Sybil Resistance on Bitsy Wiki

Proof of Work

Mon, 01 Jan 0001 00:00:00 +0000

A proof of work is a piece of data that shows its producer performed a certain amount of computation. The producer searches for a value that, combined with some fixed data and put through a hash function, gives a result below a target number. The only known way to find such a value is to try one after another, and the lower the target, the more tries it takes on average. Anyone can check a claimed value with a single hash.

Proof of Stake

Mon, 01 Jan 0001 00:00:00 +0000

Proof of stake is the practice of weighting the participants in a blockchain’s consensus by how much of the chain’s own currency each has locked up in it. A participant who wants to produce or vote on blocks, a validator, deposits coins in the ledger. The protocol then chooses block producers and counts votes in proportion to those deposits.

The deposit is the scarce thing that keeps one party from posing as many, which makes proof of stake a Sybil-resistance mechanism. A thousand validators holding one coin each carry the same weight as one validator holding a thousand.

Proof of Space

Mon, 01 Jan 0001 00:00:00 +0000

A proof of space is a way for a participant to show that it has set aside a large amount of disk storage. The participant first fills the storage with data that it generates itself and that has no other use. Later, when sent a random challenge, it looks up a small part of that data and returns a short answer, which the challenger can check quickly. Answering from the stored data is cheap. Answering without it means regenerating the data, which is slow.

Proof of Storage

Mon, 01 Jan 0001 00:00:00 +0000

A proof of storage is a protocol by which a party holding someone else’s data shows that it still holds it, without sending the data back. The holder is sent a challenge that names a few randomly chosen pieces of the file, and it returns a short answer that it could compute only from those pieces. The checker compares the answer with a small amount of information kept from when the file was first handed over.

Proof of Burn and Proof of Transfer

Mon, 01 Jan 0001 00:00:00 +0000

A proof of burn shows that someone has destroyed cryptocurrency on purpose. The coins are sent, in an ordinary transaction, to an address that nobody can spend from, and the transaction sitting permanently in that chain’s history is the proof. Anyone can check it, and the coins are gone for good. A proof of transfer is the same payment sent to other participants in place of the unspendable address.

Both turn spending into standing. They have been used in two ways: once, to hand out a new token to whoever burned an old one, and continuously, as the cost of competing to produce each block of a chain. Only the second makes them a Sybil-resistance mechanism, in which the scarce resource is currency given up, the chain’s own or another chain’s. The Bitcoin wiki’s page on the idea observes that the coins burned have so far all been mined by proof of work somewhere, so that “the ultimate source of scarcity remains the proof-of-work-mined ‘fuel’.”

Proof of Authority

Mon, 01 Jan 0001 00:00:00 +0000

Proof of authority is an arrangement in which a blockchain’s blocks are produced by a fixed list of approved signers. A block is valid if it carries the signature of a key on the list and the signer has respected the protocol’s rule for taking turns. Nobody competes for the right to produce a block and nothing is staked. What a participant needs is a place on the list, which is set when the chain starts and changed afterwards by a vote of the signers already on it.

Proof of Personhood

Mon, 01 Jan 0001 00:00:00 +0000

A proof of personhood is a credential showing that its holder is a real human being who holds no other such credential. It says nothing else about the holder. A system that hands one to each person, and counts each credential once, gives every person exactly one voice.

As a Sybil-resistance mechanism it replaces the tradable resource with the participant’s own existence. Proof of work and proof of stake give more influence to whoever commits more, and the people who proposed proof of personhood meant it as an objection to that. Bryan Ford’s requirement is that it must not be feasible “to buy more (effective) voting power with more wealth, connections, or other resources”, which, he notes, “excludes investment-based foundations for decentralized systems such as proof of work or proof of stake.”

Proof of Elapsed Time

Mon, 01 Jan 0001 00:00:00 +0000

Proof of elapsed time is a way of choosing who produces the next block by having every validator wait for a random length of time, with the shortest wait winning. The waiting is supervised by a trusted execution environment: a protected region of a processor, called an enclave, whose code the machine’s owner cannot alter and whose outputs the processor signs. The enclave draws the wait time, and when the time has passed it issues a signed certificate saying so. The validator presents that certificate as its right to publish the block.

Proof of Useful Work

Mon, 01 Jan 0001 00:00:00 +0000

A proof of useful work is a proof of work whose computation produces something of value besides the proof. Ordinary proof of work has a miner try billions of hashes whose only product is a number below a target. The proposal is to have the miner solve a problem somebody wants solved, in a form that still shows the computation was done.

The appeal is the electricity. The obstacle is that a mining puzzle needs a set of properties that useful problems rarely have, and each attempt to supply them has taken away some of the usefulness. No widely used chain runs on useful work. The idea is as old as the term proof of work: the 1999 paper that formalized proof of work also defined a “bread pudding protocol”, one in which the effort “may be reused by the verifier to achieve a separate, useful, and verifiably correct computation.”