Serial key generation xor string encryption

Question

When I do an xor on a string I get special characters

private void btnEncryptDecrypt_Click(object sender, EventArgs e)
    {
        byte[] bytes = Encoding.UTF8.GetBytes(keyTextBox.Text);
        textTextBox.Text = string.Empty;

        foreach (byte i in bytes)
        {
            byte c = Convert.ToByte(i ^ Convert.ToByte(textBox1.Text));

            textTextBox.Text = textTextBox.Text + Convert.ToChar(c);

        }         

    }

62FA7AC4 1234567890 xor to !%QV VT#7&%$#"! /.'

I want to use it for serial key generation with only alphanumeric characters I want to pass "62FA7AC4", a date and a number between 0 and 5000 and a few random dummy numbers

Input will be "62FA7AC4"+"2500"+"21/05/2018"

Output should be something like "5QBK-J4D1-8CF6-5MW2-78FZ-2FPL-4S6S-CTGB"

What am I doing Wrong?

Answer 1

Try working with numbers, not strings:

  using System.Numerics;

  ... 

  // Let's use BigInteger for arbitrary long values
  BigInteger left = BigInteger.Parse("62FA7AC4", NumberStyles.HexNumber);
  BigInteger right = BigInteger.Parse("1234567890", NumberStyles.HexNumber);

  string result = (left ^ right).ToString("X");

  Console.Write(result);

So you have

  1256AC0254

Edit: As far as I can see you want alphanumeric (i.e. Base 36 == 26 letters + 10 digits output). You can use the same approach: operate with integer(s), not string(s)

  private static Random s_Random = new Random();

  ...

  BigInteger value = BigInteger.Parse(
      "0"        +                             // we don't want negative values  
      "62FA7AC4" +                             // header 
       s_Random.Next(5001).ToString("0000") +  // random in [0..5000] range
       DateTime.Today.ToString("ddMMyyyy"),    // Date like 21052018
    NumberStyles.HexNumber);                    

Then do any xor (if you like):

 value ^= some_secret_value;

Finally represent the value in base 36:

private static String ToBase36(BigInteger value) {
  List<char> list = new List<char>();

  for (int index = 0; value > 0; value /= 36, index++) {
    if (index > 0 && index % 4 == 0)
      list.Add('-');

    BigInteger v = value % 36;

    list.Add(v < 10 ? (char)('0' + v) : (char) ('A' + v - 10));
  }

  list.Reverse();

  return string.Concat(list);
}

Test:

 BigInteger value = BigInteger.Parse(
    "0" + 
    "62FA7AC4" + 
    "2500" + 
     DateTime.Today.ToString("ddMMyyyy"), 
   NumberStyles.HexNumber);

 string result = ToBase36(value);

 Console.Write(result);

Outcome:

2443-WNC5-AVBB-M32W

Edit 2: To restore the original number

private static BigInteger FromBase36(string value) {
  BigInteger result = 0;
  BigInteger power = 1;

  for (int i = value.Length - 1; i >= 0; --i) {
    char item = value[i];

    if (item >= '0' && item <= '9') {
      result += power * (item - '0');
      power *= 36;
    }
    else if (item >= 'A' && item <= 'Z') {
      result += power * (item - 'A' + 10);
      power *= 36;
    }
    else if (item >= 'a' && item <= 'z') {
      result += power * (item - 'a' + 10);
      power *= 36;
    }
  }

  return result;
}

e.g.

BigInteger value = BigInteger.Parse(
    "0"        + 
    "62FA7AC4" + 
    "2500" + 
     DateTime.Today.ToString("ddMMyyyy"), 
   NumberStyles.HexNumber);

string result = ToBase36(value);

BigInteger back = FromBase36(result);

Console.WriteLine(string.Join(Environment.NewLine, value, result, back));

Outcome:

467412447575903165554712
2443-WNC5-AVBB-M32W
467412447575903165554712