MIT DCI's Cryptocurrency Research Review #3 (Email)

Bulletproofs, central bank digital currency, and Spice

Hello!

Welcome to Cryptocurrency Research Review, Issue No. 3! This publication (curated by the MIT Media Lab’s Digital Currency Initiative in partnership with MIT Press) crowdsources high-quality reviews of interdisciplinary research from the fields of cryptocurrency and blockchain technology. We intend this publication to help surface impactful work in the space, provide more neutral analysis of ongoing research, and to encourage interdisciplinary discussions in the community.

This is an experimental effort to figure out how best to meet the needs of cryptocurrency and blockchain technology researchers. If you have any feedback, please respond to this email directly!

Based on feedback from readers, we asked contributors to write longer, more in-depth reviews for this issue. Unfortunately, the longer reviews could not fit in an email, so we’ve sent you a snippet of each review and you can read the full reviews here:
https://mitcryptocurrencyresearch.substack.com/p/mit-dcis-cryptocurrency-research-350

We hope you enjoy this issue. Let’s get right to it!

Building, and building on, Bulletproofs

By Cathie Yun, Interstellar

Preface

In this post, I will explain how the Bulletproofs zero knowledge proof protocol works, as well as talk about the confidential asset protocol and confidential smart contract language we are building using Bulletproofs.

This post is a condensed version of previous talks and blog posts and our Bulletproofs library documentation.

Background

Zero-knowledge range proofs are a key building block for confidential payment systems, such as Confidential Transactions for Bitcoin, Monero, and Mimblewimble, Confidential Assets, and many other protocols. Range proofs allow a verifier to ensure that secret values, such as asset amounts, are nonnegative. This prevents a user from forging value by secretly using a negative amount. Since every transaction involves one or more range proofs, their efficiency, both in terms of proof size and verification time, is key to transaction performance.

In 2017, Bünz, Bootle, Boneh, Poelstra, Wuille, and Maxwell published Bulletproofs, which dramatically improves proof performance both in terms of proof size and verification time. In addition, it allows for proving a much wider class of statements than just range proofs.

Read the full article here: https://mitcryptocurrencyresearch.substack.com/p/mit-dcis-cryptocurrency-research-350

Review of ‘Proceeding with caution’ a BIS survey on central bank digital currency

Paper by Christian Barontini and Henry Holden, Jan. 8, 2019

Review by Robleh Ali, MIT Media Lab, Digital Currency Initiative

The Bank for International Settlements (BIS) recently released a survey on central bank digital currency (CBDC). This review is in two sections, the first discusses the BIS survey and the second analyses the issues facing central banks as their work on CBDC progresses.

Overall the BIS survey is a very useful insight into central banks’ thinking about CBDC. This usefulness derives from three factors; comprehensive coverage, quantifiable results and segmentation between developed economies and emerging markets.

BIS Survey

Defining CBDC

The paper starts by addressing how to define CBDC. It uses the money flower taxonomy set out in Bech and Garratt (2017) and identifies the four key properties of money as issuer, form, accessibility and technology. A subset of the technology question is the distinction between token based and account-based money.

Token-based and account-based money are differentiated in the paper by how payment verification works. By this definition in a token-based model the recipient does the verification themselves, in an account-based model an intermediary handles verification. This approach draws on a distinction from payment economics and the authors acknowledge that these definitions can vary considerably and can include other descriptions such as value-based money.

These distinctions are driven by underlying technology and there are two problems with applying this taxonomy to digital currencies:

  1. Transaction verification can both be done by recipients who run full nodes and by the mining network.

  1. Digital currencies can use both unspent transaction outputs and accounts as data models for recording transactions.

This makes digital currencies like Bitcoin hard to categorise using the money flower taxonomy because it employs this token/account method to distinguish between different forms of digital money. It may be preferable to draw a distinction between programmable and non-programmable digital money – this taxonomy would delineate more clearly between different types of digital money; bank deposits and e-money on one hand and digital currencies on the other.

In relation to CBDC, this programmable/non-programmable could also be usefully applied to different manifestations of CBDC. For example the Swedish Riksbank is proposing non-programmable CBDC as a complement to cash.

Read the full review here: https://mitcryptocurrencyresearch.substack.com/p/mit-dcis-cryptocurrency-research-350

Review of "Proving the correct execution of concurrent services in zero-knowledge”

Paper by Srinath Setty, Sebastian Angel, Trinabh Gupta, and Jonathan Lee, Oct. 2018

Review by Willy R. Vasquez, University of Texas at Austin

Introduction

In “Proving the correct execution of concurrent services in zero-knowledge” by Setty et al. [1], the authors describe a system called Spice that realizes stateful services that are both high performant and verifiable. Stateful services are services that allow clients to manage their data and perform transactions between each other. Some examples include cloud-hosted ledgers which are used to create exchanges or ride-sharing apps, interbank payment networks, and dark pools. Spice is able to achieve 500-1000 transactions per second on a cluster of 16 servers on such types of applications. By using a set-based key/value store with batched verification, they increase transaction throughput by 18,000 - 685,000x over prior work.

The contributions of Spice are:

  • A definition for the verifiable state machine (VSM) primitive: this primitive captures the requirements of a verifiable stateful service, allowing client privacy from an untrusted verifier. While VSMs were implicitly realized in previous work, the authors provide a concrete definition.

  • Batched State Verification: the batching of individual state operations for later verification, allowing the cost of individual updates to be amortized.

  • Concurrent stateful updates: the ability to handle multiple state operations at a single time, allowing for increased throughput in client requests.

  • SetKV: a concurrent verifiable key/value store that helps realize the Spice VSM. SetKV extends mechanisms for verifying memory and relies on a set-based data structure to construct the verifiable key/value store. SetKV is separate contribution for realizing the Spice VSM and could be used to build a stand-alone untrusted storage service.

The challenges of Spice are:

  • Batched verification audit: the verification step of SetKV is called an audit, which has a runtime linear in the size of the key/value store. For a large number of entries this can take a noticeable amount of time (e.g. 3.5 minutes for one million entries).

  • No client privacy from the server: Spice’s threat model is such that the server is trusted to maintain the privacy of client updates, but untrusted to perform correct computations. This threat model is consistent with banks, which maintain account balance privacy but undergo audits to ensure their operations are correct. In Spice, privacy is maintained from other actors in Spice, such as other clients or verifiers.

Read the full review here: https://mitcryptocurrencyresearch.substack.com/p/mit-dcis-cryptocurrency-research-350

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