Vigenère_cipher encode

using System;
using System.Text;

namespace Vigenère_cipher
{
    class Program
    {
        static void Main(string[] args)
        {
            const string key = "REDDIT";
            const string message = "TODAYISMYBIRTHDAY";

            string encoded = Encode(key, message);
        }

        private static string Encode(string key, string message)
        {
            var encoded = new StringBuilder(message.Length);

            int keyIndex = 0;

            for (int i = 0; i < message.Length; i++)
            {
                int keyValue = CreateAlphabeticalValueFromChar(key[keyIndex]);
                int messageValue = CreateAlphabeticalValueFromChar(message[i]);

                int encodedValue =  1+ (keyValue + messageValue) % 26;

                encoded.Append(CreateAlphabeticalValueFromInt(encodedValue));

                keyIndex = (keyIndex + 1)%key.Length;
            }

            return encoded.ToString();
        }

        private static int CreateAlphabeticalValueFromChar(char character)
        {
            return Char.ToLower(character) - 'a';
        }

        private static char CreateAlphabeticalValueFromInt(int character)
        {
            return (char) (('a' +  character)-1);
        }
    }
}