refac: Secure Improvements on the ERC20 and ERC721 tokens

contracts/ERC20.sol

@@ -1,19 +1,24 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.24;
+pragma solidity >=0.7.0 <0.9.0 ;
 
 import "./IERC20.sol";
+import "./Ownable.sol";
 
-contract ERC20 is IERC20 {
+contract ERC20 is IERC20, Ownable {
     event Transfer(address indexed from, address indexed to, uint256 value);
     event Approval(
         address indexed owner, address indexed spender, uint256 value
     );
+    event Mint(address indexed to, uint256 amount);
+    event Burn(address indexed from, uint256 amount);
+
 
     uint256 public totalSupply;
     mapping(address => uint256) public balanceOf;
     mapping(address => mapping(address => uint256)) public allowance;
     string public name;
     string public symbol;
+    uint8 public decimals;
 
     constructor(string memory _name, string memory _symbol, uint8 _decimals) {
         name = _name;
@@ -23,16 +28,19 @@ contract ERC20 is IERC20 {
 
     function transfer(address recipient, uint256 amount)
         external
+        onlyOwner
         returns (bool)
     {
+        require(recipient != address(0), "A Transfer to Address Zero!");
+        require(amount <= balanceOf[msg.sender]);
         balanceOf[msg.sender] -= amount;
         balanceOf[recipient] += amount;
         emit Transfer(msg.sender, recipient, amount);
 
         return true;
     }
 
-    function approve(address spender, uint256 amount) external returns (bool) {
+    function approve(address spender, uint256 amount) external onlyOwner returns (bool) {
         allowance[msg.sender][spender] = amount;
         emit Approval(msg.sender, spender, amount);
         return true;
@@ -42,6 +50,8 @@ contract ERC20 is IERC20 {
         external
         returns (bool)
     {
+        require(amount <= allowance[sender][msg.sender], "Transfer amount exceeds allowance");
+        require(amount <= balanceOf[msg.sender]);
         require(msg.sender != recipient, "cannot transfer to self");
         allowance[sender][msg.sender] -= amount;
         balanceOf[sender] -= amount;
@@ -50,23 +60,25 @@ contract ERC20 is IERC20 {
         return true;
     }
 
-    function _mint(address to, uint256 amount) internal {
+    function _mint(address to, uint256 amount) onlyOwner internal {
+        require(to != address(0), "You can't mint to Address Zero!");
         balanceOf[to] += amount;
         totalSupply += amount;
-        emit Transfer(address(0), to, amount);
+        emit Mint(to, amount);
     }
 
-    function _burn(address from, uint256 amount) internal {
+    function _burn(address from, uint256 amount) onlyOwner internal {
         balanceOf[from] -= amount;
         totalSupply -= amount;
-        emit Transfer(from, address(0), amount);
+        emit Burn(from, amount);
+
     }
 
-    function mint(address to, uint256 amount) external {
+    function mint(address to, uint256 amount) onlyOwner external {
         _mint(to, amount);
     }
 
-    function burn(address from, uint256 amount) external {
+    function burn(address from, uint256 amount) onlyOwner external {
         _burn(from, amount);
     }
 }

contracts/ERC721.sol

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
-pragma solidity ^0.8.24;
+pragma solidity >=0.7.0 <0.9.0 ;
+import "contracts/Ownable.sol";
+
 
 interface IERC165 {
     function supportsInterface(bytes4 interfaceID)
@@ -41,7 +43,7 @@ interface IERC721Receiver {
     ) external returns (bytes4);
 }
 
-contract ERC721 is IERC721 {
+contract ERC721 is IERC721, Ownable {
     event Transfer(
         address indexed from, address indexed to, uint256 indexed id
     );
@@ -88,7 +90,7 @@ contract ERC721 is IERC721 {
         emit ApprovalForAll(msg.sender, operator, approved);
     }
 
-    function approve(address spender, uint256 id) external {
+    function approve(address spender, uint256 id) external onlyOwner {
         address owner = _ownerOf[id];
         require(
             msg.sender == owner || isApprovedForAll[owner][msg.sender],
@@ -117,6 +119,7 @@ contract ERC721 is IERC721 {
     }
 
     function transferFrom(address from, address to, uint256 id) public {
+        require(_ownerOf[id] != address(0), "token doesn't exist");
         require(from == _ownerOf[id], "from != owner");
         require(to != address(0), "transfer to zero address");
 
@@ -133,7 +136,7 @@ contract ERC721 is IERC721 {
 
     function safeTransferFrom(address from, address to, uint256 id) external {
         transferFrom(from, to, id);
-
+        require(_isApprovedOrOwner(from, msg.sender, id), "not authorized");
         require(
             to.code.length == 0
                 || IERC721Receiver(to).onERC721Received(msg.sender, from, id, "")
@@ -149,7 +152,7 @@ contract ERC721 is IERC721 {
         bytes calldata data
     ) external {
         transferFrom(from, to, id);
-
+        require(_isApprovedOrOwner(from, msg.sender, id), "not authorized");
         require(
             to.code.length == 0
                 || IERC721Receiver(to).onERC721Received(msg.sender, from, id, data)
@@ -182,12 +185,12 @@ contract ERC721 is IERC721 {
 }
 
 contract MyNFT is ERC721 {
-    function mint(address to, uint256 id) external {
+    function mint(address to, uint256 id) external onlyOwner {
         _mint(to, id);
     }
 
-    function burn(uint256 id) external {
+    function burn(uint256 id) external onlyOwner  {
         require(msg.sender == _ownerOf[id], "not owner");
         _burn(id);
     }
-}
+}

contracts/Ownable.sol

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.7.0 <0.9.0 ;
+
+contract Ownable{
+    address public owner;
+
+    constructor() {
+        owner = msg.sender;
+    }
+
+    modifier onlyOwner () {
+        require(msg.sender == owner, "You're not the Owner!");
+        _;
+    }
+
+}