C# very simple string encryption for game

Question

I am making a small game on Unity using c#. It is basically a 2d side scrolling runner type game. I measure the time taken by each player to finish the game and it is stored on a float. I would like to put this in a string along with a salt word, and display an encrypted code to the player.

The code has to be small enough to be comfortably written down by the player. I should be able to decrypt the code which the player would send me, and figure out how much time he/she took to complete the game.

Please tell me how I should proceed in doing this. most of the algorithms i saw online are very complex and generate large blocks of encrypted text.

Thanks in advance.

It isn't possible to have an encryption which is smaller than the input and still possible to decrypt. So the best case scenario is that the code generated is the length of the data you are encrypting. — CathalMF
– CathalMF, Commented May 21, 2014 at 10:55
If you want to prevent the user changing the value, a checksum (which hashes the data and puts the hash at the end) is good as they won't know how to generate the check sum. See ISBNs for how to do this. — T. Kiley
– T. Kiley, Commented May 21, 2014 at 14:21

ThreeFx · Accepted Answer · 2014-05-21 14:05:15Z

2

Just do something like the Caesar cipher. For example:

for (int i = 0; i < message.Length(); i++)
{
    if (message[i] < 123 && message[i] > 96)
    {
        // 97 to 122 are 'a' to 'z'
        message[i] += 3;

        if (message[i] > 122)
        {
            message[i] -= 26;
        }
    }
}

EDIT: XOR-encryption:

string codeword = "word";
string res = "";
for (int i = 0; i < message.Length(); i++)
{
    res += (char)(message[i] ^ codeword[i % codeword.Length()]);
}

To decrypt it, just run the encryption again on the encrypted message.

edited May 21, 2014 at 14:05

answered May 21, 2014 at 10:53

ThreeFx

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2 Comments

xSF.AzraeL Over a year ago

Too simple. Too easy to figure out at a glance.

ThreeFx Over a year ago

You could try XOR-ing characters of a codeword and your message. That one is not easy to get but still manageable to solve.

stevepkr84 · Accepted Answer · 2014-05-21 10:56:13Z

1

Properly encrypting it will generally create large values that don't really suit your needs. You could look at something like converting the value e.g to base 36

http://www.translatorscafe.com/cafe/units-converter/numbers/calculator/decimal-to-base-36/

For example take a time of 10:50 you could translate this to something like 9991050, giving a user code of 5Y556. You could then convert it back to decimal and parse it accordingly.

This isn't really encryption as such, but it may suit your needs.

answered May 21, 2014 at 10:56

stevepkr84

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Comments

John Henckel · Accepted Answer · 2016-08-30 18:51:46Z

1

Here are some variants on the Rot13 idea. The Rot39 also scrambles only a subset of the characters, and it is not simple arithmetic like Caesar.

public static string Rot47(string input)
{
    return !string.IsNullOrEmpty(input) ? new string(input.Select(x =>
        (x >= 33 && x <= 126) ? (char)((x + 14) % 94 + 33) : x).ToArray()) : input;
}


public static string Rot13(string input)
{
    return !string.IsNullOrEmpty(input) ? new string(input.Select(x =>
        (x >= 'a' && x <= 'z') ? (char)((x - 'a' + 13) % 26 + 'a') :
        (x >= 'A' && x <= 'Z') ? (char)((x - 'A' + 13) % 26 + 'A') : x).ToArray()) : input;
}


public static string Rot39(string input)
{
    // This string contains 78 different characters in random order.
    var mix = "QDXkW<_(V?cqK.lJ>-*y&zv9prf8biYCFeMxBm6ZnG3H4OuS1UaI5TwtoA#Rs!,7d2@L^gNhj)EP$0";
    var result = (input ?? "").ToCharArray();
    for (int i = 0; i < result.Length; ++i)
    {
        int j = mix.IndexOf(result[i]);
        result[i] = (j < 0) ? result[i] : mix[(j + 39) % 78];
    }
    return new string(result);
}

It would probably take a good hacker about 15 minutes to figure out Rot39 using black box testing.

edited Aug 30, 2016 at 18:51

answered Aug 29, 2016 at 14:36

John Henckel

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1 Comment

John Henckel Over a year ago

@zaph thanks, i'll remove the %, nobody needs that :)

0xBADF00D · Accepted Answer · 2014-05-21 10:53:27Z

0

First of all, I would recommend you to implement a a mechanisms, that sends you the information needed through a secure connection automatically. So the user is not involved in this and there for not (easily) able to manipulate something.

It this is not possible for some kind of reason and you only want to avoid that everyone is able manipulate it, there are some easy to implement possibilities: Eg Rot13 algorithm or a variation. Or you can also use some other simple "replace one digit/character by an other" algorithm.

answered May 21, 2014 at 10:53

0xBADF00D

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2 Comments

xSF.AzraeL Over a year ago

Rot13 is too common and simple. I was looking for something that does not look anything like the input, and a bit more advanced.

0xBADF00D Over a year ago

You can define a dictionary that replaced one digit (if you only transmit digits) to another character. Because there are only 10 digits but a lot more usable characters, you can replace one digits by different characters and make at look more random. Bu this is also a weak algorithm.

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