## CryptoDB

### Vassilis Zikas

#### Publications

Year
Venue
Title
2021
EUROCRYPT
Clock synchronization allows parties to establish a common notion of global time by leveraging a weaker synchrony assumption, i.e., local clocks with approximately the same speed. Despite intensive investigation of the problem in the fault-tolerant distributed computing literature, existing solutions do not apply to settings where participation is unknown, e.g., the ad hoc model of Beimel et al. [EUROCRYPT 17], or is dynamically shifting over time, e.g., the fluctuating/sleepy/dynamic-availability models of Garay et al. [CRYPTO 17], Pass and Shi [ASIACRYPT 17] and Badertscher et al. CCS 18]. We show how to apply and extend ideas from the blockchain literature to devise synchronizers that work in such dynamic ad hoc settings and tolerate corrupted minorities under the standard assumption that local clocks advance at approximately the same speed. We discuss both the setting of honest-majority hashing power and that of a PKI with honest majority. Our main result is a synchronizer that is directly integrated with a new proof-of-stake (PoS) blockchain protocol, Ouroboros Chronos, which we construct and prove secure; to our knowledge, this is the first PoS blockchain protocol to rely only on local clocks, while tolerating worst-case corruption and dynamically fluctuating participation. We believe that this result might be of independent interest.
2020
EUROCRYPT
Traditional bounds on synchronous Byzantine agreement (BA) and secure multi-party computation (MPC) establish that in absence of a private correlated-randomness setup, such as a PKI, protocols can tolerate up to $t<n/3$ of the parties being malicious. The introduction of Nakamoto style'' consensus, based on Proof-of-Work (PoW) blockchains, put forth a somewhat different flavor of BA, showing that even a majority of corrupted parties can be tolerated as long as the majority of the computation resources remain at honest hands. This assumption on honest majority of some resource was also extended to other resources such as stake, space, etc., upon which blockchains achieving Nakamoto-style consensus were built that violated the $t<n/3$ bound in terms of number of party corruptions. The above state of affairs begs the question of whether the seeming mismatch is due to different goals and models, or whether the resource-restricting paradigm can be generically used to circumvent the $n/3$ lower bound. In this work we study this question and formally demonstrate how the above paradigm changes the rules of the game in cryptographic definitions. First, we abstract the core properties that the resource-restricting paradigm offers by means of a functionality {\em wrapper}, in the UC framework, which when applied to a standard point-to-point network restricts the ability (of the adversary) to send new messages. We show that such a wrapped network can be implemented using the resource-restricting paradigm---concretely, using PoWs and honest majority of computing power---and that the traditional $t<n/3$ impossibility results fail when the parties have access to such a network. Our construction is in the {\em fresh} Common Reference String (CRS) model---i.e., it assumes a CRS which becomes available to the parties at the same time as to the adversary. We then present constructions for BA and MPC, which given access to such a network tolerate $t<n/2$ corruptions without assuming a private correlated randomness setup. We also show how to remove the freshness assumption from the CRS by leveraging the power of a random oracle. Our MPC protocol achieves the standard notion of MPC security, where parties might have dedicated roles, as is for example the case in Oblivious Transfer protocols. This is in contrast to existing solutions basing MPC on PoWs, which associate roles to pseudonyms but do not link these pseudonyms with the actual parties.
2020
EUROCRYPT
An intensive effort by the cryptographic community to minimize the round complexity of secure multi-party computation (MPC) has recently led to optimal two-round protocols from minimal assumptions. Most of the proposed solutions, however, make use of a broadcast channel in every round, and it is unclear if the broadcast channel can be replaced by standard point-to-point communication in a round-preserving manner, and if so, at what cost on the resulting security. In this work, we provide a complete characterization of the trade-off between number of broadcast rounds and achievable security level for two-round MPC tolerating arbitrarily many active corruptions. Specifically, we consider all possible combinations of broadcast and point-to-point rounds against the three standard levels of security for maliciously se- cure MPC protocols, namely, security with identifiable, unanimous, and selective abort. For each of these notions and each combination of broadcast and point-to-point rounds, we provide either a tight feasibility or an infeasibility result of two-round MPC. Our feasibility results hold assuming two-round OT in the CRS model, whereas our impossibility results hold given any correlated randomness.
2020
TCC
The Global and Externalized UC frameworks [Canetti-Dodis-Pass-Walfish, TCC 07] extend the plain UC framework to additionally handle protocols that use a global setup'', namely a mechanism that is also used by entities outside the protocol. These frameworks have broad applicability: Examples include public-key infrastructures, common reference strings, shared synchronization mechanisms, global blockchains, or even abstractions such as the random oracle. However, the need to work in a specialized framework has been a source of confusion, incompatibility, and an impediment to broader use. We show how security in the presence of a global setup can be captured within the plain UC framework, thus significantly simplifying the treatment. This is done as follows: - We extend UC-emulation to the case where both the emulating protocol $\pi$ and the emulated protocol $\phi$ make subroutine calls to protocol $\gamma$ that is accessible also outside $\pi$ and $\phi$. As usual, this notion considers only a single instance of $\phi$ or $\pi$ (alongside $\gamma$). - We extend the UC theorem to hold even with respect to the new notion of UC emulation. That is, we show that if $\pi$ UC-emulates $\phi$ in the presence of $\gamma$, then $\rho^{\phi\rightarrow\pi}$ UC-emulates $\rho$ for any protocol $\rho$, even when $\rho$ uses $\gamma$ directly, and in addition calls many instances of $\phi$, all of which use the same instance of $\gamma$. We prove this extension using the existing UC theorem as a black box, thus further simplifying the treatment. We also exemplify how our treatment can be used to streamline, within the plain UC model, proofs of security of systems that involve global set-up, thus providing greater simplicity and flexibility.
2019
JOFC
When analyzing the round complexity of multi-party protocols, one often overlooks the fact that underlying resources, such as a broadcast channel, can by themselves be expensive to implement. For example, it is well known that it is impossible to implement a broadcast channel by a (deterministic) protocol in a sublinear (in the number of corrupted parties) number of rounds. The seminal works of Rabin and Ben-Or from the early 1980s demonstrated that limitations as the above can be overcome by using randomization and allowing parties to terminate at different rounds, igniting the study of protocols over point-to-point channels with probabilistic termination and expected constant round complexity. However, absent a rigorous simulation-based definition, the suggested protocols are proven secure in a property-based manner or via ad hoc simulation-based frameworks, therefore guaranteeing limited, if any, composability. In this work, we put forth the first simulation-based treatment of multi-party cryptographic protocols with probabilistic termination. We define secure multi-party computation (MPC) with probabilistic termination in the UC framework and prove a universal composition theorem for probabilistic termination protocols. Our theorem allows to compile a protocol using deterministic termination hybrids into a protocol that uses expected constant round protocols for emulating these hybrids, preserving the expected round complexity of the calling protocol. We showcase our definitions and compiler by providing the first composable protocols (with simulation-based security proofs) for the following primitives, relying on point-to-point channels: (1) expected constant round perfect Byzantine agreement, (2) expected constant round perfect parallel broadcast, and (3) perfectly secure MPC with round complexity independent of the number of parties.
2018
EUROCRYPT
2017
CRYPTO
2017
CRYPTO
2016
EUROCRYPT
2016
CRYPTO
2016
CRYPTO
2016
ASIACRYPT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
CRYPTO
2014
CRYPTO
2014
CRYPTO
2014
EPRINT
2014
EPRINT
2013
TCC
2013
TCC
2012
CRYPTO
2010
EUROCRYPT
2009
TCC
2008
TCC
2008
ASIACRYPT
2007
EPRINT
Secure function evaluation (SFE) allows a set of players to compute an arbitrary agreed function of their private inputs, even if an adversary may corrupt some of the players. Secure multi-party computation (MPC) is a generalization allowing to perform an arbitrary on-going (also called reactive or stateful) computation during which players can receive outputs and provide new inputs at intermediate stages. At Crypto~2006, Ishai \emph{et al.} considered mixed threshold adversaries that either passively corrupt some fixed number of players, or, alternatively, actively corrupt some (smaller) fixed number of players, and showed that for certain thresholds, cryptographic SFE is possible, whereas cryptographic MPC is not. However, this separation does not occur when one considers \emph{perfect} security. Actually, past work suggests that no such separation exists, as all known general protocols for perfectly secure SFE can also be used for MPC. Also, such a separation does not show up with \emph{general adversaries}, characterized by a collection of corruptible subsets of the players, when considering passive and active corruption. In this paper, we study the most general corruption model where the adversary is characterized by a collection of adversary classes, each specifying the subset of players that can be actively, passively, or fail-corrupted, respectively, and show that in this model, perfectly secure MPC separates from perfectly secure SFE. Furthermore, we derive the exact conditions on the adversary structure for the existence of perfectly secure SFE resp.~MPC, and provide efficient protocols for both cases.

Eurocrypt 2019
TCC 2018
Crypto 2017
PKC 2017
TCC 2016
Asiacrypt 2015
Crypto 2014