Paper 2024/1108

Faster Asynchronous Blockchain Consensus and MVBA

Abstract

Blockchain consensus, a.k.a. BFT SMR, are protocols enabling $n$ processes to decide on an ever-growing chain. The fastest known asynchronous one is called 2-chain VABA (PODC'21 and FC'22), and is used as fallback chain in Abraxas* (CCS'23). It has a claimed $9.5\delta$ expected latency when used for a single shot instance, a.k.a. an MVBA. We exhibit attacks breaking it. Hence, the title of the fastest asynchronous MVBA with quadratic messages complexity goes to sMVBA (CCS'22), with $10\delta$ expected latency. Our positive contributions are two new and complementary designs. $\bullet$ 2PAC (2-phase asynchronous consensus). It has a simpler and lighter chaining than in previous approaches. Instantiated with either quadratic or cubic phases of voting, it yields: 2PAC$^\text{lean}$: $+90\%$ throughput and $9.5\delta$ expected latency, with quadratic ($O(n^2)$) messages complexity. In both 2-chain VABA and sMVBA (as if chained, with pipelining), the quorum-certified transactions which were produced in the worst-case 1/3 of views with a slow leader were dumped, so the work was lost. The simpler design of 2PAC inserts such blocks in straight-line in the chain. Thus, contrary to naive uncle-referencing, this comes with no computational overhead, yielding a net $+50\%$ throughput gain over chained sMVBA. Both the remaining throughput and latency ($-0.5\delta$) gains, come from the lighter interactive construction of proofs of consistency appended to proposed blocks, compared to sMVBA. 2PAC$^\text{BIG}$: the fastest asynchronous blockchain consensus with cubic ($O(n^3)$) messages complexity. Fault-free single shot MVBA runs decide in just $4\delta$, as soon as no message is delivered more than twice faster than others: GradedDAG (SRDS'23) required furthermore no messages reordering. $\bullet$ Super Fast Pipelined Blocks. This is an upgrade of previous approaches for pipelining: in 2-chain VABA, Cordial Miners (DISC'23) and GradedDAG, a block pipelined by a leader in the middle of the view had almost twice larger latency than the non-pipelined block. Our design provides a fast path deciding the pipelined block with even smaller latency than the non-pipelined block. The fast delay is guaranteed in all executions with a fair scheduler, but remarkably, whatever the behaviors of faulty processes. Consistency is preserved by a lightweight mechanism, of one threshold signature appended per proposal. Instantiated with the previous protocols, it yields: s2PAC$^\text{lean}$, with fast decision of pipelined blocks in $4\delta$; s2PAC$^\text{BIG}$, in $3\delta$; and sGradedDAG, in $3\delta$.