Stellar Whitepaper - Stellar Consensus Protocol

Stellar Whitepaper - Stellar Consensus Protocol

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Currency
Stellar Lumens
Symbol
XLM
Founders
Jed McCaleb, Joyce Kim
A Federated Model for Internet-level Consensus
This paper introduces a new model for consensus called federated Byzantine agreement (FBA). FBA achieves robustness through quorum slices—individual trust decisions made by each node that together determine system-level quorums. Slices bind the system together much the way individual networks’ peering and transit decisions now unify the Internet.

We also present the Stellar Consensus Protocol (SCP), a construction for FBA. Like all Byzantine agreement protocols, SCP makes no assumptions about the rational behavior of attackers. Unlike prior Byzantine agreement models, which presuppose a unanimously accepted membership list, SCP enjoys open membership that promotes organic network growth. Compared to decentralized proof of-work and proof-of-stake schemes, SCP has modest computing and financial requirements, lowering the barrier to entry and potentially opening up financial systems to new participants.

Introduction
Financial infrastructure is currently a mess of closed systems. Gaps between these systems mean that transaction costs are high and money moves slowly across political and geographic boundaries. This friction has curtailed the growth of financial services, leaving billions of people under- served financially.

To solve these problems, we need financial infrastructure that supports the kind of organic growth and innovation we’ve seen from the Internet, yet still ensures the integrity of financial transactions. Historically, we have relied on high barriers to entry to ensure integrity. We trust established financial institutions and do our best to regulate them. But this exclusivity conflicts with the goal of organic growth. Growth demands new, innovative participants, who may possess only modest financial and computing resources.

We need a worldwide financial network open to anyone, so that new organizations can join and extend financial access to unserved communities. The challenge for such a network is ensuring participants record transactions correctly. With a low barrier to entry, users won’t trust providers to police themselves. With worldwide reach, providers won’t all trust a single entity to operate the network. A compelling alter- native is a decentralized system in which participants together ensure integrity by agreeing on the validity of one another’s transactions. Such agreement hinges on a mechanism for worldwide consensus.

This paper presents federated Byzantine agreement (FBA), a model suitable for worldwide consensus. In FBA, each participant knows of others it considers important. It waits for the vast majority of those others to agree on any transaction before considering the transaction settled. In turn, those important participants do not agree to the transaction until the participants they consider important agree as well, and so on. Eventually, enough of the network accepts a transaction that it becomes infeasible for an attacker to roll it back. Only then do any participants consider the transaction settled. FBA’s consensus can ensure the integrity of a financial network. Its decentralized control can spur organic growth.

This paper further presents the Stellar consensus protocol (SCP), a construction for FBA. We prove that SCP’s safety is optimal for an asynchronous protocol, in that it guarantees agreement under any node-failure scenario that admits such a guarantee. We also show that SCP is free from blocked states—in which consensus is no longer possible—unless participant failures make it impossible to satisfy trust dependencies. SCP is the first provably safe consensus mechanism to enjoy four key properties simultaneously:
  • Decentralized control. Anyone is able to participate and no central authority dictates whose approval is required for consensus.
  • Low latency. In practice, nodes can reach consensus at timescales humans expect for web or payment transactions—i.e., a few seconds at most.
  • Flexible trust. Users have the freedom to trust any combination of parties they see fit. For example, a small non-profit may play a key role in keeping much larger institutions honest.
  • Asymptotic security. Safety rests on digital signatures and hash families whose parameters can realistically be tuned to protect against adversaries with unimaginably vast computing power.
SCP has applications beyond financial markets for ensuring organizations perform important functions honestly. An example is certificate authorities (CAs), who literally hold the keys to the web. Experience shows that CAs sign incorrect certificates that get used in the wild. Several proposals address this problem through certificate transparency. Certificate transparency allows users to examine the history of certificates issued for any given entity and detect attempts by CAs to change an entity’s public key without the endorsement of the previous key. SCP holds the potential to strengthen the indelible certificate history at the core of certificate transparency. Demanding global consensus on certificate history among a decentralized group of auditors would make it harder to backpedal and override previously issued certificates.

The next section discusses previous approaches to consensus. Section 3 defines federated Byzantine agreement (FBA) and lays out notions of safety and liveness applicable in the FBA model. Section 4 discusses optimal failure resilience in an FBA system, thereby establishing the security goals for SCP. Section 5 develops federated voting, a key building block of the SCP protocol. Section 6 presents SCP itself, proving safety and freedom from blocked states. Section 7 discusses limitations of SCP. Finally, Section 8 summarizes results. For readers less familiar with mathematical notation, Appendix A defines some symbols used throughout the paper.

Summary
Byzantine agreement has long enabled distributed systems to achieve consensus with efficiency, standard cryptographic security, and flexibility in designating trusted participants. More recently, Bitcoin introduced the revolutionary notion of decentralized consensus, leading to many new systems and research challenges. This paper introduces federated Byzantine agreement (FBA), a model for achieving decentralized consensus while preserving the traditional benefits of Byzantine agreement. The key distinction between FBA and prior Byzantine agreement systems is that FBA forms quorums from participants’ individual trust decisions, allowing an organic growth model similar to that of the Internet. The Stellar Consensus Protocol (SCP) is a construction for FBA that achieves optimal safety against ill-behaved participants.

Acknowledgments
Jed McCaleb inspired this work and provided feedback, terminology suggestions, and help thinking through numerous conjectures. Jessica Collier collaborated on writing the paper. Stan Polu created the first implementation of SCP and provided invaluable corrections, suggestions, simplifications, and feedback in the process. Jelle van den Hooff provided the key idea to restructure the paper around quorum intersection and federated voting, as well as other crucial suggestions for terminology, organization, and presentation. Nicolas Barry found several bugs in the paper as he implemented the protocol, as well as identifying necessary clarifications. Ken Birman, Bekki Bolt-House, Joseph Bonneau, Mike Hamburg, Graydon Hoare, Joyce Kim, Tim Makarios, Mark Moir, Robert Morris, Lucas Ryan, and Katherine Tom slogged through drafts of the paper, identifying errors and sources of confusion as well as providing helpful suggestions. Eva Gantz provided helpful motivation and references. Winnie Lim provided guidance on figures. The reddit community and Tahoe-LAFS group pointed out a censorship weakness in an earlier version of SCP, leading to the improved nomination protocol. Finally, the author would like to thank the whole Stellar team for their support, feedback, and encouragement.