This means that Algorand must use user weights and seeds from before any

This means that algorand must use user weights and

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This means that Algorand must use user weights and seeds from before any possible forks occurred. To do this, Algorand relies on the weak synchrony assumption—namely, that in every period of length b (think of b as 1 day), there must be a strongly synchronous period of length s < b (think of s as a few hours). Under this assumption, using the block timestamps, Algorand quantizes time into b -long periods (think days), and finds the most recent block from the next- to-last complete b -long period. Algorand then uses the seed 12
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from this block, and uses user weights from the last block that was agreed upon at least b -long time before it (§5.3). Algorand takes the seed from the block from the next- to-last b -long period because the most recent b -long period may still have an unresolved fork. Such a fork would prevent users from agreeing on the seed and weights used in the recovery. However, as long as Algorand can recover within the s -long strongly synchronous period in the most recent b -long period, all users will agree on the same block from the next-to-last period (as long as their clocks are roughly synchronized). To ensure that Algorand recovers from a fork (i.e., most honest users switch to the same fork) within the s -long syn- chronous period, Algorand users repeatedly attempt to reach consensus on a fork (applying a hash function to the seed each time to produce a different set of proposers and com- mittee members), until they achieve consensus. Since, by assumption, Algorand is operating in a strongly synchronous period, it is not important whether BA returns “final” or “tentative” consensus in this case. When Algorand is recov- ering outside of a strongly synchronous period, we cannot ensure recovery within s time. 8.3 Bootstrapping Bootstrapping the system. To deploy Algorand, a com- mon genesis block must be provided to all users, along with the initial cryptographic sortition seed. The value of seed 0 specified in the genesis block is decided using distributed random number generation [ 14 ], after the public keys and weights for the initial set of participants are publicly known. Bootstrapping new users. Users that join the system need to learn the current state of the system, which is defined to be the result of a chain of BA consensus outcomes. To help users catch up, Algorand generates a certificate for ev- ery block that was agreed upon by BA (including empty blocks). The certificate is an aggregate of the votes from the last step of Binary BA () (not including the final step) that would be sufficient to allow any user to reach the same con- clusion by processing these votes (i.e., there must be at least T step · τ step + 1 votes). Importantly, the users must check the sortition hashes and proofs just like in Algorithm 6, and that all messages in the certificate are for the same Algorand round and BA step.
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  • Spring '19
  • NA
  • hash function, Cryptographic hash function, Algorand

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