The European Central Bank is preparing for the potential issuance of a central bank digital currency (CBDC), called the digital euro. A recent regulatory proposal by the European Commission defines several requirements for the digital euro, such as support for both online and offline payments. Offline payments are expected to enable cash-like privacy, local payment settlement, and the enforcement of holding limits. While other central banks have expressed similar desired functionality, achieving such offline payments poses a novel technical challenge. We observe that none of the existing research solutions, including offline E-cash schemes, are fully compliant. Proposed solutions based on secure elements offer no guarantees in case of compromise and can therefore lead to significant payment fraud. The main contribution of this paper is PayOff, a novel CBDC design motivated by the digital euro regulation, which focuses on offline payments. We analyze the security implications of local payment settlement and identify new security objectives. PayOff protects user privacy, supports complex regulations such as holding limits, and implements safeguards to increase robustness against secure element failure. Our analysis shows that PayOff provides strong privacy and identifies residual leakages that may arise in real-world deployments. Our evaluation shows that offline payments can be fast and that the central bank can handle high payment loads with moderate computing resources. However, the main limitation of PayOff is that offline payment messages and storage requirements grow in the number of payments that the sender makes or receives without going online in between.

Decentralized storage faces a fundamental trade-off between replication overhead, recovery efficiency, and security guarantees. Current approaches either rely on full replication, incurring substantial storage costs, or employ erasure-coding schemes that struggle with efficient recovery, especially under high churn. We present Walrus, a decentralized blob storage system that addresses these limitations through multiple technical innovations. At the core of Walrus is Red Stuff, a two-dimensional erasure-coding protocol that achieves high security with only a 4.5x replication factor, while providing self-healing of lost data. This means that recovery is done without centralized coordination and requires bandwidth proportional to the amount of lost data. However, Red Stuff on its own is not sufficient for Walrus, as it is designed with a static set of participants in mind. To further support decentralization, we also introduce a multi-stage epoch-change protocol that efficiently handles storage node churn while maintaining uninterrupted availability during committee transitions. Our system incorporates authenticated data structures to defend against malicious clients and ensure data consistency throughout storage and retrieval. Walrus has been deployed in production since March 2025 and has secured 686 TB of data by July 2026. We conduct an experimental evaluation of the deployed system and demonstrate that Walrus achieves practical performance at scale and outperforms the Arweave decentralized storage system.

To realize the long-standing vision of providing quality-of-service (QoS) guarantees on a public Internet, this paper introduces Hummingbird: a lightweight QoS-system that provides fine-grained inter-domain reservations for end hosts. Hummingbird enables flexible and composable reservations with end-to-end guarantees, and addresses an often overlooked, but crucial, aspect of bandwidth-reservation systems: incentivization of network providers. Hummingbird represents bandwidth reservations as tradable assets, allowing markets to emerge. These markets then ensure fair and efficient resource allocation and encourage deployment by remunerating providers. This incentivization is facilitated by decoupling reservations from network identities, which enables novel control-plane mechanisms and allows the design of a control plane based on smart contracts. Hummingbird also provides an efficient reservation data plane, which streamlines the processing on routers and thus simplifies the implementation, deployment, and traffic policing, while maintaining robust security properties. Our prototype implementation demonstrates the efficiency and scalability of Hummingbird's asset-based control plane, and our high-speed software implementation can fill a 160 Gbps link with Hummingbird packets on commodity hardware.

Due to the popularity of blockchain-based cryptocurrencies, the increasing digitalization of payments, and the constantly reducing role of cash in society, central banks have shown an increased interest in deploying central bank digital currencies (CBDCs) that could serve as a digital cash-equivalent. While most recent research on CBDCs focuses on blockchain technology, it is not clear that this choice of technology provides the optimal solution. In particular, the centralized trust model of a CBDC offers opportunities for different designs. In this paper, we depart from blockchain designs and instead build on ideas from traditional e-cash schemes. We propose a new style of building digital currencies that combines the transaction processing model of e-cash with an account-based fund management model. We argue that such a style of building digital currencies is especially well-suited to CBDCs. We also design the first such digital currency system, called Platypus, that provides strong privacy, high scalability, and expressive but simple regulation, which are all critical features for a CBDC. Platypus achieves these properties by adapting techniques similar to those used in anonymous blockchain cryptocurrencies like Zcash to fit our account model and applying them to the e-cash context.

Due to the popularity of blockchain-based cryptocurrencies, the increasing digitalization of payments, and the constantly reducing role of cash in society, central banks have shown an increased interest in deploying central bank digital currencies (CBDCs) that could serve as a digital cash-equivalent. While most recent research on CBDCs focuses on blockchain technology, it is not clear that this choice of technology provides the optimal solution. In particular, the centralized trust model of a CBDC offers opportunities for different designs. In this paper, we depart from blockchain designs and instead build on ideas from traditional e-cash schemes. We propose a new style of building digital currencies that combines the transaction processing model of e-cash with an account-based fund management model. We argue that such a style of building digital currencies is especially well-suited to CBDCs. We also design the first such digital currency system, called Platypus, that provides strong privacy, high scalability, and expressive but simple regulation, which are all critical features for a CBDC. Platypus achieves these properties by adapting techniques similar to those used in anonymous blockchain cryptocurrencies like Zcash to fit our account model and applying them to the e-cash context.

A versatile non-repudiation service that can be used directly and without application-specific modifications is desirable in many realistic use-cases. Since TLS is one of the most popular current means of secure communication, several proposals have been made for augmenting it with a general, flexible and efficient non-repudiation service. However, none of them offers sufficient robustness for scenarios that require high reliability. Also, they lack flexibility by requiring the party providing non-repudiable evidence to do so for all content transmitted within a given TLS session. In this paper, we propose ROSEN, an extension for TLS that provides non-repudiation using an efficient checkpointing mechanism that minimizes loss of evidence in the presence of faults in order to increase robustness and ensure reliability. In addition, ROSEN inherits privacy-preserving properties of prior methods and introduces selective non-repudiation which allows the party providing non-repudiable evidence to selectively and efficiently redact parts of the session so as to make them repudiable.

In this paper we propose Bitcontracts, a novel solution that enables secure and efficient execution of generic smart contracts on top of unmodified legacy cryptocurrencies like Bitcoin that do not support contracts natively. The starting point of our solution is an off-chain execution model, where the contract's issuers appoints a set of service providers to execute the contract's code. The contract's execution results are accepted if a quorum of service providers reports the same result and clients are free to choose which such contracts they trust and use. The main technical contribution of this paper is how to realize such a trust model securely and efficiently without modifying the underlying blockchain. We also identify a set of generic properties that a blockchain system must support so that expressive smart contracts can be added safely, and analyze popular existing blockchains based on these criteria.

Central banks around the world are exploring and in some cases even piloting Central Bank Digital Currencies (CBDCs).CBDCs promise to realize a broad range of new capabilities, including direct government disbursements to citizens, frictionless consumer payment and money-transfer systems, and a range of new financial instruments and monetary policy levers.CBDCs also give rise, however, to a host of challenging technical goals and design questions that are qualitatively and quantitatively different from those in existing government and consumer payment systems.A well-functioning CBDC will require an extremely resilient, secure, and performant new infrastructure, with the ability to onboard, authenticate, and support users on massive scale.It will necessitate an architecture simple enough to support modular design and rigorous security analysis, but flexible enough to accommodate current and future functional requirements and use cases.A CBDC will also in some way need to address an innate tension between privacy and transparency, protecting user data from abuse while selectively permitting data mining for end-user services, policymakers, and law enforcement investigations and interventions.In this paper, we enumerate the fundamental technical design challenges facing CBDC designers, with a particular focus on performance, privacy, and security.Through a survey of relevant academic and industry research and deployed systems, we discuss the state of the art in technologies that can address the challenges involved in successful CBDC deployment.We also present a vision of the rich range of functionalities and use cases that a well-designed CBDC platform could ultimately offer users.

Off-chain protocols (or so-called Layer 2) are heralded as a scaling solution for cryptocurrencies. One prominent approach, state channels, allows a group of parties to transact amongst themselves and the global blockchain is only used as a last resort to self-enforce any disputed transactions. To evaluate state channels as a scaling solution, we provide a proof of concept implementation for a two-player battleship game. It fits a category of applications that are not considered reasonable to execute on the blockchain, but it is widely perceived as an ideal application for off-chain protocols. We explore the minimal modifications required to deploy the battleship game as a state channel and propose a new state channel construction, Kitsune, which combines features from existing constructions. While in the optimistic case we demonstrate the battleship game can be played efficiently in a state channel, the requirement for unanimous off-chain agreement introduces new economic and time-based attacks that can render the game as unreasonable to play.

Permissionless blockchains offer many advantages but also have significant limitations including high latency. This prevents their use in important scenarios such as retail payments, where merchants should approve payments fast. Prior works have attempted to mitigate this problem by moving transactions off the chain. However, such Layer-2 solutions have their own problems: payment channels require a separate deposit towards each merchant and thus significant locked-in funds from customers; payment hubs require very large operator deposits that depend on the number of customers; and side-chains require trusted validators. In this paper, we propose Snappy, a novel solution that enables recipients, like merchants, to safely accept fast payments. In Snappy, all payments are on the chain, while small customer collaterals and moderate merchant collaterals act as payment guarantees. Besides receiving payments, merchants also act as statekeepers who collectively track and approve incoming payments using majority voting. In case of a double-spending attack, the victim merchant can recover lost funds either from the collateral of the malicious customer or a colluding statekeeper (merchant). Snappy overcomes the main problems of previous solutions: a single customer collateral can be used to shop with many merchants; merchant collaterals are independent of the number of customers; and validators do not have to be trusted. Our Ethereum prototype shows that safe, fast (<2 seconds) and cheap payments are possible on existing blockchains.

Cryptocurrencies record transactions between parties in a blockchain maintained by a peer-to-peer network. In most cryptocurrencies, transactions explicitly identify the previous transaction providing the funds they are spending, revealing the amount and sender/recipient pseudonyms. This is a considerable privacy issue. Zerocash resolves this by using zero-knowledge proofs to hide both the source, destination and amount of the transacted funds. To receive payments in Zerocash, however, the recipient must scan the blockchain, testing if each transaction is destined for them. This is not practical for mobile and other bandwidth constrained devices. In this paper, we build ZLiTE, a system that can support the so called “light clients”, which can receive transactions aided by a server equipped with a Trusted Execution Environment. Even with the use of a TEE, this is not a trivial problem. First, we must ensure that server processing the blockchain does not leak sensitive information via side channels. Second, we need to design a bandwidth efficient mechanism for the client to keep an up-to-date version of the witness needed in order to spend the funds they previously received.

In contrast to traditional contracts, cryptocurrencybased smart contracts can provide improved business automation and more transparency. However, not all cryptocurrencies support expressive contracts. For example, Bitcoin only supports a restricted scripting language that is not expressive enough to realize many contracts. Ethereum supports a Turing-complete programming language, but the types of contracts that can be implemented are still severely constrained due to gas limits. Recent research has explored ways to add contract support to legacy currencies like Bitcoin or enable more complex contracts on systems like Ethereum, but such previous solutions have significant security and functional limitations. In this paper we propose Bitcontracts, a novel solution to enable generic and expressive smart contracts on legacy cryptocurrencies. The starting point of our solution is a common off-chain execution model, where the contract’s issuers appoints a set of service providers to execute the contract’s code; the contract’s execution results are accepted if a quorum of service providers reports the same result; and clients are free to choose which such contracts they trust and use. The main technical challenge of this paper is how to realize such a trust model securely and efficiently without modifying the underlying blockchain. Bitcontracts achieves this using two main techniques. First, the state of each contract is stored on the chain which avoids the need to run expensive consensus protocols between the service providers. Second, the validity of each execution result is bound to the latest state of the chain to prevent double-spending attacks. Bitcontracts can be used to retrofit contracts to currencies like Bitcoin or to extend the contract execution capabilities of systems like Ethereum. We also identify a set of generic properties that a blockchain system must support so that expressive smart contracts can be added safely and efficiently, and analyze existing blockchains based on these criteria.

Fiat currency implemented as a blockchain can enable multiple benefits such as reduced cost compared to expensive handling of cash and better transparency for increased public trust. However, such deployments have conflicting requirements including fast payments, strong user privacy and regulatory oversight. None of the existing blockchain transaction techniques supports all of these three requirements. In this paper we design a new blockchain currency, called PRCash, that addresses the above challenge. The primary technical contribution of our work is a novel regulation mechanism for transactions that use cryptographic commitments. We enable regulation of spending limits using zero-knowledge proofs. PRCash is the first blockchain currency that provides fast payments, good level of user privacy and regulatory control at the same time.

Blockchain is being praised as a technological innovation which allows to revolutionize how society trades and interacts. This reputation is in particular attributable to its properties of allowing mutually mistrusting entities to exchange financial value and interact without relying on a trusted third party. A blockchain moreover provides an integrity protected data storage and allows to provide process transparency. In this paper we critically analyze whether a blockchain is indeed the appropriate technical solution for a particular application scenario. We differentiate between permissionless (e.g., Bitcoin/Ethereum) and permissioned (e.g. Hyperledger/Corda) blockchains and contrast their properties to those of a centrally managed database. We provide a structured methodology to determine the appropriate technical solution to solve a particular application problem. Given our methodology, we analyze in depth three use cases - Supply Chain Management, Interbank and International Payments, and Decentralized Autonomous Organizations and conclude the article with an outlook for further opportunities.

Decentralized cryptocurrencies based on blockchains provide attractive features, including user privacy and system transparency, but lack active control of money supply and capabilities for regulatory oversight, both existing features of modern monetary systems. These limitations are critical, especially if the cryptocurrency is to replace, or complement, existing fiat currencies. Centralized cryptocurrencies, on the other hand, provide controlled supply of money, but lack transparency and transferability. Finally, they provide only limited privacy guarantees, as they do not offer recipient anonymity or payment value secrecy. We propose a novel digital currency, called PRCash, where the control of money supply is centralized, money is represented as value-hiding transactions for transferability and improved privacy, and transactions are verified in a distributed manner and published to a public ledger for verifiability and transparency. Strong privacy and regulation are seemingly conflicting features, but we overcome this technical problem with a new regulation mechanism based on zero-knowledge proofs. Our implementation and evaluation shows that payments are fast and large-scale deployments practical. PRCash is the first digital currency to provide control of money supply, transparency, regulation, and privacy at the same time, and thus make its adoption as a fiat currency feasible.

We present a novel approach for automatic repair of corrupted files that applies to any common file format and does not require knowledge of its structure. Our lightweight approach modifies the execution of a file viewer instead of the file data and makes use of instrumentation and execution hijacking, two techniques from software testing. It uses a file viewer as a black box and does not require access to its source code or any knowledge about its inner workings. We present our implementation of this approach and evaluate it on corrupted PNG, JPEG, and PDF files. © 2017 The Author(s). Published by Elsevier Ltd on behalf of DFRWS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

We present a novel approach for automatic repair of corrupted files that applies to any common file format and does not require knowledge of its structure. Our lightweight approach modifies the execution of a file viewer instead of the file data and makes use of instrumentation and execution hijacking, two techniques from software testing. It uses a file viewer as a black box and does not require access to its source code or any knowledge about its inner workings. We present our implementation of this approach and evaluate it on corrupted PNG, JPEG, and PDF files.

Proof of Work (PoW) powered blockchains currently account for more than 90% of the total market capitalization of existing digital cryptocurrencies. Although the security provisions of Bitcoin have been thoroughly analysed, the security guarantees of variant (forked) PoW blockchains (which were instantiated with different parameters) have not received much attention in the literature. This opens the question whether existing security analysis of Bitcoin's PoW applies to other implementations which have been instantiated with different consensus and/or network parameters. In this paper, we introduce a novel quantitative framework to analyse the security and performance implications of various consensus and network parameters of PoW blockchains. Based on our framework, we devise optimal adversarial strategies for double-spending and selfish mining while taking into account real world constraints such as network propagation, different block sizes, block generation intervals, information propagation mechanism, and the impact of eclipse attacks. Our framework therefore allows us to capture existing PoW-based deployments as well as PoW blockchain variants that are instantiated with different parameters, and to objectively compare the tradeoffs between their performance and security provisions.