Make .txt file unreadable / uneditable

Question

I have a program which saves a little .txt file with a highscore in it:

 // Create a file to write to. 
string createHighscore = _higscore + Environment.NewLine;
File.WriteAllText(path, createText);

// Open the file to read from. 
string createHighscore = File.ReadAllText(path);

The problem is that the user can edit the file as simple as possible – with a texteditor. So I want to make the file unreadable / uneditable or encrypt it.

My thinking was that I could save the data in a resource file, but can I write in a resource file? Or save it as .dll, encrypt/decrypt it or look for a MD5-sum/hash.

Answer 1

You can't prevent the user from modifying the file. It's their computer, so they can do whatever they want (that's why the whole DRM issue is… difficult).

Since you said you're using the file to save an high-score, you have a couple of alternatives. Do note that as previously said no method will stop a really determined attacker from tampering with the value: since your application is running on the user computer he can simply decompile it, look at how you're protecting the value (gaining access to any secret used in the process) and act accordingly. But if you're willing to decompile an application, find out the protection scheme used and come up with a script/patch to get around it only to change a number only you can see, well, go for it?

Obfuscate the content

This will prevent the user from editing the file directly, but it won't stop them as soon as the obfuscation algorithm is known.

var plaintext = Encoding.UTF8.GetBytes("Hello, world.");
var encodedtext = Convert.ToBase64String(plaintext);

Save the ciphertext to the file, and reverse the process when reading the file.

Sign the content

This will not prevent the user from editing the file or seeing its content (but you don't care, an high-score is not secret) but you'll be able to detect if the user tampered with it.

var key = Encoding.UTF8.GetBytes("My secret key");
using (var algorithm = new HMACSHA512(key))
{
    var payload = Encoding.UTF8.GetBytes("Hello, world.");
    var binaryHash = algorithm.ComputeHash(payload);
    var stringHash = Convert.ToBase64String(binaryHash);
}

Save both the payload and the hash in the file, then when reading the file check if the saved hash matches a newly computed one. Your key must be kept secret.

Encrypt the content

Leverage .NET's cryptographic libraries to encrypt the content before saving it and decrypt it when reading the file.

Please take the following example with a grain of salt and spend due time to understand what everything does before implementing it (yes, you'll be using it for a trivial reason, but future you — or someone else — may not). Pay special attention on how you generate the IV and the key.

// The initialization vector MUST be changed every time a plaintext is encrypted.
// The initialization vector MUST NOT be reused a second time.
// The initialization vector CAN be saved along the ciphertext.
// See https://en.wikipedia.org/wiki/Initialization_vector for more information.
var iv = Convert.FromBase64String("9iAwvNddQvAAfLSJb+JG1A==");

// The encryption key CAN be the same for every encryption.
// The encryption key MUST NOT be saved along the ciphertext.
var key = Convert.FromBase64String("UN8/gxM+6fGD7CdAGLhgnrF0S35qQ88p+Sr9k1tzKpM=");

using (var algorithm = new AesManaged())
{
    algorithm.IV = iv;
    algorithm.Key = key;

    byte[] ciphertext;

    using (var memoryStream = new MemoryStream())
    {
        using (var encryptor = algorithm.CreateEncryptor())
        {
            using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
            {
                using (var streamWriter = new StreamWriter(cryptoStream))
                {
                    streamWriter.Write("MySuperSecretHighScore");
                }
            }
        }

        ciphertext = memoryStream.ToArray();
    }

    // Now you can serialize the ciphertext however you like.
    // Do remember to tag along the initialization vector,
    // otherwise you'll never be able to decrypt it.

    // In a real world implementation you should set algorithm.IV,
    // algorithm.Key and ciphertext, since this is an example we're
    // re-using the existing variables.
    using (var memoryStream = new MemoryStream(ciphertext))
    {
        using (var decryptor = algorithm.CreateDecryptor())
        {
            using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
            {
                using (var streamReader = new StreamReader(cryptoStream))
                {
                    // You have your "MySuperSecretHighScore" back.
                    var plaintext = streamReader.ReadToEnd();
                }
            }
        }
    }
}

Answer 2

As you seem to look for relatively low security, I'd actually recommend going for a checksum. Some pseudo-code:

string toWrite = score + "|" + md5(score+"myKey") + Environment.NewLine

If the score would be 100, this would become

100|a6b6b0a8e56e42d8dac51a4812def434

To make sure the user didn't temper with the file, you can then use:

string[] split = readString().split("|");
if (split[1] != md5(split[0]+"myKey")){
     alert("No messing with the scores!");
}else{
     alert("Your score is "+split[0]);
}

Now of course as soon as someone gets to know your key they can mess with this whatever they want, but I'd consider that beyond the scope of this question. The same risk applies to any encryption/decryption mechanism.

One of the problems, as mentioned in the comments down below, is that once someone figures out your key (through brute-forcing), they could share it and everybody will be able to very easily change their files. A way to resolve this would be to add something computer-specific to the key. For instance, the name of the user who logged in, ran through md5.

string toWrite = score + "|" + md5(score+"myKey"+md5(System.username /**or so**/)) + Environment.NewLine

This will prevent the key from being "simply shared".

Answer 3

Probably your best bet is securing the whole file using standard NT security and programmatically change the access control list to protect the whole file from being edited by unwanted users (excepting the one impersonating your own application, of course).

Cryptography can't help here because the file could be still editable using a regular text editor (for example, notepad) and the end user can corrupt the file just adding an extra character (or dropping one too).

There's an alternate approach which doesn't involve programming effort...

Tell your users that once they've manually edited the whole text file they've lost your support. At the end of the day, if you're storing this data is because it's required by your application. Corrupting it or doing the risky task of manually editing it can make your application produce errors.

Another alternate approach which involves programming effort...

Whenever you change the file from your application, you can compute a MD5 or SHA hash and store in a separate file, and once you want to read or write it again, you're going to check that the whole file produces the same hash before writing on it again.

This way, the user can still edit your file manually, but you'll know when this unexpected behavior was done by the user (unless the user also manually computes the hash whenever the file is changed...).

Answer 4

Something I have not yet seen mentioned is storing the high score on an online leader board. Obviously this solution requires a lot more development, but since you are talking about a game, you could probably make use of a third party provider like Steam, Origin, Uplay, ... This has the added advantage of leader boards not just being for your machine.