Encryption/Decryption crash

Question

I have to write a "encryption" program for my C# intro class. I'm encountering 2 problems:

When a negative is entered or it calculates a negative during the decryption process, it crashes. - it returns and unhandled exception error: System.FormatException: Input string was not in a correct format.
The decryption should be the reverse of the encryption without using the original variable. How do you reverse a remainder calculation?

I know it's part of the do while loop that's the issue, I'm just not sure how to continue to prompt for a value if a negative is entered and if encountered during the decryption calculation to keep it from crashing. Any guidance would be much appreciated. Thanks for taking a look at it!

public class MainClass
{
  public static void Main(string[] args)
    {
    int num = 0;
    int dnum = 0;
      do
      {
      Console.Write("Please enter a non-negative integer to encrypt: ");
  num = Convert.ToInt32(Console.ReadLine());
      string numstr = Convert.ToString(num);
      string num1 = Convert.ToString(numstr.Substring(0, 1));
      string num2 = Convert.ToString(numstr.Substring(1, 1));
      string num3 = Convert.ToString(numstr.Substring(2, 1));
      string num4 = Convert.ToString(numstr.Substring(3, 1));
      int enum1 = ((Convert.ToInt32(num1) + 7) % 10);
      int enum2 = ((Convert.ToInt32(num2) + 7) % 10);
      int enum3 = ((Convert.ToInt32(num3) + 7) % 10);
      int enum4 = ((Convert.ToInt32(num4) + 7) % 10);
      Console.WriteLine("Encrypted Integer: {0:D4}", (1000 * enum3 + 100 * enum4 + 10 * enum1 + enum2));

      Console.Write("Please enter a non-negative integer to decrypt: ");
      dnum = Convert.ToInt32(Console.ReadLine());
      string dnumstr = Convert.ToString(dnum);
      string num1d = Convert.ToString(dnumstr.Substring(0, 1));
      string num2d = Convert.ToString(dnumstr.Substring(1, 1));
      string num3d = Convert.ToString(dnumstr.Substring(2, 1));
      string num4d = Convert.ToString(dnumstr.Substring(3, 1));
      int dnum1 = ((Convert.ToInt32(num1d) - 7) * 10);
      int dnum2 = ((Convert.ToInt32(num2d) - 7) * 10);
      int dnum3 = ((Convert.ToInt32(num3d) - 7) * 10);
      int dnum4 = ((Convert.ToInt32(num4d) - 7) * 10);
      Console.WriteLine("Decrypted Integer: {0:D4}", (1000 * dnum1 + 100 * dnum2 + 10 * dnum3 + dnum4));
        } while (num > 0);
   } // end Main
}// end class

Simply check for a negative number and inform the user they dont know how to read your instructions - in a polite manner of course. — Wjdavis5, Sep 24 '14 at 00:51
@Heather "it crashes" is never a sufficient explanation of your problem. You **need** to provide more information about the crash. You should be running you program under the debugger, which will give you lots of tasty information about the crash (such as the call stack), that you should reproduce in your question. It makes it considerably easier to get a good answer to your question when you include such information. — spender, Sep 24 '14 at 01:00
@spender - Good point! Sorry, I'm new to this site and asking for help in this area. If a - is entered, it returns and unhandled exception error: System.FormatException: Input string was not in a correct format. The problem I'm having is I have to account for a negative even though it's probably not real world. — Heather, Sep 24 '14 at 01:14

score 0 · Accepted Answer · answered Sep 24 '14 at 03:10

The policy is, do not proceed until user enters a 'correct' input. Here is code sample, note that I use numstr[0] - an index to get the first char instead of numstr.Substring(0, 1) so the code looks cleaner.

        int num = 0;
        bool isValid = false;
        do
        {
            Console.Write("Please enter 4 digits to encrypt: ");
            string numstr = Console.ReadLine();
            if (numstr.Length == 4)
            {
                isValid = Char.IsDigit(numstr[0]) && Char.IsDigit(numstr[1]) 
                       && Char.IsDigit(numstr[2]) && Char.IsDigit(numstr[3]);
            }
            if (isValid)
                num = Convert.ToInt32(input);
        }
        while (!isValid);

As for your 2nd question, you can't use multiplication to reverse a remainder calculation ((d+7)%10), you should again use remainder operator (d+10-7)%10, the additional 10 is added to keep it from getting negative result.

There is another bug in your decryption process, you can turn to your debugger for help.

Encryption/Decryption crash

1 Answers1