Cryptographic Tools for Privacy Preservation

Data permeates every aspect of our daily life and it is the backbone of our digitalized society. Smartphones, smartwatches and many more smart devices measure, collect, modify and share data in what is known as the Internet of Things.Often, these devices don’t have enough computation power/storage space thus out-sourcing some aspects of the data management to the Cloud. Outsourcing computation/storage to a third party poses natural questions regarding the security and privacy of the shared sensitive data.Intuitively, Cryptography is a toolset of primitives/protocols of which security prop- erties are formally proven while Privacy typically captures additional social/legislative requirements that relate more to the concept of “trust” between people, “how” data is used and/or “who” has access to data. This thesis separates the concepts by introducing an abstract model that classifies data leaks into different types of breaches. Each class represents a specific requirement/goal related to cryptography, e.g. confidentiality or integrity, or related to privacy, e.g. liability, sensitive data management and more.The thesis contains cryptographic tools designed to provide privacy guarantees for different application scenarios. In more details, the thesis:(a) defines new encryption schemes that provide formal privacy guarantees such as theoretical privacy definitions like Differential Privacy (DP), or concrete privacy-oriented applications covered by existing regulations such as the European General Data Protection Regulation (GDPR);(b) proposes new tools and procedures for providing verifiable computation’s guarantees in concrete scenarios for post-quantum cryptography or generalisation of signature schemes;(c) proposes a methodology for utilising Machine Learning (ML) for analysing the effective security and privacy of a crypto-tool and, dually, proposes a secure primitive that allows computing specific ML algorithm in a privacy-preserving way;(d) provides an alternative protocol for secure communication between two parties, based on the idea of communicating in a periodically timed fashion

Towards practicalization of blockchain-based decentralized applications

Blockchain can be defined as an immutable ledger for recording transactions, maintained in a distributed network of mutually untrusting peers. Blockchain technology has been widely applied to various fields beyond its initial usage of cryptocurrency. However, blockchain itself is insufficient to meet all the desired security or efficiency requirements for diversified application scenarios. This dissertation focuses on two core functionalities that blockchain provides, i.e., robust storage and reliable computation. Three concrete application scenarios including Internet of Things (IoT), cybersecurity management (CSM), and peer-to-peer (P2P) content delivery network (CDN) are utilized to elaborate the general design principles for these two main functionalities. Among them, the IoT and CSM applications involve the design of blockchain-based robust storage and management while the P2P CDN requires reliable computation. Such general design principles derived from disparate application scenarios have the potential to realize practicalization of many other blockchain-enabled decentralized applications. In the IoT application, blockchain-based decentralized data management is capable of handling faulty nodes, as designed in the cybersecurity application. But an important issue lies in the interaction between external network and blockchain network, i.e., external clients must rely on a relay node to communicate with the full nodes in the blockchain. Compromization of such relay nodes may result in a security breach and even a blockage of IoT sensors from the network. Therefore, a censorship-resistant blockchain-based decentralized IoT management system is proposed. Experimental results from proof-of-concept implementation and deployment in a real distributed environment show the feasibility and effectiveness in achieving censorship resistance. The CSM application incorporates blockchain to provide robust storage of historical cybersecurity data so that with a certain level of cyber intelligence, a defender can determine if a network has been compromised and to what extent. The CSM functions can be categorized into three classes: Network-centric (N-CSM), Tools-centric (T-CSM) and Application-centric (A-CSM). The cyber intelligence identifies new attackers, victims, or defense capabilities. Moreover, a decentralized storage network (DSN) is integrated to reduce on-chain storage costs without undermining its robustness. Experiments with the prototype implementation and real-world cyber datasets show that the blockchain-based CSM solution is effective and efficient. The P2P CDN application explores and utilizes the functionality of reliable computation that blockchain empowers. Particularly, P2P CDN is promising to provide benefits including cost-saving and scalable peak-demand handling compared with centralized CDNs. However, reliable P2P delivery requires proper enforcement of delivery fairness. Unfortunately, most existing studies on delivery fairness are based on non-cooperative game-theoretic assumptions that are arguably unrealistic in the ad-hoc P2P setting. To address this issue, an expressive security requirement for desired fair P2P content delivery is defined and two efficient approaches based on blockchain for P2P downloading and P2P streaming are proposed. The proposed system guarantees the fairness for each party even when all others collude to arbitrarily misbehave and achieves asymptotically optimal on-chain costs and optimal delivery communication

Intrusion-Tolerant Middleware: the MAFTIA approach

The pervasive interconnection of systems all over the world has given computer services a significant socio-economic value, which can be affected both by accidental faults and by malicious activity. It would be appealing to address both problems in a seamless manner, through a common approach to security and dependability. This is the proposal of intrusion tolerance, where it is assumed that systems remain to some extent faulty and/or vulnerable and subject to attacks that can be successful, the idea being to ensure that the overall system nevertheless remains secure and operational. In this paper, we report some of the advances made in the European project MAFTIA, namely in what concerns a basis of concepts unifying security and dependability, and a modular and versatile architecture, featuring several intrusion-tolerant middleware building blocks. We describe new architectural constructs and algorithmic strategies, such as: the use of trusted components at several levels of abstraction; new randomization techniques; new replica control and access control algorithms. The paper concludes by exemplifying the construction of intrusion-tolerant applications on the MAFTIA middleware, through a transaction support servic

Distributed Key Generation and Its Applications

Numerous cryptographic applications require a trusted authority to hold a secret. With a plethora of malicious attacks over the Internet, however, it is difficult to establish and maintain such an authority in online systems. Secret-sharing schemes attempt to solve this problem by distributing the required trust to hold and use the secret over multiple servers; however, they still require a trusted {\em dealer} to choose and share the secret, and have problems related to single points of failure and key escrow. A distributed key generation (DKG) scheme overcomes these hurdles by removing the requirement of a dealer in secret sharing. A (threshold) DKG scheme achieves this using a complete distribution of the trust among a number of servers such that any subset of servers of size greater than a given threshold can reveal or use the shared secret, while any smaller subset cannot. In this thesis, we make contributions to DKG in the computational security setting and describe three applications of it. We first define a constant-size commitment scheme for univariate polynomials over finite fields and use it to reduce the size of broadcasts required for DKG protocols in the synchronous communication model by a linear factor. Further, we observe that the existing (synchronous) DKG protocols do not provide a liveness guarantee over the Internet and design the first DKG protocol for use over the Internet. Observing the necessity of long-term stability, we then present proactive security and group modification protocols for our DKG system. We also demonstrate the practicality of our DKG protocol over the Internet by testing our implementation over PlanetLab. For the applications, we use our DKG protocol to define IND-ID-CCA secure distributed private-key generators (PKGs) for three important identity-based encryption (IBE) schemes: Boneh and Franklin's BF-IBE, Sakai and Kasahara's SK-IBE, and Boneh and Boyen's BB1-IBE. These IBE schemes cover all three important IBE frameworks: full-domain-hash IBEs, exponent-inversion IBEs and commutative-blinding IBEs respectively, and our distributed PKG constructions can easily be modified for other IBE schemes in these frameworks. As the second application, we use our distributed PKG for BF-IBE to define an onion routing circuit construction mechanism in the identity-based setting, which solves the scalability problem in single-pass onion routing circuit construction without hampering forward secrecy. As the final application, we use our DKG implementation to design a threshold signature architecture for quorum-based distributed hash tables and use it to define two robust communication protocols in these peer-to-peer systems

Data governance through a multi-DLT architecture in view of the GDPR

The centralization of control over the processing of personal data threatens the privacy of individuals due to the lack of transparency and the obstruction of easy access to their data. Individuals need the tools to effectively exercise their rights, enshrined in regulations such as the European Union General Data Protection Regulation (GDPR). Having direct control over the flow of their personal data would not only favor their privacy but also a “data altruism”, as supported by the new European proposal for a Data Governance Act. In this work, we propose a multi-layered architecture for the management of personal information based on the use of distributed ledger technologies (DLTs). After an in-depth analysis of the tensions between the GDPR and DLTs, we propose the following components: (1) a personal data storage based on a (possibly decentralized) file storage (DFS) to guarantee data sovereignty to individuals, confidentiality and data portability; (2) a DLT-based authorization system to control access to data through two distributed mechanisms, i.e. secret sharing (SS) and threshold proxy re-encryption (TPRE); (3) an audit system based on a second DLT. Furthermore, we provide a prototype implementation built upon an Ethereum private blockchain, InterPlanetary File System (IPFS) and Sia and we evaluate its performance in terms of response time

Authentication and Key Management Automation in Decentralized Secure Email and Messaging via Low-Entropy Secrets

We revisit the problem of entity authentication in decentralized end-to-end encrypted email and secure messaging to propose a practical and self-sustaining cryptographic solution based on password-authenticated key exchange (PAKE). This not only allows users to authenticate each other via shared low-entropy secrets, e.g., memorable words, without a public key infrastructure or a trusted third party, but it also paves the way for automation and a series of cryptographic enhancements; improves security by minimizing the impact of human error and potentially improves usability. First, we study a few vulnerabilities in voice-based out-of-band authentication, in particular a combinatorial attack against lazy users, which we analyze in the context of a secure email solution. Next, we propose solving the problem of secure equality test using PAKE to achieve entity authentication and to establish a shared high-entropy secret key. Our solution lends itself to offline settings, compatible with the inherently asynchronous nature of email and modern messaging systems. The suggested approach enables enhancements in key management such as automated key renewal and future key pair authentications, multi-device synchronization, secure secret storage and retrieval, and the possibility of post-quantum security as well as facilitating forward secrecy and deniability in a primarily symmetric-key setting. We also discuss the use of auditable PAKEs for mitigating a class of online guess and abort attacks in authentication protocols

Secure Schemes for Semi-Trusted Environment

In recent years, two distributed system technologies have emerged: Peer-to-Peer (P2P) and cloud computing. For the former, the computers at the edge of networks share their resources, i.e., computing power, data, and network bandwidth, and obtain resources from other peers in the same community. Although this technology enables efficiency, scalability, and availability at low cost of ownership and maintenance, peers defined as ``like each other'' are not wholly controlled by one another or by the same authority. In addition, resources and functionality in P2P systems depend on peer contribution, i.e., storing, computing, routing, etc. These specific aspects raise security concerns and attacks that many researchers try to address. Most solutions proposed by researchers rely on public-key certificates from an external Certificate Authority (CA) or a centralized Public Key Infrastructure (PKI). However, both CA and PKI are contradictory to fully decentralized P2P systems that are self-organizing and infrastructureless. To avoid this contradiction, this thesis concerns the provisioning of public-key certificates in P2P communities, which is a crucial foundation for securing P2P functionalities and applications. We create a framework, named the Self-Organizing and Self-Healing CA group (SOHCG), that can provide certificates without a centralized Trusted Third Party (TTP). In our framework, a CA group is initialized in a Content Addressable Network (CAN) by trusted bootstrap nodes and then grows to a mature state by itself. Based on our group management policies and predefined parameters, the membership in a CA group is dynamic and has a uniform distribution over the P2P community; the size of a CA group is kept to a level that balances performance and acceptable security. The muticast group over an underlying CA group is constructed to reduce communication and computation overhead from collaboration among CA members. To maintain the quality of the CA group, the honest majority of members is maintained by a Byzantine agreement algorithm, and all shares are refreshed gradually and continuously. Our CA framework has been designed to meet all design goals, being self-organizing, self-healing, scalable, resilient, and efficient. A security analysis shows that the framework enables key registration and certificate issue with resistance to external attacks, i.e., node impersonation, man-in-the-middle (MITM), Sybil, and a specific form of DoS, as well as internal attacks, i.e., CA functionality interference and CA group subversion. Cloud computing is the most recent evolution of distributed systems that enable shared resources like P2P systems. Unlike P2P systems, cloud entities are asymmetric in roles like client-server models, i.e., end-users collaborate with Cloud Service Providers (CSPs) through Web interfaces or Web portals. Cloud computing is a combination of technologies, e.g., SOA services, virtualization, grid computing, clustering, P2P overlay networks, management automation, and the Internet, etc. With these technologies, cloud computing can deliver services with specific properties: on-demand self-service, broad network access, resource pooling, rapid elasticity, measured services. However, theses core technologies have their own intrinsic vulnerabilities, so they induce specific attacks to cloud computing. Furthermore, since public clouds are a form of outsourcing, the security of users' resources must rely on CSPs' administration. This situation raises two crucial security concerns for users: locking data into a single CSP and losing control of resources. Providing inter-operations between Application Service Providers (ASPs) and untrusted cloud storage is a countermeasure that can protect users from lock-in with a vendor and losing control of their data. To meet the above challenge, this thesis proposed a new authorization scheme, named OAuth and ABE based authorization (AAuth), that is built on the OAuth standard and leverages Ciphertext-Policy Attribute Based Encryption (CP-ABE) and ElGamal-like masks to construct ABE-based tokens. The ABE-tokens can facilitate a user-centric approach, end-to-end encryption and end-to-end authorization in semi-trusted clouds. With these facilities, owners can take control of their data resting in semi-untrusted clouds and safely use services from unknown ASPs. To this end, our scheme divides the attribute universe into two disjointed sets: confined attributes defined by owners to limit the lifetime and scope of tokens and descriptive attributes defined by authority(s) to certify the characteristic of ASPs. Security analysis shows that AAuth maintains the same security level as the original CP-ABE scheme and protects users from exposing their credentials to ASP, as OAuth does. Moreover, AAuth can resist both external and internal attacks, including untrusted cloud storage. Since most cryptographic functions are delegated from owners to CSPs, AAuth gains computing power from clouds. In our extensive simulation, AAuth's greater overhead was balanced by greater security than OAuth's. Furthermore, our scheme works seamlessly with storage providers by retaining the providers' APIs in the usual way

HAIL: A High-Availability and Integrity Layer for Cloud Storage

We introduce HAIL (High-Availability and Integrity Layer), a distributed cryptographic system that permits a set of servers to prove to a client that a stored file is intact and retrievable. HAIL strengthens, formally unifies, and streamlines distinct approaches from the cryptographic and distributed-systems communities. Proofs in HAIL are efficiently computable by servers and highly compact---typically tens or hundreds of bytes, irrespective of file size. HAIL cryptographically verifies and reactively reallocates file shares. It is robust against an active, mobile adversary, i.e., one that may progressively corrupt the full set of servers. We propose a strong, formal adversarial model for HAIL, and rigorous analysis and parameter choices. We show how HAIL improves on the security and efficiency of existing tools, like Proofs of Retrievability (PORs) deployed on individual servers. We also report on a prototype implementation

SoK: Understanding BFT Consensus in the Age of Blockchains

Blockchain as an enabler to current Internet infrastructure has provided many unique features and revolutionized current distributed systems into a new era. Its decentralization, immutability, and transparency have attracted many applications to adopt the design philosophy of blockchain and customize various replicated solutions. Under the hood of blockchain, consensus protocols play the most important role to achieve distributed replication systems. The distributed system community has extensively studied the technical components of consensus to reach agreement among a group of nodes. Due to trust issues, it is hard to design a resilient system in practical situations because of the existence of various faults. Byzantine fault-tolerant (BFT) state machine replication (SMR) is regarded as an ideal candidate that can tolerate arbitrary faulty behaviors. However, the inherent complexity of BFT consensus protocols and their rapid evolution makes it hard to practically adapt themselves into application domains. There are many excellent Byzantine-based replicated solutions and ideas that have been contributed to improving performance, availability, or resource efficiency. This paper conducts a systematic and comprehensive study on BFT consensus protocols with a specific focus on the blockchain era. We explore both general principles and practical schemes to achieve consensus under Byzantine settings. We then survey, compare, and categorize the state-of-the-art solutions to understand BFT consensus in detail. For each representative protocol, we conduct an in-depth discussion of its most important architectural building blocks as well as the key techniques they used. We aim that this paper can provide system researchers and developers a concrete view of the current design landscape and help them find solutions to concrete problems. Finally, we present several critical challenges and some potential research directions to advance the research on exploring BFT consensus protocols in the age of blockchains