Ethereum Cookbook

How to do it…

1. Programs in Ethereum have to be deterministic since they run on multiple nodes, and everyone running the EVM has to get the same result. This prevents the implementation of a true random number generator for Ethereum-based DApps.
2. A common method of implementation for the random number generator is by using deterministic, miner-defined parameters such as difficulty or timestamp:

pragma solidity^0.4.24;// Contract to generate random number// Uses timestamp and difficulty// Not recommendedcontract RNG {    function generateRandom() view returns (uint) {        return uint256(keccak256(            abi.encodePacked(block.timestamp, block.difficulty)        ));    }}

3. Modify this further to generate a random number from the block hash of a future block, which ...