Trustless communication across distributed ledgers: impossibility and practical solutions

Since the advent of Bitcoin as the first decentralized digital currency in 2008, a plethora of distributed ledgers has been created, differing in design and purpose. Considering the heterogeneous nature of these systems, it is safe to say there shall not be ``one coin to rule them all". However, despite the growing and thriving ecosystem, blockchains continue to operate almost exclusively in complete isolation from one another: by design, blockchain protocols provide no means by which to communicate or exchange data with external systems. To this date, centralized providers hence remain the preferred route to exchange assets and information across blockchains~-- undermining the very nature of decentralized currencies. The contribution of this thesis is threefold. First, we critically evaluate the (im)possibilty, requirements, and challenges of cross-chain communication by contributing the first systematization of this field. We formalize the problem of Cross-Chain Communication (CCC) and show it is impossible without a trusted third party by relating CCC to the Fair Exchange problem. With this impossibility 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 then present XCLAIM, the first generic framework for transferring assets and information across permissionless distributed ledgers without relying on a centralized third party. XCLAIM leverages so-called cryptocurrency-backed assets, blockchain-based assets one-to-one backed by other cryptocurrencies, such as Bitcoin-backed tokens on Ethereum. Through the secure issuance, transfer, and redemption of these assets, users can perform cross-chain exchanges in a financially trustless and non-interactive manner, overcoming the limitations of existing solutions. To ensure the security of user funds, XCLAIM relies on collateralization of intermediaries and a proof-or-punishment approach, enforced via smart contracts equipped with cross-chain light clients, so-called chain relays. XCLAIM has been adopted in practice, among others by the Polkadot blockchain, as a bridge to Bitcoin and other cryptocurrencies. Finally, we contribute to advancing the state of the art in cross-chain light clients. We develop TxChain, a novel mechanism to significantly reduce storage and bandwidth costs of modern blockchain light clients using contingent transaction aggregation, and apply our scheme to Bitcoin and Ethereum individually, as well as in the cross-chain setting.Open Acces

SoK: Communication Across Distributed Ledgers

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

Critical Perspectives on Provable Security: Fifteen Years of Another Look Papers

We give an overview of our critiques of “proofs” of security and a guide to our papers on the subject that have appeared over the past decade and a half. We also provide numerous additional examples and a few updates and errata

University of Dayton Flyer News, Vol. 15, No. 16 (Jan. 12, 1968)

Student-run newspaper of the University of Dayto

Optimizing polynomial convolution for NTRUEncrypt

NTRUEncrypt is one of the most promising candidates for quantum-safe cryptography. In this paper, we focus on the NTRU743 paramter set. We give a report on all known attacks against this parameter set and show that it delivers 256 bits of security against classical attackers and 128 bits of security against quantum attackers. We then present a parameter-dependent optimization using a tailored hierarchy of multipli- cation algorithms as well as the Intel AVX2 instructions, and show that this optimization is constant-time. Our implementation is two to three times faster than the reference implementation of NTRUEncrypt

Deliberative Democracy in the EU. Countering Populism with Participation and Debate. CEPS Paperback

Elections are the preferred way to freely transfer power from one term to the next and from one political party or coalition to another. They are an essential element of democracy. But if the process of power transfer is corrupted, democracy risks collapse. Reliance on voters, civil society organisations and neutral observers to fully exercise their freedoms as laid down in international human rights conventions is an integral part of holding democratic elections. Without free, fair and regular elections, liberal democracy is inconceivable. Elections are no guarantee that democracy will take root and hold, however. If the history of political participation in Europe over the past 800 years is anything to go by, successful attempts at gaining voice have been patchy, while leaders’ attempts to silence these voices and consolidate their own power have been almost constant (Blockmans, 2020). Recent developments in certain EU member states have again shown us that democratically elected leaders will try and use majoritarian rule to curb freedoms, overstep the constitutional limits of their powers, protect the interests of their cronies and recycle themselves through seemingly free and fair elections. In their recent book How Democracies Die, two Harvard professors of politics write: “Since the end of the Cold War, most democratic breakdowns have been caused not by generals and soldiers but by elected governments themselves” (Levitsky and Ziblatt, 2018)

Do the Securities Laws Promote Short-Termism?

Since 1970, the Securities and Exchange Commission (SEC) has required public companies to file reports summarizing their financial performance on a quarterly basis. Such mandatory quarterly disclosure has recently been criticized as incentivizing corporations to deliver short-term results rather than developing sustainable, long-term strategies. This Article examines the origins of the quarterly reporting system to assess whether the SEC should reduce the frequency of periodic filings. It concludes that much of the pressure on public companies to deliver short-term results emerged as the market increasingly focused on earnings projections issued by research analysts. The pressure to meet such projections can distort the behavior of public companies, but such distortions will only be significant in certain circumstances. Because it is unclear that the quarterly reporting system substantially impacts company incentives, the SEC should pursue modest reforms rather than take the radical step of eliminating quarterly disclosure. Quarterly disclosure is one example of how securities law tends to promote the short-term interests of transacting investors. In contrast, corporate law, which mediates the interests of shareholders, often gives managers the discretion to consider long-term interests. Strong securities law can be balanced by weak corporate law