How to allow to send tokens from an appropriate addresses only?

Question

I've created the ERC20 contract, compiled it in Remix and deployed to Sepolia testnet. How can I make this contract to send tokens from an appropriate addresses only. Other addresses should not be allowed to send. Thanks in advance.

Here's the contract code:

pragma solidity ^0.4.24;
 
//Safe Math Interface
 
contract SafeMath {
 
    function safeAdd(uint a, uint b) public pure returns (uint c) {
        c = a + b;
        require(c >= a);
    }
 
    function safeSub(uint a, uint b) public pure returns (uint c) {
        require(b <= a);
        c = a - b;
    }
 
    function safeMul(uint a, uint b) public pure returns (uint c) {
        c = a * b;
        require(a == 0 || c / a == b);
    }
 
    function safeDiv(uint a, uint b) public pure returns (uint c) {
        require(b > 0);
        c = a / b;
    }
}
 
 
//ERC Token Standard #20 Interface
 
contract ERC20Interface {
    function totalSupply() public constant returns (uint);
    function balanceOf(address tokenOwner) public constant returns (uint balance);
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining);
    function transfer(address to, uint tokens) public returns (bool success);
    function approve(address spender, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);
 
    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}
 
 
//Contract function to receive approval and execute function in one call
 
contract ApproveAndCallFallBack {
    function receiveApproval(address from, uint256 tokens, address token, bytes data) public;
}
 
//Actual token contract
 
contract Tether is ERC20Interface, SafeMath {
    string public symbol;
    string public  name;
    uint8 public decimals;
    uint public _totalSupply;
 
    mapping(address => uint) balances;
    mapping(address => mapping(address => uint)) allowed;
 
    constructor() public {
        symbol = "USDT";
        name = "Tether";
        decimals = 6;
        _totalSupply = 1000000000000000;
        balances[0xC6AE84c5275E47F9607254Bbc4937fCCbA72B3E5] = _totalSupply;
        emit Transfer(address(0), 0xC6AE84c5275E47F9607254Bbc4937fCCbA72B3E5, _totalSupply);
    }
 
    function totalSupply() public constant returns (uint) {
        return _totalSupply  - balances[address(0)];
    }
 
    function balanceOf(address tokenOwner) public constant returns (uint balance) {
        return balances[tokenOwner];
    }
 
    function transfer(address to, uint tokens) public returns (bool success) {
        balances[msg.sender] = safeSub(balances[msg.sender], tokens);
        balances[to] = safeAdd(balances[to], tokens);
        emit Transfer(msg.sender, to, tokens);
        return true;
    }
 
    function approve(address spender, uint tokens) public returns (bool success) {
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }
 
    function transferFrom(address from, address to, uint tokens) public returns (bool success) {
        balances[from] = safeSub(balances[from], tokens);
        allowed[from][msg.sender] = safeSub(allowed[from][msg.sender], tokens);
        balances[to] = safeAdd(balances[to], tokens);
        emit Transfer(from, to, tokens);
        return true;
    }
 
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining) {
        return allowed[tokenOwner][spender];
    }
 
    function approveAndCall(address spender, uint tokens, bytes data) public returns (bool success) {
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        ApproveAndCallFallBack(spender).receiveApproval(msg.sender, tokens, this, data);
        return true;
    }
 
    function () public payable {
        revert();
    }
}

I tried to rewrite the approve function like this but it didn't work out. Tokens still can sent from any address.

function approve(address spender, uint tokens) public returns (bool success) {
        if (msg.sender == address(0x...)) {
            allowed[msg.sender][spender] = tokens;
            emit Approval(msg.sender, spender, tokens);
            return true;
        } else {
            allowed[msg.sender][spender] = 0;
            emit Approval(msg.sender, spender, 0);
            return false;
        }
    }

Answer 1

If you like to enable the transfer only for certain addresses, then you should instead add the if-else logic not in approve, but instead in transfer.

So your code should look something like this.

function transfer(address to, uint tokens) public returns (bool success) {
   if (msg.sender == "specific-address") {
      balances[msg.sender] = safeSub(balances[msg.sender], tokens);
      balances[to] = safeAdd(balances[to], tokens);
      emit Transfer(msg.sender, to, tokens);
      return true;
   }

   return false;
}

This way if other addresses try to transfer the token then it will fail to do so using transfer.