A cryptocurrency is a decentralised digital currency that utilises blockchain technology to remove the role of a central authority. Monero is one of the cryptocurrencies that improves its anonymity by employing privacy-preserving cryptographic techniques, such as linkable ring signature. In this thesis, we explore three areas in Monero system that can cause anonymity problems. These areas are Monero transaction creation protocol, Monero protocol update, and Monero third-party services. We identify attack schemes to reduce honest users' transaction anonymity. We then investigate the impact of Monero protocol updates to transaction anonymity. Lastly, we study wallet service providers that can trace Monero transactions and mining pools that leak information.
Tao Feng, Hongmei Pei, Rong Ma, Youliang Tian · 5 authors
Data privacy is important to the security of our society, and enabling authorized users to query this data efficiently is facing more challenge. Recently, blockchain has gained extensive attention with its prominent characteristics as public, distributed, decentration and chronological characteristics. However, the transaction information on the blockchain is open to all nodes, the transaction information update operation is even more transparent. And the leakage of transaction information will cause huge losses to the transaction party. In response to these problems, this paper combines hierarchical attribute encryption with linear secret sharing, and proposes a blockchain data privacy protection control scheme based on searchable attribute encryption, which solves the privacy exposure problem in traditional blockchain transactions. The user’s access control is implemented by the verification nodes, which avoids the security risks of submitting private keys and access structures to the blockchain network. Associating the private key component with the random identity of the user node in the blockchain can solve the collusion problem. In addition, authorized users can quickly search and supervise transaction information through searchable encryption. The improved algorithm ensures the security of keywords. Finally, based on the DBDH hypothesis, the security of the scheme is proved in the random prediction model.
The insurance business plays a quite significant role in people's lives, but in the process of claim settlement, there are still various frauds such that the insurance companies' refusal to compensate or customers' malicious fraud to obtain compensation. Therefore, it is very important to ensure fair and just claims. In this paper, by combining the blockchain technology and the ciphertext-policy attribute-based encryption system, we build a scheme for secure storage and update for insurance records under the InterPlanetary File System (IPFS) storage environment in the insurance system. In this scheme, we use the fog node to outsource encryption of insurance records to improve the efficiency of the staff; In addition, we store encrypted insurance records on IPFS to ensure the security of the storage platform and avoid the single point failure of the centralized mechanism. In addition, we use the immutability of the blockchain to achieve the non-repudiation of both insurance companies and the client. The security proof shows that the proposed scheme can achieve selective security against selected keyword attacks. Our scheme is efficient and feasible under performance analysis and real data set experiments.
As a quite attractive secure search mechanism in cloud environments, searchable encryption allows encrypted files to be searched by keyword and does not reveal any information about original data files. However, most existing searchable encryption schemes only support single keyword ciphertext retrieval, and they cannot resist against inside keyword guessing attacks. Besides, the previous schemes rarely focus on integrity verification and fair transactions without any third party. Focusing on these problems, we propose a multi-keyword certificateless searchable public key authenticated encryption scheme based on blockchain. We use certificateless cryptosystem to encrypt keywords, which avoids the problems of certificate management in traditional cryptosystem and key escrow in identity-based cryptosystem. Our scheme also supports multi-keyword search, which locates encrypted files precisely and returns the desired files. Moreover, we upload the real encrypted files to the cloud server, while the encrypted indexes are put in blockchain, which ensures the anti-tampering, integrity and traceability of the encrypted indexes. The anti-tampering of blockchain also ensures that users can receive accurate search results without any third party verification. Furthermore, we utilize smart contract to track monetary rewards, which enables fair transactions between data owners and users without any trusted third party. We prove that the proposed scheme is secure against inside keyword guessing attacks in the random oracle model. Finally, our performance evaluation shows that the proposed scheme has higher computational performance than other related schemes.
In online crowdsourcing services, credible accountability mechanisms are crucial for guaranteeing a good interactive environment. However, the crowdsourcing systems are established in virtual environments, the identities of the participants are various and complicated, the systems could scarcely identify malicious nodes automatically. So it is very hard to preserve the complete evidence of malicious behaviors and investigate relevant legal responsibilities. Blockchain is regarded as a very promising solution to these problems because it possesses characteristics of decentration, non-modifiability and traceability. However, a main challenge is to design an applicable blockchain consensus algorithm which can reach an agreement on credibility of participants automatically, prevent transaction data from tampering, and trace to the source of malicious behaviors. In this paper, an improved Proof-of-Trust (PoT) consensus scheme is proposed with the underlying technology of blockchain, which is properly to the crowdsourcing service scenarios. Firstly, this PoT consensus selects nodes with high credibility using subjective logic reputation algorithm. Only selected nodes have the chance to generate blocks, participate in verification, and claim crowdsourcing tasks. Secondly, the choice scheme of generate-block nodes is further optimized through the unpredictability of timestamp and digital signature. Moreover, an incentive mechanism based on game theory is designed in this consensus. With this mechanism, candidate nodes prefer to give honest verification results rather than engage in collusion with malicious nodes. The analysis and simulation results demonstrate the effectiveness, feasibility and scalability of the proposed approach.
James Bartusek, Andrea Coladangelo, Dakshita Khurana, Fermi Ma
We investigate the round complexity of maliciously-secure two-party quantum computation (2PQC) with setup, and obtain the following results:
- A three-message protocol (two-message if only one party receives output) in the common random string (CRS) model assuming classical two-message oblivious transfer (OT) with post-quantum malicious security. This round complexity is optimal for the sequential communication setting. Under the additional assumption of reusable malicious designated-verifier non-interactive zero-knowledge (MDV-NIZK) arguments for NP, our techniques give an MDV-NIZK for QMA. Each of the assumptions mentioned above is known from the quantum hardness of learning with errors (QLWE).
- A protocol with two simultaneous rounds of communication, in a quantum preprocessing model, assuming sub-exponential QLWE. In fact, we construct a three-round protocol in the CRS model with only two rounds of online communication, which implies the above result. Along the way, we develop a new delayed technique that we call simulation via teleportation, which may be useful in other settings.
In addition, we perform a preliminary investigation into barriers and possible approaches for two-round 2PQC in the CRS model, including an impossibility result for a natural class of simulators, and a proof-of-concept construction from a strong form of quantum virtual black-box (VBB) obfuscation.
Prior to our work, maliciously-secure 2PQC required round complexity linear in the size of the quantum circuit.
Max Hoffmann, Michael Klooß, Markus Raiber, Andy Rupp
Abstract Black-box accumulation (BBA) is a building block which enables a privacy-preserving implementation of point collection and redemption, a functionality required in a variety of user-centric applications including loyalty programs, incentive systems, and mobile payments. By definition, BBA+ schemes (Hartung et al. CCS ‘17) offer strong privacy and security guarantees, such as unlinkability of transactions and correctness of the balance flows of all (even malicious) users. Unfortunately, the instantiation of BBA+ presented at CCS ‘17 is, on modern smartphones, just fast enough for comfortable use. It is too slow for wearables, let alone smart-cards. Moreover, it lacks a crucial property: For the sake of efficiency, the user’s balance is presented in the clear when points are deducted. This may allow to track owners by just observing revealed balances, even though privacy is otherwise guaranteed. The authors intentionally forgo the use of costly range proofs, which would remedy this problem. We present an instantiation of BBA+ with some extensions following a different technical approach which significantly improves efficiency. To this end, we get rid of pairing groups, rely on different zero-knowledge and fast range proofs, along with a slightly modified version of Baldimtsi-Lysyanskaya blind signatures (CCS ‘13). Our prototype implementation with range proofs (for 16 bit balances) outperforms BBA+ without range proofs by a factor of 2.5. Moreover, we give estimates showing that smart-card implementations are within reach.
Mohammad Madine, Khaled Salah, Raja Jayaraman, Ibrar Yaqoob · 7 authors
Patients are becoming aware of the importance of taking secure control and managing access over their medical data, thereby leading to the rise in the adoption of personal health record (PHR) systems. However, today's PHR systems fall short in providing secure and trustable data sharing and access facilities to patients when they are in emergency situations or temporarily incapacitated. Also, the existing PHR systems are centralized and vulnerable to the single point of failure problem. Integrating PHR systems with blockchain technology can help to overcome such limitations. In this paper, we propose a blockchain-based PHR architecture that employs smart contracts to implement multi-party authorization (MPA) and threshold cryptographic schemes to automate secure and trustable medical data sharing and access in PHR systems. Moreover, we mitigate the limited storage and computation capabilities of blockchain by using InterPlanetary File System (IPFS) storage and reputation-governed trusted oracles into the proposed architecture. MPA and threshold cryptographic schemes allow the patient to split and share a secret key with a set of trusted parties, such as the healthcare regulatory agency, guardians, and hospitals, in such a way that they can collectively decide on sharing medical data on behalf of patients. We present algorithms along with their full smart contract function implementation details. We evaluate the robustness and performance of our solution by performing correctness verification and cost analysis. Furthermore, we evaluate the proposed approach in terms of security, generalization, and limitation aspects to find out its feasibility and practicality. We make our smart contract code publicly available on GitHub.
Merkle trees are ubiquitous in blockchains and other distributed ledger technologies (DLTs). They guarantee that the involved systems are referring to the same binary tree, even if each of them knows only the cryptographic hash of the root. Inclusion proofs allow knowledgeable systems to share subtrees with other systems and the latter can verify the subtrees' authenticity. Often, blockchains and DLTs use data structures more complicated than binary trees; authenticated data structures generalize Merkle trees to such structures. We show how to formally define and reason about authenticated data structures, their inclusion proofs, and operations thereon as datatypes in Isabelle/HOL. The construction lives in the symbolic model, i.e., we assume that no hash collisions occur. Our approach is modular and allows us to construct complicated trees from reusable building blocks, which we call Merkle functors. Merkle functors include sums, products, and function spaces and are closed under composition and least fixpoints. As a practical application, we model the hierarchical transactions of Canton, a practical interoperability protocol for distributed ledgers, as authenticated data structures. This is a first step towards formalizing the Canton protocol and verifying its integrity and security guarantees.
Custody is a core financial service in which the custodian holds in safekeeping assets on behalf of the client. Although traditional custody service is typically endorsed by centralized authorities, decentralized custody scheme has become technically feasible since the emergence of digital assets, and furthermore it is badly needed by new applications such as blockchain and DeFi (Decentralized Finance). In this work, we propose a framework of decentralized asset custody scheme that is able to support a large number of custodians and safely hold customer assets of multiple times value of the total security deposit. The proposed custody scheme distributes custodians and assets into many custodian groups via combinatorial designs and random sampling, where each group fully controls the assigned assets. Since every custodian group is small, the overhead cost is significantly reduced. The liveness is also improved because even a single alive group would be able to process transactions. The security of this custody scheme is guaranteed in the game-theoretic sense, such that any adversary corrupting a bounded fraction of custodians cannot move assets more than his own security deposit. We further analyze the security and performance of our constructions, and give explicit examples with concrete numbers and figures for a better understanding of our results.
Mahmoud El-Gayyar, Hany F. ElYamany, Katarina Grolinger, Miriam A. M. Capretz · 5 authors
A federated identity is a single identity that enables users to access multiple services across a network of business parties. Such identities are subject to various threats and attacks and face diverse challenges including identity leaks, centralised management, auditing limitations, and long breach investigation processes. This paper proposes a framework aimed at automating and decentralising the generation and auditing of a robust and secured blockchain-based federated identity in a marketplace. Business parties participating in the marketplace form the nodes of a distributed blockchain network and participate in the creation of federated identities. Users of this network can access services provided by any one of the participating parties using a single federated identity. All transactions are fully audited in the blockchain, meaning that participating parties can monitor access to their service and users can trace the use of their identities. The proposed framework has been evaluated using two blockchain technologies (Ethereum and Hyperledger Fabric) to measure its performance in public and permissioned blockchain environments.
The sharing of electronic health records (EHRs) has shown great advantages in the accurate treatment of patients and the development of medical institutions. However, it is easy to cause some security problems in the process of medical data sharing. Generally, after a patient's EHRs are generated by different medical institutions, they are outsourced to the cloud server (CS) by the authorized medical institutions for storage, which causes the patient to lose control of EHRs. Moreover, malicious medical institutions and semi-trusted cloud servers may collude to tamper with EHRs to seek benefits, which threatens the integrity of EHRs. Therefore, we propose a blockchain-assisted verifiable outsourced attribute-based signcryption scheme (BVOABSC) which realizes the secure sharing of EHRs in a multi-authority cloud storge environment. Firstly, we use the attribute-based signcryption algorithm to realize the confidentiality and unforgeability of the EHRs and protect the privacy of the signer. Secondly, it greatly reduces the computational burden of users by using verifiable outsourcing computation mechanism. Most of the designcryption calculation is performed by the cloud server, and the correctness of the generated partial designcryption ciphertext is verified by users. Furthermore, we use blockchain technology to protect outsourced EHRs from tampering by illegal users. Specifically, each operation on outsourced EHRs is stored as a transaction on the public blockchain, which ensures that EHRs cannot be modified. At the same time, the auditor can verify the integrity of the outsourced EHRs by checking the corresponding transactions. In addition, the smart contract created by the patient can solve the problems in cloud storage, such as tampering EHRs and returning incorrect results. Finally, security analysis and performance evaluation show that the proposed BVOABSC scheme satisfies stronger security and higher efficiency than similar schemes.