gistfile1.cs

using System;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        string maskString = Console.ReadLine();

        int maskAsInt = 0;
        for (int i = 0; i < maskString.Length; i++)
        {
            maskAsInt += (int)maskString[i];
        }

        int maskInt = getOnlyNumber(maskAsInt);

        string encrypted = Console.ReadLine();
        StringBuilder output = new StringBuilder();

        for (int i = 0; i < encrypted.Length; i++)
        {
            if ((int)encrypted[i] % maskInt == 0)
            {
                output.Append((char)((int)encrypted[i] + maskInt));
            }
            else
            {
                output.Append((char)((int)encrypted[i] - maskInt));
            }
        }

        for (int i = 0; i < output.Length; i++)
        {
            Console.Write(output[output.Length - 1 - i]);
        }
        Console.WriteLine();
    }

    private static int getOnlyNumber(int maskString)
    {
        int sum = 0;

        while (maskString > 0)
        {
            sum += maskString % 10;
            maskString /= 10;
        }

        if (sum >= 10)
        {
            return getOnlyNumber(sum);
        }

        return sum;
    }
}