Ethernaut - 18. Magic Number

Difficulty: 🌕🌕🌕🌑🌑

To solve this level, you only need to provide the Ethernaut with a Solver, a contract that responds to whatIsTheMeaningOfLife() with the right number.
Easy right? Well… there’s a catch.
The solver’s code needs to be really tiny. Really reaaaaaallly tiny. Like freakin’ really really itty-bitty tiny: 10 opcodes at most.
Hint: Perhaps its time to leave the comfort of the Solidity compiler momentarily, and build this one by hand O_o. That’s right: Raw EVM bytecode.
Good luck!

Contract

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

contract MagicNum {

  address public solver;

  constructor() public {}

  function setSolver(address _solver) public {
    solver = _solver;
  }

  /*
    ____________/\\\_______/\\\\\\\\\_____        
     __________/\\\\\_____/\\\///////\\\___       
      ________/\\\/\\\____\///______\//\\\__      
       ______/\\\/\/\\\______________/\\\/___     
        ____/\\\/__\/\\\___________/\\\//_____    
         __/\\\\\\\\\\\\\\\\_____/\\\//________   
          _\///////////\\\//____/\\\/___________  
           ___________\/\\\_____/\\\\\\\\\\\\\\\_ 
            ___________\///_____\///////////////__
  */
}

Writeup

1. Get new instance.
2. Create a new contract

    // SPDX-License-Identifier: MIT
    pragma solidity ^0.6.0;
   
    contract MagicNumberCracker{
        constructor() public{
            assembly{
                mstore(0x00, 0x602a60005260206000f3)
                return(0x16, 0x0a)
            }
        }
    }
   

   How 0x602a60005260206000f3 come from ?

   1. PUSH(0x2a) –> 0x602a (Push 42 onto the stack)
   2. PUSH(0x00) –> 0x6000 (Push memory slot 00 to stack)
   3. MSTORE –> 0x52 (Store 42 to memory slot 00)
   4. PUSH(0x20) –> 0x6020 (Memory slot size is 32 bytes)
   5. PUSH(0x80) –> 0x6000 (Value is stored at moemory slot 00)
   6. RETURN –> 0xf3 (Return value which is stored at memory 00 with sizeof 32 bytes)
3. Compile and Deploy.
4. Set Solver :

   
    await contract.setSolver('MAGICNUMBERCRACKER_CONTRACT_ADDRESS')
   
   

5. Submit instance ξ( ✿＞◡❛)

Reference

• ethervm
• Solidity- Layout in memory
• The Ethereum Virtual Machine