Proof-of-X

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General

Goal: Secure updating of a state

Block time: trade off between time


Proof of Work

  • Right to create new block: be the first to solve a puzzle based on consumption of a resource external to the system
  • Miners choose one of all possible forks to contribute to (or splits among chains but with reduced efficiency per chain)

Light Clients

Algorithm:

  • Download block headers from peers, consisting of
    • Merkle root
    • Hash value from previous block
    • Nonce
  • Verify proof-of-work for each block header: hash(Merkle root, previous hash, nonce) < target (based on difficulty)
  • Consider longest chain (the one with the most work in it) as the true chain
  • To find UTXOs:
    • (optional) Build Bloom filter for the desired transaction (a Bloom filter allows for testing set membership; it can have false positives but no false negatives)
    • Send request to a full node: "Please give me all partial Merkle branches that match the Bloom filter for the given block"
    • Full node sends the matching transactions and the other needed hashes from the Merkle tree (see, e.g., | Merkle tree)

Problem: Depends on the willingness of full nodes to provide the requested information and they don't have an incentive for doing so

Advantages

Disadvantages

Proof of Stake

Light Clients

Advantages

Disadvantages

  • Nothing at stake: Miners (voters) can vote on all forks of a blockchain