Instant Block Confirmation in the Sleepy Model

Blockchain protocols suffer from an interesting conundrum: owning stake in the Blockchain doesn't necessarily mean that the party is willing to participate in day to day operations. This leads to large quantities of stake being owned by parties who do not actually participate in the growth of the blockchain, reducing its security. Pass and Shi [23] captured this concern in the sleepy model, and subsequent work by Pass et al. [5] extended their results into a full Proof of Stake blockchain protocol which can continue to securely progress even when the majority of parties may be offline. However, their protocol requires 10 or more blocks to be added after a transaction first appears in the ledger for it to be confirmed. On the other hand, existing Byzantine Agreement based blockchain protocols such as Algorand [6,7,14] confirm transactions as soon as they appear in the ledger, but are unable to progress when users are not online when mandated. The main question we address is: Do there exist blockchain protocols which can continue to securely progress even when the majority of parties (resp. stake) may be offline, and confirm transactions as soon as they appear in the ledger? Our main result shows the answer to this question to be "yes". We present a Proof of Stake blockchain protocol which continues to securely progress so long as more than half of the online stake is controlled by honest parties, and instantly confirms transactions upon appearance in the ledger.