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Accountable Liveness

Speaker: Tim Roughgarden (Columbia University/a16z crypto) Co-authors: Andrew Lewis-Pye (London School of Economics), Joachim Neu (a16z crypto), Luca Zanolini (Ethereum Foundation) Paper: https://eprint.iacr.org/2025/693.pdf Tweetstorm: https://x.com/jneu_net/status/1917948... Abstract: When a blockchain protocol stalls (i.e., suffers a liveness violation), which validators should be held accountable? This paper defines the notion of accountable liveness and characterizes the assumptions under which it can be attained by protocols that are optimally resilient in partial synchrony: an honest majority of validators and "majority synchrony" are both necessary and sufficient for accountable liveness. Contents: (0:00) Introduction (4:55) Problem definition (atomic/total-order broadcast) (6:58) Consistency and liveness (7:50) Partial synchrony (11:41) Goal: guarantees beyond optimal resilience in partial synchrony. (13:04) Accountable safety and three cool facts about it (18:17) Defining accountable liveness (21:55) Accountable liveness: overview of challenges (26:41) Accountable liveness is impossible in "half-synchrony" (29:54) Proof sketch (34:33) Revised goal: accountable liveness in executions that satisfy stronger timing assumptions (38:00) Quick review of Tendermint (41:24) Adding a "blaming primitive" to Tendermint (46:01) Why liveness failures in synchronous executions cause Byzantine validators to be blamed (48:35) Accountable liveness is impossible without an honest majority (50:31) Proof sketch (55:28) The coolest thing that could be true (57:46) Defining majority synchrony (1:02:26) Main result: one can augment Tendermint (for example) to achieve accountable liveness with an honest majority in all majority synchronous executions (1:03:40) Analysis, part 1: properties of the blaming primitive (1:08:06) Analysis, part 2: an incomplete blaming function (1:13:50) Analysis, part 3: a revised blaming function (1:20:53) Analysis, part 4: proof of the key lemma (1:28:03) Dealing with Byzantine leaders via super-views (1:31:51) Summary of contributions