Encryption/Decryption crash

Question

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

1. 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.
2. 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

Answer 1

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.