The decentralized cryptocurrency Bitcoin has experienced great success but also encountered many challenges. One of the challenges has been the long confirmation time. Another challenge is the lack of incentives at certain steps of the protocol, raising concerns for transaction withholding, selfish mining, etc. To address these challenges, we propose Solida, a decentralized blockchain protocol based on reconfigurable Byzantine consensus augmented by proof-of-work. Solida improves on Bitcoin in confirmation time, and provides safety and liveness assuming the adversary control less than (roughly) one-third of the total mining power.
@cite_19: The Bitcoin system only provides eventual consistency. For everyday life, the time to confirm a Bitcoin transaction is prohibitively slow. In this paper we propose a new system, built on the Bitcoin blockchain, which enables strong consistency. Our system, PeerCensus, acts as a certification authority, manages peer identities in a peer-to-peer network, and ultimately enhances Bitcoin and similar systems with strong consistency. Our extensive analysis shows that PeerCensus is in a secure state with high probability. We also show how Discoin, a Bitcoin variant that decouples block creation and transaction confirmation, can be built on top of PeerCensus, enabling real-time payments. Unlike Bitcoin, once transactions in Discoin are committed, they stay committed.
@cite_11: 
@cite_7: Large-scale peer-to-peer systems face security threats from faulty or hostile remote computing elements. To resist these threats, many such systems employ redundancy. However, if a single faulty entity can present multiple identities, it can control a substantial fraction of the system, thereby undermining this redundancy. One approach to preventing these "Sybil attacks" is to have a trusted agency certify identities. This paper shows that, without a logically centralized authority, Sybil attacks are always possible except under extreme and unrealistic assumptions of resource parity and coordination among entities.
@cite_8: While showing great promise, Bitcoin requires users to wait tens of minutes for transactions to commit, and even then, offering only probabilistic guarantees. This paper introduces ByzCoin, a novel Byzantine consensus protocol that leverages scalable collective signing to commit Bitcoin transactions irreversibly within seconds. ByzCoin achieves Byzantine consensus while preserving Bitcoin's open membership by dynamically forming hash power-proportionate consensus groups that represent recently-successful block miners. ByzCoin employs communication trees to optimize transaction commitment and verification under normal operation while guaranteeing safety and liveness under Byzantine faults, up to a near-optimal tolerance of f faulty group members among 3f + 2 total. ByzCoin mitigates double spending and selfish mining attacks by producing collectively signed transaction blocks within one minute of transaction submission. Tree-structured communication further reduces this latency to less than 30 seconds. Due to these optimizations, ByzCoin achieves a throughput higher than PayPal currently handles, with a confirmation latency of 15-20 seconds.
To our knowledge, PeerCensus @cite_19 is the first work to envision a committee that approves transactions using PBFT. It makes the observation that Byzantine consensus enables fast transaction confirmation. The description and pseudocode are very high level. Under our best interpretation, it seems that the committee is responsible for reconfiguring itself using Byzantine consensus, but no detail is given on reconfiguration. ByzCoin @cite_8 employs multi-signatures to improve the scalability of PBFT. The reconfiguration algorithm in ByzCoin seems to be left open with multiple options. The description in the paper @cite_8 is quite terse, and can be interpreted in multiple ways. A later blog post @cite_11 suggests adopting hybrid consensus @cite_7 . If ByzCoin goes this path, then our comparison to hybrid consensus applies to ByzCoin as well. Through private communication, we learned that ByzCoin designers also had in mind a PBFT-style reconfiguration protocol, which seems to involve extra steps not described in the paper. We will have to wait to see a detailed published reconfiguration protocol to compare with ByzCoin properly. Pass and Shi proposed hybrid consensus @cite_7 , which we have compared to in detail in .