Community Resources

In Proof-of-Stake (PoS) and permissioned blockchains, a committee of verifiers agrees and sign every new block of transactions. These blocks are validated, propagated, and stored by all users in the network. However, posterior corruptions pose a common threat to these designs, because the adversary can corrupt committee verifiers after they certified a block and use their signing keys to certify a different block. Designing efficient and secure digital signatures for use in PoS blockchains can substantially reduce bandwidth, storage and computing requirements from nodes, thereby enabling more efficient applications. This paper was peer-reviewed and will appear at USENIX Security 2020.

Algorand uses a new Byzantine Agreement (BA) protocol to reach consensus among users on the next set of transactions. To scale the consensus to many users, Algorand uses a novel mechanism based on Verifiable Random Functions that allows users to privately check whether they are selected to participate in the BA to agree on the next set of transactions, and to include a proof of their selection in their network messages. In Algorand’s BA protocol, users do not keep any private state except for their private keys, which allows Algorand to replace participants immediately after they send a message. This mitigates targeted attacks on chosen participants after their identity is revealed.

We present a simple Byzantine agreement protocol with leader election, that works under > 2/3 honest majority and does not rely on the participants having synchronized clocks. When honest messages are delivered within a bounded worst-case delay, agreement is reached in expected constant number of steps when the elected leader is malicious, and is reached after two steps when the elected leader is honest. Our protocol is resilient to arbitrary network partitions with unknown length, and recovers fast after the partition is resolved and bounded message delay is restored.

A public ledger is a tamperproof sequence of data that can be read and augmented by everyone. Public ledgers have innumerable and compelling uses. They can secure, in plain sight, all kinds of transactions —such as titles, sales, and payments— in the exact order in which they occur. Public ledgers not only curb corruption, but also enable very sophisticated applications —such as cryptocurrencies and smart contracts. They stand to revolutionize the way a democratic society operates. As currently implemented, however, they scale poorly and cannot achieve their potential. Algorand is a truly democratic and efficient way to implement a public ledger.