Liveness goal In addition to safety Algorand also makes progress ie allows new

Liveness goal in addition to safety algorand also

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Liveness goal. In addition to safety, Algorand also makes progress (i.e., allows new transactions to be added to the log) under additional assumptions about network reachability that we describe below. Algorand aims to reach consensus on a new set of transactions within roughly one minute. Assumptions. Algorand makes standard cryptographic assumptions such as public-key signatures and hash func- tions. Algorand assumes that honest users run bug-free software. As mentioned earlier, Algorand assumes that the fraction of money held by honest users is above some thresh- old h (a constant greater than 2 / 3 ), but that an adversary can participate in Algorand and own some money. We believe that this assumption is reasonable, since it means that in order to successfully attack Algorand, the attacker must in- vest substantial financial resources in it. Algorand assumes that an adversary can corrupt targeted users, but that an adversary cannot corrupt a large number of users that hold a significant fraction of the money (i.e., the amount of money held by honest, non-compromised users must remain over the threshold). To achieve liveness, Algorand makes a “strong synchrony” assumption that most honest users (e.g., 95%) can send mes- sages that will be received by most other honest users (e.g., 95%) within a known time bound. This assumption allows the adversary to control the network of a few honest users, but does not allow the adversary to manipulate the network at a large scale, and does not allow network partitions. Algorand achieves safety with a “weak synchrony” as- sumption: the network can be asynchronous (i.e., entirely controlled by the adversary) for a long but bounded period of time (e.g., at most 1 day or 1 week). After an asynchrony period, the network must be strongly synchronous for a rea- sonably long period again (e.g., a few hours or a day) for Algorand to ensure safety. More formally, the weak syn- chrony assumption is that in every period of length b (think of b as a day or a week), there must be a strongly synchronous period of length s < b (an s of a few hours suffices). Finally, Algorand assumes loosely synchronized clocks across all users (e.g., using NTP) in order to recover liveness after weak synchrony. Specifically, the clocks must be close enough in order for most honest users to kick off the recovery protocol at approximately the same time. If the clocks are out of sync, the recovery protocol does not succeed. 4 OVERVIEW Algorand requires each user to have a public key. Algorand maintains a log of transactions, called a blockchain. Each transaction is a payment signed by one user’s public key transferring money to another user’s public key. Algorand grows the blockchain in asynchronous rounds , similar to Bitcoin. In every round, a new block, containing a set of transactions and a pointer to the previous block, is appended to the blockchain. In the rest of this paper, we refer to Algo- rand software running on a user’s computer as that user. Algorand users communicate through a gossip protocol.
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  • Spring '19
  • NA
  • hash function, Cryptographic hash function, Algorand

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