> For the complete documentation index, see [llms.txt](https://ret2basic.gitbook.io/ctfnote/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ret2basic.gitbook.io/ctfnote/web3-security-research/foundry/authentication.md).

# Authentication

{% embed url="<https://youtu.be/gYwO3Jbi4O4>" %}
Authentication
{% endembed %}

The target is a wallet contract `Wallet.sol`:

```solidity
pragma solidity ^0.8.18;

contract Wallet {
    address payable public owner;

    constructor() payable {
        owner = payable(msg.sender);
    }

    receive() external payable {}

    function withdraw(uint256 _amount) external {
        require(msg.sender == owner, "caller is not owner");
        payable(msg.sender).transfer(_amount);
    }

    function setOwner(address _owner) external {
        require(msg.sender == owner, "caller is not owner");
        owner = payable(_owner);
    }
}
```

Create a test file `Auth.t.sol`:

```solidity
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.18;

import "forge-std/Test.sol";
import {Wallet} from "../src/Wallet.sol";

contract AuthTest is Test {
    Wallet public wallet;

    function setUp() public {
        wallet = new Wallet();
    }

    function testSetOwner() public {
        wallet.setOwner(address(1));
        assertEq(wallet.owner(), address(1));
    }
}
```

This `AuthTest` contract can call `wallet.setOwner()` because it was set as the `owner` in the constructor.

<figure><img src="/files/UMpCal5kCpedBNs2zUFJ" alt=""><figcaption><p>testSetOwner()</p></figcaption></figure>

Write a fail test case:

```solidity
function testFailNotOwner() public {
    vm.prank(address(1));
    wallet.setOwner(address(1));
    assertEq(wallet.owner(), address(1));
}
```

`vm.prank(address(1))` means "for the next call, pretend we are `address(1)`". This test case would fail because `address(1)` is not the owner so that it cannot call `wallet.setOwner()`.

<figure><img src="/files/aCVqaBYlzwtiQUb8hFmB" alt=""><figcaption><p>testFailNotOwner()</p></figcaption></figure>

An extension of `vm.prank()` is the `vm.startPrank()` and `vm.endPrank()` pair. The impersonation will start on `vm.startPrank()` until `vm.endPrank()` is executed.

```solidity
function testFailSetOwnerAgain() public {
    // msg.sender = address(this)
    wallet.setOwner(address(1));

    vm.startPrank(address(1));

    // msg.sender = address(1)
    wallet.setOwner(address(1));
    wallet.setOwner(address(1));
    wallet.setOwner(address(1));

    vm.stopPrank();

    // msg.sender = address(this) -> revert because address(this) is not the owner anymore
    wallet.setOwner(address(1));
}
```

<figure><img src="/files/cWUfr5gPAzVJRsYvBtcX" alt=""><figcaption><p>testFailSetOwnerAgain()</p></figcaption></figure>


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