DetectiveBoev.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class DetectiveBoev
{
    public static int SumDigits(int value)
    {
        int sum = 0;
        while (value != 0)
        {
            int rem;
            value = Math.DivRem(value, 10, out rem);
            sum += rem;
        }
        return sum;
    }

    public static string Reverse(string s)
    {
        char[] charArray = s.ToCharArray();
        Array.Reverse(charArray);
        return new string(charArray);
    }

    static void Main(string[] args)
    {
        string codeword = Console.ReadLine();
        string message = Console.ReadLine();
        //string message = "|wfvohyfqX#hodzwclV";
        int[] codewordvalues = new int[codeword.Length];

        for (int c = 0; c < codeword.Length; c++)
        {
            codewordvalues[c] = Convert.ToInt32(codeword[c]);
        }

        int codevaluesum = codewordvalues.Sum();
        int mask = codevaluesum;
        while (mask > 9)
        {
            mask = SumDigits(mask);
        }

        //  Console.WriteLine("Mask value: " +mask);     

        int[] messagevalues = new int[message.Length];
        for (int i = 0; i < message.Length; i++)
        {
            messagevalues[i] = Convert.ToInt16(message[i]);
        }
        //int ca = 0;
        //do
        //{
        //    ca++;
        //    messagevalues[ca] = Convert.ToInt16(message[ca]);
            
        //    // Console.WriteLine(message[ca] + "=" + messagevalues[ca]);
        //}
        //while (ca < message.Length - 1);

        int[] dectyptmessagevalues = new int[message.Length];

        for (int c1 = 0; c1 < message.Length; c1++)
        {
            if (messagevalues[c1] % mask == 0)
            {
                dectyptmessagevalues[c1] = messagevalues[c1] + mask;
            }
            if (messagevalues[c1] % mask != 0)
            {
                dectyptmessagevalues[c1] = messagevalues[c1] - mask;
            }
        }
        char[] encodedmessage = new char[message.Length];

        for (int count = 0; count < message.Length; count++)
        {
            encodedmessage[count] = Convert.ToChar(dectyptmessagevalues[count]);
        }

        string finalmessage_reverse = new string(encodedmessage);
        // Console.WriteLine(finalmessage_reverse);
        string final_result = Reverse(finalmessage_reverse);
        Console.WriteLine(final_result);

    }
}