FruitChains: A Fair Blockchain

The five-minute read

One action, walked through

1. Miners search for fruit and for blocks, at different difficulties.
2. A fruit hangs off a recent block and carries transactions.
3. A later block stabilises which fruit count.
4. Rewards over a segment of length tied to the security parameter are shared so that honest hash power is paid approximately in proportion.
5. A coalition below half the hash power cannot, in the theorem, gain more than a small factor by deviating.

The argument, unpacked

What has to be true

• Honest majority of hash power for the Nakamoto-style safety claims.
• Parameters, including the window, set as the paper requires for the approximation.
• Rewards and fees are actually distributed the way the fairness proof assumes.
• The hash function and the network model match a longest-chain argument.

What happened after the paper

FruitChains is the standard citation when a design claims to have fixed miner fairness. Check whether the design pays over a window and separates fruit from blocks. A new name for the longest chain does not inherit the theorem.

What to check before you use the idea

• What is the fruit, and what is the block?
• Over what window is fairness claimed?
• Does the attacker bound include transaction reordering, or only withholding?
• Is the citation used to justify a fee or a yield? It should not be.

Terms

Fruit

A lower-difficulty record that carries transactions and is later stabilised by the chain.

Approximate fairness

An honest coalition's reward share stays close to its hash-power share over a long segment.

The problem the paper names

If a miner can earn more by withholding blocks than by publishing them, the incentive story of the longest chain is incomplete. The paper wants approximate fairness: a fraction of the hash power should get about that fraction of the rewards.

What the design proposes

• Fruit are easier to mine than blocks and carry the transactions.
• Blocks still form a chain and also stabilise which fruit count.
• Rewards are spread across a window of the chain rather than paid only to the block finder.

How the mechanism is specified

• The fairness claim is approximate, over a long enough segment, and assumes an honest majority of hash power.
• The paper also argues that lower reward variance reduces the need to join a pool. That is a consequence it states, not a measurement of later pools.
• Consistency and liveness are meant to match Nakamoto under honest majority.

What this page does not treat as proven

• Fairness in the paper is about block rewards, not about transaction ordering or MEV.
• A protocol that pays more evenly can still be captured by a pool that coordinates the hash power.
• This does not set a fee, a price, or a yield.

Why a venture studio still reads it

When someone says miners are paid 'fairly', ask fair against which deviation. FruitChains answers selfish withholding. It does not answer a builder who reorders transactions inside an honest block.