## Erkan Tairi

Dipl.-Ing.

Roles
• PreDoc Researcher
Projects (at TU Wien)
Publications (at TU Wien)
2022
• Foundations of Coin Mixing Services
Glaeser, N., Maffei, M., Malavolta, G., Moreno-Sanchez, P., Tairi, E., & Thyagarajan, S. A. (2022). Foundations of Coin Mixing Services. In CCS ’22: Proceedings of the 2022 ACM SIGSAC Conference on Computer and Communications Security (pp. 1259–1273). Association for Computing Machinery.
Abstract: Coin mixing services allow users to mix their cryptocurrency coins and thus enable unlinkable payments in a way that prevents tracking of honest users' coins by both the service provider and the users themselves. The easy bootstrapping of new users and backwards compatibility with cryptocurrencies (such as Bitcoin) with limited support for scripts are attractive features of this architecture, which has recently gained considerable attention in both academia and industry. A recent work of Tairi et al. [IEEE S&P 2021] formalizes the notion of a coin mixing service and proposes A2L, a new cryptographic protocol that simultaneously achieves high efficiency and interoperability. In this work, we identify a gap in their formal model and substantiate the issue by showing two concrete counterexamples: we show how to construct two encryption schemes that satisfy their definitions but lead to a completely insecure system. To amend this situation, we investigate secure constructions of coin mixing services. First, we develop the notion of blind conditional signatures (BCS), which acts as the cryptographic core for coin mixing services. We propose game-based security definitions for BCS and propose A2L+, a modified version of the protocol by Tairi et al. that satisfies our security definitions. Our analysis is in an idealized model (akin to the algebraic group model) and assumes the hardness of the one-more discrete logarithm problem. Finally, we propose A2L-UC, another construction of BCS that achieves the stronger notion of UC-security (in the standard model), albeit with a significant increase in computation cost. This suggests that constructing a coin mixing service protocol secure under composition requires more complex cryptographic machinery than initially thought.
2021
• Post-Quantum Adaptor Signature for Privacy-Preserving Off-Chain Payments
Tairi, E., Moreno-Sanchez, P., & Maffei, M. (2021). Post-Quantum Adaptor Signature for Privacy-Preserving Off-Chain Payments. In Financial Cryptography and Data Security (pp. 131–150).
Abstract: Cryptographic objects with updating capabilities have been proposed by Bellare, Goldreich and Goldwasser (CRYPTO'94) under the umbrella of incremental cryptography. They have recently seen increased interest, motivated by theoretical questions (Ananth et al., EC'17) as well as concrete practical motivations (Lehmann et al., EC'18; Groth et al. CRYPTO'18; Kloo{\ss} et al., EC'19). In this work, the form of updatability we are particularly interested in is that primitives are key-updatable \textit{and} allow to update old" cryptographic objects, e.g., signatures or message authentication codes, from the old" key to the updated key at the same time without requiring full access to the new key (i.e., only via a so-called update token). Inspired by the rigorous study of updatable encryption by Lehmann and Tackmann (EC'18) and Boyd et al. (CRYPTO'20), we introduce a definitional framework for updatable signatures (USs) and message authentication codes (UMACs). We discuss several applications demonstrating that such primitives can be useful in practical applications, especially around key rotation in various domains, as well as serve as building blocks in other cryptographic schemes. We then turn to constructions and our focus there is on ones that are secure and practically efficient. In particular, we provide generic constructions from key-homomorphic primitives (signatures and PRFs) as well as direct constructions. This allows us to instantiate these primitives from various assumptions such as DDH or CDH (latter in bilinear groups), or the (R)LWE and the SIS assumptions. As an example, we obtain highly practical US schemes from BLS signatures or UMAC schemes from the Naor-Pinkas-Reingold PRF.