For this challenge, you simply had to reverse DotNet.exe using an automated tool (I used ILSpy). Then, in the main function, you could see the following code:
Console.WriteLine("Greetings challenger! Step right up and try your shot at gaining the flag!");
Console.WriteLine("You'll have to know the pascode to unlock the prize:");
string value = Console.ReadLine();
long num = Convert.ToInt64(value);
long num2 = 53129566096L;
long num3 = 65535655351L;
if ((num ^ num2) == num3)
{
Console.WriteLine("yay");
}
else
{
Console.WriteLine("Incorrect, try again!");
}
You can clearly see that the passcode needs to be num3 ^ num2 which is 13371337255.
And there’s the flag: flag{I'll create a GUI interface using visual basic...see if I can track an IP address.}