The possibility of forks also makes it difficult for new users to bootstrap

The possibility of forks also makes it difficult for

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Bitcoin’s proof-of-work approach and inherit its limitations. The possibility of forks also makes it difficult for new users to bootstrap securely: an adversary that isolates the user’s network can convince the user to use a particular fork of the blockchain [29]. By relying on Byzantine agreement, Algorand eliminates the possibility of forks, and avoids the need to reason about mining strategies [ 8 , 25 , 47 ]. As a result, transactions are confirmed on the order of a minute. To make the Byzan- tine agreement robust to Sybil attacks, Algorand associates weights with users according to the money they hold. Other techniques have been proposed in the past to resist Sybil attacks in Byzantine-agreement-based cryptocurrencies, in- cluding having participants submit security deposits and punishing those who deviate from the protocol [13]. Byzantine consensus. Byzantine agreement protocols have been used to replicate a service across a small group of servers, such as in PBFT [ 15 ]. Follow-on work has shown how to make Byzantine fault tolerance perform well and scale to dozens of servers [ 1 , 17 , 34 ]. One downside of Byzan- tine fault tolerance protocols used in this setting is that they require a fixed set of servers to be determined ahead of time; allowing anyone to join the set of servers would open up the protocols to Sybil attacks. These protocols also do not scale to the large number of users targeted by Algorand. BA is a Byzantine consensus protocol that does not rely on a fixed set of servers, which avoids the possibility of targeted attacks on well-known servers. By weighing users according to their currency balance, BA allows users to join the cryptocur- rency without risking Sybil attacks, as long as the fraction of the money held by honest users is at least a constant greater than 2/3. BA ’s design also allows it to scale to many users (e.g., 500,000 shown in our evaluation) using VRFs to fairly select a random committee. Most Byzantine consensus protocols require more than 2 / 3 of servers to be honest, and Algorand’s BA inherits this limitation (in the form of 2 / 3 of the money being held by honest users). BFT2F [ 36 ] shows that it is possible to achieve “fork -consensus” with just over half of the servers being honest, but fork -consensus would allow an adver- sary to double-spend on the two forked blockchains, which Algorand avoids. 2
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Honey Badger [ 40 ] demonstrated how Byzantine fault tol- erance can be used to build a cryptocurrency. Specifically, Honey Badger designates a set of servers to be in charge of reaching consensus on the set of approved transactions. This allows Honey Badger to reach consensus within 5 min- utes and achieve a throughput of 200 KBytes/sec of data appended to the ledger using 10 MByte blocks and 104 par- ticipating servers. One downside of this design is that the cryptocurrency is no longer decentralized; there are a fixed set of servers chosen when the system is first configured.
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  • Spring '19
  • NA
  • hash function, Cryptographic hash function, Algorand

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