If I use private variable, can attacker or anyone be able to know what the variable is?

For example.

I declare a private variable named "secret" without initialize its value.

Later after I created the contract, I then interacted with my contract by set the "secret" variable to "hello". Suppose I did this using ether wallet with metamask.


In theory, I know that anyone can't get my private secret by interacting with my Secret contract. But since I set the secret to "hello" in the previous transaction. Will the word "hello" get record in a log file, transaction, or somewhere which can be dig up?

I'm just trying to understand how to keep a secret inside my contract.

This is my example contract

pragma solidity ^0.4.15;

 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permission"
contract Ownable {

    address public owner;

     * @dev The Ownable constructor sets the original 'owner' of the contract to the sender
     * account.
    function Ownable() internal {
        owner = msg.sender;

     *  @dev Throws if called by any account other than the owner.
    modifier onlyOwner() {
        require(msg.sender == owner);


contract Secret is Ownable {

  string private secret;

  function Secret() public {
    secret = "";

  function setSecret(string _secret) public onlyOwner {
    secret = _secret;


All data stored inside a smart contract is stored in the blockchain.

All data in the blockchain is publicly accessible.

Making a variable private just means it will not be included in your ABI. Anyone can still determine its value.

To store a secret on the blockchain, you must encrypt it off-chain and store the decryption key off-chain.

  • 1
    Basically, everything in my contract as well as all the variables are visible to the world. – Geeroz Oct 13 '17 at 8:32

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