In leakage resilient cryptography, there is a seemingly inherent restraint on the ability of the adversary that it cannot get access to the leakage oracle after the challenge. Recently, a series of works made a breakthrough to consider a postchallenge leakage. They presented achievable public key encryption (PKE) schemes which are semantically secure against after-the-fact leakage in the split-state model. This model puts a more acceptable constraint on adversary’s ability that the adversary cannot query the leakage of secret states as a whole but the functions of several parts separately instead of prechallenge query only. To obtain security against chosen ciphertext attack (CCA) for PKE schemes against after-the-fact leakage attack (AFL), existing works followed the paradigm of “double encryption” which needs noninteractive zero knowledge (NIZK) proofs in the encryption algorithm. We present an alternative way to achieve AFL-CCA security via lossy trapdoor functions (LTFs) without NIZK proofs. First, we formalize the definition of LTFs secure against AFL (AFLR-LTFs) and all-but-one variants (ABO). Then, we show how to realize this primitive in the split-state model. This primitive can be used to construct AFLR-CCA secure PKE scheme in the same way as the method of “CCA from LTFs” in traditional sense.
Machine learning is promising, but it often needs to process vast amounts of\nsensitive data which raises concerns about privacy. In this white-paper, we\nintroduce Substra, a distributed framework for privacy-preserving, traceable\nand collaborative Machine Learning. Substra gathers data providers and\nalgorithm designers into a network of nodes that can train models on demand but\nunder advanced permission regimes. To guarantee data privacy, Substra\nimplements distributed learning: the data never leave their nodes; only\nalgorithms, predictive models and non-sensitive metadata are exchanged on the\nnetwork. The computations are orchestrated by a Distributed Ledger Technology\nwhich guarantees traceability and authenticity of information without needing\nto trust a third party. Although originally developed for Healthcare\napplications, Substra is not data, algorithm or programming language specific.\nIt supports many types of computation plans including parallel computation plan\ncommonly used in Federated Learning. With appropriate guidelines, it can be\ndeployed for numerous Machine Learning use-cases with data or algorithm\nproviders where trust is limited.\n
In the big data era, data are envisioned as critical resources with various values, e.g., business intelligence, management efficiency, and financial evaluations. Data sharing is always mandatory for value exchanges and profit promotion. Currently, certain big data markets have been created for facilitating data dissemination and coordinating data transaction, but we have to assume that such centralized management of data sharing must be trustworthy for data privacy and sharing fairness, which very likely imposes limitations such as joining admission, sharing efficiency, and extra costly commissions. To avoid these weaknesses, in this paper, we propose a blockchain-based fair data exchange scheme, called FaDe. FaDe can enable de-centralized data sharing in an autonomous manner, especially guaranteeing trade fairness, sharing efficiency, data privacy, and exchanging automation. A fairness protocol based on bit commitment is proposed. An algorithm based on blockchain script architecture for a smart contract, e.g., by a bitcoin virtual machine, is also proposed and implemented. Extensive analysis justifies that the proposed scheme can guarantee data exchanging without a trusted third party fairly, efficiently, and automatically.
With the increasing interest in connected vehicles along with electrification opportunities, there is an ongoing effort to automate the charging process of electric vehicles (EVs) through their capabilities to communicate with the infrastructure and each other. However, charging EVs takes time and thus in-advance scheduling is needed. As this process is done frequently due to limited mileage of EVs, it may expose the locations and charging pattern of the EV to the service providers, raising privacy concerns for their users. Nevertheless, the EV still needs to be authenticated to charging providers, which means some information will need to be provided anyway. While there have been many studies to address the problem of privacy-preserving authentication for vehicular networks, such solutions will be void if charging payments are made through traditional means. In this thesis, we tackle this problem by utilizing distributed applications enabled by Blockchain and smart contracts. We adapt zero-knowledge proofs to Blockchain for enabling privacy-preserving authentication while removing the need for a central authority. We introduce two approaches, one using a token-based mechanism and another utilizing the Pederson Commitment scheme to realize anonymous authentication. We also describe a protocol for the whole process which includes scheduling and charging operations. The evaluation of the proposed approaches indicates that the overhead of this process is affordable to enable real-time charging operations for connected EVs.
Ethereum is a distributed blockchain that can execute smart contracts, which inter-communicate and perform transactions automatically. The execution of smart contracts is paid in the form of gas, which is a monetary unit used in the Ethereum blockchain. The Ethereum Virtual Machine (EVM) provides the metering capability for smart contract execution. Instruction costs vary depending on the instruction type and the approximate computational resources required to execute the instruction on the network. The cost of gas is adjusted using transaction fees to ensure adequate payment of the network. In this work, we highlight the "real" economics of smart contracts. We show that the actual costs of executing smart contracts are disproportionate to the computational costs and that this gap is continuously widening. We show that the gas cost-model of the underlying EVM instruction-set is wrongly modeled. Specifically, the computational cost for the SLOAD instruction increases with the length of the blockchain. Our proposed performance model estimates gas usage and execution time of a smart contract at a given block-height. The new gas-cost model incorporates the block-height to eliminate irregularities in the Ethereum gas calculations. Our findings are based on extensive experiments over the entire history of the EVM blockchain.
Georgia Avarikioti, Antoine Desjardins, Eleftherios Kokoris-Kogias, Roger Wattenhofer
Sharding distributed ledgers is a promising on-chain solution for scaling blockchains but lacks formal grounds, nurturing skepticism on whether such complex systems can scale blockchains securely. We fill this gap by introducing the first formal framework as well as a roadmap to robust sharding. In particular, we first define the properties sharded distributed ledgers should fulfill. We build upon and extend the Bitcoin backbone protocol by defining consistency and scalability. Consistency encompasses the need for atomic execution of cross-shard transactions to preserve safety, whereas scalability encapsulates the speedup a sharded system can gain in comparison to a non-sharded system. Using our model, we explore the limitations of sharding. We show that a sharded ledger with $n$ participants cannot scale under a fully adaptive adversary, but it can scale up to $m$ shards where $n=c'm\log m$, under an epoch-adaptive adversary; the constant $c'$ encompasses the trade-off between security and scalability. This is possible only if the sharded ledgers create succinct proofs of the valid state updates at every epoch. We leverage our results to identify the sufficient components for robust sharding, which we incorporate in a protocol abstraction termed Divide & Scale. To demonstrate the power of our framework, we analyze the most prominent sharded blockchains (Elastico, Monoxide, OmniLedger, RapidChain) and pinpoint where they fail to meet the desired properties.
Georgia Avarikioti, Eleftherios Kokoris-Kogias, Roger Wattenhofer
Sharding distributed ledgers is the most promising on-chain solution for scaling blockchain technology. In this work, we define and analyze the properties a sharded distributed ledger should fulfill. More specifically, we show that a sharded blockchain cannot be scalable under a fully adaptive adversary, but it can scale up to $O(n/\log n)$ under an epoch-adaptive adversary. This is possible only if the distributed ledger creates succinct proofs of the valid state updates at the end of each epoch. Our model builds upon and extends the Bitcoin backbone protocol by defining consistency and scalability. Consistency encompasses the need for atomic execution of cross-shard transactions to preserve safety, whereas scalability encapsulates the speedup a sharded system can gain in comparison to a non-sharded system. We introduce a protocol abstraction and highlight the sufficient components for secure and efficient sharding in our model. In order to show the power of our framework, we analyze the most prominent shared blockchains (Elastico, Monoxide, OmniLedger, RapidChain) and pinpoint where they fail to meet the desired properties.
Blockchain is an emerging technology that promises many exciting applications in various fields, including financial, medical, energy, and logistics management. However, there are still some limitations in the existing blockchain framework that prevents its widespread adoption in the commercial world. One important limitation is the storage requirement, wherein each blockchain node has to store a copy of the distributed ledger. Thus, as the number of transactions increases, this storage requirement grows quadratically, eventually limiting the scalability of a blockchain system. Moreover, the public ledger in a blockchain framework allows anyone in the network to audit the transaction, which may not be favourable in some privacy-sensitive applications. In this paper, a secret-sharing scheme is proposed to reduce the size of the blockchain transactions. Each transaction block is divided into t parts, and the size of each part is 1/ t size of transaction block. We use the secret-sharing mechanism to share t parts into n shares. Hence, each node stores not one transaction but one share in the blockchain system. The proposed scheme can eventually reduce the storage cost of a blockchain transaction by 1/ t without introducing an additional recovery communication cost; however, robustness is reduced in node failure as a tradeoff. Meanwhile, the proposed scheme was more efficient and secure compared to other state-of-the-art schemes that aim to reduce blockchain storage for industrial big data.
Jonathan Passerat‐Palmbach, Tyler Farnan, Robert Miller, Marielle S. Gross · 6 authors
We propose a novel architecture for federated learning within healthcare consortia. At the heart of the solution is a unique integration of privacy preserving technologies, built upon native enterprise blockchain components available in the Ethereum ecosystem. We show how the specific characteristics and challenges of healthcare consortia informed our design choices, notably the conception of a new Secure Aggregation protocol assembled with a protected hardware component and an encryption toolkit native to Ethereum. Our architecture also brings in a privacy preserving audit trail that logs events in the network without revealing identities.
Marco Baldi, Franco Chiaraluce, Migelan Kodra, Luca Spalazzi
We consider a blockchain-based protocol for the certification of academic credentials named Blockcerts, which is currently used worldwide for validating digital certificates of competence compliant with the Open Badges standard. We study the certification steps that are performed by the Blockcerts protocol to validate a certificate, and find that they are vulnerable to a certain type of impersonation attacks. More in detail, authentication of the issuing institution is performed by retrieving an unauthenticated issuer profile online, and comparing some data reported there with those included in the issued certificate. We show that, by fabricating a fake issuer profile and generating a suitably altered certificate, an attacker is able to impersonate a legitimate issuer and can produce certificates that cannot be distinguished from originals by the Blockcerts validation procedure. We also propose some possible countermeasures against an attack of this type, which require the use of a classic public key infrastructure or a decentralized identity system integrated with the Blockcerts protocol.
Muneeb Ul Hassan, Mubashir Husain Rehmani, Jinjun Chen
Blockchain has received a widespread attention because of its decentralized, tamper-proof, and transparent nature. Blockchain works over the principle of distributed, secured, and shared ledger, which is used to record, and track data within a decentralized network. This technology has successfully replaced certain systems of economic transactions in organizations and has the potential to overtake various industrial business models in future. Blockchain works over peer-to-peer (P2P) phenomenon for its operation and does not require any trusted-third party authorization for data tracking and storage. The information stored in blockchain is distributed throughout the decentralized network and is usually protected using cryptographic hash functions. Since the beginning of blockchain technology, its use in different applications is increasing exponentially, but this increased use has also raised some questions regarding privacy and security of data being stored in it. Protecting privacy of blockchain data using data perturbation strategy such as differential privacy could be a novel approach to overcome privacy issues in blockchain. In this article, we cover the topic of integration of differential privacy in each layer of blockchain and in certain blockchain based scenarios. Moreover, we highlight some future challenges and application scenarios in which integration of differential privacy in blockchain can produce fruitful results.
The Nakamoto longest chain protocol has served Bitcoin well in its decade long existence. It is remarkably simple and uses only basic cryptographic primitives, but its proof-of-work framework is energy wasting. Proof-of-stake (PoS) protocols are an energy efficient alternative; however they are significantly complicated and promise weaker security guarantees. An effort to mimic the Nakamoto protocol directly in the PoS setting is made in [10, 11] with security shown only for a class of purely private attacks. In this paper we demonstrate a new, and fatal, attack on the protocol of [10, 11]. This attack motivates the design of a new family of Nakamoto-style longest chain PoS protocols, with a formal proof of their security against all possible attacks in a general security model.
The proof-of-stake (PoS) protocols aim to reduce the unnecessary computing power waste seen in Bitcoin. Various practical and provably secure designs have been proposed, like Ouroboros Praos (Eurocrypt 2018) and Snow White (FC 2019). However, the essential security property of unpredictability in these protocols remains insufficiently explored. This paper delves into this property in the cryptographic setting to achieve the "best possible" unpredictability for PoS. We first present an impossibility result for all PoS protocols under the single-extension design framework, where each honest player extends one chain per round. The state-of-the-art permissionless PoS protocols (e.g., Praos, Snow White, and more), are all under this single-extension framework. Our impossibility result states that, if a single-extension PoS protocol achieves the best possible unpredictability, then this protocol cannot be proven secure unless more than 73% of stake is honest. To overcome this impossibility, we introduce a new design framework called multi-extension PoS, allowing each honest player to extend multiple chains using a greedy strategy in a round. This strategy allows us to construct a class of PoS protocols that achieve the best possible unpredictability. It is noteworthy that these protocols can be proven secure, assuming a much smaller fraction (e.g., 57%) of stake to be honest.
Les primitives lossy trapdoor, preuve à divulgation nulle de connaissance et applications Dans cette thèse, nous étudions deux primitives différentes : les lossy trapdoor functions (LTF) et les systèmes de preuve à divulgation nulle de connaissance. Les LTFs sont des familles de fonctions dans lesquelles les fonctions injectives et les fonctions lossy sont calculatoirement indistinguables. Depuis leur introduction, elles se sont avérées utiles pour la construction de diverses primitives cryptographiques. Nous donnons dans cette thèse des constructions efficaces d’une variante de la LTF : le filtre algébrique lossy. Avec cette variante, nous pouvons améliorer l’efficacité du schéma de chiffrement KDM-CCA et extracteur flous. Dans la deuxième partie de cette thèse, nous étudions les constructions de systèmes de preuve à divulgation nullle de connaissance. Nous donnons la première signature d’anneau de taille logarithmique avec la sécurité étroite en utilisant une variante de Groth-Kolhweiz Σ-protocole dans le modèle de l’oracle aléatoire. Nous proposons également une nouvelle construction d’arguments à divulgation nulle de connaissance non-intéractive et à vérifieur désigné (DVNIZK) sous l’hypothèse de réseaux Euclidiens. En utilisant cette nouvelle construction, nous construisons un système de vote basé sur les réseaux Euclidiens dans le modèle standard.
Tien Tuan Anh Dinh, Anwitaman Datta, Beng Chin Ooi
Research in blockchain systems has mainly focused on improving security and bridging the performance gaps between blockchains and databases. Despite many promising results, we observe a worrying trend that the blockchain landscape is fragmented in which many systems exist in silos. Apart from a handful of general-purpose blockchains, such as Ethereum or Hyperledger Fabric, there are hundreds of others designed for specific applications and typically do not talk to each other. In this paper, we describe our vision of interoperable blockchains. We argue that supporting interaction among different blockchains requires overcoming challenges that go beyond data standardization. The underlying problem is to allow smart contracts running in different blockchains to communicate. We discuss three open problems: access control, general cross-chain transactions, and cross-chain communication. We describe partial solutions to some of these problems in the literature. Finally, we propose a novel design to overcome these challenges.
Online social networks (OSNs) are becoming more and more prevalent in people's life, but they face the problem of privacy leakage due to the centralized data management mechanism. The emergence of distributed OSNs (DOSNs) can solve this privacy issue, yet they bring inefficiencies in providing the main functionalities, such as access control and data availability. In this article, in view of the above-mentioned challenges encountered in OSNs and DOSNs, we exploit the emerging blockchain technique to design a new DOSN framework that integrates the advantages of both traditional centralized OSNs and DOSNs. By combining smart contracts, we use the blockchain as a trusted server to provide central control services. Meanwhile, we separate the storage services so that users have complete control over their data. In the experiment, we use real-world data sets to verify the effectiveness of the proposed framework.
Threat intelligence sharing is posited as an important aid to help counter cybersecurity attacks and a number of threat intelligence sharing communities exist. There is a general consensus that many challenges remain to be overcome to achieve fully effective sharing, including concerns about privacy, negative publicity, policy/legal issues and expense of sharing, amongst others. One recent trend undertaken to address this is the use of decentralized blockchain based sharing architectures. However while these platforms can help increase sharing effectiveness they do not fully address all of the above challenges. In particular, issues around trust are not satisfactorily solved by current approaches. In this paper, we describe a novel trust enhancement framework -TITAN- for decentralized sharing based on the use of P2P reputation systems to address open trust issues. Our design uses blockchain and Trusted Execution Environment technologies to ensure security, integrity and privacy in the operation of the threat intelligence sharing reputation system.
In any registration process, it is a hassle to always bring physical documents. Not only that, the process is elongated if they are lost, and will contribute to identity theft if unauthorized persons have access to those documents. Therefore, the objective of this paper is to generate a decentralized system, similar to the concept of blockchain, to allow registered persons to access user’s personal records. To further elaborate, this system is designed to be used by three consumers, which are: user, authority, and third-party (requester). Nowadays, majority of systems are susceptible to massive data breaches. However, some researches have theorized that blockchain technology may solve this problem, such as one that uses a real-world example, Aadhaar. In conclusion, this project enhances that the area of blockchain identity is vital in helping society gain control of their personal details. Since most of researches are focused on storage system of businesses using blockchain, personal identities of the people should be digitized on the blockchain as well. An identity verification system that is entirely owned by the individual will increase trust that the data is genuine and reliable.
The widespread recognition of the smart contracts has established their importance in the landscape of next generation blockchain technology. However, writing a correct smart contract is notoriously difficult. Moreover, once a state-changing transaction is confirmed by the network, the result is immutable. For this reason, it is crucial to perform a thorough testing of a smart contract application before its deployment. This paper's focus is on the test coverage criteria for smart contracts, which are objective rules that measure test quality. We analyze the unique characteristics of the Ethereum smart contract program model as compared to the conventional program model. To capture essential control flow behaviors of smart contracts, we propose the notions of whole transaction basis path set and bounded transaction interaction. The former is a limited set of linearly independent inter-procedural paths from which the potentially infinite paths of Ethereum transactions can be constructed by linear combination, while the latter is the permutations of transactions within a certain bound. Based on these two notions, we define a family of path-based test coverage criteria. Algorithms are given to the generation of coverage requirements. A case study is conducted to compare the effectiveness of the proposed test coverage criteria with random testing and statement coverage testing.
The fake certificate is a special global problem in today's digital age. Thousands of universities and educational institutions around the world do not exist but can release hundreds of millions of fake degrees. Verifying the integrity of qualifications is a real challenge for today's employers. Applying the anti-data modifying properties of blockchain technology, this study proposes a solution issuing and verifying digital certificates called EUniCert to solve this problem. By changing the design and integrating new consensus algorithm used in Ethereum platform into the Unicoin network that was used to verify and store the information related to the issued digital certificate, the EUniCert improves the latency to validate transactions as well as the number of verified blocks in the blockchain network compared to the previous solution that we have proposed. We implement a simple blockchain system to illustrate the management operation of the digital certificates on the ethereum platform. Besides, we conduct a simulation to evaluate the performance of our proposal compared with the previous system. The result is that the average latency decreases by 3.27 times as well as the number of verified blocks increases by 11% compared with the previous system.
Public blockchains like Ethereum use Merkle trees to verify transactions received from untrusted servers before applying them to the blockchain. We empirically show that the low throughput of such blockchains is due to the I/O bottleneck associated with using Merkle trees for processing transactions. We present RAINBLOCK, a new architecture for public blockchains that increases throughput without affecting security. RAINBLOCK achieves this by tackling the I/O bottleneck on two fronts: first, decoupling transaction processing from I/O, and removing I/O from the critical path; second, reducing I/O amplification by customizing storage for blockchains. RAINBLOCK uses a novel variant of the Merkle tree, the Distributed Sharded Merkle tree (DSM-TREE) to store system state. We evaluate RAINBLOCK using workloads based on public Ethereum traces (including smart contracts) and show that RAINBLOCK processes 20K transactions per second in a geo-distributed setting with four regions spread across three continents.
Lei Yang, Vivek Bagaria, Gerui Wang, Mohammad Alizadeh · 7 authors
Bitcoin is the first fully decentralized permissionless blockchain protocol and achieves a high level of security: the ledger it maintains has guaranteed liveness and consistency properties as long as the adversary has less compute power than the honest nodes. However, its throughput is only 7 transactions per second and the confirmation latency can be up to hours. Prism is a new blockchain protocol which is designed to achieve a natural scaling of Bitcoin's performance while maintaining its full security guarantees. We present an implementation of Prism which achieves a throughput of 70,000 transactions per second and confirmation latencies of tens of seconds.
There is a preconception that a blockchain needs consensus. But consensus is a powerful distributed property with a remarkably high price tag. So one may wonder whether consensus is at all needed.
We introduce a new blockchain architecture called ABC that functions despite not establishing consensus, and comes with an array of advantages: ABC is permissionless, deterministic, and resilient to complete asynchrony. ABC features finality and does not rely on costly proof-of-work.
Without establishing consensus, ABC cannot support certain applications, in particular smart contracts that are open for interaction with unknown agents. However, our system is an advantageous solution for many important use cases, such as cryptocurrencies like Bitcoin.
We introduce a new permissionless blockchain architecture called ABC. ABC is completely asynchronous, and does rely on neither randomness nor proof-of-work. ABC can be parallelized, and transactions have finality within one round trip of communication. However, ABC satisfies only a relaxed form of consensus by introducing a weaker termination property. Without full consensus, ABC cannot support certain applications, in particular ABC cannot support general smart contracts. However, many important applications do not need general smart contracts, and ABC is a better solution for these applications. In particular, ABC can implement the functionality of a cryptocurrency like Bitcoin, replacing Bitcoin's energy-hungry proof-of-work with a proof-of-stake validation.