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Random Number Generator in Solidity using keccak256

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  • Difficulty Level : Medium
  • Last Updated : 11 May, 2022
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Random numbers are numbers that occur in a sequence with the following two mandatory conditions:

  1. The values are uniformly distributed over a set interval.
  2. One cannot predict future values based on past outputs.

Example: In the below example, we have created a contract with a function that will generate a random number. Below is the step by step description of the entire process to be followed.

Step 1: Take the instance of now, the msg.sender, and an incrementing nonce.

Step 2: “pack” the inputs and use keccak256() to convert into 256-bit hash.

Step 3: Convert that hash to an uint, and then use % 100 to take only the last 2 digits. This will give us a totally random number between 0 and 99.


// Solidity program to
// demonstrate on how
// to generate a random number
pragma solidity ^0.6.6;
// Creating a contract
contract GeeksForGeeksRandom
// Initializing the state variable
uint randNonce = 0;
// Defining a function to generate
// a random number
function randMod(uint _modulus) internal returns(uint)
   // increase nonce
   return uint(keccak256(abi.encodePacked(now,
                                          randNonce))) % _







Note: The alias “now” for block.timestamp  in  the line below  was removed in the version 0.7.0 ,  you can use block.timestamp in place of now

return uint(keccak256(abi.encodePacked(now, msg.sender, randNonce))) % _modulus;

Possible attacks with this approach:

In Ethereum, all nodes are trying to solve the problem and verify the transaction. Once a node verifies it, it broadcasts it to the network.


Suppose we create a DApp where we flip a coin where the head is the winning side. We use the above function to predict heads or tails. If I were running a node, I could publish a transaction only to my own node and not share it. I will run the randMod function or the coin flip function until I will and will only share the transaction after I have won.


One way to solve this would be to use an oracle to access a random number function from outside the Ethereum blockchain. There are other cryptographic algorithms and third party functions that can be utilized, but they are not safe or should be audited.


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