The Engineer's Guide to Blockchain Finality

Finality for off-chain applications cannot safely rely on a simple block delay, because a re-org can expunge a transaction that appeared final and enable double-spend attacks. The article says Juno and Pathfinder, two L2 clients, either did not check finality or used block delays incorrectly, and that fixes were published in Juno v0.4.0 and Pathfinder v0.6.2 after disclosure. For probabilistic proof-of-work chains, it recommends setting block delays from historical reorg data and from the cost of a 25% attack, with Crypto51 used to calculate delays for $75k deposits as of June 2023. Bitcoin, for example, requires two blocks (about 20 minutes), while Ethereum PoW requires 23,600 blocks (about 89 hours).