Chainspace is a decentralized infrastructure, known as a distributed ledger, that supports user defined smart contracts and executes user-supplied transactions on their objects. The correct execution of smart contract transactions is verifiable by all. The system is scalable, by sharding state and the execution of transactions, and using S-BAC, a distributed commit protocol, to guarantee consistency. Chainspace is secure against subsets of nodes trying to compromise its integrity or availability properties through Byzantine Fault Tolerance (BFT), and extremely highauditability, non-repudiation and 'blockchain' techniques. Even when BFT fails, auditing mechanisms are in place to trace malicious participants. We present the design, rationale, and details of Chainspace; we argue through evaluating an implementation of the system about its scaling and other features; we illustrate a number of privacy-friendly smart contracts for smart metering, polling and banking and measure their performance.

Coconut is a novel selective disclosure credential scheme supporting distributed threshold issuance, public and private attributes, re-randomization, and multiple unlinkable selective attribute revelations. Coconut integrates with blockchains to ensure confidentiality, authenticity and availability even when a subset of credential issuing authorities are malicious or offline. We implement and evaluate a generic Coconut smart contract library for Chainspace and Ethereum; and present three applications related to anonymous payments, electronic petitions, and distribution of proxies for censorship resistance. Coconut uses short and computationally efficient credentials, and our evaluation shows that most Coconut cryptographic primitives take just a few milliseconds on average, with verification taking the longest time (10 milliseconds).request Alberto: @George, Describe how the CoCoNut authorities can also be Chainspace nodes (to make clear the potential of CoCoNut when deeply related to blockchains), since we actually built all of this to have credentials in smart contracts. V. EVALUATION A. Primitives evaluationThe signature scheme has been implemented in python using the two crypo libraries petlib [1] and bplib [2]. The bilinear pairing works over a curve, using OpenSSL as the arithmetic backend. We have released the code as an open-source project on GitHub. 3 . Table I shows the mean (µ) and standard deviation ( 2 ) of the execution of each procedure described in section section II. Each entry is the result of 10,000 measured on an Octa-core Dell desktop computer, 3.6GHz Intel Xeon. This table shows that signing is much faster than verifying signatures (about 15 times faster for the scheme working on clear messages, therefore set to 32 bytes (for SHA-2). The size of a signature is 132 bytes. The highest transaction size appears when the user ask a signature on a hidden message. This comes from the fact that the proof s associated with the message is approximately 318 bytes; the proof v is only 157 bytes.Alberto: Update the above B. System evaluation Alberto: @Bano, test system on AWS (n authority): client latency vs t -ask n signatures (and n blind signatures) and check the time it takes to hear back from t authorities. VI. COMPARISON WITH RELATED WORKSAlberto: discuss crypto related works Alberto: compare results (speed and size) with alternatives to see why it is cool stuff; not many scheme have actually been 3 https://github.com/asonnino/coconut plies that the adversary needs to corrupt at least t authorities for this attack to happen. This property also prevents a single authority from taking the user money and disappear without issuing any token. À Ã Õ OE oe --"

Light clients, also known as Simple Payment Verification (SPV) clients, are nodes which only download a small portion of the data in a blockchain, and use indirect means to verify that a given chain is valid. Typically, instead of validating block data, they assume that the chain favoured by the blockchain's consensus algorithm only contains valid blocks, and that the majority of block producers are honest. By allowing such clients to receive fraud proofs generated by fully validating nodes that show that a block violates the protocol rules, and combining this with probabilistic sampling techniques to verify that all of the data in a block actually is available to be downloaded, we can eliminate the honest-majority assumption for block validity, and instead make much weaker assumptions about a minimum number of honest nodes that rebroadcast data. Fraud and data availability proofs are key to enabling on-chain scaling of blockchains (e.g., via sharding or bigger blocks) while maintaining a strong assurance that on-chain data is available and valid. We present, implement, and evaluate a novel fraud and data availability proof system.

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Since the inception of Bitcoin, a plethora of distributed ledgers differing in design and purpose has been created. While by design, blockchains provide no means to securely communicate with external systems, numerous attempts towards trustless cross-chain communication have been proposed over the years. Today, cross-chain communication (CCC) plays a fundamental role in cryptocurrency exchanges, scalability efforts via sharding, extension of existing systems through sidechains, and bootstrapping of new blockchains. Unfortunately, existing proposals are designed ad-hoc for specific use-cases, making it hard to gain confidence in their correctness and composability. We provide the first systematic exposition of cross-chain communication protocols. We formalize the underlying research problem and show that CCC is impossible without a trusted third party, contrary to common beliefs in the blockchain community. With this result in mind, we develop a framework to design new and evaluate existing CCC protocols, focusing on the inherent trust assumptions thereof, and derive a classification covering the field of cross-chain communication to date. We conclude by discussing open challenges for CCC research and the implications of interoperability on the security and privacy of blockchains.

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Transparency is crucial in security-critical applications that rely on authoritative information, as it provides a robust mechanism for holding these authorities accountable for their actions. A number of solutions have emerged in recent years that provide transparency in the setting of certificate issuance, and Bitcoin provides an example of how to enforce transparency in a financial setting. In this work we shift to a new setting, the distribution of software package binaries, and present a system for so-called "binary transparency. " Our solution, Contour, uses proactive methods for providing transparency, privacy, and availability, even in the face of persistent man-in-the-middle attacks. We also demonstrate, via benchmarks and a test deployment for the Debian software repository, that Contour is the only system for binary transparency that satisfies the efficiency and coordination requirements that would make it possible to deploy today.Full version of an extended abstract published in CBT 2018.