How to find value of non-public state variable

Question

I understand that public state variables automatically creates a getter function when deployed, its value can be accessed via the getter function.

However, if a state variable is not public, how can we access the value? I am doing this for experiment purpose. Since everything is on the blockchain, even if a state variable is not public, we should still be able to find its value in the contract storage right?

I am running my contracts on Ganache, I saw one of the RPC for ganache is eth_getStorageAt(), maybe somehow it can be accomplished here?

eth_getStorageAt seems to return very raw information, if we can access the non-public state variable values, how do we locate it in the storage?

Answer 1

A contract with two storage variables, one public and one private.

pragma solidity 0.7.0;

contract Test {
    uint256 public a = 10;
    uint256 private b = 20;
}

Variable a is stored at slot 0 and can be accessed using

web3.eth.getStorageAt(
    "address of contract",
    0,
    (
        err,
        slot_content
    ) => console.log(err, slot_content)
)

Variable b is stored at slot 1 and can be accessed using

web3.eth.getStorageAt(
    "address of contract",
    1,
    (
        err,
        slot_content
    ) => console.log(err, slot_content)
)