Algorand

Use byzantine consensus/agreements in block chains

• Replace the Proof-of-works consensus protocols in Bitcoin
• Pros
  • higher throughput
  • no forks
  • no mining
• Cons
  • liveness
    • malicious
    • concurrent proposals
  • membership management
    • join/leave
    • Sybil attacks

Sortition based on Verifiable Random Function (VRF)

• Sortition:
  • proposer
  • committee
• given public/private key pair (pk, sk), VRF(sk, seed) generates a hash and a proof
  • use the hash to deterministically decide whether the user is a proposer or a committee member
  • seed is in the previous block

BA agreements

• Vote and CountVote
  • over threshold in the first round: final consensus
  • else: tentative consensus
    • fork: could achieve tentative consensus on two blocks, why? (network asynchrony)

Compare to Stellar

• a node in Stellar selects a number of servers it trusts as the consensus group, compared to VRF used in Algorand
• BA in Stellar is ballot-based, a node first tries to find out what value is potentially chosen before