vigenere cypher, C#

vigenere cypher

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        static int mod(int a,int b)
        {
            int abs = a < 0 ? -a:a;
            return a < 0? b - abs:(a > b? a - b:a);
        }
        public static void Main(string[] args)
        {
            int lc = 'a'; 
            string key = Console.ReadLine().ToLower();//line 0 of input
            string txt = Console.ReadLine().ToLower();//line 1 of input
            /*{
                string pr = txt;
                do{pr = Console.ReadLine();txt += '\n' + (pr != "" ? pr:"");}while (pr != "");
            }*/
            int ind = 0;
            foreach(char c in txt)
            {
                int ke = key[ind] - lc;
                int ch = mod(ke - (c - lc),26);
                if (char.IsLetter(c)){Console.Write((char)(ch + lc)); ind ++;}else{Console.Write(c);}
                ind = ind % key.Length;
            }
        }
    }
}


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