If I understand correctly, the smart contracts are run by the miners. What incentive do the miners have to execute the smart contracts correctly? What stops a malicious miner from changing the result, or a simply lazy miner from doing a simpler computation?

In bitcoin, the proof-of-work is hard to calculate but easy to verify. I don't expect this to be true of all contracts.

2 Answers 2


Every full node will run the same contract with the same input (with the goal of checking the output).

So, when a miner runs a contract and notes its execution in a published block, every full node then runs the same contract (with same inputs) for themselves, in order to verify the output of the miner. If the output if different, then the block is rejected.

Therefore, the miner's incentive is that the block will be rejected if the miner does not process the contract correctly or maliciously - the consensus process!


Every full node verifies every contract execution, so they will not relay invalid blocks, and miners will not mine on top of invalid blocks.

This does mean that calculation on Ethereum is relatively inefficient, since the computation must be carried out by every node, but there are techniques for moving computation off the blockchain in a secure manner.

  • Okay, so smart contracts are intended to be cheap then. Right? Apr 17, 2016 at 23:47
  • There is a very small amount of computation available per block, and thus computation can be somewhat expensive (not exorbitantly so, still under $0.01 per transaction). Thus, it is in the designer's best interest to only put critical components on the blockchain, usually the money-handling components. Apr 18, 2016 at 0:00
  • Is the small amount of computation a hard limit? If so, how does the technique you linked of having the computation be run on-chain if there is a dispute work? Apr 18, 2016 at 0:03
  • 1
    The amount of computation allowed per block (as measured in gas and called the block gas limit) is voted up or down by miners. So it is flexible, and will increase as supply of computing power increases. Another thing that can be done to get around the limit, is that large computations can be broken up across multiple blocks, if they need to be run on-chain Apr 18, 2016 at 0:06
  • Hopefully cryptographic obfusication becomes practical at some point. It would allow computations to be run and signed in such a way that it couldn't be altered. Apr 18, 2016 at 0:09

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