5 Search Results for "Karakostas, Dimitris"

Document
DOI: 10.4230/LIPIcs.DISC.2025.2
On the Efficiency of Dynamic Transaction Scheduling in Blockchain Sharding

Authors: Ramesh Adhikari, Costas Busch, and Miroslav Popovic

Published in: LIPIcs, Volume 356, 39th International Symposium on Distributed Computing (DISC 2025)

Abstract
Sharding is a technique to speed up transaction processing in blockchains, where the n processing nodes in the blockchain are divided into s disjoint groups (shards) that can process transactions in parallel. We study dynamic scheduling problems on a shard graph G_s where transactions arrive online over time and are not known in advance. Each transaction may access at most k shards, and we denote by d the worst distance between a transaction and its accessing (destination) shards (the parameter d is unknown to the shards). To handle different values of d, we assume a locality sensitive decomposition of G_s into clusters of shards, where every cluster has a leader shard that schedules transactions for the cluster. We first examine the simpler case of the stateless model, where leaders are not aware of the current state of the transaction accounts, and we prove a O(d log² s ⋅ min{k, √s}) competitive ratio for latency. We then consider the stateful model, where leader shards gather the current state of accounts, and we prove a O(log s⋅ min{k, √s}+log² s) competitive ratio for latency. Each leader calculates the schedule in polynomial time for each transaction that it processes. We show that for any ε > 0, approximating the optimal schedule within a (min{k, √s})^{1 -ε} factor is NP-hard. Hence, our bound for the stateful model is within a poly-log factor from the best possibly achievable. To the best of our knowledge, this is the first work to establish provably efficient dynamic scheduling algorithms for blockchain sharding systems.
Cite as

Ramesh Adhikari, Costas Busch, and Miroslav Popovic. On the Efficiency of Dynamic Transaction Scheduling in Blockchain Sharding. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 2:1-2:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{adhikari_et_al:LIPIcs.DISC.2025.2,
  author =	{Adhikari, Ramesh and Busch, Costas and Popovic, Miroslav},
  title =	{{On the Efficiency of Dynamic Transaction Scheduling in Blockchain Sharding}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{2:1--2:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-402-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{356},
  editor =	{Kowalski, Dariusz R.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2025.2},
  URN =		{urn:nbn:de:0030-drops-248191},
  doi =		{10.4230/LIPIcs.DISC.2025.2},
  annote =	{Keywords: Blockchain, Blockchain Sharding, Dynamic Transaction Scheduling}
}
Document
DOI: 10.4230/LIPIcs.AFT.2025.19
Two-Tier Black-Box Blockchains and Application to Instant Layer-1 Payments

Authors: Michele Ciampi, Yun Lu, Rafail Ostrovsky, and Vassilis Zikas

Published in: LIPIcs, Volume 354, 7th Conference on Advances in Financial Technologies (AFT 2025)

Abstract
Common blockchain protocols are monolithic, i.e., their security relies on a single assumption, e.g., honest majority of hashing power (Bitcoin) or stake (Cardano, Algorand, Ethereum). In contrast, so-called optimistic approaches (Thunderella, Meshcash) rely on a combination of assumptions to achieve faster transaction liveness. We revisit, redesign, and augment the optimistic paradigm to a tiered approach. Our design assumes a primary (Tier 1) and a secondary (Tier 2, also referred to as fallback) blockchain, and achieves full security also in a tiered fashion: If the assumption underpinning the primary chain holds, then we guarantee safety, liveness and censorship resistance, irrespectively of the status of the fallback chain. And even if the primary assumption fails, all security properties are still satisfied (albeit with a temporary slow down) provided the fallback assumption holds. To our knowledge, no existing optimistic or tiered approach preserves both safety and liveness when any one of its underlying blockchain (assumptions) fails. The above is achieved by a new detection-and-recovery mechanism that links the two blockchains, so that any violation of safety, liveness, or censorship resistance on the (faster) primary blockchain is temporary - it is swiftly detected and recovered on the secondary chain - and thus cannot result in a persistent fork or halt of the blockchain ledger. We instantiate the above paradigm using a primary chain based on proof of reputation (PoR) and a fallback chain based on proof of stake (PoS). Our construction uses the PoR and PoS blockchains in a mostly black-box manner - where rather than assuming a concrete construction we distil abstract properties on the two blockchains that are sufficient for applying our tiered methodology. In fact, choosing reputation as the resource of the primary chain opens the door to an incentive mechanism - which we devise and analyze - that tokenizes reputation in order to deter cheating and boost participation (on both the primary/PoR and the fallback/PoS blockchain). As we demonstrate, such tokenization in combination with interpreting reputation as a built-in system-wide credit score, allows for embedding in our two-tiered methodology a novel mechanism which provides collateral-free, multi-use payment-channel-like functionality where payments can be instantly confirmed.
Cite as

Michele Ciampi, Yun Lu, Rafail Ostrovsky, and Vassilis Zikas. Two-Tier Black-Box Blockchains and Application to Instant Layer-1 Payments. In 7th Conference on Advances in Financial Technologies (AFT 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 354, pp. 19:1-19:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{ciampi_et_al:LIPIcs.AFT.2025.19,
  author =	{Ciampi, Michele and Lu, Yun and Ostrovsky, Rafail and Zikas, Vassilis},
  title =	{{Two-Tier Black-Box Blockchains and Application to Instant Layer-1 Payments}},
  booktitle =	{7th Conference on Advances in Financial Technologies (AFT 2025)},
  pages =	{19:1--19:24},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-400-0},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{354},
  editor =	{Avarikioti, Zeta and Christin, Nicolas},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.AFT.2025.19},
  URN =		{urn:nbn:de:0030-drops-247380},
  doi =		{10.4230/LIPIcs.AFT.2025.19},
  annote =	{Keywords: Fault tolerant blockchain, instantly confirmed payments}
}
Document
DOI: 10.4230/LIPIcs.AFT.2025.22
Single-Token vs Two-Token Blockchain Tokenomics

Authors: Aggelos Kiayias, Philip Lazos, and Paolo Penna

Published in: LIPIcs, Volume 354, 7th Conference on Advances in Financial Technologies (AFT 2025)

Abstract
We study long-term equilibria that arise in the token monetary policy, or tokenomics, design of proof-of-stake (PoS) blockchain systems that engage utility maximizing users and validators. Validators are system maintainers who get rewarded with tokens for performing the work necessary for the system to function properly, while users compete and pay with such tokens for getting a desired portion of the system service. We study how the system service provision and suitable rewards schemes together can lead to equilibria with the following desirable characteristics (1) viability: the system keeps parties engaged, (2) decentralization and skin-in-the-game: multiple sufficiently invested validators are participating, (3) stability: the price path of the underlying token used to transact with the system does not change widely over time, and (4) feasibility: the mechanism is easy to implement as a smart contract, e.g., it does not require a fiat reserve on-chain to perform token buybacks or to perform bookkeeping of exponentially growing token holdings. Our analysis enables us to put forward a novel generic mechanism for blockchain monetary policy that we call quantitative rewarding (QR). We investigate how to implement QR in single-token and two-token proof of stake (PoS) blockchain systems. The latter are systems that utilize one token for the users to pay the transaction fees and a different token for the validators to participate in the PoS protocol and get rewarded. Our approach demonstrates a concrete advantage of the two-token setting in terms of the ability of the QR mechanism to be realized effectively and provide good equilibria. Our analysis also reveals an inherent limitation of the single token setting in terms of implementing an effective blockchain monetary policy - a distinction that is, to the best of our knowledge, highlighted for the first time.
Cite as

Aggelos Kiayias, Philip Lazos, and Paolo Penna. Single-Token vs Two-Token Blockchain Tokenomics. In 7th Conference on Advances in Financial Technologies (AFT 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 354, pp. 22:1-22:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{kiayias_et_al:LIPIcs.AFT.2025.22,
  author =	{Kiayias, Aggelos and Lazos, Philip and Penna, Paolo},
  title =	{{Single-Token vs Two-Token Blockchain Tokenomics}},
  booktitle =	{7th Conference on Advances in Financial Technologies (AFT 2025)},
  pages =	{22:1--22:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-400-0},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{354},
  editor =	{Avarikioti, Zeta and Christin, Nicolas},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.AFT.2025.22},
  URN =		{urn:nbn:de:0030-drops-247412},
  doi =		{10.4230/LIPIcs.AFT.2025.22},
  annote =	{Keywords: Blockchain, tokenomics, buyback, equilibria, price path, stable price, discounted game, dual-token, proof-of-stake, validator}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2024.7
Incentive Compatibility of Ethereum’s PoS Consensus Protocol

Authors: Ulysse Pavloff, Yackolley Amoussou-Guenou, and Sara Tucci-Piergiovanni

Published in: LIPIcs, Volume 324, 28th International Conference on Principles of Distributed Systems (OPODIS 2024)

Abstract
This paper investigates whether following the fork-choice rule in the Ethereum PoS consensus protocol constitutes a Nash equilibrium - i.e., whether the protocol that maintains the canonical chain in Ethereum is incentive-compatible. Specifically, we explore whether selfish participants may attempt to manipulate the fork-choice rule by forking out previous blocks and capturing the rewards associated with those blocks. Our analysis considers two strategies for participants: the obedient strategy, which adheres to the prescribed protocol, and the cunning strategy, which attempts to manipulate the fork-choice rule to gain more rewards. We evaluate the conditions under which selfish participants might deviate from the obedient strategy. We found that, in a synchronous system, following the prescribed fork-choice rule is incentive-compatible. However, in an eventually synchronous system, the protocol is eventually incentive-compatible - that is, only a limited number of proposers will find it profitable to fork the chain during the synchronous period. After this sequence of cunning proposers, subsequent proposers will find it more profitable to follow the protocol.
Cite as

Ulysse Pavloff, Yackolley Amoussou-Guenou, and Sara Tucci-Piergiovanni. Incentive Compatibility of Ethereum’s PoS Consensus Protocol. In 28th International Conference on Principles of Distributed Systems (OPODIS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 324, pp. 7:1-7:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{pavloff_et_al:LIPIcs.OPODIS.2024.7,
  author =	{Pavloff, Ulysse and Amoussou-Guenou, Yackolley and Tucci-Piergiovanni, Sara},
  title =	{{Incentive Compatibility of Ethereum’s PoS Consensus Protocol}},
  booktitle =	{28th International Conference on Principles of Distributed Systems (OPODIS 2024)},
  pages =	{7:1--7:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-360-7},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{324},
  editor =	{Bonomi, Silvia and Galletta, Letterio and Rivi\`{e}re, Etienne and Schiavoni, Valerio},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2024.7},
  URN =		{urn:nbn:de:0030-drops-225431},
  doi =		{10.4230/LIPIcs.OPODIS.2024.7},
  annote =	{Keywords: Ethereum PoS, Game Theory, Block Reward}
}
Document
DOI: 10.4230/OASIcs.Tokenomics.2019.7
Cryptocurrency Egalitarianism: A Quantitative Approach

Authors: Dimitris Karakostas, Aggelos Kiayias, Christos Nasikas, and Dionysis Zindros

Published in: OASIcs, Volume 71, International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2019)

Abstract
Since the invention of Bitcoin one decade ago, numerous cryptocurrencies have sprung into existence. Among these, proof-of-work is the most common mechanism for achieving consensus, whilst a number of coins have adopted "ASIC-resistance" as a desirable property, claiming to be more "egalitarian," where egalitarianism refers to the power of each coin to participate in the creation of new coins. While proof-of-work consensus dominates the space, several new cryptocurrencies employ alternative consensus, such as proof-of-stake in which block minting opportunities are based on monetary ownership. A core criticism of proof-of-stake revolves around it being less egalitarian by making the rich richer, as opposed to proof-of-work in which everyone can contribute equally according to their computational power. In this paper, we give the first quantitative definition of a cryptocurrency’s egalitarianism. Based on our definition, we measure the egalitarianism of popular cryptocurrencies that (may or may not) employ ASIC-resistance, among them Bitcoin, Ethereum, Litecoin, and Monero. Our simulations show, as expected, that ASIC-resistance increases a cryptocurrency’s egalitarianism. We also measure the egalitarianism of a stake-based protocol, Ouroboros, and a hybrid proof-of-stake/proof-of-work cryptocurrency, Decred. We show that stake-based cryptocurrencies, under correctly selected parameters, can be perfectly egalitarian, perhaps contradicting folklore belief.
Cite as

Dimitris Karakostas, Aggelos Kiayias, Christos Nasikas, and Dionysis Zindros. Cryptocurrency Egalitarianism: A Quantitative Approach. In International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2019). Open Access Series in Informatics (OASIcs), Volume 71, pp. 7:1-7:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{karakostas_et_al:OASIcs.Tokenomics.2019.7,
  author =	{Karakostas, Dimitris and Kiayias, Aggelos and Nasikas, Christos and Zindros, Dionysis},
  title =	{{Cryptocurrency Egalitarianism: A Quantitative Approach}},
  booktitle =	{International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2019)},
  pages =	{7:1--7:21},
  series =	{Open Access Series in Informatics (OASIcs)},
  ISBN =	{978-3-95977-108-5},
  ISSN =	{2190-6807},
  year =	{2020},
  volume =	{71},
  editor =	{Danos, Vincent and Herlihy, Maurice and Potop-Butucaru, Maria and Prat, Julien and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/OASIcs.Tokenomics.2019.7},
  URN =		{urn:nbn:de:0030-drops-119715},
  doi =		{10.4230/OASIcs.Tokenomics.2019.7},
  annote =	{Keywords: blockchain, egalitarianism, cryptocurrency, economics, proof-of-work, proof-of-stake}
}
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