Uniswap is regarded as the leading decentralized exchange (DEX) platform, allowing users to trade (i.e., swap) one asset for another without intermediaries, thanks to blockchain technology. The platform consists of smart contracts holding asset reserves, called liquidity pools, and its widespread adoption constitutes an opportunity for assessing whether the promises of decentralization of DEXes are effectively fulfilled in practice. For this reason, in this paper we propose an analysis of the Uniswap ecosystem, focusing on the implementation of version 3 on the Ethereum blockchain. First, we study liquidity pools and their core operations, aiming to quantify the overall activity levels. Next, we focus on liquidity provision to assess whether users diversify their participation across multiple pools. We then analyze asset swapping activity, identifying the most active pools and the most frequently traded assets. Finally, we perform a complex network analysis of multi-hop swaps, namely trades routed through multiple pools. We represent them as a graph and study its topological properties. Results show that pool creations and activity have grown steadily, but many pools trigger few operations and remain active for at most two weeks. Also, trader activity concentrates in pools involving the Wrapped Ether asset and major stablecoins, while about 60% of liquidity providers hold only a single position, thus participating in only one pool. Finally, network analysis suggested a preferential attachment phenomenon, with a few pools handling most swaps and acting symmetrically as both entry and exit bridges.

Non-Fungible Tokens (NFTs) are promising additions to the blockchain universe. After Ethereum introduced the NFT concept in 2015, it quickly rose in popularity, triggering other public blockchains to support their own versions of these new types of token by extending their smart contract capabilities to enable minting of these new tokens in their ecosystems. Ethereum is the reference for NFT development, establishing the token standards widely used today. The popularity of NFTs in Ethereum created a rich application ecosystem that also revealed limitations in scalability and throughput for this network. An attempt to solve these issues resulted in the creation of Flow, a new NFT-centric blockchain that claims to solve the scalability issues that plagued Ethereum. This paper presents the alternative NFT architecture introduced by Flow and how it compares with Ethereum’s version.

This paper considers the application scenario of Intellectual Property (IP) management, a business process yet to fully embrace digitisation and the advantages it brings. We propose to leverage Distributed Ledger Technology (DLT) to digitise license agreements management by providing automated and trustworthy royalty computation, transaction execution, and payment distribution. This can be achieved by employing smart licenses, i.e., bundles of smart contracts implementing the royalty logic of license agreements. To provide scalability, flexibility, and resilience, we propose to deploy smart licenses on a network of networks model, i.e., a set of heterogeneous networks potentially running different DLT protocols and connected by cross-chain information exchange protocols. A novel advantage of the network of networks approach is that it allows for the use of private values for royalty computation, which is impossible in the traditional model. Of course, supporting private DLT networks requires privacy-preserving cross-chain schemes, a still open problem in the literature. This is why we present two alternative privacy-preserving cross-chain schemes for our considered application scenario of license agreements management, one based on Homomorphic Encryption (HE) and the other on Zero-Knowledge (ZK) proofs. Besides discussing their theoretical advantages and drawbacks, we present an experimental evaluation of a prototype implementation of smart licenses based on both schemes.

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Decentralized applications, the driving force behind the new Web3 paradigm, require continuous access to blockchain data. Their adoption, however, is hindered by the constantly increasing size of blockchains and the sequential scan nature of their read operations, which introduce a clear inefficiency bottleneck. Also, the growing amount of data recorded on the blockchain makes resource-constrained light nodes dependent on untrusted full nodes for fetching information, with a consequent need for query authentication protocols ensuring result integrity. Motivated by these reasons, in this paper we propose the skip index, an indexing data structure that allows users to quickly retrieve information simultaneously from multiple blocks of a blockchain. Our solution is also designed to be used as an authenticated data structure to guarantee the integrity of query results for light nodes. We discuss the theoretical properties of skip indices, propose efficient algorithms for their construction and querying, and detail their computational complexity. Finally, we assess the effectiveness of our proposal through an experimental evaluation on the Ethereum blockchain. As a reference use case, we focus on the popular CryptoKitties application and simulate a scenario where users seek to retrieve the events generated by the service. Our experimental results suggest that the use of skip indices offers a constant multiplicative speedup, thanks to search times that are at most logarithmic within a chosen search window. This allows to reduce the number of visited blocks by up to two orders of magnitude if compared to the naive sequential approach currently in use. • We propose the skip index, a data structure for efficient blockchain data retrieval. • The skip index provides guarantees about the integrity of query results. • We devise efficient algorithms to construct and query skip indices. • Compared to a sequential scan, skip indices offer a constant multiplicative speedup. • Skip indices experimentally provide a speedup of up to two orders of magnitude.

Non-Fungible Tokens (NFTs) are currently used in a large number of scenarios, from digital art to the metaverse, to trace the ownership of assets exchanged between users. However, most NFT defining standards, such as the widely adopted ERC 721 for the Ethereum protocol, have been designed with immutable assets only in mind. As such, they are not suitable for representing assets with features that may need to be updated during their lifetime. To overcome this issue, in the literature have been proposed new models that properly represent and protect mutable assets through NFTs, such as Non Fungible Mutable Tokens, NMTs. In this paper, we expanded the NMT model with a security support meant to protect assets’ features updates through access control policies that are defined by the asset creator and the current asset owner and enforced during the assets’ lifetime. Policing updates is of paramount importance, because it protects the asset from unintended updates that could greatly alter the asset itself and its value. The main contributions of this paper are a detailed description of the NMT smart contracts architecture and internal dependencies, as well as an experimental validation of NMTs by providing the implementation of a NMT representing a wearable (a jacket) in Decentraland, a popular metaverse environment.

Attribute-Based Access Control (ABAC) systems are commonly used for protecting resources from unauthorized accesses. They perform the access decision process taking into account a set of attributes describing the features of the subjects executing the accesses, the resources being accessed, and the environment. In this paper, we propose to enhance traditional ABAC systems by integrating them with the novel Self Sovereign Identity (SSI) paradigm. More precisely, we propose a novel access control framework, leveraging the well known eXtensible Access Control Markup Language (XACML) standard for ABAC systems, which allows to exploit SSI credentials as sources of users’ attributes for policy evaluation. Finally, we showcase a prototype implementation of our proposal, and we analyze its performance when evaluating XACML policies integrating SSI credentials with a variable number of attributes.

For the viability of future “mission-critical” applications such as remote surgery and connected cars, the customers must trust the network connection and operators must adhere to the Service Level Agreement (SLA). The key to enabling trust between the customer and the operator is the transparency and accountability of the SLA. That is, the operators ensure the transparent and appropriate execution of SLA clauses (e.g., Quality of Service (QoS) and compensations if the SLA is violated). In this work, we argue that today’s network is highly volatile. Therefore, it is convenient for the operators to provide service guarantees for short-term rather than traditional long-term methods. Consequently, we advocate short-term and dynamic service contracts considering spatial and temporal characteristics rather than typical long-term contracts. We propose a Distributed Ledger Technology (DLT)-focused end-to-end transparent, accountable and automated resource provisioning system architecture in which are installed as smart contracts. In our architecture, resources can be requested and allocated dynamically and automatically with Quality-of-Service (QoS) monitoring. Our architecture is scalable through a side-channel based QoS monitoring protocol that guarantees data integrity and minimises the Permissioned Distributed Ledgers (PDL) updates. To measure the viability of our proposal, we first evaluate resource provisioning in the context of network slicing. Then we assess the DLT performance for smart contract execution, in both permissioned and permission-less settings. In the end, we evaluate the monitoring tools and compare and contrast sketches and bloomfilters, and justify our choice of bloomfilters.

The Self-Sovereign Identity (SSI) is a novel paradigm aimed at giving back users sovereignty over their digital identities. Adopting the SSI approach prevents users to have a distinct identity for each service they use, instead, use a unique decentralised identity for all the services they need to access. However, to really benefit from the SSI advantages, an actual decentralised implementation is needed to fit the specific requirements and limits of decentralised architectures, such as blockchain. To this aim, this paper proposes Algorand Identity (AlgoID), a new SSI framework for the Algorand blockchain which differs from the already existing one, because it is fully blockchain based, i.e., it exploits Algorand itself for the storage of the data identity and as the registry location. The proposed framework has been completely implemented and validated through experiments, showing that the time required to execute the framework operations is acceptably low in realistic use cases.

Recent years have witnessed, especially in Europe, a shift aimed at bringing users back at the center of digital systems. This has driven innovation towards the affirmation of decentralized systems, in line with the Self Sovereign Identity paradigm. User control over the consumption and disclosure of their data is a key topic of such drive. In this paper we show how it is possible to apply this increasingly popular concept to a traditionally centralized and opaque digital process: Access Control systems. To this aim we expand the XACML standard for Attribute Based Access Control systems with the novel concept of private attributes, i.e. attributes whose values should not be disclosed while still contributing to a policy evaluation result after user consent. Basing our proposal on blockchain systems, we show how to leverage smart contracts and zero knowledge proofs to allow for transparent policies evaluation without disclosing the value of such sensible attributes. Beside formalizing our goals, presenting the system architecture, and discussing its advantages and drawbacks with respect to the traditional model, we provide a reference example to show our proposal innovative capabilities and provide a prototype experimental evaluation to prove its feasibility.

The Lightning Network is a P2P overlay that allows two Bitcoin users to create a new payment channel with a transaction, and later exchange cryptocurrency on that channel independently from the Bitcoin blockchain, improving Bitcoin’s scalability issues and making it more suitable for frequent micropayments. Moreover, the channels can also be used to route payments between two users who do not share a direct channel themselves.In this paper we study the topology of the Lightning Network in a given time period, performing a number of analysis on the channels and on the nodes composing the network. We study the churn rate, we evaluate centrality measurements and the clustering coefficient to evaluate the network connectivity, and finally we analyze the presence of a pattern that we named "bouquet". The importance of this pattern is proven by the finding that, removing specific nodes of the "bouquets" (1% of the total) causes the disconnection from the largest component of about 41% nodes.

The viability of new mission-critical networked applications such as connected cars or remote surgery is heavily dependent on the availability of truly customized network services at a Quality of Service (QoS) level that both the network operator and the customer can agree on. This is difficult to achieve in today’s mainly "best effort" Internet. Even if a level of service were to be agreed upon between a consumer and an operator, it is important for both parties to be able to scalably and impartially monitor the quality of service delivered in order to enforce the service level agreement (SLA). Building upon a recently proposed architecture for automated negotiation of SLAs using smart contracts, we develop a low overhead solution for monitoring these SLAs and arranging automated payments based on the smart contracts. Our solution uses cryptographically secure bloom filters to create succinct summaries of the data exchanged over fine-grained epochs. We then use a state channel-based design for both parties to quickly and scalably agree and sign off on the data that was delivered in each epoch, making it possible to monitor and enforce at run time the agreed upon QoS levels.

This work proposes to exploit blockchain technology to define Access Control systems that guarantee the auditability of access control policies evaluation. The key idea of our proposal is to codify attribute-based Access Control policies as smart contracts and deploy them on a blockchain, hence transforming the policy evaluation process into a completely distributed smart contract execution. Not only the policies, but also the attributes required for their evaluation are managed by smart contracts deployed on the blockchain. The auditability property derives from the immutability and transparency properties of blockchain technology. This paper not only presents the proposed Access Control system in general, but also its application to the innovative reference scenario where the resources to be protected are themselves smart contracts. To prove the feasibility of our approach, we present a reference implementation exploiting XACML policies and Solidity written smart contracts deployed on the Ethereum blockchain. Finally, we evaluate the system performances through a set of experimental results, and we discuss the advantages and drawbacks of our proposal.

BITCOIN is a novel decentralized cryptocurrency system which has recently received a great attention from a wider audience. An interesting and unique feature of this system is that the complete list of all the transactions occurred from its inception is publicly available. This enables the investigation of funds movements to uncover interesting properties of the BITCOIN economy. In this paper we present a set of analyses of the user graph, i.e. the graph obtained by an heuristic clustering of the graph of BITCOIN transactions. Our analyses consider an up-to-date BITCOIN blockchain, as in December 2015, after the exponential explosion of the number of transactions occurred in the last two years. The set of analyses we defined includes, among others, the analysis of the time evolution of BITCOIN network, the verification of the "rich get richer" conjecture and the detection of the nodes which are critical for the network connectivity.