Bad Randomness in Solidity Smart Contracts

Introduction

Bad randomness, often referred to as the “nothing is secret” attack, is a vulnerability in Solidity smart contracts deployed on the Ethereum blockchain. This attack vector exploits the pseudo-random nature of certain variables and functions in Solidity, allowing an attacker to predict or influence the outcome of smart contract operations that rely on randomness.

Vulnerability Points

1. Using Block Hash for Randomness

Code Example:

solidityCopy code
function randomNumber() public view returns (uint) {
    return uint(keccak256(abi.encodePacked(block.difficulty))) % 10;
}

Attack Vector:

An attacker can monitor the block difficulty and potentially influence it by colluding with miners. By doing so, the attacker can predict the output of the randomNumber() function.

2. Using Block Timestamp for Randomness

Code Example:

solidityCopy code
function randomNumber() public view returns (uint) {
    return uint(keccak256(abi.encodePacked(block.timestamp))) % 10;
}

Attack Vector:

An attacker who controls a miner can manipulate the block timestamp, thereby influencing the output of the randomNumber() function.

3. Using Block Number for Randomness

Code Example:

solidityCopy code
function play() public payable {
    require(msg.value >= 1 ether);
    if (block.blockhash(blockNumber) % 2 == 0) {
        msg.sender.transfer(this.balance);
    }
}

Attack Vector:

An attacker can create a malicious contract that checks if the current block number is a winner. If so, it calls the vulnerable smart contract, ensuring a win since the block number remains constant within the same transaction.

Real-World Impact

• Loss of more than 400 ETH in SmartBillions Lottery.
• TheRun

Mitigation

1. Use hardware random number generators (RNGs) for generating truly random numbers.
2. Utilize decentralized randomness beacons like Chainlink’s VRF.

Conclusion

Bad randomness is an attack vector in Solidity smart contracts. Developers must be cautious when implementing randomness and should opt for secure and unpredictable sources to mitigate this risk.