## CryptoDB

### Sven Schäge

#### Publications

Year
Venue
Title
2021
EUROCRYPT
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.
2021
CRYPTO
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.
2021
TCC
The existence of one-way functions implies secure digital sig- natures, but not public-key encryption (at least in a black-box setting). Somewhat surprisingly, though, efficient public-key encryption schemes appear to be much easier to construct from concrete algebraic assumptions (such as the factoring of Diffie-Hellman-like assumptions) than efficient digital signature schemes. In this work, we provide one reason for this apparent difficulty to construct efficient signature schemes. Specifically, we prove that a wide range of algebraic signature schemes (in which verification essentially checks a number of linear equations over a group) fall to conceptually surprisingly simple linear algebra attacks. In fact, we prove that in an algebraic signature scheme, sufficiently many signatures can be linearly combined to a signature of a fresh message. We present attacks both in known-order and hidden-order groups (although in hidden-order settings, we have to restrict our definition of algebraic signatures a little). More explicitly, we show: – the insecurity of all algebraic signature schemes in Maurer’s generic group model, as long as the signature schemes do not rely on other cryptographic assumptions, such as hash functions. – the insecurity of a natural class of signatures in hidden-order groups, where verification consists of linear equations over group elements. We believe that this highlights the crucial role of public verifiability in digital signature schemes. Namely, while public-key encryption schemes do not require any publicly verifiable structure on ciphertexts, it is exactly this structure on signatures that invites attacks like ours and makes it hard to construct efficient signatures.
2020
PKC
We propose $mathsf {FO_mathsf {AKE}}$ , 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. Dealing with imperfect schemes is one of the major difficulties in a setting involving active attacks. Our direct construction, when applied to schemes such as the submissions to the recent NIST post-quantum competition, is more natural than previous AKE transformations. 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 competition, e.g., ones based on codes and lattices. $mathsf {FO_mathsf {AKE}}$ can be seen as a generalisation of the well known Fujisaki-Okamoto transformation (for building actively secure PKE from passively secure PKE) to the AKE setting. As a helper result, we also provide a security proof for the Fujisaki-Okamoto transformation in the QROM for PKE with non-perfect correctness which is tighter and tolerates a larger correctness error than previous proofs.
2020
PKC
In this paper, we present a strong, formal, and general-purpose cryptographic model for privacy-preserving authenticated key exchange (PPAKE) protocols. PPAKE protocols are secure in the traditional AKE sense but additionally guarantee the confidentiality of the identities used in communication sessions. Our model has several useful and novel features, among others: it is a proper extension of classical AKE models, guarantees in a strong sense that the confidentiality of session keys is independent from the secrecy of the used identities, and it is the first to support what we call dynamic modes, where the responsibility of selecting the identities of the communication partners may vary over several protocol runs. We show the validity of our model by applying it to the cryptographic core of IPsec IKEv2 with signature-based authentication where the need for dynamic modes is practically well-motivated. In our analysis, we not only show that this protocol provides strong classical AKE security guarantees but also that the identities that are used by the parties remain hidden in successful protocol runs. Historically, the Internet Key Exchange (IKE) protocol was the first real-world AKE to incorporate privacy-preserving techniques. However, lately privacy-preserving techniques have gained renewed interest in the design process of important protocols like TLS 1.3 (with encrypted SNI) and NOISE. We believe that our new model can be a solid foundation to analyze these and other practical protocols with respect to their privacy guarantees, in particular, in the now so wide-spread scenario where multiple virtual servers are hosted on a single machine.
2017
JOFC
2016
EUROCRYPT
2015
JOFC
2015
PKC
2014
PKC
2012
CRYPTO
2012
PKC
2011
EUROCRYPT
2010
ASIACRYPT