# Sample PERC20 contract

There is a sample PERC20 (Private ERC20) contract with disabled logs and signature check in the `balanceOf` function

Contract developers should avoid adding sensitive information to logs, such as transfer amounts, sender and recipient, etc. But you still can use them for some debugging or contract-wide events, for example, contract initialization.

Also, you should be careful with public view functions that can also leak some sensitive information, for example, user balance. In the example, we're using a `msg.sender` check to verify if the caller is the wallet owner. If this function was called from EOA (Externally Owned Account), EVM will try to recover `msg.sender` from provided signature within `eth_call`.

Code of that contract you can see below (full example you can find here: <https://github.com/SigmaGmbH/swisstronik-tutorials/blob/main/PERC20_interaction/contracts/PERC20Sample.sol>)

```solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

import "./PERC20.sol";

/**
 * @dev Sample implementation of the {PERC20} contract.
 */
contract PERC20Sample is PERC20 {
    constructor() PERC20("Sample PERC20", "pSWTR") {}

    /// @dev Wraps SWTR to PSWTR.
    receive() external payable {
        _mint(_msgSender(), msg.value);
    }

    /**
     * @dev Regular `balanceOf` function marked as internal, so we override it to extend visibility  
     */ 
    function balanceOf(address account) public view override returns (uint256) {
        // This function should be called by EOA using signed `eth_call` to make EVM able to
        // extract original sender of this request. In case of regular (non-signed) `eth_call`
        // msg.sender will be empty address (0x0000000000000000000000000000000000000000).
        require(msg.sender == account, "PERC20Sample: msg.sender != account");

        // If msg.sender is correct we return the balance
        return _balances[account];
    }

    /**
     * @dev Regular `allowance` function marked as internal, so we override it to extend visibility  
     */
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        // This function should be called by EOA using signed `eth_call` to make EVM able to
        // extract original sender of this request. In case of regular (non-signed) `eth_call`
        // msg.sender will be empty address (0x0000000000000000000000000000000000000000)
        require(msg.sender == spender, "PERC20Sample: msg.sender != account");
        
        // If msg.sender is correct we return the allowance
        return _allowances[owner][spender];
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.swisstronik.com/swisstronik-docs/development/guides/deployment-and-interaction-perc-20/sample-perc20-contract.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.