Is it possible to create unpredictable random number with Solidity?

Question

I have been researching this topic lately and most of the resources are saying that it's impossible to generate a random number on-chain. However, I have come across a post saying that this function can generate a unpredictable number. Is there any way to predict this function?

function rand() public view returns(uint256) {
    uint256 seed = uint256(keccak256(abi.encodePacked(
        block.timestamp + block.difficulty +
        ((uint256(keccak256(abi.encodePacked(block.coinbase)))) / (now)) +
        block.gaslimit + 
        ((uint256(keccak256(abi.encodePacked(msg.sender)))) / (now)) +
        block.number
    )));

    return (seed - ((seed / 1000) * 1000));
}

Petr Hejda Petr Hejda · Accepted Answer · 2021-04-26T10:48:23

Is there any way to predict this function?

A miner can publish their block number with a timestamp that fits together with other criteria:

difficulty is given for some time
coinbase is given for even longer period of time
msg.sender (the real transaction sender and signer) can be one of the miner's addresses

There's no way how to generate truly indeterminable random number on-chain. But you can use an oracle that returns an off-chain generated "random" number.

Is it possible to create unpredictable random number with Solidity?

1 Answers