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# Random Number Generator in Solidity using keccak256

• Difficulty Level : Medium
• Last Updated : 24 Feb, 2023

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 in a specific range. Below is the step by step description of the entire process to be followed.

Step 1: Take the instance of block.timestamp, 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  modulus (%100) to take only the last 2 digits. This will give us a totally random number between 0 and 99.

## Solidity

 `// SPDX-License-Identifier: GPL-3.0``pragma solidity >=0.4.22 <0.9.0; ``/// @title A contract for demonstrating random number generation in specific range``/// @author Jitendra Kumar``/// @notice For now, this contract just show how to generate a random number in specific range using keccak256``contract` `GeeksForGeeksRandom``{``    ``// Initializing the state variable``    ``uint randNonce = 0;` `    ``// Defining a function to generate``    ``// a random number``    ``function randMod(uint _modulus) external returns(uint)``    ``{``        ``// increase nonce``        ``randNonce++;``        ``return` `uint(keccak256(abi.encodePacked(block.timestamp,msg.sender,randNonce))) % _modulus;``    ``}``}`

Output: Remix Console Output

Step 4: If you do not want to get the random number in specific range than use the below smart contract which returns a large random number.

## Solidity

 `// SPDX-License-Identifier: GPL-3.0``pragma solidity >=0.4.22 <0.9.0; ``/// @title A contract for demonstrate random number generation``/// @author Jitendra Kumar``/// @notice For now, this contract just show how to generate a random number in Solidity using keccak256``contract` `GeeksForGeeksRandom``{``    ``// Initializing the state variable``    ``uint randNonce = 0;` `    ``// Defining a function to generate``    ``// a random number``    ``function randMod() external returns(uint)``    ``{``        ``// increase nonce``        ``randNonce++;``        ``return` `uint(keccak256(abi.encodePacked(block.timestamp,msg.sender,randNonce)));``    ``}``}`

Output: Remix Console Output

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

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 win, 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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