Insecure-Randomness

Smart Contract Security -Insecure Randomness

In most programming languages, if we want to generate a random number it is super trivial to do so by using built-in methods. But in Solidity it is extermly difficult to generate a secure random number number

The Attack

A common technique to generate a random number is to hash together the block.timestamp and block.number - 1 as they seem random, while they are not.

Why the block.timestamp and block.number - 1 seem random input?

• At the time of calling a function*__creating a transaction*, the block is not yet mined, and it's mining time is unkown so the block.timestamp is hard to be guessed. Same thing with block.number - 1.

To demonstrate in a practical example, a vulnerable contract is created in ./Lottery-attack.sol. The contract's logic is simple:

Someone calls the guess function and passes a uint _guess parameter, the function calculates a *randomAnswer* number by hashing along the *block.timestamp* and *block.number - 1*.

If the randomAnswer equals the _guess the msg.sender receives the contract's balance.

However, the Attack contract may exploit easily the Lottery contract. Yes, it is hard for us*__humans* to predict the block.timestamp and block.number - 1, but for a smart contratc it is easy. The attacker invokes the attack function, this function calculates the randomAnswer which will be exactly the same that will be caluclated in Lottery.guessToWin, because both transactions will be mined in the same block.