Forensic support is an important property of BFT protocols that addresses the other side of security: what happens when the number of malicious parties exceeds the allowable threshold? In a previous post, we systematically studied different BFT protocols to assess their ability to detect and prove malicious behavior when safety is violated. We learned that protocols such as PBFT and HotStuff with ${\sf poly}(n)$ communication have strong forensic support, meaning...
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What are Blockchains Useful for, Really?
Blockchains, or the decentralized ledger, are touted as the next big disruptive technology, as big as the Internet was in the 90s. What are these blockchains useful for, really? While there are relevant use cases, many examples people use that are either far too academic to be useful or are scenarios where blockchains are not the right solution in the first place. Thus, in this post, I am trying to...
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Pairing-based Anonymous Credentials and the Power of Re-randomization
David Chaum wrote in 1985:
Large-scale automated transaction systems are imminent. The architecture chosen for these systems may have a long-term impact on the centralization of our economic system, on some of our basic liberties, and even on our democracy. The initial choice of direction will gather economic and societal momentum, making reversal increasingly less likely.
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Player Replaceability - Towards Adaptive Security and Sub-quadratic Communication Simultaneously (Part II)
This is part II of a two-part post on player-replaceability. Part I can be found here.
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Player Replaceability - Towards Adaptive Security and Sub-quadratic Communication Simultaneously (Part I)
This is part I of a two-part post on the concept of player-replaceability.
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Responsiveness under omission failures
In this post, we discuss log replication responsiveness in the context of omission failures. We show how to transform the protocol in our previous post to a multi-shot version of Paxos for omission faults. The Byzantine failure case uses similar ideas and is covered in the next post of this series.
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Set Replication - fault tolerance without total ordering
While state machine replication is the gold standard for implementing any (public) ideal functionality, its power comes at the cost of needing to totally order all transactions and as a consequence solve (Byzantine) agreement. In some cases this overhead is unnecessary.
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What is Responsiveness?
In asynchronous protocols, latency to commit is a function of the actual maximum network delay $\delta$. In synchronous protocols, message delay is bounded by $\Delta$, and for $n/3 \leq f < n/2$, the $\Delta$ bound is used to obtain both safety and liveness. In partial synchrony, message delay is bounded by $\Delta$ after GST, and the $\Delta$ bound is used to obtain liveness.
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What about Validity?
Perhaps the archetypical trilemma is consensus - it requires three properties: agreement, liveness, and validity. Getting any two is easy, but all three together is what makes consensus such a facinating problem that continues to create new challenges even after 40 years of research.
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Two Round HotStuff
In the first part of this post we describe a single-shot variation of Two Round HotStuff (see HotStuff v1 paper, march 2018 and this march 2018 post) using Locked Broadcast that follows a similar path as our previous posts on Paxos and Linear PBFT.
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Linear PBFT: a gentle introduction to Practical Byzantine Fault Tolerance
PBFT is a foundational multi-year project lead by Barbara Liskov and her students, obtaining major advances in both the theory and practice of Byzantine Fault Tolerance. The PBFT conference version, journal version, Castro’s thesis, Liskov’s talk, and follow up work on BASE are all required reading for anyone who wants to deeply understand BFT systems.
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From Single-Shot Agreement to State Machine Replication
In this post we explore the path from Single-Shot Agreement, via Write-Once Registers, to Log Replication, and finally to State Machine Replication. We begin by defining all four problems assuming minority omission failures and partial synchrony. This post continues our previous posts on Paxos from Recoverable Broadcast and on State Machine Replication.
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On Paxos from Recoverable Broadcast
There are many ways to learn about the Paxos protocol (see Lampson, Cachin, Howard, Howard 2, Guerraoui, Kladov, Krzyzanowski, Lamport, Wikipedia and many more). The emphasis of this post is on a decomposition of Paxos for omission failures that will later help when we do a similar decomposition for Byzantine failures (for PBFT and HotStuff).
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Provable Broadcast
We explore a family of broadcast protocols in the authenticated setting in which a designated sender wants to create a delivery-certificate of its input value. After describing the base protocol we call Provable Broadcast ($PB$), we explore the surprising power of simply running $PB$ two times in a row, then three times, and finally four times in a row.
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What is a Blockchain?
TLDR: a Blockchain is a trusted coordination mechanism;
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Dining Cryptographers and the additivity of polynomial secret sharing
David Chaum’s dining cryptographer problem is a pioneering work on the foundations of privacy. It shows the amazing power of information-theoretic Secure Multi Party Computation. The original paper from 1988 is super accessible and fun to read. Many systems in the last 20 years for anonymity and privacy-preserving communication are based on the Dining Cryptographers problem. Herbivore, Dissent, Riposte, Blinder, and many others.
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The BGW Verifiable Secret Sharing Protocol
In this post, we present the classic Ben-or, Goldwasser, and Wigderson, 1988 (BGW) Verifiable Secret Sharing protocol (VSS) with the simplifications of Feldman, 1988. The analysis and notation in this post are based on the full proof of the BGW MPC protocol of Asharov and Lindell. This post is a continuation of our previous posts on secret sharing for passive and crash failures.
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Polynomial Secret Sharing with crash failures
We continue our series on polynomial secret sharing. In the previous post of this series we discussed secret sharing with a passive adversary. In this post we assume crash failures and in later posts we will extend to malicious failures. As before, we must assume parties have private channels: the adversary cannot see the content of messages sent between two non-faulty parties.
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A new Dolev-Reischuk style Lower Bound
In a previous post we discussed Crusader Broadcast and showed a $O(n^2)$ words, $O(1)$ time solution for $f<n$ and assuming a PKI. In this post, we overview a new Dolev-Reischuk style bower bound (see our full paper):
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He-HTLC - Revisiting Incentives in HTLC
Hashed Time-locked Contracts (HTLC) find many useful applications in the L2 Layer such as the lightning network and atomic swaps. In this post, we will focus on discussing protocols for implementing HTLC when taking into consideration incentives for parties in the system. We will discuss a line of work — WHF’19, MAD-HTLC, He-HTLC — towards developing an HTLC protocol secure in the presence of rational parties.
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