converting keccak from old version: pragma solidity 0.4.19; [duplicate]

Question

I am trying to compile following code:

pragma solidity 0.5.9;
contract test{
//https://github.com/ethereum/solidity/issues/3469

address payable owner;
mapping (address => bool) pd;
uint public sS;
function lnA(address addr) public view returns(uint n) {
        // 1 in 8 chance
        n = uint(keccak256(uint(addr), sS)[0]) % 8;

    }
}

solc keccakSyntaxerr.sol

keccakSyntaxerr.sol:10:18: Error: Wrong argument count for function call: 2 arguments given but expected 1. This function requires a single bytes argument. Use abi.encodePacked(...) to obtain the pre-0.5.0 behaviour or abi.encode(...) to use ABI encoding.

n = uint(keccak256(uint(addr), sS)[0]) % 8; ^-----------------------^ keccakSyntaxerr.sol:10:13: Error: Explicit type conversion not allowed from "bytes1" to "uint256". n = uint(keccak256(uint(addr), sS)[0]) % 8; ^--------------------------------^

How to convert the above syntax into lastest Solidity versions? Somebody please guide me.

Zulfi.

Answer 1

You have to covert the arguments to a bytes. abi.encodePacked(args) is a way to do it.

pragma solidity 0.5.9;
contract test{
    //https://github.com/ethereum/solidity/issues/3469

    address payable owner;
    mapping (address => bool) pd;
    uint public sS;

    function lnA(address addr) public view returns(uint8 n) {
        // 1 in 8 chance
        n = uint8(keccak256(abi.encodePacked(addr, sS))[0]) % 8;
    }
}

I dropped uint(addr) because padding the address with extra zeroes isn't needed for the "1 in 8" objective described in the remark.

As a PSA: Be sure you fully understand the attack vectors for a game based on randomness if that is the goal. This is entirely predictable and an adversary can brute force it to get a desired result.

Hope it helps.

UPDATE

The 1 byte step doesn't add any entropy to this (there isn't any). You can:

// 1 in 8 chance
n = uint(keccak256(abi.encodePacked(addr, sS))) % 8;

Where n is a uint. That may save a little gas or be a little more readable.