifically, we prove the security against impersonation under the $q$-(co-)SDH assumption with a tight reduction. Besides the set commitment scheme, which may be of independent interest, our security models can serve as a foundation for the design of future ABC systems

Compact Structure-preserving Signatures with Almost Tight Security

In structure-preserving cryptography, every building block shares the same bilinear groups. These groups must be generated for a specific, a prior fixed security level, and thus it is vital that the security reduction of all involved building blocks is as tight as possible. In this work, we present the first generic construction of structure-preserving signature schemes whose reduction cost is independent of the number of signing queries. Its chosen-message security is almost tightly reduced to the chosen-plaintext security of a structure-preserving public-key encryption scheme and the security of Groth-Sahai proof system. Technically, we adapt the adaptive partitioning technique by Hofheinz (Eurocrypt 2017) to the setting of structure-preserving signature schemes. To achieve a structure-preserving scheme, our new variant of the adaptive partitioning technique relies only on generic group operations in the scheme itself. Interestingly, however, we will use non-generic operations during our security analysis. Instantiated over asymmetric bilinear groups, the security of our concrete scheme is reduced to the external Diffie-Hellman assumption with linear reduction cost in the security parameter, independently of the number of signing queries. The signatures in our schemes consist of a larger number of group elements than those in other non-tight schemes, but can be verified faster, assuming their security reduction loss is compensated by increasing the security parameter to the next standard level