Cómo hacer un token que coincida con una cadena

Quiero crear 15 tokens cada vez que el releaseToken()método recibe la cadena hello. Intenté crear esta condición pero no funciona. Este es el código que he escrito hasta ahora:

pragma solidity ^0.4.11;

import './IERC20.sol';
import './SafeMath.sol';

contract testcoin{

    using SafeMath for uint256;

    uint public _totalSupply = 0;

    string public constant symbol = "TES";
    string public constant name = "testcoin";
    uint8 public constant decimals = 3;
    string public contractString = 'hello';

    uint256 public constant RATE = 500;
    address public owner;

    mapping(address => uint256) balances;
    mapping(address => mapping(address => uint256)) allowed; 

    function releaseToken(string _string)  payable{
        require(keccak256(_string) == keccak256(contractString));
        createTokens();
    }

    function testcoin(){
        owner = msg.sender;
    }

    function totalSupply() constant returns (uint256 totalSupply){
        return _totalSupply;
    }

    function balanceOf(address _owner) constant returns (uint256 balance){
        return balances[_owner];
    }

    function transfer(address _to, uint256 _value) returns (bool success){
        require(
            balances[msg.sender] >= _value
            && _value > 0
        );
        balances[msg.sender] -= _value;
        balances[_to] += _value;
        Transfer(msg.sender, _to, _value);
        return true;
    }

    function createTokens() payable {
        uint256 tokens = msg.value.mul(RATE);
        balances[msg.sender] = balances[msg.sender].add(tokens);
        _totalSupply = _totalSupply.add(tokens);
        owner.transfer(msg.value);
    }

    function totalSupply() constant returns (uint256 totalSupply){
        return _totalSupply;
    }

    function balanceOf(address _owner) constant returns (uint256 balance){
        return balances[_owner];
    }

    function transfer(address _to, uint256 _value) returns (bool success){
        require(
            balances[msg.sender] >= _value
            && _value > 0
        );
        balances[msg.sender] = balances[msg.sender].sub(_value); //changed
        balances[_to] += _value;
        Transfer(msg.sender, _to, _value);
        return true;
    }

    function transferFrom(address _from, address _to, uint256 _value) returns (bool success){
        require(
            allowed[_from][msg.sender] >= _value
            && balances[_from] >= _value
            && _value > 0
        );
        balances[_from] -= _value;
        balances[_to] += _value;
        allowed[_from][msg.sender] -= _value;
        Transfer(_from, _to, _value);
        return true;
    }

    function approve(address _spender, uint256 _value) returns (bool success){
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }

    function allowance(address _owner, address _spender) constant returns (uint256 remaining){
        return allowed[_owner][_spender];
    }

    event Transfer(address indexed _from, address indexed _to, uint256 _value);

    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

¿Dónde debo arreglarlo? Gracias por su respuesta.