Vigenere cipher C#

using System;

namespace ConsoleApp10
{
    class Program
    {
        public static void Main()
        {
            char[] s = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };
            string key = "b";
            for (int i = 0; i < s.Length; i++)
            {
                VigenereEncrypt(s, key);
                Console.WriteLine(s);
            }

            Console.WriteLine();

            for (int i = 0; i < s.Length; i++)
            {
                VigenereDecrypt(s, key);
                Console.WriteLine(s);
            }

        }

        static void VigenereEncrypt(char[] s, string key)
        {
            for (int i = 0; i < s.Length; i++) s[i] = Char.ToUpper(s[i]);
            key = key.ToUpper();
            int j = 0;
            for (int i = 0; i < s.Length; i++)
            {
                if (Char.IsLetter(s[i]))
                {
                    s[i] = (char)(s[i] + key[j] - 'A');

                    if (s[i] > 'Z')
                    {
                        s[i] = (char)(s[i] - 'Z' + 'A' - 1);
                    }
                }

                j += 1 == key.Length ? 0 : j + 1;
            }
        }

        static void VigenereDecrypt(char[] s, string key)
        {
            for (int i = 0; i < s.Length; i++)
                s[i] = Char.ToUpper(s[i]);
                key = key.ToUpper();
            int j = 0;
            for (int i = 0; i < s.Length; i++)
            {
                if (Char.IsLetter(s[i]))
                {
                    s[i] = s[i] >= key[j] ? (char)(s[i] - key[j] + 'A') : (char)('A' + ('Z' - key[j] + s[i] - 'A') + 1);
                }

                j +=1 == key.Length ? 0 : j + 1;
            }
        }
    }
}